Food microbiology is the study of the microorganisms that inhabit, create, or contaminate food which causing food spoilage.

1. FOOD SPOILAGE
P. Karthika
Assistant Professor
Department of Food Science and Nutrition
Periyar University, Salem - 11

2. Definition
• Food spoilage means the original nutritional value,
texture, flavor of the food are damaged, the food
become harmful to people and unsuitable to eat.

3. Basic Types of Food Spoilage
•Appearance: when a food “looks bad”
1. Microbial growth - mycelia or colonies visible on surface
development of cloudiness in liquids
2. Changes in food color due to heme or chlorophyll breakdown
colony pigments, growth of mycelia, etc
• Textural changes (feel)
1. Slime formation - due primarily to surface accumulation of
microbial cells also be a manifestation of tissue degradation
2. Tissue softening due to enzymatic degradation (e.g. soft rot in
veggies)
• Changes in taste and odor
1.Development of
Nitrogenous compounds (ammonia, amines, etc.)
Sulfides
Organic acids

4. Types of Food Decay
• Putrefaction
• Fermentation
• Rancidity
▫ Microbial rancidity
▫ Hydrolytic rancidity
▫ Oxidative rancidity

5. Putrefaction
• It is anaerobic breakdown of proteins, with the
production of foul-smelling compounds such as
hydrogen sulfide and amines.

6. FERMENTATION
• Fermentation is a metabolic process in which an
organism converts a carbohydrate, such as starch or
a sugar, into an alcohol or an acid. For example, yeast
performs fermentation to obtain energy by converting
sugar into alcohol. Bacteria perform fermentation,
converting carbohydrates into lactic acid. The study of
fermentation is called zymology.

7. Rancidity
• Rancidity is the complete or
incomplete oxidation or
hydrolysis of fats and oils
when exposed to air, light,
moisture or by bacterial
action, resulting in unpleasant
taste and odor.

8. Microbial rancidity
• Rancidity is the complete or incomplete oxidation or
hydrolysis of fats and oils when exposed to air, light,
moisture or by bacterial action, resulting in unpleasant
taste and odor. Specifically, it is
the hydrolysis or autoxidation of fats into short-
chain aldehydes and ketones, which are objectionable in
taste and odor

9. Hydrolytic rancidity
• Fatty acids formed through hydrolysis of the lipid(fat) by the
water which it contains. Some of the liberated fatty acids are
volatile, and some have very unpleasant odours and
flavours.

10. Oxidative rancidity
• Oxidative rancidity is associated with the degradation by
oxygen in the air. The double bonds of an unsaturated fatty
acid can be cleaved by free-radical reactions involving
molecular oxygen. This reaction causes the release of
malodorous and highly volatile aldehydes and ketones. Because
of the nature of free-radical reactions, the reaction is catalyzed
by sunlight.

11. Sources
• Ubiquitous microorganisms
• From natural sources like soil, water, air
• Special source of contamination
• Spoiled raw materials
• Biofilm on surface of equipment
• Human’s personal hygiene

12. Contamination of various food items
Cereals and its product
• Cereal grains and cereal products normally contain
several genera of bacteria, molds, and yeasts.
• Bacteria can also cause spoilage of cereals, but yeasts
cause few spoilage problems.
• The molds associated with cereal grains can be
divided into two groups: the field molds and the
storage molds.
• The main genera of molds causing spoilage during
storage of cereal grains are Aspergillus and Penicillium.

13. Sugar and its products
• Sugar products: Sucrose (cane and beet sugar),
molasses, syrups, maple sap and sugar, honey and
candy. Sugar gets spoiled only under improper storage
conditions such as high humidity.
SUCROSE:
Leuconostoc and Bacillus;
representatives of the genera
Micrococcus, Flavobacterium,
Alcaligenes, Xanthomonas,
Pseudomonas, Erwinia, and
Enterobactor; a variety of
yeast, chiefly in the genera
Saccharomyces, Candida and
Pichia; and a few moulds.

