The following points highlight the two main factors that lead to the spoilage of food. The factors are: 1. Storage Conditions 2. Food’s Own Chemical Properties.
Factor # 1. Storage Conditions:
Temperature and oxygen are considered two most important factors that invite microbial contamination resulting in spoilage of foods in storage conditions.
1. Role of temperature:
Foods stored at below -17°C remain free from microbial growth and a slow decrease in their population may even take place. Above this temperature, the presence and multiplication of microorganisms in food is usually recorded.
This is the reason why refrigerated foods are subject to spoilage by microorganisms. Food and food items stored at room temperature or in warm conditions remain open for spoilage by mesophilic and thermophilic microorganisms.
2. Role of oxygen:
Aerobic and anaerobic conditions play an important role in determining the kinds of microorganisms which can multiply and spoil various food and food-items in storage conditions. If oxygen is available, various aerobic bacteria and moulds cause spoilage chiefly surface spoilage, whereas if the conditions are anaerobic the spoilage is caused by anaerobic, bacteria like Clostridium spp., etc.
Factor # 2. Food’s Own Chemical Properties:
The chemical conditions of foods influence the type of microorganisms which can grow over and within it and hence determine the nature of changes that would be brought by the spoilage action of contaminating microorganisms. Four major chemical conditions of food, e.g., composition, acidity, moisture, and osmotic concentration are of major importance in this type of spoilage.
1. Chemical composition:
(i) Foods rich in proteins are degraded by proteolytic microorganisms. Proteins are degraded into its various components due to the action of especially gram-negative, spore forming bacteria, e.g., Proteus, Pseudomonas, some cocci, etc.
Protein foods + Proteolytic microbes → Amino acids + Amines + Ammonia + Hydrogen sulfide
(ii) Foods rich in carbohydrates are degraded by carbohydrate fermenting microorganisms, particularly yeasts and moulds. Bacteria like Micrococcus, Leuconostoc, and Streptococcus can also degrade carbohydrates.
Carbohydrate foods + Carbohydrate fermenting microorganisms → Acids + Alcohols + Gases
(iii) Foods rich in fats are attacked by relatively few microorganisms such as moulds and some gram- negative bacteria. These microorganisms are, therefore, lipolytic in nature.
Fatty foods + Lipolytic microorganisms →Fatty acids + Glycerol
Generally the fruits are acid foods (pH below 4.5) while nearly all vegetables, fish, meats, and milk-products are non-acid (pH above 4.5). Since the pH of the acid foods (fruits) is sufficiently low, they do not allow bacterial growth and subsequent spoilage. They are spoiled mainly by yeasts and moulds. Contrary to this, non-acid foods have sufficiently high pH and are spoiled mainly by bacteria.
3. Moisture and osmotic concentration:
Average 13% free water is required in food for usual microbial growth. This is the reason why the foods of high sugar and salt concentrations do not allow most of the microorganisms to grow. But, specific microbial growths cannot be over-ruled. 65-70% sugar concentration is required to prevent mould-growth and 50% to prevent bacterial and yeast growth.