Growth of microbes and their propagation in the food is one of the important factors to food spoilage. They decrease the nutritive value and wholesomeness of food. Hence, the knowledge of the factors that favour or inhibit growth of microorganisms is essential to an understanding of the principles of food spoilage or preservation. The various factors that affect the growth of microbes are given as follows:
Nutrients and other constituents in food: Nutrients in food, their kinds and proportions, are all important in determining what organism is most likely to grow. Proteins are degraded by the proteolytic enzymes present in aerobic and anaerobic microbes like bacteria and moulds. The aerobic degradation does not result into objectionable products, but anaerobic degradation of protein may result into awful odour and is called putrefaction. Smell of hydrogen sulphide (H2S) and ammonia (NH3) is common during putrefaction.
Carbohydrates are the most preferred nutrients for microbial growth and are affected primarily by yeast and moulds. Microbes degrade carbohydrates in food resulting in the production of alcohol, lactic acid, acetic acid, propionic acid, butyric acid etc.
Microbes break down fats in food resulting in the production of fatty acids and glycerol. Fats are mainly digested by moulds and few gram negative bacteria. Organic acids are mostly present as salts in food. These are oxidized to carbonates by the microbes making the food alkaline.
Acidity: Foods having pH below 4.5 and above 4.5 are classified as acidic and non-acidic foods, respectively. Acidic foods are mainly spoiled by moulds and yeasts whereas the non-acidic foods are particularly attacked by bacterial spoilage.
Water content: Microbes require at least 13 per cent free water for their growth and it is best expressed in terms of available water or water activity (aw). The aw of pure water is 1.0 and many bacteria, yeasts and moulds require 0.91, 0.88 and 0.80 aw, respectively. Foods preserved with salt or sugar concentration do not support growth of most microbes. For example salt concentration of 5-15 per cent inhibits growth of bacteria whereas many moulds and some yeasts can tolerate more than 15 per cent. Sugar concentration of 65 per cent and above is required to inhibit mould growth whereas 50 per cent do not allow bacteria and most yeasts.
Physical and environmental factors: Temperature of food affects the growth of microbes. Temperature in the range 20-50°C is most suitable for the growth of microbes. Most of the bacterial pathogens of fruits and vegetables will grow between 6 and 35°C. Some fungi (Botrytis cinerea) will survive and even grow at low temperatures, 1°C, on agricultural produce, whereas Botryodiplodia theobromae or Aspergillus niger cause losses in warm regions.
Availability of oxygen: Anaerobic micro-organisms are mainly involved in the spoilage of air tight canned food under anaerobic conditions. Oxygen is necessary for mould growth.
Presence of inhibitory substances: Inhibitory substances originally present in food, added purposely or accidentally or developed thereby prevent growth of microorganisms.
Biological structure: The biological structure of food has a protective function against food spoilage. The inner parts of whole, healthy tissues of living plants and animals are either sterile or low in microbial content. An increase in exposed surface due to peeling, skinning, chopping or mincing distributes spoilage.