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ACKNOWLEGDEMENT
First and fore most I would like extend my heartfelt
gratitude to the Management, Munnar Catering
College, Sooryanelli, Munnar for giving me the
opportunity for studying the post in the college.
I would like to express my sincere thanks to our
respected principal and all my respected faculties of
Munnar Catering College for their support and
encouragement.
I would like to express my sincere thanks to Mr.
Charles Jayakumar and project co-ordinator for his
kind support and guidance rendered for the
preparation for this project report
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DECLARATION
I, GIVEES BIJU JACOB, student of Munnar Catering
College, Kerala hereby declare that the project
report entitled on FOOD PRODUCTION. Submitted to
NCHMCT partially fulfillment of the requirements of
the award of degree of B.Sc in HHA is a record of the
guidance of Mr. CHARLES JAYAKUMAR during the
academic year of 2014-2017
INSTITUTE: - MUNNAR CATERING COLLEG
MUNNAR
DATE :- GIVEES BIJU JACOB
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YEAST
INTRODUCTION
Yeasts are actually microbial eukaryotes which belong to as comycetes
that are good source of vitamin Band protein. Yeasts are plant-like unicellular
fungith riving on every living organism. Being living organism fungi require
warmth, water, albumen or nitrogenous material and sugars to remain alive
Yeasts are typically spherical, oval or cylindrical in shape and a single cell
of Saccharomyces. Cerevisiae (a mold which ferments the sugar in cereal) is
around 8 µm in diameter. Every cell has a double-layered wall, which is
porous to certain substances and in this way food fabric is taken into the cell
and metabolites leave it. Yeast is made up of many tiny, single-celled plants,
which grow by budding and each bud breaking away from the parent cell and
forming new buds. Though most yeast replicate only as single cells, under a
number of circumstances some yeasts can figure out as filaments.
The conditions required for growth are warmth (optimum 25-30 °C),
moisture and food (starch plus a small amount of sugar). Refrigeration slows
down the growth so that yeast can be kept for a limited period of time. When
the yeast is used, the conditions and the utensils should be kept lukewarm to
obtain the best results. As soon as the yeast has been added to the dough or
batter, the yeast begins to feed on the starch in the mixture, forming sugar,
alcohol and carbon dioxide. The bubbles of CO2 cause the dough to expand.
The dough must be "kneaded" thoroughly to distribute the bubbles evenly and
then left to rise again, usually to about double its original volume.
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If the mixture is left too long, acid produced by the oxidation of the alcohol
results in taste sour of the product.
Yeasts thrive in habitats where sugars are present, such as fruits,
flowers and bark of trees. However, saleable yeasts of today are fairly
different from wild strains due to genetic treatment, allowing them to grow in
inappropriate situations. The enzymes which are created by the yeast cells
and act as natal catalysts in the fermentation process are maltase, invertase
and zymase complex. Maltase has the aptitude to alter maltose, which is
formed by starch degradation by alpha- and beta-amylases, to glucose and
acts when the supply of simple sugars has been bushed. Invertase converts
sucrose to glucose and fructose, while the doings of the zymase complex
fallouts in the change of glucose, fructose and other simple sugars into carbon
dioxide and ethanol. It is the carbon dioxide which raises the dough during
fermentation .
Properties of yeast
Preservatives are commonly used in breads because economic losses
from bread spoilage caused by bacteria or by moulds are substantial. Ropy
spoilage is caused mainly by Bacillus subtilis and Bacillus licheniformis, the
spores of which contaminate raw materials such as flour, bread improvers,
yeast, etc., and survive baking temperatures .
Ropy spoilage in bread is first detected by an odour similar to that of
pineapple. Later, the crumb becomes discoloured, soft and sticky to the touch,
which makes the bread inedible. The deterioration of bread texture is due to
slime being formed as a result of the combined effect of the proteolytic and
amylolytic enzymes produced by some bacillus strains that results in slime
formation.
The full extent of losses caused by ropy spoilage of bread is difficult to
quantify, because the condition is often misidentified as sour or rotten
spoilage caused by failed dough leavening or an insufficient bake.
Consumption of ropy bread may cause illness if bacteria are present at ≥108
cfu/g . Ropiness can develop very rapidly under warm and humid conditions,
so it is a common problem in the warm climates of Mediterranean countries,
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Africa and Australia . Bacillus spore numbers can be controlled by ensuring
raw material quality, good sanitation and cooling of production and storage
environments. Spore germination and growth in bread can be inhibited by
chemical preservatives such as propionic and acetic acids, although the
current trend is to reduce the levels of these substances. Acetic acid adversely
affects the organoleptic quality of baked products, while propionic acid has
been reported to cause irritability, restlessness, inattention and sleep
disturbance in some children
Alternative antimicrobial systems to prevent bread spoilage are therefore
required. The yeast used for bread manufacturing is Saccharomyces
cerevisiae, often referred to as simply baker’s yeast. It converts the
fermentable sugars present in the dough into carbon dioxide and ethanol as
the main products.
The fermentation intensity depends on the form of the yeast and the
availability of fermentable sugars in the flour, including maltose produced by
starch hydrolysis. During the bread-making process, baker's yeast is
uncovered to many environmental stresses such as air- drying, freeze–thaw,
and high-sucrose concentrations. Yeast cells worn for bread making must
acclimatize to different sucrose concentrations during dough-fermentation
processes. In exacting, sweet dough (high-sugar
dough) contains up to roughly 30 % sucrose per
weight of flour. Such high- sucrose
concentrations apply harsh osmotic stress that
badly damages cellular mechanism and hold
back the optimal fermentation aptitude of yeast.
To evade lethal injury, baker's yeast cells want to
get osmotolerance, but the progress of
osmotolerant baker's yeast strains will require knowledge of the molecular
mechanism concerned in high-sucrose stress lenience, for example, by the
introduction of stress proteins, the buildup of stress protectants, and the
variations in membrane composition.
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When elevated osmotic pressure is felt, S. cerevisiae cells collect
glycerol and trehalose. Microarray examination and genome-wide screening
using a removal strain group exposed that the metabolism of glycerol and
trehalose, both of which are recognized as osmoprotectants, is significant for
high-sucrose stress tolerance. In reply to osmotic stress, proline is
accumulated in many plant and bacterial cells as an osmoprotectant . During a
variety of stresses, yeast cells encourage glycerol or trehalose production, but
the intracellular proline level is not augmented under a range of stress
circumstances. Proline has many functions in vitro, such as protein and
membrane stabilization, decreasing the Tm of DNA, and scavenging of hasty
oxygen species (ROS), but the mechanisms of these functions in vivo are not
well understood. Sacharomyces cerevisiae cells that collect proline, and the
engineered strains effectively indicated improved lenience to many stresses,
counting freezing, desiccation, oxidation and ethanol. With respect to high
osmotic pressure, it was found that the proline oxidase-deficient strain, which
had a considerably elevated proline level, was obviously more osmotolerant
than were other strains in the existence of 1 M NaCl . Recently, it was found
that proline accumulating baker's yeast retained higher-level fermentation
aptitude in the frozen dough than that of the wild-type strain. Based on these
results, it is concluded that it is possible that proline collection confers
tolerance to high- sucrose stress on baker's yeast. For the application of
recombinant yeasts for marketable use, self-cloning yeast that has no foreign
genes or DNA sequences apart from yeast DNA might be more satisfactory for
consumers than a genetically modified yeast.
There is no doubt that folate (vitamin B9) has a vital role in primary cell
processes, such as nucleic acid and amino acid biosynthesis. Inadequate folate
intake may lead to the typical folate insufficiency disease megaloblastic
anaemia and greater risks for neural tube defects as well as other
malformations. In addition, the useful role of folate for more than a few other
diseases such as cardiovascular diseases , Alzheimer's disease and some forms
of cancer is under careful examination.
Humans are, in contrast to yeasts and plants, auxotrophic for this
vitamin and must therefore satisfy their needs by the diet. For a large portion
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of humankind though, it is very tough to sustain the daily intake on sufficient
levels. One striking idea to boost folate intake is to employ biotechnology to
improve the concentration of ordinary folates in food-as opposed to
supplement food by using man-made folates or use supplementation by
tablets.
