microbiology section

1. Yeast in food spoilage
Mohammed Mashhood b
1st M.sc microbiology

2. What is food spoilage ?
• Food spoilage means the original
nutritional value, texture, flavour
of the food are damaged, the
food become harmful to people
and unsuitable to eat.
• Loss of one or more normal
characteristics in food is
considered to be due to spoilage

3. yeast

4. General characteristics of yeast
Yeast are eukaryotic single celled microorganism
Size 3-4 µm
Reproduce asexually by mitosis and usually show budding
Ability to ferment sugars for the production of ethanol
They are usually unicellular and exist as colonies and used in
baking process.

5. Continue
Budding yeast referred as the “true yeasts” are members of
the phylum Ascomycota and order Saccharomycetals
Minimum aw for growth of yeast lies between 0.88 to 0.94
optimum temperature for yeast growth lies between 25C and
30C .
Acid environment [ pH 4.5-6.0] will favour the yeast growth.

6. • Moisture loss
2. Enzyme
action
3. Microbial
contamination
Causes of food spoilage

7. Moisture loss
Most easily demonstrated in fruit and vegetables
which contain large amount of water
After harvesting they continue to respire.
Moisture is lost through skin and leaves
Causes wrinkling ,shrinkage and limpness
Moisture loss can also occur in the foods [eg;
meat, fish, cheese] because of evaporation

8. Action of enzyme
Ripening
Enzymes cause food to ripen, then become over-ripe
and eventually decay. Starch changes to sugar, colour
changes and texture softens.
• Browning
When certain foods are cut and the surface exposed to
air, enzymes cause them to turn brown. E.g. Apples,
• Enzymatic rotting
Enzymes in fish cause deterioration even at low
temperatures.

9. Microbial contamination
Yeast, mould and bacteria are the microbes that cause
food spoilage.
Food at the optimum temperature is the ideal place for
microbes to grow.
Yeast and mould spoil the outside of food and can be seen.
If the food is eaten it will probably be harmless, unless they
make mycotoxins.
Bacteria cause spoilage by making toxins in food - not
visible.

10. Yeast in food spoilage
Yeasts can be found in a wide variety of environments, such
as in plants, animal Products, soil, water and insects
Yeasts can utilize a variety of substrates such as pectines
and other carbohydrates, organic acids, proteins and lipids
Moreover, yeasts are relatively tolerant to low pH, low
water activity, low temperature and the presence of
preservatives.

11. Some yeasts can utilize food ingredients, such as organic
acids like lactic, citric and acetic acids, that are generally
considered to have an inhibitory effect on the growth of
many microorganisms.
Even common preservatives such as benzoate, propionate
and sorbate can be utilized by some yeast species
Beneficial yeasts
Yeasts are well known for their beneficial contributions
in the fermentation 0f alcoholic beverages, bread and
other products

12. Yeasts are exploited for the production of metabolic
products (eg. Enzymes, lipids and vitamins)
• Yeasts are also involved in spoilage of food products.
Contamination of foods and beverages by yeasts has been
extensively reported
• Common cases of food spoilage by yeasts are those
reported in sandwiches and salads in retail outlets

13. Contemporary work has also reported the occurrence of
yeasts in fresh seafood, packaged meats, delicatessen
salads and in fresh and frozen vegetables

14. SPECIES FOODs affected
 Zygosaccharomyces rouxii Confectionery, fruit concentrates
 Zygosaccharomyces bailii Soft drinks, sauces, fruit juice, wine,
ciders and syrups
 Saccharomyces cerevisiae Soft drinks and fruit juices
 Debaryomyces hansenii Cured meats and brined products
 Brettanomyces bruxellensis Beer and wine, fruit yoghurts
Some important spoilage yeast

15. Spoilage of foods and beverages by yeasts causes undesirable
changes in these products
These changes are mostly of a sensory nature, recognizable in
the product's appearance as a powdery or slimy coating on
solid products or as a film on, or unnatural turbidity in, liquid
products.
They may also include off-flavours or odours not natural to the
product

16. Yeast species which are considered spoilers in certain foods may
be regarded as normal microflora in other foods
Eg.The yeasts candida lipolytica and kluyveromyces fragilis are
spoilers of margarine, causing rancidity, and yogurt, causing fruity
flavour and fermentation, respectively
On the other hand, the proteolytic properties of C. lipolytica and
K.Fragils have proved useful in cheese ripening

