Factors Affecting Microbial Growth in Food

Factors Affecting Microbial Growth in Food

	Factors Affecting Microbial Growth in Food - A variety of factors such as the pH, moisture content, oxidation reduction potential, nutrients, etc., influence microbial activity in foods. For example, each organism has an optimum pH for growth. In general, yeast and fungi are more acid tolerant than bacteria. Most yeasts favour a pH around 4-4.5 while fungi can tolerate a pH much below, that. Most bacteria favour a pH around 7.0. Thus, both the growth as well as their survival in foods depends on the pH of the food material. The pH of foods varies; some may be neutral while others may be acidic.
Acid foods (pH below 4.5) are not readily spoiled by bacteria but are susceptible to spoilage by yeast and molds. Foods may have a low pH either because of inherent acidity as in fruits and soft drinks or develop acidity as a result of microbial activity as in lactic acid fermentation. Microorganisms have an absolute, demand for water and the optimum level, of moisture required for growth varies with, the organisms. The water requirement is expressed in terms of available water or water/activity (aw), which is the vapour pressure of the solution divided by the vapour pressure of the solvent. This is equal to the vapour pressure of the solutions in water divided by vapour pressure of the water.
The aw for pure water is 1.00. Each organism has a maximal, optimal and a minimal aw for growth. Most bacteria grow well in a medium of aw activity around 0.995 to 0.998. Molds differ considerably in the optimal aw. For example, Rhizopus sp., has an optimal aw of 0.995-0.980, while Penicillium sp., and has an optimal aw of 0.9935. Also, each mold has an optimal as well as a range of aw for growth. The aw value of a food is affected by the vapour pressure of solutes such as sugars, salts, hydrophilic colloids or gels. An increase in the concentration of sugars and salts allows the water to be tied up and also causes the removal of water from the microbial cells, The aw value of the food therefore, determines to considerable extent the type of organism that can grow in it.
The oxygen tension or partial pressure of oxygen and the reducing and oxidizing power of the food (O-R potential) influences the growth of organisms. In relation to oxygen, bacteria can be aerobic, anaerobic or facultative, while fungi are mostly aerobic. Yeast are aerobic or facultatively anaerobic. A high O-R potential favours the growth of aerobic and facultative organisms. Sometimes growth of an aerobe may reduce the O-R potential of food to restrain the growth of other organisms. Most fresh animal and plant foods have a low O-R potential in their interior but have a higher O-R outside. Thus, a fresh piece of meat could support the growth of aerobic organisms in the exterior and the growth of anaerobic organisms inside.
	Nutrients in food, their kind and proportions determine the type of organism that will grow. Also, microorganisms vary in their ability to use nutrients. The presence of easily utilizable nutrients will encourage faster growth and quicker damage. For example, a food with easily utilizable sugars will allow better growth than one which contains polysaccharides. Also, the concentration of the sugars will determine the type and extent of growth, since it affects both the osmotic pressure and the aw- Generally, yeasts and molds are more resistant to high concentrations of sugar than bacteria.

Most foods contain enough peptides and amino acids that they can meet the nitrogen requirement of most organisms found in foods. Some organisms are also proteolytic and can grow on proteins found in the food. The mineral requirement of microorganisms is generally met by the food and this is not a limiting factor. Some foods may contain antibacterial substances which may prevent bacterial growth and food spoilage. For example, egg is rich in lysozyme and thus, even if the bacteria cross the outer shell (If the egg, they are destroyed by the lysozyme before they can cause any damage.

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