Baker's yeast, Saccharomyces cerevisiae, has been found to generally
contain a relatively high amount of folate per weight. Seyoum and Selhub
described a total folate content of 24.5 µg/g of dry matter of yeast while
Patring and Jastrebova reported 35.2 µg/g. Folates from yeast obviously add
to the finishing folate content in yeast fermented foodstuffs, such as bread and
kefir In wheat bread folate levels were improved 2.5 times when using yeast,
in place of baking powder, as leavening agent .
The Story of Yeast
Yeast in History
Man used yeast before he knew how to write. Hieroglyphics suggest that
the ancient Egyptian civilizations were using living yeast and the process of
fermentation to cause their bread to rise over 5,000 years ago. Of course, they
didn’t know what was responsible for the leavening process, and probably
l looked upon the chemical action of yeast as a
mysterious and unreal phenomenon. Leaven,
mentioned in the Bible, was a soft, dough-type
medium kept from one bread baking session to
another. A small portion of this dough was used to
start or leaven each new lot of bread dough.
It is believed that since early times, leavening
mixtures for bread making were formed by natural
contaminants in flour such as wild yeast and
lactobacilli, organisms also present in milk.
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Yeast Today
Centuries later, scientific research revealed that yeast is actually a tiny
microorganism, visible only with a microscope. The chemical action and
growth of yeast that causes dough to rise then became apparent.
What Is Yeast?
Yeast is a tiny form of fungi scientists refer to as “microorganisms”. They
are egg -shaped cells that can only be seen with a microscope. It takes
20,000,000,000 (twenty billion) yeast cells to weigh one gram or 1/28 of an
ounce.
A tiny organism with a long name
The scientific name for one species of yeast is SACCHAROMYCES CEREVISIAE,
or sugar-eating fungus. This name is derived from the Latin word “cerevisiae”,
which means “brewer”. A very long name for such a tiny organism! This strain
of yeast is very strong and capable of fermentation, the process that causes
bread dough to rise.
A fungus with a sweet tooth
Yeast cells digest food to obtain energy for growth. Their favorite food is sugar
in its various forms: sucrose (beet or cane sugar), fructose and glucose (found
in honey, molasses, maple syrup and fruit), and maltose (derived from starch
in flour).
The process, alcoholic fermentation, produces useful end products, carbon
dioxide and ethyl alcohol, which are released by the yeast cells into the
surrounding liquid. This is how alcoholic drinks are produced from starch
containing flours. For example, barley flour is used for making beer and
wheat, corn and other grains are used for making whiskey.
Fermentation in nature
Fermentation occurs naturally in nature. For instance, many berries break
open in late fall when they are overripe and full of sugar. Natural yeast
organisms, so small they cannot be seen with the naked eye, lodge on the
surface of these berries, which then become fermented and alcoholic.
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Wine and bread making
In commercial fermentation of grape juice for the production of wine, the
carbon dioxide gas escapes from the solution. Evidence of gas can be seen in
the heavy foam caps in fermenting wine tanks.
In bread baking, when yeast ferments the sugars available from the flour and
from added sugar, the carbon dioxide gas cannot escape because the dough is
elastic and stretchable. As a result of this expanding gas, the dough inflates.
Thus, the term “yeast-leavened breads” was added to the vocabulary of the
world of baking.
Types of Yeast
When you hear the word “yeast”, what do you think of? No doubt you think of
the type of yeast used in baking breads. However, through the selection of
strains and development of propagation techniques, more specific
applications of yeast are now being found in
many different industries, including
brewing, malting, farming (animal feeds),
pharmaceuticals and dietetics.
what is Baker’s Yeast?
Baker’s yeast is the type of yeast used in
home and commercial bread baking. It is widely available in a number of
forms, including Compressed Yeast, Active Dry Yeast and Quick-Rise Yeast.
Baker’s Compressed Yeast
This yeast is sold to the commercial and retail baking industry throughout the
United States. It comes in one pound and five pound cakes and crumbled fifty
pound bags. In order to achieve the solid formulation, “cream yeast” is
pumped into presses where the excess water is removed. Once pressed, the
resulting “cake yeast” is transferred to mixers, to assure uniformity, and then
to extruders where the proper lengths and weights are cut. After the cakes are
wrapped or bagged, they are stored in refrigerated rooms to await shipment.
Compressed Yeast is also called “wet yeast” or “fresh yeast”. It is
traditionally sold to consumers in two sizes: 2 ounce and 8 ounce blocks. The
yeast is characterized by a high moisture content (about 70%). It is perishable
10. 10
and should be stored under refrigeration at all times. The shelf life is about 8
weeks from packaging.
Active Dry Yeast
This yeast is processed one step further than Compressed Yeast. It is extruded
into noodle form, loaded onto a conveyor belt and passed through a series of
drying chambers where warm air is blown through the yeast. The yeast
emerges with a moisture content of about 8% as compared to 70% moisture
in Compressed Yeast. Due to the low moisture content, the yeast is in a semi-
dormant state. Therefore, it can stand long periods of exposure with little
effect on its ultimate baking activity. The packages are nitrogen flushed to
extend the shelf life which is about one year from packaging. This yeast is sold
in a 3-pack strip and a 4-ounce jar.
Quick-Rise Yeast
This is a high-activity yeast strain created by protoplast fusion, the scientific
technique of combining two separate yeast strains into a superior single
strain. The manufacturing process is the same as for Active Dry Yeast, except
that ascorbic acid is added as a dough conditioner or enhancer. It is also
available in nitrogen flushed 3-strip packages and 4-ounce jars.
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Ingredients Affecting Fermentation
FLOUR
White or whole grain flours may be used for making yeast breads. When
mixed with liquid and kneaded, flour develops enough gluten to support the
carbon dioxide produced by the yeast. Gluten is the elastic substance formed
when the protein of flour is exposed to liquids. Kneading develops the gluten,
making it stronger so it can hold in the gases formed by the action of yeast.
The gluten then stretches, trapping the bubbles and building the framework of
the bread.
FAT
Fats such as butter, margarine, shortening, oils and cheese are used in breads
to add tenderness, moisture and flavor. They also make the gluten strands
slippery so the yeast gases can expand easier.
LIQUIDS
Water and milk are the most commonly used liquids in breads. Breads made
with water only will have a more open texture, a more wheaty flavor and a
crispier crust. Milk creates breads which are richer and have a velvety texture;
crusts are softer and will brown more quickly due to the sugar and butterfat
in milk.
SUGAR
Sugar provides food for the yeast to grow, adds flavor and helps in the
browning of the crust. However, if the concentration is too high, more yeast or
longer proofing times may be required.
SALT
Salt controls the speed at which the dough rises by reducing the osmotic
pressure of the dough and controlling the gasses power of the yeast. Salt also
strengthens the structure of the dough and adds the necessary flavor to a
baked product.
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NOTE: Bread flavor is formed in the crust as it reaches a temperature of 302°
to 356°F while the internal bread temperature does not exceed 210°F. The
higher temperature causes the sugar in the dough to caramelize, creating a
fruity or winy odor. Removal of the crust too soon after baking prevents the
crumb from absorbing the crust flavor, causing it to taste different
*** Because the yeast is surrounded by flour, it is able to tolerate higher
liquid temperatures.
Temperature Requirements of Yeast
Type of Baker’s Yeast
Compressed Yeast
Active Dry Yeast
Quick-Rise Yeast
Yeast Dissolved Directly in
Liquids
80 - 90°F
Yeast Mixed First with Flour***
-----
110 - 115°F
110 - 115°F
120 - 130°F
120 - 130°F
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What is Nutritional Yeast?
Nutritional Yeast is powdered yeast without leavening power, marketed for
its protein and vitamin content. This type of yeast is available in both powder
and pill form.
How nutritional yeast is made
“Cream yeast” is heated by means of a heat exchanger and held at
pasteurization temperatures for a period long enough to kill the yeast. During
this holding period, all necessary vitamins are added to meet the
requirements of the specific type of nutritional yeast produced. The yeast is
then drum dried before it is ground and shipped to consumers. The drying
process assures that all the cells are killed in order for the full nutritional
benefits to be available.
What Are Nutritional Yeast Flakes?
Nutritional yeast is sourced from whey, blackstrap molasses or wood pulp.