17. Foods spoiled by yeasts have so far never been reported to
cause human disease.
 Infections from the few known pathogenic yeasts, such as
candida albicans and Cryptococcus neoformans, are
generally not transmitted through foods

18. spoilage yeasts in the wine industry

19. Origin of spoilage yeasts in the wine industry
The wine production environment may be divided in two
fundamental parts
1. Vineyard
2. winery

20. Vineyard

21. vineyard
 Microbial communities in vineyards and on grape surfaces may
be summarised as follows.
1. Mature sound grapes Harbour microbial populations at levels of
103-105 CFU/g
2. Insects are the principal vectors for the transportation of yeasts
3. Yeast colonisation on grapes is influenced by the degree of
ripeness of the bunch

22. 5. The occurrence and growth of microorganisms on the skin of
the grapes is affected by the rainfall, temperature, grape
variety, and application of agrochemicals
6. Yeasts are mainly localised in area of grape surface where
some juice might escape and are embedded in a fruit secrete
7. Oxidative basidiomycetous yeasts, without any ecological
interest sporobolomyces, cryptococcus, rhodotorula, and
filobasidium—are mostly prevalent in the
vineyardenvironment

23. winery

24. Considering the winery environment, two sections are
relevant:
1. Winemaking and bulk wine storage
2. The bottling line

25. Winemaking and wine
storage
The yeast population of wineries is quite different from that of
grapes, particularly due to the high proportion of S. Cerevisiae
Besides S. Cerevisiae, other species frequently Recovered
from wine or grape juice contact surfaces Tank walls,
crushers, presses, floor, winery walls, Pipes, etc

26. P. Membranifaciens, P. Anomala, and candida spp are known for
film formation on the surface of bulk wines in unfilled containers
Yeast species regarded as the most dangerous to wines, ie,
Dekkera/brettanomyces spp., Z. Bailii, and S. Ludwigii, are
seldom detected in yeast studies performed in wineries.

27. Bottling line
Wine bottling is a critical operation since, with the exception of
hot bottling, it is the last contamination source before wine is
released to the market
In most dry red wines, yeast contamination during bottling is
not serious
Wines with residual sugar and for some dry white wines, it
can be very serious, being responsible for a major part of
the microbiological problems in bottled wines.

28. Critical points
Some authors have studied the critical points of bottling lines.
The outlet side of the sterilising filter, the filler, in particular, the
bell rubbers and rubber spacers, the corker, in particular, the
bells/cork jaws and cork hopper, the bottle sterilizer, the bottle
mouth, and the air inside the bottling room are important critical
points.
 Packaging materials such as bottles, corks, and rip-caps are
generally not significant contamination sources, because they are
frequently infected by fungi, spore forming bacteria, and
adventitious yeasts, which do not survive in wine. However, they
can be important sources of spoilage yeasts when wine is
improperly stored for long periods in a humid and contaminated
winery environment

29. fruits
 Fruit is a fertilized and ripened ovary.
 Fruits supply vitamins, minerals, sugars, organic acids
and dietary fibre
Fruits are used to make soft drinks, sauce ,pickles ,Jam
etc
 Fruits are prone to postharvest deterioration caused
by microorganisms. It was estimated that about 20%
of all fruits and vegetables produced is lost each year
due to spoilage

30. Fruits are nutrient rich substrates but the pH of fruits
does not favor bacterial growth. As a result, yeasts and
molds are more important than bacteria in the spoilage
of fruits.
Several genera of yeasts can be found on fruit.
High moisture content (high aw)
pH range of 3-5
High concentration soluble carbohydrates
 0verall a very nice source for yeast growth
Yeast association are directly reflective of harvesting and
handling practices

31. Dairy product
Yeasts can grow well at the low pH of cultured products
such as in buttermilk and sour cream and can produce off-
flavours described as fermented or yeasty
Additionally, yeasts can metabolize diacetyl in these
products thereby leading to a yogurt-like flavour
Contamination of cottage cheese with the common
yeast Geotrichum candidum often results in a decrease
of diacetyl content Geotrichum candidum reduced by
52–56% diacetyl concentrations in low fat cottage
cheese after 15–19 days of storage at 4–7◦C

