Md. Humayun Kobir Dept: Agro Product processing Technology Jashore University Of Science and Technology,

1. Food spoilage

2. Significance of Microbes in Foods
• Microorganisms, in relation to food, can have one of these
4 roles:
1. Food spoilage
2. Food poisoning
3. Food preservation
4. Food production

3. Food spoilage
• Food spoilage is the process in which food
deteriorates to the point that it is not
edible to humans or its quality of edibility
becomes reduced.
• Various external forces are responsible for
the spoilage of food.
• Signs of food spoilage may include an
appearance different from the food in its
fresh form, such as a change in color, a
change in texture, an unpleasant odor, or
an undesirable taste. The item may become
softer than normal.
• If mold occurs, it is often visible externally
on the item.

4. Food spoilage
• Some spoiled foods are harmless to eat,
and may simply be diminished in
quality. But foods exhibiting certain
types of spoilage may be harmful to
consume.
• Uncooked or under-cooked animal flesh
that spoils is typically quite toxic, and
consumption can result in serious
illness or death.
• The toxic effects from consuming
spoiled food are known colloquially as
"food poisoning", and more properly as
"foodborne illness.”

5. Food spoilage

6. Cause of spoilage
• Food spoilage refers to undesirable changes occurring in food due to the influence of
air, heat, light, moisture, which foster the growth of microorganisms.
• Foods take different period of time to lose their natural form through spoilage.
• Spoilage may be due to one or more of the followings:

7. Physical changes
• Physical factors such as those
caused by freezing, burning,
drying, pressure etc. are
responsible for the damages and
spoilage of fresh foods.

8. Chemical reactions
• Chemical food spoilage is an
unwanted quality change in a
foodstuff, such as staling,
discoloration, the development
of off-flavors and odors (e.g.
rancidity), and thinning of
sauces.
• It can be caused, for example, by
enzymic or microbial activity,
oxidation or external tainting.

9. Growth and activity of microorganisms
• Harvested crops decompose from the moment they are harvested due
to attacks from microorganisms. Various bacteria can be responsible
for the spoilage of food.
• When bacteria breaks down the food, acids and other waste products
are created in the process. While the bacteria itself may or may not be
harmful, the waste products may be unpleasant to taste or may even
be harmful to one's health.

10. Growth and activity of microorganisms
Microbial food spoilage can be
caused by:
➢growing microbes
➢microbial enzymes
• Bacteria, yeast and molds multiply
in food
• Viruses and parasites do not
multiply in food.
• Microorganisms must multiply and
attain certain level of viable cell per
g, ml or cm²
• 10ᶺ5: some bacterial exotoxins can
be detected
• 10ᶺ6-7: changes in odor and color
• 10ᶺ8-9: changes in texture, slime
formation
• Slime: composed of multiple
species of bacteria and their
secreted polysaccharide matrix and
bodily fluids.

11. Detection of food spoilage
Examples of action of bacteria involved in
food spoilage:
➢Lactic acid formation: Lactobacillus,
Leuconostoc.
➢Lipolysis: Pseudomonas, Alcaligenes,
Serratia, Micrococcus.
➢Pigment formation: Flavobacterium,
Serratia, Micrococcus.
➢Gas formation: Leuconostoc,
Lactobacillus, Proteus.
➢Slime or rope formation: Enterobacter,
Streptococcus.
Examples of action of molds
involved in food spoilage:
• Moulds: Some strains produce
mycotoxins under certain
conditions
• Aspergillus produces aflatoxin,
ochrtoxin, citrinin and patulin
• Fusarium-HT2
• Cladosporium-deoxynivalenol
• Alternaria-brefeldin, tentoxin

12. Detection of food spoilage
Mycotoxins
➢Hepatotoxins: aflatoxins, sporidesmins,
luteoskyrin
➢Nephrotoxins: ochratoxin, citrinin
➢GIT toxins: trichocetens
➢Neuro- and myotoxins: tremorgens,
citreoviridin
➢Dermatotoxins: verukarins, psoralen,
sporidesmins, trichocetes
➢Respiratory tract toxins: patulin
Food at risk of mycotoxins:
• Grains and grain products -
many mycotoxin types
• Peanuts, nuts and pulses –
aflatoxin
• Fruits and vegetables (raw
and preserved) – patulin
• Milk and milk products -
aflatoxin

13. Classification of foods by ease of
spoilage
1. Stable or non perishable foods:
• These foods which do not spoil unless handled carelessly include such
products as sugar, flour and dry beans.
2. Semi perishable foods:
• If these foods are properly handled and stored, they will remain
unspoiled for a fairly long period. Ex: Potatoes, apples etc.
3. Perishable foods:
• It includes most important daily foods that spoil readily unless special
preservative methods are used. Ex: Meats, fish, poultry, milk, eggs, fruits
and vegetables.