“Nutritional yeast is yeast that is grown on molasses or a similar habitat, much
like brewer's yeast, a byproduct from producing beer,” says Brandice Lardner,
an NASM-certified personal trainer and nutrition coach with One by One
Nutrition. “When the yeast is harvested, it is washed and then dried with heat
so that it stops growing.” After this heat treatment, it’s crumbled or flaked and
packaged for the consumer.
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The Basic Nutritional Yeast Making Process
A) “Seed yeast” is grown in small flasks
B) “Seed yeast” is transferred to 1,000 gallon tanks for fermentation (becomes “stock yeast”).
C) Alcohol from fermentation separated from “stock yeast”
D) “Stock yeast” moved to refrigerated tanks (“Trade Fermenter”) for fermentation cultivation
E) Sterilized molasses, air and nutrients are added to “stock yeast”
F) When yeast is ready to be harvested, fermented yeast liquid is passed through a
“Separator” to produce “cream yeast”
G) “Cream yeast” is heated and pasteurized, necessary vitamins are added
H) Vitamin-enhanced “cream yeast” is cooled
I) “Cream yeast” is drum dried
J) “Cream yeast” is put through a grinder to produce a fine powder
K) Nutritional yeast is packaged and ready for shipping
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What is Brewer’s Yeast?
Brewer’s Yeast is a dried, inactive yeast that has no fermenting power. It is
sold for its nutritional qualities as it is very high in at least 10 separate B-
vitamin factors, including:
Thiamin Biotin
Riboflavin Choline
Niacin Inositol
Pyridoxine Inositol
Pantothenic Acid Folic acid
Paraminobenzoic Acid
Brewer’s yeast is a by-product of the brewing
industry. After 5-10 succeeding beer
fermentations, the yeast, due to increasing
contamination, loses its viability and activity and is
no longer acceptable for making beer. The yeast then becomes surplus and
can be used for the production of food flavors, feed formulations or as
nutritional yeast food. Over the years, the term “brewer’s yeast” has become
generic. Primary grown baker’s yeast (not a by-product of the same strain of
yeast used by bakers to make bread) is often sold as brewer’s yeast because
the term is familiar to the consumer. The processing and drying of this yeast is
carefully controlled so it remains inactive, making it easily digestible and
yielding valuable amounts of B- complex vitamins and protein for
assimilation.
Another form of brewer’s yeast is labeled as “DEBITTERED Brewer’s Yeast”,
due to neutralization of the bitter flavor of hops. This form of yeast is almost
always certain to be brewer’s yeast. However, due to a lack of standards
governing this particular label, product packaging may claim to contain
“brewer’s yeast” and actually contain inactive baker’s yeast.
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ADVANTAGES AND DISADVANTAGES OF YEAST
ADVANTAGES
Products are more digestible because there are microorganisms that can
decompose flour and gluten more completely (reducing also glycemic index
and postprandial insulin level), removing creation of molds (so products last
longer)
Mother yeast makes vitamins and minerals from the flour more assailable
than a buy yeast; products release more fragrance; products have a wonderful
natural taste.
DISADVANTAGES
I think there’s only one disadvantage about using mother yeast and it’s the
long rising time.
But as Balzac said, “All human power is a compound of time and patience”, so
we head back at what is natural, smell genuine fragrance and flavors that our
grandparents used to have every day.
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Experimental structure on fermentation
Henry and Saini described that the most significant carbohydrates from
flour influencing the loaf volume are glucose, fructose and sucrose. The
arrangement in which these different carbohydrates are fermented by
Saccharomyces cerevisiae is not at random, but rather is based on a specific
pecking order, glucose being the preferred sugar. It is considered that glucose
decreases the uptake of fructose because both sugars are imported by the
same carriers, which have a greater empathy for glucose than for fructose.
Of the above mentioned carbohydrates, sucrose is changed almost right
away to glucose and fructose, due to the effective invertase of yeast. When the
concentration of glucose and fructose is elevated enough, the maltose
concentration in dough is also mounted due to amylase, a starch debasing
enzyme in flour, which is continuously generating new glucose and maltose in
flour starch. When glucose and fructose are ended, the maltose concentration
begins to lessen, making difficult for yeast cells to hydrolyze since they do not
have the essential enzymatic tools in time, working methods and techniques
such as thin layer chromatography
MOLDS
Molds have both positive and negative effects on the food industry the same
way that yeasts do. Some molds are perfectly safe to eat and, in some cases,
even desirable (the classic example
would be cheese made with mold,
such as blue, Brie, Camembert, and
Gorgonzola). Other molds can be
quite toxic and may produce allergic
reactions and respiratory problems,
or produce poisonous substances
called mycotoxins. Aspergillus mold,
for instance, which is most often
found on meat and poultry (as well as
environmentally), can cause an
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infection called aspergillosis, which is actually a group of illnesses ranging
from mild to severe lung infections, or even whole-body infections. One of the
greatest concerns regarding mold in food is the mycotoxins that some
varieties produce. One of the most researched mycotoxins is aflatoxin, a
cancer-causing poison.
YEAST DOUGH PREPARATION
Yeast breads and rills can be prepared by traditional “hand “methods.
However, larger quantities and faster turnover times are often required. Yeast
breads and rolls can also be prepared through an bread making. The steps
involved in making yeast breads vary depending on the type of dough used
and the item being produced. However, the same general apply to all yeast
dough products.
Scaling ingredients.
Mixing and kneading.
Fermentation.
Dividing dough.
Rounding dough.
Bench rest.
Shaping dough.
Panning dough.
Final proofing.
Baking dough.
Cooling dough.
Packaging dough.
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Keeping the following quality guidelines in mind when producing yeast
breads and rolls:
Maintain personal cleanliness at all times.
Keep utensils, materials and machinery clean and I good working order.
Use the best quality ingredients.
Read all formulas carefully and measure ingredients.
Maintain the appropriate environmental temperatures.
Regulate dough temperature.
Serve only freshly baked and properly stored yeast products.
Mixing method
There are three basic methods of mixing yeast dough ingredients : the straight
dough method, and the sponge method. Each of these methods gives its own
characteristics to the finished product. Each method also affect the activity of
the yeast and the formation of the gluten.
Straight dough method
You will use the straight dough methods to mix the ingredients for most basic
breads. The straight dough methods call for mixing all the ingredients
together in a single step. Ingredients may be mixed by hand or with a bench
mixer.
In dough mixed by the straight dough method, the yeast begins acting on all
the ingredients immediately. As you continue mixing or working the dough,
the glutens develops
Modified straight dough method
The modified straight dough method breaks the straight dough methods into
steps. These steps allow for a more even distribution of sugar and fats
throughout the dough. This modification is commonly used when preparing
rich dough.
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Dissolve the yeast in part of the water
Combine the fat, sugar, milk, solids, and flavorings.
Mix well, but do not whip
Add eggs one at a time, as they are absorbed into the mixture.
Add the rest of the liquids and mix briefly.
Add the flour and the dissolved yeast last.
Mix until a smooth dough forms.
Sponge methods
Some yeast products, such as crusty
hearth breads or sweeter dough,
benefit from the yeast to develop
separately before it is mixed with the
other ingredients. This method results
in a more intense flavor and lighter,
airy texture. The sponge method make
a very soft, moist and absorbent
dough.
Here are the basic steps:
Combine 50% water with 50% flour.
Add to this mixture to promote faster yeast growth
Baking yeast dough
Baking is the process that changes dough into breads or
rolls through the application of heat. Oven temperature
and baking time are determined by five factors.
Dough type : young, under fermented dough
required cooler oven temperature , higher humidity ,
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and longer baking times that fully proofed dough. Old , over fermented
dough require higher oven temperature, less humidity, and shorter
baking times.
Dough richness: lean dough requires higher oven temperatures, less
humidity, and shorter baking times.
Portion size: Smaller items, such as rolls require shorter baking times
that larger item, such as loaves.
Desired color: the desired color of the crust often depends on the tastes
of the customer. Higher oven temperatures and longer baking times
generally yield a darker crust color than lower temperature and shorter
baking times. An egg wash can add color to a crust that must be baked at
a low temperature or for a short time.
