There are various types of interactions between microorganisms and other living organisms. These interactions are natural, constant and also play a significant role in maintaining the ecological balance and stability of the biogeochemical cycle of the nature. Mostly, the food products we consume are obtained from plants and animals and also these foods are rich in variety of microorganisms which may or may not be pathogenic to humans. Growth of microorganisms in food depends on various different parameters which can be broadly classified as INTRINSIC and EXTRINSIC factors.
INTRINSIC FACTORS –
These are the factors that are present in the food substance in which the microorganism is growing, or it may be said as the internal factors of that particular food substrate. Various intrinsic factors are-
- Hydrogen Ion Concentration – All the microorganisms have minimal, maximal, or optimal pH for their growth and survival. Thus, the growth of microorganisms in food is affected by the pH of the food material. Foods may be classified as low pH or high pH foods. Most fruits, fermented foods come under high acid foods whereas most vegetables, meat, fish and milk are low acid foods. pH range of different microorganisms are :
• Molds – 1.5-9.0
• Yeasts – 2.0-8.5
• Gram-positive bacteria – 4.0-8.5
• Gram-negative bacteria – 4.5-9.0
- Water activity or moisture content – Water activity can be defined as the measure of availability of water present in any substance which can be used for biological functions and it also gives an idea of free water present in any food product. Water is an excellent requirement for microorganism for their growth. It has been observed and noted that the water activity of fresh food substances is 0.99. Also bacteria require more water activity i.e. free water in any food substrate for their growth than molds and yeasts. If specifically studied, it will be observed that gram-negative bacteria have relatively higher water requirements than gram-positive bacteria. Free water in any food substance is an essential requirement for the growth of microorganisms. Water activity of any food can be reduced by various absorption techniques to reduce the affect of spoilage by microorganisms.
- Redox Potential – Redox potential can be defined as the reducing and oxidizing power of food and it also greatly influences the growth of microorganisms in food. The concentration of oxygen present in any food sample determines the type of microorganism that will grow in it. Like, aerobic microbes require the oxygen whereas anaerobic microbes can also grow in lack of oxygen. So, it can be said aerobes grow at positive O-R potential whereas anaerobes grow at negative O-R potential.
- Composition of nutrients – Food composition is also an another intrinsic factor which influences the growth of microorganisms in food. There are 5 major nutrients group which are counted i.e. carbohydrates, proteins, lipids, vitamins and minerals, amount of each varies with the type of food and so the type and growth of microorganisms. Bacteria require the most nutrient requirement than yeasts and molds. Microorganisms utilize large complex nutrient molecules and convert them into smaller molecules. For e.g. there are some proteolytic bacteria which acts on proteins and hydrolyze it. Also some microbes convert lipids into glycerol with the help of an enzyme lipase. Some microorganisms which require vitamins for their growth are called as fastidious microorganisms.
- Inhibitory substances – There are a number of inhibitory substances that are present in foods by their origin which naturally prevent the growth of microorganisms in them. For e.g. some plants contain essential oils possessing antimicrobial properties. Milk also contains several antimicrobials like lactoferrin, conglutinin, etc.
- Biological structures – The natural structure of some foods have the remarkable excellence in controlling the entry as well as the growth of microorganisms in or on them. It can be noted as the natural covering of some foods which prevents the entry of microbes. So, the inner parts of the healthy tissues are sterile and possess very less microbial count. For e.g. skin of egg, rind on fruits, etc.