fermentation notes

1. Fermentation

2. • Microorganisms are the key building blocks of
fermented foods and beverages.The final
produt of fermentation is the result of unique
chemical,physical and biological interactions
between specific organisms and food.The goal
of fermentation is to provide specific taste and
texture to food.

3. • Microorganisms necessary in food fermentation
(starters) may be added as pure cultures or
mixed cultures or in some instances no cultures
may be added if desired microorganisms are
known to be present in sufficient numbers in the
original raw material. Controlled starter cultures
are employed in the manufacture of certain dairy
products,bread,wines,distilled liquors etc.. which
transform spontaneous fermentations into well-
controlled industrial operations.

4. GENERAL PRINCIPLES OF CULTURE
MAINTENANCE AND PREPARATION
• 1.SELECTION OF CULTURES
• Is done on the basis of their stability and
ability to produce desired products
efficiently.Selection is the most commonly
used method for the improvement of
strains.Selection can be made from new
strains isolated from the environment, from
existing strains or following mutation of
strains by various means.

5. 2.MAINTENANCE OF ACTIVITY OF
CULTURE
• Once a satisfactory culture has been obtained ,it must be kept
pure and active.This can be achieved by
• Periodic transfer of the culture into proper culture medium
• Incubation until the culture reaches the maximal stationary
phase of growth and then
• Storage at a temperature low enough to prevent further growth.
• Stock cultures are preserved by lyophilization(freeze drying) and
freezing in liquid nitrogen.Other methods include sealing of tube
cultures with paraffin oil and keeping at room temperature on
agar slants with 1 percent NaCl. Dry Spore stock on sterilized
soil can be used to preserve spores of bacteria or molds of long
periods .

6. 3.MAINTENANCE OF PURITY OF
CULTURES
• Regular checks of purity is done by
• Microscopy;which shows the contaminant in
large numbers and difference in appearance
from the desired organisms.
• Tests for the presence of substances not
produced by the desired organism.For eg: for
catalase , in a culture of catalase negative lactic
acid bacteria as the presence of catalase –
positive contaminants .

7. 4.PREPARATION OF CULTURES
• Is usually prepared from a mother culture or
from stock culture.The mother cultures can be
used to inoculate a larger quantity of culture
medium to produce the mass or bulk culture
to be used in the fermentation process.
• The properties of a good culture include :
• Only desired microorganisms

8. • Uniform in microbialnumbers,proportions(if a
mixed culture) and activity from day to day.
• Active in producing desired products
• Has adequate resistance to unfavourable
conditions.
• Temperature and time of incubation are
adjusted so that the culture will be ready at the
time when it is needed.

9. 5.ACTIVITY OF CULTURE
• This is judged by the rate of growth and
production of desired products.It should be
good if the mother or the intermediate culture
is satisfactory and culture medium,incubation
time and temperature are optimal.

10. Fermentation and metabolism basics
• Fermentations are energy-yielding reactions in
which an organic molecule is the electron
acceptor.
• In lactic acid fermentation, Pyruvic acid serve as
electron acceptor forming lactic acid.
• In ethanolic pathway, acetaldehyde formed by
decarboxylation of pyruvate is the electron
recipient forming ethanol.
• Examples of non fermentative pathways-
malolactic fermentation , used in wine making is
a decarboxylation reaction.

11. SUGAR METABOLISM
• Homofermentation
• More than 90% of the substrate is converted to
lactic acid during homofermentative metabolism.
• Lactic acid bacteria are obligate fermentors and
cannot obtain energy by oxidative or respiratory
processes. For homofermentative lactic acid bacteria,
hexoses are metabolized via the enzymes of the
glycolytic Embden-Meyerhoff pathway, which yields
two molecules of Pyruvate and two moles of ATP per
mole of hexose.

12. • Key enzymes:
• Aldolase.
• Lactate dehydrogenase.
• Examples of homofermentative LAB:
Lactococcus lactis, Streptococus thermophilus,
Lactobacillus helveticus,L.delbrueckii
subsp.bulgaricus,Pediococcus
sp.,Tetragenococcus.

13. • Heterofermentation
• Heterofermentative LAB metabolize hexoses via
Phosphoketolase pathway. Approximately
equimolar amounts of lactate,acetate, ethanol
and CO2 are produced along with only one mole
of ATP per hexose.
• Key enzymes:
• Phosphoketolase ( instead of Aldolase)
• Acetaldehyde dehydrogenase.
• Alcohol dehydrogenase.

14. • Examples :
• L.mesenteroides subsp. cremoris.
• Leuconostoc lactis.
• L. mesenteroides subsp.mesenteroides
• Leuconostoc kimchii.
• O.oeni.
• Lactobacillus sanfranciscans.

15. PROTEIN METABOLISM
• The proteolytic system in Lactic Acid Bacteria
utilizes, milk protein casein as primary
substrate.the process involves:
• Proteinase system- hydrolysis of casein by
Proteinases to form Peptides.
• Peptide transport systems- which helps in the
transport of peptides to cells.
• Peptidases –peptides are hydrolysed by
intracellular peptidases to form free
aminoacids.

16. CITRATE FERMENTATION
• Several species of LAB can ferment citrate and
produce Diacetyl, which act as an important
flavouring agent .
• Enzymes involved:
• Citrate permease- helps in transport of citrate to
cells.
• citrate lyase- conversion of citrate to acetate
which is released into the medium and
oxaloacetate which is further decarboxylated to
form Pyruvate and CO2.

17. PROPIONIC ACID FERMENTATION
• P.freuderenchii utilizes lactate as carbon
source producing propionate,acetate and CO2.
The metabolic pathway involves several metal
containing enzymes and vitamin cofactors.

18. • METABOLISM OF MOLDS
• Fungal metabolism involves the production of
numerous proteolytic , amylolytic and lipolytic
enzymes. These enzymatic end products serve as
substrates for further metabolism.
• Eg; the blue mold, Penicillium roqueforti, grow in
cheese,cause proteolysis by the involvement of
proteinases,endopeptidases and exopeptidases.
The amino acids are metabolized via deaminases
and decarboxylases releasing amines,ammonia and
other flavor compounds.

19. • Lipolysis- lipases hydrolyse fat using lipase, releasing free
fatty acids like Butyric and Caproic acids. Metabolism of
free fatty acids produce Methylketones which produce
characteristic flavor and aroma of blue cheese.
• Brie cheese mold, P.camemberti, produce Proteinases
and Lipases which diffuse through the cheese generating
amino and free fatty acids. Metabolism of the amino
acids results in the formation of ammonia, methanethiol
and other sulfur compounds.
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