Weather : oven temperature may need to be adjusted to compensate for
less than ideal temperature and humidity condition during dough
preparation, altitude ,or the location of the baking site above sea level,
affects baking too the moisture in dough evaporates more slowly at
higher altitudes such as those found in mountainous area. Oven
temperature may be increased slightly to prevent the dough from
expanding too much and breaking down the cell structure in the bread
Formulas will list the ideal oven temperature and baking time. Slight
adjustment may be necessary. Appropriate placement of pan in the oven
is also important, air and heat must be allowed to circulate freely
around the pans. This can be accomplished by placing pans at the
appropriate distance from the heating element. Crowding the oven
slows baking time and result in unevenly baked items.
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Baking with steam
Breads with thin , crispy such as French and Italian loaves, benefits
from the addition of steam to the oven during
baking. The steam keeps the crumb soft while
adding a glossy shine to the surface. As the sugar
in the crust caramelize a thin, crispy is formed.
Some bakery ovens are equipped to inject a
desired amount of steam into oven for several
second depending on the type of bread and the
formula. In oven without steam injection a pan can
be added with just enough water so the water evaporates during the
early stages of baking.
Cooling & storing yeast products
Once a yeast dough product is removed from the oven it must be cooled
and stored properly to maintain the highest possible quality.
Remove yeast products
from their pans immediately .
Place them on cooling rack
or screens at room temperature . one
exception is rolls baking on sheet. These may
be left on the sheets to cool, if they are well
spaced
Cool yeast products
completely before slicing or wrapping
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Glazing : In some cases, you will brush melted butter or shortening or a
glaze onto a hot yeast dough product immediately after removing it
from the oven. Sweet dough products such as coffee cake and Danish
pastry may be glazed with a mixture of water and sugar or corn syrup
while they are still warm.
Staling prevention: yeast dough products begin the process of staling as
they are baked. Staling causes yeast dough products to lose their
freshness. During staling the crust becomes moist and tough while the
interior crumb of the bread to lose flavor. There are several procedures
for slowing the staling process.
Additions to dough: depending on the formula, ingredients
such as malt syrup may be added to the mixing process to
help slow staling. Commercial bakeries may also add
chemical such as monoglycerides and calcium propionates
to lengthen shelf life.
Avoid refrigeration : refrigeration speeds up the staling
process of yeast breads.
Proper packing and storage : do not warp product while they are still
warm. Most bread should not be kept for more than one day in a
foodservice operation. If you are keeping them longer than tightly in
moisture proof wrapping and store them in a freezer to prevent staling.
Warp items with thin, crisp crusts, such as French baguettes, in paper.
They will lose their characteristic crunchiness and becomes soggy if
wrapped I paper or plastic. Sweet dough products can be packaged in a
pastry box or wrapped in plastic.
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Role of yeast in bread making
Yeast-fermented breads in Europe and the United States are mostly based
on wheat, although rye is commonly used for some popular bread types in
Scandinavia and other northern European countries. A large variety of bread
is produced, and the European
tradition for consumption of bread
seems to spread over the world,
including regions like South-East
Asia and Africa, as a result of the
strong impact of European eating
habits. Traditional bread making in
Africa does not seem to be widely
known, but strong traditions exist
for fermentation of cereals, with
yeast playing a signifi- cant role,
especially in co-cultures with lactic
acid bacteria, as was also the case
in the past in Europe. For Sudan, 11 types of sorghum and millet bread are
described by Dirar and in other parts of Africa various cereal dough
fermentations, like kenkey made from fermented maize dough in Ghana, play
a substantial role in the daily food intake. A review on yeast in traditional
African food is given by Jespersen .
In most cases, the dominant yeast appears to be S. cerevisiae Meyen
ex E.C. Hansen with the taxonomic delimitation given by Vaughan- Martini and
Martini . It is considered the principal species responsible for cereal
fermentation and bread making as well as alcoholic fermentations, except for
fermentation of lager beer. Several methods based on molecular techniques
have been reported for subspecies typing of Saccharomyces spp., one of the
most popular methods has been determination of chromosome length
polymorphism
25. 25
As chromosome length polymorphism is evident among the isolates, the
technique clearly shows that several subspecies are involved in the
fermentation process. Further, the observed chromosome profiles are quite
similar to chromosome profiles observed for baker’s yeast. According to
general experience, the genomic stability of individual strains of baker’s yeast
appears to be high.
Other molecular methods which seem to be well ac- cepted for
species recognition and clarification of phylogenetic relationships are based
on sequencing of r DNA, such as the ribosomal internal transcribed spacer
(ITS) region, or sequencing of the Dl/D2 domain of the large subunit (26S) r
DNA. Functional genomics and proteomics are also valuable tools in
clarification of strain differentiation and elucidation of the relationship
between geno- and phenotypes.
Mixed cultures of yeast species may occur in bakeries depending on the
type of flour used and other conditions employed. Other than S. cerevisiae
species are of interest, because they may be better suited for applications
where S. cerevisiae cannot meet desired technological properties, like resis-
tance to osmotic stress, freezing and thawing . An alternative baker’s yeast
could be Kazachstania exigua (previously Saccharomyces exiguus) Rees ex
E.C. Hansen or its anamorph form Candida holmii (Jorgensen), according to
several investigations as reviewed by Jenson. Another candidate is
Torulaspora delbrueckii (Lindner) and its anamorph form Candida colliculosa
(Hartmann) because of its high
osmotolerance and resis-tance to freezing.
Candida krusei and its teleomorph form
Issatchenkia orientalis are often seen in
microbial successions with S. cerevisiae in
dough fermentations where it appears to
take over when the conditions become too inhibitory, probably too acid, to S.
cerevisiae. Molecular typing has indicated that the strains observed are
different from pathogenic strains of krusei , a point which should also be
considered when selecting new strains, although the yeast is supposed to be
killed during baking. Other indigenous fermented cereals in Africa and in
26. 26
smaller traditional bakeries in Europe which still rely upon sponta- neous
dough fermentations may be considered as sources for alternative cultures or
for cultures to be applied in mixed cultures with S. cerevisiae. The cultures
may not only be selected to ensure efficient bread leavening, but also for
thepurpose of adding aroma characteristics to the bread.
Tomer et a1 specifically investigated the effect on volatile compounds in
fermentation studies with S. cerevisiae and Candida guilliermondii without
specifying which of the two species varieties (var. guilliermondii or var.
membranifaciens ) was used. They concluded that S. cerevisiae produced the
highest number of aroma components and,in general, the yeast examined
produced more flavour components than the
lactic acid bacteria investigated. yeast, and
higher carbohydrates (dextrins) which S.
cerevisiae cannot ferment. Maltose is present at
the highest level and the faster the yeast can
ferment maltose, the faster the fermentation will
occur. Fast fermentation is one of the most
desired properties of baker’s yeast. Of the three
major sugars, glucose is preferentially utilised
by S. cerevisiae, but efficient fermentation
requires the rapid utilisation of both maltose
and maltotriose. Gene dosage studies performed with laboratory strains of
yeast have shown that the transport of maltose into the cell may be the
ratelimiting step in the utilisation of this sugar. Information on the maltose
and maltotriose trans- porter genes present in brewer’s yeast is therefore of
some value in selecting suitable strains and in predicting fermentation
performance. Maltose utilisation in S. cerevisiae is conferred by anyone of five
MAL loci, MALI to MAL4 and MAL6. Each locus consists of three genes: gene
1encodes a maltose transporter, gene 2 encodes a maltase (a glucosidase) and
gene 3 encodes a transcriptional activator of the other two genes. Thus, for
example, the maltose transporter gene at the MAL6 locus is designated
MAL61. The five MAL loci each map to a different yeast chromosome, as
follows: MAL1, chromosome VII; MAL2, chromosome III; MAL3, chromosome
27. 27
II; MAL4, chromosome XI; MAL6, chromosome VIII. The MAL loci exhibit a
very high degree of homology and are telomere linked, suggesting that they
evolved by translocation from telo- meric regions of different chromosomes.
Since a fully functional or partial allele of the MAL1 locus is found in all strains
of S. cerevisiae, this locus has been proposed as the progenitor of the other
MAL loci.
It should be mentioned that lactic acid bacteria are often found in high
levels during bread fermentations and the microbial interactions be- tween
baker’s yeast and these bacteria can be very important, in particular in
sourdough breads, as reviewed by Hammes and Ganzle and Hammes et a.
