Food Microbiology

Control Of Microbial Growth In Foods

Because they are nutrient rich, foods are excellent environment for the growth of microorganisms. However, growth of microorganisms in food can be controlled by two ways. One is by adding preservatives in the food, called intrinsic factors, and by manipulating the storage environment of food, called extrinsic factors.

Intrinsic factors or food related factors include pH, moisture content, water activity or availability, oxidation-reduction potential, physical structure of the food, available nutrients, and the possible presence of natural anti microbial agents.

Extrinsic or environmental factors include temperature, relative humidity, gases (CO2, O2) present, and the types and numbers of microorganisms present in the food.

Intrinsic Factors

Food composition is a critical intrinsic factor that influences microbial growth. If a food  rich in carbohydrates, fungal, rather then bacteria, growth predominates and spoilage does not result in major odors. Thus foods such as bread, jams, and some fruits first show spoilage by fungal growth. In contrast, when a food contain protein and fats in large amount (e.g. butter and meat), bacterial growth can produce a variety of foul odors. This anaerobic breakdown of proteins yields foul-smelling amine compounds and is called putrefaction. The production of short – chained fatty acids from fats renders butter rancid and foul smelling.

The pH of food is another critical factor. Low pH favors the growth of yeasts and molds. In neutral or alkaline pH foods, such as meats, bacteria are more dominant in spoilage and putrefaction. Depending on the major constituent in food, different types of spoilage may occur.

The availability of water in foods also affect the growth of microorganisms to colonize foods. Simply by drying a feed, one can control or eliminate spoilage processes. Water, even if present, can be made less available by adding solutes such as sugar and salt.

When large quantity of salt or sugar are added to food, most microorganisms are dehydrated by the hypertonic condition and cannot grow.

Even under these adverse conditions, osmophilic and xerophilic microorganisms may spoil food. osmophilic microorganisms grow best in or on media with a high osmotic concentration, where as xerophilic microorganismsprefer a low water availability and may not grow under high water conditions.

The oxidation – reduction potential of a food also influences spoilage. When meat products, especially broths, are cooked, they often have lower oxidation – reduction potentials – that is, they present a reducing environment for microbial growth. These products with their readily available amino acids, peptides, and growth factors are ideal media for the growth of anaerobes, including Clostridium.

Many food contain natural anti microbial substances, including complex chemical inhibitors and enzymes. Coumarins found in fruits and vegetables exhibit antimicrobial substances. Eggs are rich in the enzyme lysozyme that can lose the cell walls of contaminating gram-positive bacteria.

Herbs and spices often possess significant antimicrobial substances; generally fungi are more sensitive than most bacteria. Sage and rosemary are two of the most antimicrobial spices. Aldehydes and phenolic compounds that inhibit microbial growth are found in cinnamon, mustard, and oregano. Other important inhibitors are garlic, which contains allicia, cloves, which have eugenol, and basil, which contains rosmarinic acid.

Extrinsic Factors

Temperature and relative humidity are important extrinsic factors in determining whether a food will spoil. At higher relative humidities microbial growth is initiated more rapidly, even at low temperature. When drier foods are placed in moist environments, moisture absorption can occur on the food surface, eventually allowing microbial growth.The atmosphere in which food is stored also is important. The observation that food storage atmosphere is important has led to the development of modified atmosphere packaging (MAP).  Modern shrink – wrap materials and vacuum technology make it possible to package foods with controlled technology make it possible to package foods with controlled atmospheres. These materials are largely impermeable to oxygen. This prolongs shelf – life by a factor of two to five times compared to the some product packed in air.

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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