14. Microbial growth
• Competition between the different kinds of bacteria, yeasts and molds in a
food ordinarily determines which one will outgrow the others and cause its
characteristic type of spoilage.
• If conditions are favorable for all, bacteria usually grow faster than yeasts and
yeasts faster than molds.
• Microorganisms are not always antagonistic or antibiotic to each other,
however and may sometimes be symbiotic i.e., mutually helpful. Sometimes
they are synergistic i.e., when growing together they may be able to bring
about changes such as fermentations.
• Metabiosis in which when one organism makes conditions favorable for
growth of the second. Both organisms may be growing at the same time, but
more commonly one succeed the other. Ex: Raw milk at room temperature
normally first supports an acid fermentation by Streptococcus lactis and coli
form bacteria until the bacteria are inhibited by the acid they have produced.
Next the acid tolerant Lactobacilli increase the acidity further until they are
stopped.
• Then film yeasts and molds grow over the top, finally reducing the acidity so
that proteolytic bacteria can become active.

15. Chemical change caused by
microorganisms
• Different chemical changes are possible because great variety of organic
compounds are present in foods and numerous kinds of microorganisms
that can decompose them may grow in the food.
• Following changes are observed in foods.

16. Chemical change caused by
microorganisms
Changes in Nitrogenous organic compounds:
• Most of the nitrogen in foods is in the form of proteins. Proteins are
hydrolyzed to polypeptides, simpler peptides or amino acids before they
can serve as nitrogenous food for most organisms.
• Proteinases catalyze the hydrolysis of proteins to peptides gives bitter
taste to foods.
• Peptidases catalyze the hydrolysis of polypeptides to simpler peptides
and finally to amino acids.
Proteinase Peptidase Peptidase
Protein Polypeptides Peptide Amino Acids

17. Chemical change caused by microorganisms
• Anaerobic decomposition of proteins, peptides or amino acids result
in the production of obnoxious odors called putrefaction.
• Putrefaction results in foul smelling, Sulphur containing products such
as hydrogen, methyl and ethyl sulfides and mercaptans, plus
ammonia, amines (Ex: histamine, tyramine, piperidine, putrescine and
cadaverine), indole, skatole and fatty acids.
• When microorganisms act on amino acids, they may deaminate them,
decarboxylate them or both. Ex: Escherichia coli produces glyoxylic
acid, acetic acid, and ammonia from glycine.

18. Chemical change caused by microorganisms
Changes in Nitrogenous organic compounds:
• Pseudomonas produces methylamine and
CO2 ,Clostridia gives acetic acid, ammonia,
methane from alanine these three
organisms produces
1. α - Keto acid, ammonia and CO2
2. Acetic acid, ammonia and CO2
3. Propionic acid, acetic acid ammonia and
CO2 respectively.
• Desulfatomaculum nigrificans* an obligate
anaerobe, can reduce sulphate to sulphide
and produces H2S from cystine.
• Other nitrogenous compounds
include amides, amides and
urea from which ammonia is
the principal product.
• Guanidine and creatine which
yield urea and ammonia.
• Amines, purines and
pyrimidines which may yield
ammonia, CO2 and organic
acids.

19. Chemical change caused by
microorganisms
Changes in Non nitrogenous organic compounds:
• Main non nitrogenous foods for microorganisms, mostly used to obtain
energy but possibly serving as source of carbon, include carbohydrates,
organic acids, aldehydes and ketones, alcohols, glycosides, cyclic
compounds and lipids.

20. Chemical change caused by microorganisms
Carbohydrates:
• Carbohydrates act as energy source by microorganisms. Complex, di, tri or
polysaccharides usually are hydrolyzed to simple sugars before utilization.
• A monosaccharide (glucose) aerobically would be oxidized to carbon-dioxide and water.
• Glucose anaerobically decompose to -
a. An alcoholic fermentation by yeasts with ethanol and CO2 as the principal products.
b. A simple lactic fermentation as by homo-fermentative lactic acid bacteria.
c. A mixed lactic fermentation by hetero-fermentative lactic acid bacteria with lactic
and acetic acids, ethanol, glycerol and CO2 as the chief products.
d. The coli type of fermentation as by coliform bacteria with lactic, acetic formic acids,
ethanol, CO2, H2 etc.
e. The propionic acid fermentation by propionic bacterium
f. Butyric - butyl - isopropyl fermentations yields butyric and acetic acids, CO2 & H2.

21. Chemical change caused by microorganisms
Organic acids:
• Organic acids usually occurring in foods as salts are oxidized by
organisms to carbonates, causing medium to become alkaline.
Aerobically the organic acids may be oxidized completely to CO2 and
water. Saturated fatty acids or ketonic derivatives are degraded to acetic
acid.
• Other compounds:
• Alcohols usually oxidized to the corresponding organic acids. Ethanol to
acetic acid; Acetaldehyde to acetic acid.

22. Chemical change caused by microorganisms
Lipids:
• Fats are hydrolyzed to glycerol and fatty acids by lipase.
• Phospholipids may be degraded to their constituent phosphate, glycerol,
fatty acids and nitrogenous base.
Pectic substances:
• Protopectin in plants converted to pectin.
• Pectin is a water soluble polymer of galacturonic acids.
• Pectin esterase causes hydrolysis of the methyl ester linkage of pectin to
yield pectin acid and methanol.

23. Insects and animal
• Worms, bugs, weevils, fruit flies,
moths cause extensive damage
to food and reduce its nutritional
value and make it unfit for
human consumption.
• Some domestic animals may
spoiled foods by adding saliva
and germs followed by microbial
actions.

24. Sources of contamination
• Soil water sewage air or dust, skin of animals, food handlers, food
vessels, animal feed.

25. Thanks