Food microbiology

1. Physical and chemical changes
during food spoilage

2. What is food spoilage?
• Food spoilage is the process of changing the physical and chemical
properties of the food so that it becomes unfit for consumption.
• Food spoilage is any undesirable change in food.
• Most natural foods have a limited life: for example, fish, meat, milk and
bread are perishable foods, which means they have a short storage life and
they easily spoil.
• Other foods also decompose eventually, even though they keep for a
considerably longer time.
• The main cause of food spoilage is invasion by microorganisms such as
fungi and bacteria.
Food
Food
deterioration Economic loss
Food spoilage

3. Food groups
Highly Perishable
Meat
Fruit
Milk
Vegetables
Eggs
Semi perishable
Potatoes
Nuts
Flour
Stable (non-perishable)
Rice
Dry beans
Classification of Foods on the Basis of
Stability

4. Food nutrients
Food
Vitamins
Proteins
Lipids
Human Growth
Energy
Building materials
Carbohydrate

5. Major causes of food spoilage
Physical
• Temperature
• R.H.
• Light
• Mechanical damage
Chemical
• Enzymatic reaction
• Non enzymatic reactions
• Rancidity
• Chemical interaction
• Bacteria
• Yeast
• Molds
Others
Microorganisms
• Insects
• Rodents
• Animals
• Birds

6. Spoilage
The food may become unacceptable due to the following factors:
1) Growth and activities of micro-organisms principally bacteria, yeasts and molds
(This is by far the most important and common cause of food spoilage).
2) Activities of food enzymes (enzymatic browning is a common example).
3) Infestation by insects, parasites and rodents.
Biological hazards

7. Types of Spoilage
4) Chemical changes in a food (i.e. not catalyzed by enzymes of the tissues or of
micro-organisms). For example the chemical oxidation of fats producing rancidity as
well as non-enzymatic browning reactions in foods like Maillard Browning.
5) Physical changes or damages such as those caused by freezing (freezer burn), by
drying (caking) etc.
6) Presence of foreign bodies.
Physical hazards

8. Chemical changes
Chemical changes in a food (i.e. not catalyzed by enzymes of the tissues or of micro-
organisms). For example the chemical oxidation of fats producing rancidity as well as
non-enzymatic browning reactions in foods like Maillard Browning.
Chemical reactions take place in the presence of atmospheric oxygen and sunlight.
Two major chemical changes, which occur during the processing and storage of foods,
are lipid oxidation and non-enzymatic browning which deteriorate sensory quality,
colour and flavour .

9. Chemical changes
Lipid oxidation is influenced by light, oxygen, high temperature and the presence of
iron and copper, and water activity. Control of these factors can significantly reduce the
extent of lipid oxidation or rancidity in foods.
Non-enzymatic browning is one of the major causes of deterioration which takes place
during frying, cooking, storage of dried and concentrated foods through Maillard,
caramelization and ascorbic acid oxidation.

10. Chemical changes
Maillard reaction occurs due to reactions between reducing sugars and amino acids in
the presence of heat and results in formation of black brown insoluble pigments.
Caramelization of sugars occurs in presence of high heat and low moisture content in
the food. Oxidation of fatty acids to other chemicals like aldehydes, ketones, alcohols
and esters also results in off-flavours.

11. Chemical changes
Maillard reaction occurs due to reactions between reducing sugars and amino acids in
the presence of heat and results in formation of black brown insoluble pigments.
1912. Louis Camille Maillard a
French physician and chemist

12. Chemical changes
Maillard reaction occurs due to reactions between reducing sugars and amino acids in
the presence of heat and results in formation of black brown insoluble pigments.

13. Chemical changes
Maillard reaction occurs due to reactions between reducing sugars and amino acids in
the presence of heat and results in formation of black brown insoluble pigments.

14. Chemical changes
Caramelization of sugars occurs in presence of high heat and low moisture content in
the food.

15. Chemical changes
lipid oxidation or rancidity in foods.

16. Chemical changes
RANCIDITY
Hydrolytic Rancidity Oxidative Rancidity
Hydrolytic Rancidity
Triacylglycerol Glycerol Free fatty acids
(Volatile bad odour)

17. Chemical changes
Oxidative Rancidity

18. Chemical changes
Ascorbic acid
Chemical name for vitamin C is ascorbic acid, it’s chemical formula is C6H8O6.
As the name says, ascorbic acid is an acid. This means it can release protons (H+) and
lower the pH of a water solution.
By lowering the pH of a solution, it becomes more sour. Once ascorbic acid has given
away/released its proton, the name changes to ascorbate.
This reaction is reversible though, when conditions permit the proton can sit back on
the ascorbate.

19. Chemical changes
Ascorbic acid oxidation
‘donation’ of two electrons (e–), ascorbic acid also releases two protons. This will result
in a new stable molecule: dehydroascorbic acid

20. Chemical changes
 Hydroxylation of monophenol to o-diphenol
 Dehydrogenation of o- diphenol to o-quinone
ENZYMATIC BROWNING Polyphenol
oxidase
Phenolic
substrate
O2
O2
O2
O2
O2
O2
O2
Phenolic
substrate
Polyphenol
oxidase

21. Physical changes
Physical changes or damages such as those caused by freezing (freezer burn), by
drying (caking) etc.
Freezer Burn
Freezer burn is a condition that occurs when frozen food has been damaged by
dehydration and oxidation, due to air reaching the food. It is generally caused by food
not being securely wrapped in air-tight packaging.