Recent studies have made use of culture independent molecular methods for
identification of the microbiota of bread fermentations and yeast population
dynamics.
Examples:
Egg bread
Potato breads
White breads
Whole grain bread
White bread
Sour dough bread
28. 28
Role Yeasts in dairy products
In 1680 yeasts were discovered by the Dutch scientist Antonie van
Leeuwenhoek. During the second half of the nineteenth century, the French
biochemist Louis Pasteur showed that yeasts were responsible for the
conversion of sugar to ethanol and
carbon dioxide . It was only with
the development of a technique to
isolate pure cultures on solid media
by Robert Koch that it became
possible to select yeast strains on
the basis of their fermentative
characteristics. Yeasts may be
defined as unicellular fungi
reproducing by budding or fission.
Some authors regard yeasts merely as fungi that produce unicellular growth,
but that are otherwise not different from filamentous fungi. Consequently,
yeasts are ascomycetous or basidiomycetous fungi that reproduce
vegetatively by budding or fission and are capable of forming sexual states
that are not enclosed in a fruiting body.
Fungi for which no sexual stage is known, are traditionally included into the
deuteromycetes or fungi imperfecti . However, due to recent molecular
studies these asexual yeasts are currently placed in their proper phylogenetic
position . At present, approximately 800 yeast species are recognized, but only
a few are commonly used or isolated.
Yeasts are the most important microorganisms
ever exploited by man, because they have been
used during several thousand of years for the
production of a wide range of foods such as bread,
wine, beer and kefyr, and more recently for the
production of ethanol for fuel, biochemicals for the
pharmaceutical industry and many other
substances. Yeasts present on fruits, vegetables, equipments, in homemade
29. 29
starters, and in all kinds of raw biological ma terial, such as milk, are
responsible for the occurrence of sponaneous fermentations. As an example,
the raw milk and the environment of a cheese factory, such as the brine, are
important sources for yeast contamination of the cheese surface. During
ripening of smeared cheeses, these yeasts are indispensable.
Yeasts and dairy products
Dairy products provide a unique ecological niche for the selection and growth
of specific yeast species . Relatively few yeast species occur in dairy products
such as milk, cream, yoghurt, butter, cheese and kefyr. Dairy yeasts share a
number of physiological and biochemical characteristics, such as the
fermentation or assimilation of lactose, a high proteolytic or lipolytic activity,
utilization of lactic or citric acids, growth at low temperatures and tolerance
to elevated salt concentrations. Among the most important dairy yeasts
areKluyveromyces marxianus, Kluyveromyces lactis, Debaryomyces hansenii
Yarrowia lipolytica and Saccharomyces cerevisiae. Microbial interactions in
which yeasts play a role are
1. The inhibition of disadvantageous microorganisms, such as pathogens (e. g.
Clostridium butyricum and C. tyrobutyricum)
2. The synergistic effects with lactic acid bacteria , and
3. Fermentations. During the maturation of surface-ripened cheeses,
Debaryomyces hansenii utilizes lactic acid resulting in an increase of the pH,
which increases growth of Brevibacterium linens.
30. 30
Yeasts are important in the production of kefyr and related fermentation
products , but also in the making of cheeses, such as blue cheeses and white
mould cheeses. Yeasts play an important role as spoilage organisms in dairy
products because of their ability to grow at low temperatures and low pH
values, their resistance to physicochemical stresses and their metabolic
activities. For that reason, fermentative yeasts are often responsible for the
spoilage of yoghurt and other sour milks. Blowing of packages is an
undesirable consequence of yeast fermentation. Yeast spoilage is particularly
important in fermented milk products
and cheeses, and less in fresh or
pasteurized milk, cream and butter.In
fruit yoghurts, yeasts may be
introduced by non-dairy ingredients
such as fruits, sugar, honey and nuts .
More generally, spoilage yeast can be
introduced during the entire
production chain, ranging from the
farm, dairy plant, to the final product.
Hygiene and sanitation measures are important to control contamination of
dairy products with yeasts . Dairy products may also be infected by human
pathogenic yeasts, which are usually not transmitted through food.
31. 31
Role of yeast in Kefyr production
The history of kefyr
Kefyr is an acidic, mildly alcoholic and very ancient fermented milk
beverage originating from the northern slopes of Caucasus, more specifically,
from the village of Karatschajeff (2500 m) at the foot of Elbrus (5600 m) . The
root of the name “Kefyr” can be referred to
the Turkish word “kef” meaning pleasant,
delightful, well-being, making drunken,
fermenting, or to the word “kiaf” meaning
froth, or to the Caucasian word “kefy”
meaning best quality. All these different
meanings reveal a distinctive feature of
kefyr, i. e., it undergoes both a lactic acid and
an alcoholic fermentation, and the latter is
due to yeasts. The altitude of the region of
origin and, therefore, the rather low
temperatures, led to a selection of
mesophilic microorganisms.
For a long time, the manufacture of kefyr has been known only to members
of the Ossete and Karabbiner tribes. They prepared kefyr from either cow,
sheep or goat milk in bags of goat hides. In day time, due to the rather cold
climate, the sacks were subjected to sunlight and at night, they were taken
into the house and hung at the door. Every person who passed by, had to kick
the sack in order to mix the content. Fresh milk was added when some of the
fermented milk was removed, providing a continuous natural fermentation .
Depending on the outside temperature, the product could be quite different.
Low temperature led to a relatively high concentration of ethanol (up to 1 %)
and carbon dioxide, where as an elevated temperature to a more acidic
product .The actual starter culture of kefyr are the kefyr grains. But until
today, nobody really knows where and how these grains appeared. Legends
and presumptions are the only sources for an explanation of their formation.
32. 32
KUNTZE and DUITSCHAEVER et al. refer to the above-described
manufacturing procedure of kefyr. During the ongoing spontaneous
fermentation of kefyr, cauliflower-like aggregates are formed, consisting of a
matrix of polysaccharides and coagulated proteins, in which a variety of
microorganisms is embedded. BOUROUNOFF has reported on a saga of the
Caucasian people. The grains are said to originate from another fermented
milk, called “Ayran” which is similar to kefyr.
Ayranis made by natural souring of the milk in oak vats, or sacks of goat
hides, with pieces of either calf’s or camel’s stomach. The grains have been
collected from the walls of the vats and are added directly to fresh milk. The
new sour milk, kefyr, is found to be much more pleasant than ayran.
According to another story, the grains were found by shepherds in the bushes
of the high mountains as a gift from heaven . The best known and most
legendary explanation for the origin of the grains, also called “grains of the
prophet“, is reported by PODWYSSOZKI and KOROLEVA. Allah himself gave
the first grains to a chosen tribe
as a symbol of immortality.
According to another version,
Mohammed was the bearer of the
grains and he told the people how
the grains have to be used. He
strictly forbade the secret of kefyr
preparation or the grains to be
given away. Otherwise, if
unbelievers got hold of the grains,
these would lose their magic and healing power. This legend explains why the
method of kefyr preparation has been kept secret for so long. Until now
nobody has been able to disclose the secret of the formation of kefyr grains.
Dr. G. DZHOGAN in 1867 has been the first one who reported on the beneficial
effects of kefyr in the treatment of intestinal and stomach diseases. This was
the end of the secrecy of kefyr and the start of its spreading through Europe.
The owner of the Moscow Dairy got the idea to produce kefyr on an industrial
scale. To obtain the grains, he sent a beautiful woman, one of his workers, to
33. 33
the Caucasian tribes. She was kindly received by their prince but did not get
the grains. On her way back, she was kidnapped by the mountain people to
become the prince’s wife. The woman was then freed by the gendarmes and as
a compen- sation, the prince had to give her 10 pounds of “Mohammed grains”
. This is the story of how the grains started to move westwards. In the former
USSR and in Bavaria, kefyr started to be produced on an industrial scale in the
1930’s.
The yeast flora of kefyr
The sharp acid and yeasty flavour together with the prickling sensation
contributed by the carbon dioxide can be considered as the typical flavour of
kefyr play a leading role in the development of the characteristic taste and
aroma because of their ability to ferment carbon
sources releasing ethanol and carbon dioxide .
