Food Microbiology & Spoilage

What is food spoilage?

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

As well as food spoilage is defined as “damage or injury to food rendering in unsuitable for human consumption”.

Spoilage commences in food as soon as it is harvested, taken from the sea or slaughtered.

In most cases there does not need to be an evident sign of spoilage, the food might look normal and only after eating it or by careful bacteriological and toxicological investigation, one is able to realize the defect.

Causes of food spoilage

1. Growth and activity of microorganisms

  • Bacteria, yeasts and molds are microorganisms that cause food spoilage. They produce various enzymes that decompose the various constituents of food.

2. Enzyme activity

  • Action of enzymes found inherently in plant or animal tissues start the decomposition of various food components after death of plant or animal.

3. Chemical reactions

  • These are reactions that are not catalyzed by enzymes (ex:- oxidation of fat).

4. Vermin

Vermin includes ants, rats, cockroaches, mice, birds, larval stages of some insects. Vermin are important due to:

  • Aesthetic aspect of their presence.
  • Possible transmission of pathogenic agents.
  • Consumption of food.

5. Physical changes

  • These include those changes caused by freezing, burning, drying, pressure, etc…

As well as these are the factors results in damage and accelerates the spoilage

  • Poor personal & kitchen hygiene practices.
  • Poor temperature controlling practices.
  • Unsuitable packing and storing practices.
  • Rough handling.

Signs of food spoilage

  • Off odors
  • Discoloration
  • Slime
  • Stickiness
  • Mould growth
  • Change in texture
  • Unusual texture
  • The production of gas
  • Blown cans or packs
Moulds in a Bread

Microbial spoilage of food

  • Bacteria, yeasts and molds (Fungi) are the major causes of food spoilage.
  • They produce various enzymes that decompose the various constituents of food.
  • MO are the major causes of spoilage of foods with reduced water activity (ex:- dry cereals and cereal product).

Sources of microorganisms in food

The primary sources of microorganisms in food include:-

  1. Soil and water.
  2. Plant and plant products.
  3. Food utensils.
  4. Intestinal tract of man and animals.
  5. Food handlers.
  6. Animal skins.
  7. Air and dust.

Action of bacteria involved in food spoilage

  • Lactic acid formation:- Lactobacillus, Leuconostoc
  • Lipolysis:- Pseudomonas, Alcaligenes, Serratia, Micrococcus
  • Pigment formation:- Flavobacterium, Serratia, Micrococcus
  • Gas formation:- Lactobacillus, Leuconostoc, Proteus

Factors affecting microbial growth in food

1. Intrinsic factors

These are inherent in the food. They include:-

  • Hydrogen ion concentration (pH).
  • Moisture content.
  • Nutrient content of the food.
  • Antimicrobial substances.
  • Biological structures.

Hydrogen ion concentration (pH)

  • Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 to 7.5 .
  • Some bacteria can grow within a pH range of 4.5 and 9.0 (ex:- salmonella).
  • Other microorganisms especially yeasts, molds and some other bacteria grow within a wide pH range (ex:- molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and 8.5).
  • Microorganisms that are able to grow in acid environment are called “acidophilic microorganisms”.
  • These microorganisms are able to grow at pH of around 2.0 .
  • Other microorganisms such as “vibrio cholerae” are sensitive to acids and prefer alkaline conditions.
  • Most bacteria are killed in strong acid or strong alkaline environment except “mycobacteria”.

Moisture content

  • The effect of moisture is in terms of water activity:- “the amount of free water in a food medium”.
  • The amount of free water is important for growth of microorganisms.
  • If there is lack of this free water, microorganisms will not grow.

“free water means the water we using for cooking and drinking, germs and bacteria can grow very easily in this water”.

Nutrients content of the food

  • Microorganisms require proteins, carbohydrates, lipids, water, energy, nitrogen, sulfur, phosphorus, vitamins, and minerals for growth.
  • Various foods have specific nutrients that help in microbial growth.
  • Foods such as milk, meat and eggs contain a number of nutrients that are required by microorganisms.
  • These foods are hence susceptible to microbial spoilage.

Antimicrobial substances

  • Antimicrobial substances in food inhibit microbial growth.
  • Various foods have inherent antimicrobial substances that prevent (inhibit) microbial attack.
  • Such inhibitors are like lactinin and anti-coliform factors in milk and lysozyme in eggs.

Biological structures

  • Some foods have biological structures that prevent microbial entry.
  • For example, meat has skin and other membranes that prevent microbial entry.
  • Eggs have shell and inner membranes that prevent yolk and egg white from infection.

2. Extrinsic factors

•These are factors external to the food that affect microbial growth. That include:-

  • Temperature of storage.
  • Presence and concentration of gases in the environment.
  • Relative humidity of food storage environment.


  • The growth of microorganisms is affected by the environmental temperatures.
  • Various microorganisms are able to grow at certain temperatures and not others.
  • Bacteria can therefore be divided into the following groups depending upon their optimum temperature of growth.
  • Microorganisms thrive at a wide range of temperatures, they have colonized different natural environments and have adapted to extreme temperatures.
    • Psychrophiles grow best in the temperature range of 0 – 15 °C.
    • Psychrotrophs thrive between 4°C to 25 °C.
    • Mesophiles grow best at moderate temperatures in the range of 20 °C to about 45 °C.
    • Thermophiles 45 °C – 75 °C.
    • Hyperthemophiles are adapted to life at temperatures above 80 °C.

The effect of temperature on microbial growth also depends upon other environmental conditions such as:-

  • Growth factors in the nutrient medium.
  • pH of the food.
  • Water activity.

Concentration of gases in the environment

  • This relates to the presence and concentration of gases in the food environment.
  • Various microorganisms require for growth, either high oxygen tension (aerobic), low oxygen tension (micro-aerobic) or absence of oxygen (anaerobic).
  • Some microorganisms may grow either in high oxygen tension or in the absence of oxygen (facultative anaerobes).

Foods affected by various groups

  • Anaerobic or facultatively anaerobic sporeformers are most likely to grow in canned foods.
  • Microaerophilic bacteria are most likely to grow in vacuum packed foods since they have low oxygen tension.
  • Aerobic bacteria are likely to grow on the surface of raw meat.


01/05/2020 by @cheflakey

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