Microbiology of food

MICROBIOLOGY OF FOOD

Food is an indispensable item for all living organisms. All food items are associated with microorganisms in one form or other. Foods get contaminated during handling, harvest, transport and storage. Foods also get contaminated due to the methods of food collection, cooking and preparation. Food forms an ideal culture medium for the growth and multiplication of microorganisms. Some of the microorganisms particularly pathogenic forms causing dreadful diseases and food poi-soning by their secretions. There are some microorganisms, which are useful for the preparation of different types of food and beverages. The microorganisms themselves play an important role in formation of food (eg.) single cell protein and mushrooms.

Common food items

Foods may be classified as a) fresh foods, b) preserved foods, c) canned foods, d) processed foods, e) fermented food products.

Common food items as follows:

Fruits and vegetables

Milk

egg

Meat

Fish

Poultry

Bread

Pickles

Syrup and juices

Products from milk, vegetables and fruits

Sources of microorganisms in foods

Foods receive the population of microorganisms from soil, plants, cooking vessels, by the use of contaminated water for washing and cooking and also due to unhygienic habitats of food handlers, intestinal tract of humans and animals, animal feeds, animal hides, air and dust.

Factors that influence the growth of the microorganisms

Many factors that influence the growth of the microorganisms in food. Some of the factors are intrinsic and some others are extrinsic.

a)     Intrinsic factors: The intrinsic factors include pH, moisture content, oxidation-reduction potential, nutrient status, antimicrobial constitu-ents and biological structures.

a)     Intrinsic factors: The intrinsic factors include pH, moisture content, oxidation-reduction potential, nutrient status, antimicrobial constitu-ents and biological structures.

i.                  pH : It has been well established that most of the microorgan-isms grow best at pH values around 7.0, while few grow below 4.0. Bacteria grow at more pH than molds and yeasts.

ii.                  Moisture content: The preservation of foods by drying is a direct consequence of removal of moisture, without which microorganisms do not grow. The water requirements of microorganisms should be defined in terms of the water activity (aw) in the environment. Water activity is defined by the ratio of the water vapour pressure of food substrate to the vapour pressure of pure water at the same tempera-ture (aw=P/po). The aw of most fresh food is above 0.99. The minimum value of aw for the growth of the microor-ganisms in foods should be around 0.86.

iii.                  Oxidation reduction potential: The O/R potential of a sub-strate may be defined generally as the ease with substrategains electrons. When an element or compound loses elec-trons, the substrate is said to be oxidized, while a substrate that gains electrons becomes reduced.

iv.                  Nutrient content: In order to grow and function normally, the microorganisms of importance in foods require water, source of energy, source of nitrogen, vitamins and related growth fac-tors and minerals.

v.                  Antimicrobial constituents: The stability of foods against attack by microorganisms is due to the presence of certain naturally occurring substances that have been shown to have antimicro-bial activity. Some species contain essential oils that possess antimicrobial activity. Among these are eugenol in cloves, alli-cin in garlic, cinnamic aldehyde and eugenol in cinanmon.

b)          Extrinsic factors: These include those properties of the storageenvironment that affect both the foods and their microorganism. The following extrinsic factors affect the growth of microorgan-isms: Storage temperature, pH, presence and concentration of gases in the environment.

Causes of food spoilage

Food spoilage refers to the process where the food is made useless, bad and unfit for eating. It alters the chemical proportion ap-pearance, texture, colour, taste, flavour, odour and stability of the food. The altered food is called spoiled food. Food is spoiled by many fac-tors such as a) microorganisms, b) insects, c) rough handling, d) trans-port, e) improper storage, f) enzyme activity, g) unhygienic conditions.

Causes of food poisoning

Food poisoning refers to the toxicity introduced into food by microorganism and their products. Food poisoning is caused by various factors as follows:

a)           Poisons derived from plant and animal sources.

b)    Such standard chemicals added to the food.

c)     Excess use of preservation in food.

d)    Presence of higher population of microorganisms in food

e)     Toxins produced by various types of microorganisms.

Types of food poisoning

There are two types of food poisoning.

1)          Food intoxication. eg) Botulism, Staphylococcal food poi-soning.

2)          Food infection. eg) Shigellosis (Bacillary dysentry), Enteropathogen Escherichia, Cholera, Brucellosis.

Food borne diseases

       The common food borne diseases are Botulism, Staphylococcal food poisoning, enterococcus food poisoning, Traveller’s diarrhoea, Mycotoxicosis, Sligellosis, Enteropathogenic Escherichia, Cholera, Brucellosis, Tuberculosis and Tularemia.

i)             Botulism : Botulism is a food borne disease due to exotoxinproduced by the bacteriumsources of this disease are canned food and preserved foods. This disease affects the nervous system so it is called neurotoxin.

ii)          Staphylococcal food poisoning or staphylococcal entero-toxemia : The causative organism for the disease isStaphylococ-cus aureus. The main sources for the disease are potato salad,cream-filled bakery goods and dry skim milk. The disease is char-acterized by sudden nausea, vomiting and diarrhoea.

Enterococcus food poisoning : The causative organism forthis disease is trepcococcus faecalis. It is frequently found in the intestinal tract of human and animal. The disease is characterized by nausea, frequently vomiting, colicky pain and diarrhoea.