However, to obtan the best flavour, the count of
yeasts should reach at least . Also, the flavour
characteristics are very much determined by the
yeast species present in kefyr. Several working
groups have reported on the yeast count in the
grains and in the resulting kefyr, as well as in
commercial kefyr products. The microbial counts
in grain depend strongly on the method applied for their determination. By
direct microscopic counting, 108 yeasts/g grain are detected, whereas by
plate counting only 106 to 107 cfu/g areAfter adding the grains to milk and
stirring, a number of 105 cfu/ml milk is found
In kefyr prepared with grains, the amount of yeasts is very similar to that
in the grain itself . In the fermented milk made with kefyr (without using
grains) the yeast count is 105 cfu/ml . Commercial kefyr samples differ
strongly from traditionally prepared kefyr. Many samples contain no yeasts at
all, in others the count reaches 106 cfu/ml . Manufacturers usually try to keep
the yeast number as low as possible to avoid blowing of the packages. In
addition, there are no compelling regulations on the composition of the kefyr
34. 34
microflora except for the International Dairy Federation Standard that
proposes a minimal yeast count of 104 cfu/g in kefyr .
A question often debated is whether all yeasts found belong to the specific
kefyr yeast flora, or, if this is not the case, which yeasts must be considered as
contaminants. Usually, yeasts found in kefyr are the same as those species
causing spoilage in other milk products . Some authors claim that only yeasts
fermenting lactose should be considered as specific for the kefyr flora because
of their leading role in the alcoholic fermentation. Never theless, a high
percentage of the yeasts found in kefyr are lactose negative .
The first who examined the microbial flora of kefyr grains was KERN. He
showed that a symbiosis between a yeast and a bacterium is involved. The
yeast is Saccharomyces cerevisiae, a species that does not ferment lactose.
shows the yeasts that have been isolated from kefyr grains, and the frequency
of isolation of the yeast species from kefyr
grains or kefyr products. The role of yeasts is
not only limited to their contribution to kefyr
flavour. For example, LA RIVIÈRE reported that
appreciable growth of L. brevis occurs only in
the presence of a yeast. Therefore, yeasts also
promote the symbiosis between
microorganisms by providing lactic acid bacteria with growth stimulants. On
the other hand, lactic acid bacteria produce which splits lactose into glucose
and galactose. Nearly all the yeasts are able to utilise either glucose or
galactose or both.
35. 35
Role of yeast in cheese production
To make cheese, milk from domestic animals is transformed into a
coagulum by the action of rennet and of LAB. Then, water is expelled by
physical and microbial interactions in or- der to concentrate casein and fat
selectively. During the ripening period,
casein, fat and carbon sources are
metabolized in a complex process by
enzymes of the microorganisms from the
starter culture. The end product is a cheese
with characteristic flavour, taste, consistency
and shape. According to their consistency,
cheeses have been classified into extra-hard,
hard, semi-hard, semi-soft, soft and fresh
cheeses. Cheeses may also be grouped by the
raw material, fat content, the ripening, etc.
The rich and fertile agricultural area situated
between the rivers Euphrates and Tigris in
Iraq is known to be the cradle of civilisation. The staple foods were mainly
bread and cheese. Remnants of material found during an archaeological
survey, proved to have been cheese made from the milk of either cows or
goats. From carvings and other findings it is also assumed that milk was
stored in skin bags where a fermentation process took place. Most probably,
either yoghurt, laban, koumiss or kefyr was produced, or the whey was
drained off through a cloth or a perforated bowl and the solid curd then
salted. The whey was usually used as a refreshing drink. The early coagulants
for milk that were applied in addition to the fermentation process, were the
juice of fig tree, vinegar and milk clotting enzymes from the stomach of hare
or kid. The first written references to cheese can be found in the bible, and
later Homer, Herodotus and others also referred to cheese . The spread of
cheese-making has probably followed the same paths as bread. This
geographical migration has resulted in new varieties of cheese. At present,
2000 names applied to cheese can be found in the literature.
36. 36
The yeast flora of cheese
A large number of varieties of cheese are characterized, as mentioned
above, by the development of a specific surface microflora that is generally
composed of moulds, yeasts,
micrococci and coryneform bacteria.
Yeasts, therefore, are frequently found
within the microflora of many types of
cheese. Their occurrence is not
unexpected because of their tolerance
to low pH and moisture, high salt
concentration and low storage
temperatures . Also, they are widely
dispersed in the dairy environment
and appear as natural contaminants in
the raw milk, the air, the dairy
utensils, the brine, and in smear water
. The brine, being one of the most
important sources of contamination, may be the
vector of several yeast species such as
Debaryomyces hansenii, Candida versatilis,
Kluyveromyces marxianus, Saccharomyces
cerevisiae, Torulaspora delbrueckii, Trichosporon
cutaneum var. cutaneum and Yarrowia lipolytica .
The following species are found in raw milk: D.
hansenii, Clavispora lusitaniae, Tr. cutaneum var.
cutaneum, Rhodotorula mucilaginosa and K.
marxianus . The utilization of lactic acid and the
formation of alkaline metabolites by yeasts results
in an increase of the pH value, which enables the
growth of less acid tolerant microorganisms such as the micrococci and
coryneform bacteria .
37. 37
In the first few days of ripening, the yeast count on the surface of the
cheese increases very rapidly until it reaches a maximum after 10 days . The
numbers can increase to 106–109 cfu/g or 107–108 cfu/cm2 . There after,
the population remains at a nearly different types of cheese such as Cheddar,
Gouda, Danablu, Roquefort, Tilsit, Tête de Moine, Gruyère, Münster, Brie,
Camembert and many others
are listed. The composition of
the yeast flora of young cheese
seems to be rather
heterogeneous and de-pends
strongly on the cheese plant in
which it has beenproduced . In
the cheese prior to brining,
lactose positive species such as
K. lactis, K. marxianus and T.
delbrueckii are pre- dominant. These species most probably also contribute to
the characteristic open texture of blue-veined cheeses. The technology of
cheese ripening also has an impact on the composition of the yeast flora.
38. 38
Role of yeast in vinegar production
Some foods that contain vinegar are mustard, mayonnaise, barbecue sauces,
salad dressing, pickles and
mayonnaise. If you are highly
sensitive to yeast, you should
avoid foods that may contain
some form of vinegar products in
them, such as deviled eggs or
potato salad.
The use of vinegar to flavor food is
centuries old. It has also been
used as a medicine, a corrosive
agent, and as a preservative. In the Middle Ages, alchemists poured vinegar
onto lead in order to create lead acetate. Called "sugar of lead," it was added to
sour cider until it became clear that ingesting the sweetened cider proved
deadly.
By the Renaissance era, vinegar-making was a lucrative business in France.
Flavored with pepper, clovers, roses, fennel, and
raspberries, the country was producing close to
150 scented and flavored vinegars. Production of
vinegar was also burgeoning in Great Britain. It
became so profitable that a 1673 Act of Parliament
established a tax on so-called vinegar-beer. In the
early days of the United States, the production of
cider vinegar was a cornerstone of farm and
domestic economy, bringing three times the price
of traditional hard cider.
39. 39
The transformation of wine or fruit juice to vinegar
is a chemical process in which ethyl alcohol
undergoes partial oxidation that results in the
formation of acetaldehyde. In the third stage, the
acetaldehyde is converted into acetic acid. The
chemical reaction is as follows: CH 3 CH 2
OH=2HCH 3 CHO=CH 3 COOH.
Historically, several processes have been employed
to make vinegar. In the slow, or natural, process, vats of cider are allowed to
sit open at room temperature. During a period of several months, the fruit
juices ferment into alcohol and then oxidize into acetic acid.
The French Orleans process is also called the continuous method. Fruit juice is
periodically added to small batches of vinegar and stored in wooden barrels.
As the fresh juice sours, it is skimmed off the top.
Both the slow and continuous methods require several months to produce
vinegar. In the modern commercial production of vinegar, the generator
method and the submerged fermentation method are employed. These
methods are based on the goal of infusing as much oxygen as possible into the
alcohol product.
The submerged fermentation
method
The submerged fermentation method is
commonly used in the production of wine
vinegars. Production plants are filled with
large stainless steel tanks called acetators.
The acetators are fitted with centrifugal
pumps in the bottom that pump air bubbles
into the tank in much the same way that an
40. 40
aquarium pump does.
