Microbial growth and food spoilage results from microbes altering food, rendering it unsuitable for consumption. Different foods undergo specific spoilage processes due to their unique microbial loads and compositions. Approximately 1/3 of the world's food is lost to spoilage each year. Minimizing contamination through good management practices and preservation procedures can help reduce food spoilage. Different types of microbes degrade the chemical components of foods in characteristic ways, impacting appearance, flavor, and other qualities. Specific foods are prone to spoilage by certain microorganisms depending on their nutrients and conditions.

1. Food Spoilage

2. Microbial Growth and Food Spoilage
• food spoilage
• results from growth of microbes in food
• alters food visibly and in other ways, rendering it unsuitable for
consumption
• involves predictable succession of microbes
• different foods undergo different types of spoilage processes
• toxins are sometimes produced
• algal toxins may contaminate shellfish and finfish

3. • Change in appearance, smell or taste
• Economic loss
• Reduced quality
• Higher prices
• E.g. fruits, vegetables, meat, dairy products contains-
• Microorganisms- airborne, soil borne, food borne, waterborne, human
handling

4. Food Spoilage
•Approximately 1/3rd of all food manufactured in
world is lost to spoilage
•Microbial content of foods (microbial load):
qualitative (which bugs) and quantitative (how many
bugs)
•Shelf life
•Non-perishable foods (pasta)
•Semiperishable foods (bread)
•Perishable foods (eggs)

5. General Principles
•Minimize contamination by:
•Good management processes
•Acceptable sanitary practices
•Rapid movement of food through processing plant
•Well-tested preservation procedures

6. Spoilage
•Meat
•Cutting board contamination
•Conveyor belts
•Temperature
•Failure to distribute quickly
•Fecal bacteria from intestines
•Fish
•Polluted waters
•Transportation boxes

7. Spoilage
•Poultry and Eggs
•Human contact
•Penetration by bacteria
•Milk and Dairy Products
•Lactobacillus and Streptococcus species that survive
pasteurization (sour milk)
•Breads
•Spores and fungi that survive baking
•Grains
•Fungi produce toxins

8. Chemistry in Microbial Spoilage of Foods
• Food (Plant or Animal) composed of-
carbohydrates
proteins
fats
vitamins and minerals
• These serve as good media for the growth of many different microorganisms
• Microorganisms produces changes in food-
appearance
flavour
odour and some other qualities
These changes are due to degradation of the chemical constituents of the
food.

9. Carbohydrate foods
(plant products)
+
Carbohydrate fermenting
microorganisms
• Acids
• Alcohol
• Gases
Protein foods
(animal products)
+
Proteolytic microorganisms
• Amino acids
• Amines
• Ammonia
• H2S
Fatty foods
(oils and fats)
+
Lipolytic microorganisms
• Fatty acids
• Glycerol
Mechanism of microbial degradation of different kinds of foods

10. Microbiology of Spoilage of Specific Foods
Cereal Grains
- wheat, corn, rye, rice
• Microbes enter from field or storage environment
• High in carbohydrates and proteins
• But microbes do not grow due to low aw if harvested and stored properly
• Under moist conditions- microorganisms is stimulated and cause spoilage.
• Tropical climates- common moulds-
• Rhizopus stolonifera
• Penicillium
• Aspergillus
• Alternaria
• Fusarium

11. Bread
-staple baked food made up of
flour, water and salt
leavened by yeast Saccharomyces cerevisiae
• Ingredients used are generally the source of spoilage microorganisms
• Most contaminants are killed during baking (100’C for 9mins)
• Some molds and endospores producing bacteria survive
• Rhizopus nigricans; commonly called the bread mold fungus
Low water activity prevents growth of bacteria.
Exceptions-
Aerobic, spore-forming rods of genus Bacillus
B. subtilis, B. pumilis and B. mesentericus

12. Cakes
-Molds
Sources- ingredients- sugar, nuts and spices
Airborne
Human handling
High humidity
Bacterial spoilage is rare due to high sugar conc.

13. Fresh Vegetables
-88% water
-8.6% carbohydrates
-9% proteins
-0.3% fats
-<1% vitamins, nucleic acids and other constituents
All these support the growth of molds, yeasts and bacteria.
Fresh Fruits
<85% water
0.9% proteins
0.5% fat
13% carbohydrates

14. Juices
-pH of 4.0 or below (fruit juices)
-pH of 4.5 or above (vegetable juices)
-Lactobacillus, Leuconostoc and Acetobacter are common spoilage agents.

15. Assignment
Write short notes microbial spoilage of food for:
1. Cereal Grains
2. Bread
3. Fruits and Vegetables
4. Juices
5. Meat- Raw meat; Fresh meat; Ground meats
6. Poultry
7. Eggs
8. Fish
9. Milk
10. Tea
11. Coffee
12. Cocoa
• Constituents /Compositions
• Common spoilage condition
• Common spoilage agents
Note: Scientific names should be
in italics or underlined if hand
written.