32. Yeasts are a major cause of spoilage of yogurt and fermented milks
in which the low pH provides a selective environment for their growth
Yogurts produced under conditions of good manufacturing practices
should contain no more than 10 yeast cells and should have a shelf
life of 3–4 weeks at 5◦C
Yogurts having initial counts of >100 CFU/g tend to spoil quickly
Yeasty and fermented off-flavours and gassy appearance are often
detected when yeasts grow to 105
–106
CFU/g

33. • The low pH and the nutritional profile of most cheeses are
favourable for the growth of spoilage yeasts
• Surface moisture, often containing lactic acid, peptides, and
amino acids, favours rapid growth
• Some proteolytic yeast strains produce sulfides, resulting in an
egg odour
• Common contaminating yeasts of cheeses include candida
spp., Kluyveromyces marxianus, Geotrichum candidum,
Debaryomyces hansenii, and Pichia spp

34. Giudici, masini, and caggia (1996) studied the role of galactose
in the spoilage of yogurt by yeasts and concluded that
galactose, which results from lactose hydrolysis by the lactic
starter cultures, was fermented by galactose-positive strains of
yeasts such as Saccharomyces cerevisiae and Hansenula
anomala.

35. Bakery product
• The most widely recognised role of yeast in bakery is as
leaning agent in breads and other fermented goods such as
crumpets
• In many bakery products the water activity [aw] is the most
important single factor affecting the type and rate of
spoilage
• Thus many product we are concerned with yeasts which are
capable of growing at low aw.

36. • There are broadly two type of yeast spoilage of bakery
products and ingredients
1. The visible yeast spoilage
2. Fermentative yeast spoilage

37. Visible yeast spoilage
• Many yeast can produce visible growth on the surface of bakery
products , particularly those product with high aw
• The appearance of some of these yeasts has led to the
descriptive term ‘chalk mould’
• Chalk moulds and other visible yeast are most easily seen on
dark products
• Main sources of chalk moulds are the slicing machine and the
bread cooler . Both these machines are complex and have
inaccessible part which are hard to clean and sanitize
effectively

38. Saccharomycopsis fibuligera was most common
yeast in blade oil
The next common yeast species was pi .burtonni
followed by c. sake and c. parapsilosis
c. parapsilosis was one of the common
filamentous yeast in bakery
 the optimum growth temperature for most of
these visible spoilage yeast is 15-35C
The optimum pH about 4.5-4.8

39. Fermentative yeast spoilage
The fermentative spoilage of wide range of products and ingredients
which is manifested by alcoholic, estery or other odour and/or visible
evidence of gas production such as bubbling in jams and fondats or
expansion of flexible packaging
Fermentative yeast spoilage can occur in a wide range of bakery
material but most common in those with high sugar content and low
water activity.
These include jams marshmallow or buttercreams used as fillings, or
marzipan, fondants and icings used as coatings

40. Foods Main characteristics Main yeast species associated with spoilage
Mayonnaise Tomatoes and fruits
PH
Low <4.5
saccharomyces sp.,zygosoccharomyces bailii, Z.
bisporus
Fermented vegetable Medium 4.5-5.3 Rhodotorula sp., Debaryomyces sp., pichia sp.,
candida sp.
Pickles
Cottage cheese
C. parapsilosis, C. canadensis, other Candida sp., p.
Milk, Red meat, Canned
vegetables
High 5.5-7.0 Candida sp.,Kluy.
marxianus,Crypt.ftavus,R.hodotorula sp. Candida sp.,
Debaryomyces sp., Rhodotorula sp
Dried beef, cooked, sausages Salt Candida sp., D. hansenii
Wine, beer and must Alcohol Candida sp., D. hansenii, Issatchenkia orientalis, Kluy
sp
Fruit juices, processed fruis Sugar Kloeckera sp., Hanseniaspora sp., Candida sp.,
Hansenula

41. Reference
1.Food microbiology- by k. vijaya Ramesh
2. Fundamental Food Microbiology, Fourth Edition
- - By Bibek Ray, Arun Bhunia
3. Food and Nutrition By Anita Tull
4. Development and implementation of molecular typing techniques
for identification of food spoilage yeasts by prof .Dr. J.A. van Ginkel
5. Yeasts Associated with Spoilage of Some Selected Fruits in Sokoto
Metropolis – Ibrahim, M. and Sada, M. D.
6. Microbiological Spoilage of Dairy Products
-Loralyn H. Ledenbach and Robert T. Marshall
7. https://www.researchgate.net/publication/21280668