As the pump stirs the alcohol, acetozym nutrients are piped into the tank.
The nutrients spur the growth of acetobacters on the oxygen bubbles. A
heater in the tank keeps the temperature between 80 and 100°F (26-38°C).
Within a matter of hours, the alcohol product has been converted into
vinegar. The vinegar is piped from the acetators to a plate-and-frame filtering
machine. The stainless steel plates press the alcohol through paper filters to
remove any sediment, usually about 3% of the total product. The sediment is
flushed into a drain while the filtered vinegar moves to the dilution station.
41. 41
Yeasts in Alcoholic Beverage Fermentations
The production of alcoholic beverages from fermentable carbon sources by
yeast is the oldest andmost economically important of all biotechnologies.
Yeast plays a vital role in the production of all alcoholic beverages and the
selection of suitable yeast strains is essential not only to maximise alcohol
yield, but also to
maintain beverage
sensory quality.
The yeast species that
dominates in the
production of alcoholic
beverages worldwide is
Saccharomyces
cerevisiae, and the particular strains of this species employed in fermentation
exert a profound influence on the flavour and aroma characteristics of
different beverages. For large-scale beverage fermentations, as in brewing,
winemaking and distilled spirit production, pure cultures of selected strains of
S. cerevisiae are usually used.
These strains are either sourced in house or supplied from yeast producing
companies. In smaller-scale (artisanal) processes, spontaneous fermentations
may be allowed to occur that rely on indigenous microbiological flora (wild
yeasts and bacteria) present in the raw material and in the production facility.
For example, this would be typical in small distilleries in Mexico (for Tequila
and Mezcal production) and in Brazil (for Cachaça production).
In some types of alcoholic beverage fermentations, non-S. cerevisiae yeasts
may be employed either as starter cultures, or occur naturally. For example, in
winemaking the S. cerevisiae yeast strain used
42. 42
Yeast’s Role in Beer Making
Yeast works hard but really enjoys itself. This little, single-cell organism, one
of the simplest forms of plant life, is responsible for carrying out the
fermentation process in beer making, thereby providing one of life’s simplest
forms of pleasure (and its production of carbon dioxide is what causes bread
dough to rise).
Many brewers consider their yeast to be their most secret ingredient and
often guard its identity
jealously, calling it a
proprietary ingredient. Yeast is
in the fungus family and,
because of its cell-splitting
capabilities, is self-
reproducing. Yeast has a
voracious appetite for sweet
liquids and produces abundant
quantities of alcohol (ethanol) and carbon dioxide in exchange for a good
meal.
The vast majority of beer contains between 4 and 6 percent alcohol, but
occasionally, brewers make beer with higher alcohol contents. In these beers,
after reaching a level of 8 or 10 percent alcohol by volume, the beer yeast falls
into a stupor, and fermentation is effectively over. When the brew master
wants higher alcohol levels, he uses hardy champagne yeast to do the job.
Ale yeast has a lineage that reaches into antiquity — wild, airborne strains
did the trick. Yeast wasn’t even considered an ingredient in beer until its role
in fermentation was discovered and understood. (This discovery began with
the invention of the microscope in the early 1700s and was furthered by Louis
Pasteur nearly a century later.) The genetically engineered lager yeast variety
was perfected only in the mid-1800s. This factoid isn’t all that important
except that before this discovery, brewers couldn’t make what’s now called a
43. 43
lager by plan. They had to brew ale, ferment and store it at cold temperatures,
and hope for the best.
Since the late 1800s, numerous pure yeast strains more than 500 different
types have been isolated, identified, and cultured. Commercial yeast banks
inventory these strains in the form of sterile
slants (test tubes), and some individual
breweries keep their own sterile cultures on
hand for future brews.
Yeast can also take credit for the classification
of the beer style. Brew masters pick a yeast
according to the recipe or the style of beer
they want to make. Yeast is identified as either
an ale yeast or a lager yeast.
Ale yeast, which is a top-fermenting strain, works best in warm
temperatures (60 to 75 degrees Fahrenheit, 15 to 24 degrees Celsius).
Lager yeast, which is a bottom-fermenting strain, performs best in
cooler temperatures (38 to 52 degrees Fahrenheit, 3 to 11 degrees
Celsius).
44. 44
Role of yeast in wine making
Yeast is an unsung hero. It hangs around, invisible to the eye (40,000 of them
can fit on the head of a pin!) and then after we crush grapes, it goes to work
turning their sugars into alcohol. There would be no wine without yeasts. In
fact, there would be no alcohol at all.
Winemakers use either the ambient (naturally
present) or cultivated yeasts when making wine.
The yeasts take all the natural sugars in the
grapes, convert them to alcohol, and in the
process give off CO2 and a lot of heat. The yeasts
continue to work until they are either stopped
by the winemaker (usually by shocking the wine
with a dose of sulphur dioxide or by cooling the
wine down to a temperature where the yeasts
cannot survive, thus stopping the conversion
process and stabilizing the wine) or they
actually kill themselves off: yeasts cannot live in
an environment of over 16% alcohol, so once the
wine reaches this level, the alcohol actually kills
the yeast which created it. I think any parent
would tell you that this is simply a metaphor for having children
Assuming that the yeasts were not killed off by high alcohol levels or
stabilization, yeasts continue their job even after they have converted a wine’s
sugars into alcohol. In the barrel, they interact with the oak itself, sometimes
even absorbing some of the harsher tannin flavors that a new oak barrel can
impart on wine. Their interactions add even more complexity to the wine.
After the yeasts have done their job, they still remain in the wine as dead
particles. They must be removed in order for the winemaker to create the
final, particle-free product; however, some winemakers actually leave these
dead yeasts (called the lees) in the barrels for four months to a year because it
gives the wine a nice full mouth feel. On some bottles of wine you will see the
45. 45
phrase “surlie”, which translate from French as “on the lees”, meaning that the
winemaker chose to leave these yeasts in with the wine after they expired to
impart that fleshy flavor into the wine.
Fermentation requires two things : sugars and yeasts.
A ripe organic grape is full of natural sugars and there are wild yeasts living
on its skin. As soon as the skin of the
grape is broken, fermentation can
begin. To make wine, all the
winemaker has to do is collect his
grapes and gently crush them,
releasing the sugary juice and
exposing it to the
yeasts.Fermentation will continue
until all the sugar has been turned
into alcohol or the level of alcohol in
the juice reaches around fifteen percent, whichever is sooner.At around fifteen
percent alcohol, the yeasts will die naturally and any left over sugars will
remain in the wine.
Yeasts
A natural wine is fermented only with the wild yeasts native to its terroir.
Yeast strains vary widely from place to place and contribute significantly to
the odour of the finished wine. The yeasts indigenous to a particular area are
an important part of what gives its wines their character.Conventionally
grown grapes have little or no wild yeast living on their skin.The winemaker
will kill whatever yeast remains with sulphur dioxide, and reseed the grapes
with a single strain of commercially produced yeast.Wines fermented in this
way have less personality, all using the same few commercial yeast strains,
and are less an expression of their terroir. This is one reason they taste so
similar.They are also less complex, as each of the many wild yeasts present on
an organic grape will contribute something to the finished wine.
46. 46
Sugars
The level of alcohol in the finished wine is determined by the level of sugar in
the grapes from which it is made.More sugar means there is more for the
yeast to convert into alcohol.Grapes grown further north see less sun and
therefore contain less stored sugar than those grown in the south.
Traditionally, therefore, northern wines contain a lower level of alcohol.
Chaptalization is a way of boosting the level of alcohol in the finished wine by
adding sugar to the juice during fermentation. The technique is named after
Jean Antoine Chaptal , Napoleon's minister for agriculture, who is said to have
invented it.
47. 47
Yeast in Nonalcoholic beverages
A number of sweet carbonated beverages can be produced by the same
methods as beer, except the fermentation is stopped sooner, producing
carbon dioxide, but only trace amounts of alcohol, leaving a significant amount
of residual sugar in the drink.
Root beer, originally made by Native Americans, commercialized in the
United States by Charles Elmer Hires and especially popular during
Prohibition
Kvass, a fermented drink made from rye, popular in Eastern Europe. It has a
recognizable, but low alcoholic content.