14. Molasses and Syrups:
• Microbial spoilage of molasses is not common,
although it is difficult to sterilize by heat because of
the protective effect of the sugar.
Honey
• Spoilage of honey is osmophilic yeast: species of
Zyggosaccharomyces, such as Z. mellis, richteri, or
nussbaumeri, or Torula (Cryptococcus) mellis. Species
of Penicillium and Mucor develop slowly

15. Candy
• High sugar and low moisture content –
Less chance to microbial growth
• Yeast growing in these candies develops
a gas pressure which may disrupt the
entire candy or more often will push out
some of syrup or fondant through a
weak spot in chocolate coating.
• Often this weak spot is on the poorly
covered bottom of the chocolate coating

16. Vegetables and fruits
• The organisms break down pectins, giving rise to a soft, mushy
consistency, sometimes a bad odour and a water soaked
appearance
• Sour rot/ watery rot of vegetable is caused by Geotrichum
candidum and other organism and vegetable thus affected,
include bean, carrot, lettuce, cabbage, onion, garlic, radish,
cauliflower, tomato and turnip
• Rhizopus soft rot in carrot, potato, cabbage, cucumber, pumpkin,
watermelon, radish and tomato is caused by Rhizopus stolonifer
making the vegetables soft and mushy.

17. Meats and its products
• Fresh meat is spoiled by microflora coming from animals lymph
nodes, intestinal tract, hide and from the handling equipment
and personnel engaged in this business.
• Refrigerator stored the spoilage is only on the surface caused
by bacteria from external sources
• Fungal spoilage as Mucos, Rhizopus and Thamnidium which
produce whiskers on beef, Penicillium which produces green
patches and Cladosporium which causes black spot

18. • Poultry is mostly spoiled by bacteria of the genus
Pseudomonas. Poultry mostly spoiled by bacteria of the
genus pseudomonas.
• Poultry spoilage occurs mostly on the surface as the
inner portions of poultry tissue are generally sterile
• Fresh poultry stored in humid conditions is susceptible
to aerobic bacterial growth. As spoilage occurs, off-
odours and slimy appearance develop.
Poultry

19. Fish
• Fish of both fresh-water and salt-water contain high levels of
proteins and nitrogenous constituent with low fat content and
practically no carbohydrates
• Fresh fish and iced fish are spoiled by bacteria while salted and
dried fish are spoiled by fungi
• Pseudomonas, Acinetobacter and Maroxella species are involved
in the bacterial spoilage. In addition intestinal bacteria of the
fish and native proteolytic enzymes also contribute to the
spoilage and develop off-odours and sliminess.

20. Egg
• Internally the egg white is not susceptible to microbial spoilage
because of its high pH of about 9.3 and because of the presence of
ingredients which function as antimicrobial agents
• Egg yolk is an excellent medium for the growth of most
microorganism because of its neutral pH (6.8) and bacteria can
grow in the yolk producing hydrogen sulphide and other foul
smelling compounds and also cause the yolk to become runny and
discoloured

21. • Storing eggs in highly humid conditions favors
the growth of microorganisms on the surface and
facilitates microbial penetration
• At very low humidity egg lose water and become
undesirable
• Most common bacterial spoilage of egg is rotting.
Green rot caused by Pseudomonas sp., black rot by
Aeromonas, Pseudomonas proteus sp., colourless
rot by Acinetobacter and Pseudomonas, red rot by
Serratia and custard rot by Acinetobactor and
Pseudomonas

22. Milk and its products
• Milk , cream, butter and cheese are easily susceptible to
microbial spoilage.
• Milk is an excellent growth medium for a variety of
spoilage organisms.
• Fresh raw milk and refrigerated raw milk contain many
bacteria.
• Pasteurization eliminates many of these species.
• Heat resistant streptococci cause spoilage of pasteurized
milk. They produce lactic acid by fermenting lactose and
decrease the pH to about 4.5 causing curdling of the
milk.

23. • Butter is not highly perishable but still it undergoes microbial
spoilage
• Two important types of spoilage of butter include surface taint/
putridity and rancidity
• Putridity ----------- caused bad odours
• Rancidity ----------- occurs due to hydrolysis of butter fat resulting
in the formation of free fatty acids.
• Cheese, particularly cottage cheese, gets spoiled to a condition
called slimy curd.

24. Canned Foods
• Canning is one of important method of packaging
food for long term storage. Normally food is stored in
metallic containers along with heat treatment.
• The heat treatment varies depending upon type of
food.
• There is always a chance that microorganisms may
survive if the heat treatment is not proper thereby
leading to spoilage of food.

25. Causes of Spoilage in Cans
• Chemical spoilage
▫ Increased storage temperature.
▫ Increased acidity of food
▫ Improper exhaust
▫ Presence of soluble sulfur and phosphorous compounds
▫ Improper timing and lacquering of can at internal surfaces
• Biological spoilage
▫ Leakage of can
▫ faulty retort operations, excessive microbial load and
contamination in product, change in consistency of the
product.

26. Microbes
• Enterococcus
• Streptococcus thermophilus
• Micrococcus
• Lactobacillus
• Leuconostoc
• Microbacterium
• Clostridium thermosaccharolyticum
• Aspergilus and Penicillium