Kombucha, a fermented sweetened tea. Yeast in symbiosis with acetic acid
bacteria is used in its preparation. Species of yeasts found in the tea can vary,
and may include: Brettanomyces bruxellensis, Candida stellate,
Schizosaccharomyces pombe, Torulaspora delbrueckii and
Zygosaccharomyces bailii.Also popular in Eastern Europe and some former
Soviet republics under the name chajnyj grib , which means "tea mushroom".
Kefir and kumis are made by fermenting milk with yeast and bacteria.
Mauby (Spanish: mabí), made by fermenting sugar with the wild yeasts
naturally present on the bark of the Colubrina elliptica tree, popular in the
Caribbean
48. 48
Role of yeast in carbon dioxide
The importance of CO2 injection cannot be overemphasized for growing
beautiful planted aquaria.
Sounds complicated? Actually it's easy and cheap! Aquarium companies sell
extremely overpriced CO2 setups costing at least $200 for no frills models.
These consist of a high pressure tank of CO2 and a pressure regulator, as well
as a reaction chamber where the CO2 is dissolved in the water.
The setups on the my aquaria cost about $5 for supplies that will last a year
(this includes 2 liters of Coke that you get to drink). Here's the idea (which is
due to Thomas Narten off of internet as far as I
know). CO2 dissolves into (and escapes out of)
water very quickly, so we need a way to produce
bubbles of CO2 and to hold them in contact with a
fast flowing stream of water so the CO2 has time
to dissolve.
CO2 is produced by yeast fermenting sugar into
alcohol, so take a 2-liter soda bottle and fill it with
lukewarm water to about 2" from the bottom of
where the screw cap would be. Pour the
measured water into a bucket and add
approximately 2 cups sugar and 1/4 teaspoon baking yeast (e.g. Fleishmann's
brand from the baking section of Safeway). Stir until both are dissolved,
especially the yeast which is harder to dissolve than the sugar. Pour this stuff
back in the bottle and fill to the point it normally would be filled with soda.
An empty 2 liter soda bottle
7 cups of lukewarm water
2 cups of white sugar
1/4 teaspoon granular bread yeast
49. 49
Drill a hole in the center of the top of the cap which is just wide enough to
tightly fit a piece of aquarium airline tubing into it, and glue the tubing into
place with aquarium silicone sealant. Leave the cap off the bottle to dry for a
day. Then screw on the cap and put the other end of the air tube into the
intake tube of the filter, so that the CO2 will bubble into the filter. The CO2
may start bubbling the next day, or maybe not for up to 3 days. The bubbles
get sucked into the pump propeller and some end up in the filter sponge
where they slowly dissolve into the water where the plants can use it for
photosynthesis.
This mixture usually lasts about a month before you have to mix a new batch
(more sugar makes it last longer; more yeast makes it bubble faster but it will
run out quickly). Watch for when the bubbles are no longer produced, at
which time you'll have a nasty alcoholic swill left in the bottle which I don't
recommend drinking. Keep the opened yeast packets in the refrigerator in the
meantime or the remaining yeast will die.
Some people have to worry about the CO2 lowering the pH of the aquarium
water, but Davis water is so hard that the pH will hardly fluctuate at all. If your
water is soft the watch the pH closely during the first few hours of bubbling,
and harden the water with calcium carbonate (crushed coral) in the filter if
the pH gets near 6. I have gotten readings consistently above the
recommended level on my CO2 test kit (Tetra), with no apparent harm to the
fish. When using CO2 you must have a cover on the tank and avoid using
accessories which mix air into the water (the plants add plenty of oxygen to
the water), as the dissolved CO2 levels will fall quickly. If you have tried
growing plants for a long time with no luck, you will be amazed at the
incredible growth that results.
50. 50
Role of yeast in medicines
Fungi make an extraordinarily important contribution to managing disease in
humans and other animals. At the beginning of the 21st century, Fungi were
involved in the industrial processing of more than 10 of the 20 most profitable
products used in human medicine. Two anti-cholesterol statins, the antibiotic
penicillin and the
immunosuppressant
cyclosporine A are among the top
10. Each of these has a turnover
in excess of $1 billion annually.
Drug discovery continues. The
following have recently been
approved for human use:
Micafungin is an antifungal agent;
mycophenolate is used to prevent tissue rejection; rosuvastatin is use to
reduce cholesterol; and cefditoren as an antibiotic.
Fungi are extremely useful organisms in biotechnology. Fungi construct
unique complex molecules using established metabolic pathways. Different
taxa produce sets of related molecules, each with slightly different final
products. Metabolites formed along the metabolic pathway may also be
biologically active. In addition, the final compounds are often released into the
environment. Manipulation of the genome, and environmental conditions
during formation of compounds, enable the optimization of product
formation.
On the negative side, single isolates of fungi in manufacture may lose their
capacity to form or release the target molecules. Indeed, the target compound
may only be expressed under specific conditions, or at a specific point in the
life cycle of the fungus. It is amazing that so many biologically active
compounds have been discovered and taken to the point where they are
medically important. Indeed, attempts to 'discover' new and exciting
molecules remains the core activity of many research groups.
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The role of fungi was established early in history. Yeasts have been used in
the making of bread and alcohol since the beginning of civilization. LINK In
modern times, the discovery of penicillin marked the beginning of a new
approach to microbial diseases in human health. More recent approaches
include the application of hydrophobias to surfaces leading to
biocompatibility of implants, and to emulsion formation improving drug
delivery. The established importance of fungi is being expanding way beyond
their capacity to transform and protect.
Antibiotics From yeast
In 1941, penicillin from the fungus Penicillium chrysogenum was first used
successfully to treat an infection caused by a bacterium. Use of penicillin
revolutionized the treatment of pathogenic disease. Many formally fatal
diseases caused by bacteria became treatable, and new forms of medical
intervention were possible.
When penicillin was first produced, the concentration of active ingredient
was approximately 1
microgram per ml of broth
solution. Today, improved
strains and highly
developed fermentation
technologies produce more
than 700 micrograms per
ml of active ingredient.
In the early broths, several
closely related molecules were present. These molecules are beta lactam rings
fused to five-membered thiazolidine rings, with a side chain. The side chain
can be chemically modified to provide slightly different properties to the
compound.
52. 52
The natural penicillin have a number of disadvantages. They are destroyed in
the acid stomach, and so cannot be used orally. They are sensitive to beta
lactamases, which are produced by resistant bacteria, thus reducing their
effectiveness. Also, they only act on gram positive bacteria.
Modifications to manufacturing conditions have resulted in the development
of oral forms. However, antibiotic resistance among bacteria is becoming an
extremely important aspect determining the long-term use of all antibiotics.
Cephalosporin also contains the beta lactam ring. The original fungus found
to produce the compounds was a Cephalosporium, hence the name. As with
penicillin, the cephalosporin antibiotics have a number of disadvantages.
Industrial modification of the active ingredients has reduced these problems.
The only broadly useful antifungal agent from fungi is griseofulvin. The
original source was Penicillium griseofulvin. Griseofulvin is fungistatic, rather
than fungicidal. It is used for the treatment of dermatophytes, as it
accumulates in the hair and skin following topical application.
More recently, several new groups have been developed. Strobilurins target
the ubihydroquinone oxidation Centre, and in mammals, the compound from
fungi is immediately excreted. Basidiomycetes, especially from tropical
regions, produce an enormous diversity of these compounds.
Sordarins are structurally complex molecules that show a remarkably narrow
range of action against yeasts and yeast-like fungi. The compounds inhibit
protein biosynthesis and so may become important agents against a number
of fungal pathogens of humans.
Echinocandins are cyclic peptides with a long fatty acid side chain. They
target cell wall formation. Semi-synthetic members of the group of
compounds include pneumocandins which are in use in humans.
53. 53
Questions
1. What is different between yeast and mold?
2. What is the importance of yeast in food process?
3. Name the active forms of yeast and discuss each of
them?
4. What is the history of yeast?
5. What is brewer’s yeast?
6. How long do yeast to rice the dough?
7. Can we store yeast in freezer?
8. What is nutritional yeast?
9. What if the recipe didn’t have liquid to dissolve the
yeast ?
10. What the best way to active or proof the dry yeast ?
