Food is rendered unfit for human consumption due to the physical and chemical changes taking place in it as a result of the influences of air, heat, light and moisture, which foster the growth of micro­organisms. Foods take different periods of time to lose their natural form through spoilage. Perishable foods like meat, fish, milk and many fruits and vegetables begin to deteriorate immediately unless properly preserved.

Semi-perishable foods like eggs, onions and potatoes can be kept for several weeks in a cool dry place. Non­perishable foods like cereals, pulses and nuts can be stored for long periods of time. The major causes of food spoilage are discussed here.


The word micro-organisms refers to different microscopic forms of both plant an animal life such as moulds, yeast, bacteria and even viruses. Among these, moulds, yeast and bacteria may be used to produce desirable changes in foods, but mostly they are agents of food spoilage.

1. Bacteria

Although variations occur in the size, shape and structure of bacteria by altering the environment in which they grow, yet in general only three main shapes of bacteria exist: spheres called cocci, rods called bacilli and twisted rods called spirilla. They also vary in their requirements for food, moisture, pH (acid-base balance), temperature and oxygen. Bacteria are capable of withstanding ex­tremes of temperature. They may be classified according to the temperature ranges into three general groups.


Therefore, psychrophilic bacteria are those organisms which play an important part in the spoilage of food in the refrigerator and in cold storages, Kneaded dought left in the refrigerator shows grey or black specks use to the activity of psychrophilic bacteria. The food and canning industry and milk processing plants are greatly affected by thermophilic bacteria-those organisms which are capable of with­standing high temperatures.

Since bacteria may be aerobic or anaero­bic, they are likely to flourish anywhere and everywhere. Some of them may cause food spoilage while others may cause food poisoning and diseases borne through food. How intensive a bacterial attack may be can be gauged by the magnitude of their strength and the numbers they exist in. The weight of the bacterium depends upon its size and density.

It has been calculated that it would take five billion bacteria to weigh one gram. Thus a drop of milk may harbour a number of bacteria exceeding the entire human population of the city of New Delhi and yet have room to spare !

2. Moulds

Moulds are multicellular, filamentous fungi that contain spores which can spread through the air and start new mould plants. When these spores find a favourable environment, they germinate and produce a fluffy growth, often white or gray, but sometimes bluish green, red, orange or some other colour depending upon the variety of the mould.


Most moulds grow between 25°-30°C in warm damp places; some moulds can grow even at refrigerator temperature. While most bacteria spoil foods which are neutral in reaction, moulds thrive in an environment where the pH is too low for usual bacterial activity. Organic acids which the bacteria in general cannot tolerate, may be metabolised by moulds as a source of energy, and these acids may be oxidised to carbon dixoide and water. As the acids are oxidised, the pH may rise to the point where bacterial growth may become possible.

Even a high osmotic pressure does not deter the growth of fungi, as is seen by the mould growth on the surface of jellies and jams which have a high sugar content. They also grow on acid foods such as lemon and on neutral foods such as bread and other starchy foods which are spoilt by the Rhizopus species during the summer months. The green fuzz seen on decaying fruits is usually a member of the penicillium genus. Most moulds are not harmful.

A small proportion of moulds found on food stuffs is capable of producing toxic materials known as mycotoxins. The best known of these are the aflatoxins produced by moulds growing on peanuts, ragi, wheat and millet which have not been dried as soon as they are harvested. Research has shown that some moulds also carry carcinogens in them.

3. Yeasts

Yeasts are considered to be those unicellular chlorophyll-free fungi that permanently maintain a unicellular growth form, not developing mycelia. They require for their growth, water and a source of energy which is usually sugar. Many foods which are not usually attacked by bacteria because of the low pH are readily accepted by yeasts as a growing ground because of the low pH level which they can tolerate and thrive upon.


The growth is most rapid at temperatures between 25°-30°C. Since sugar serves as a source of energy to yeast, it is generally found in places where sugar is available. Yeasts find their way into the ground when they are washed or blown from the surface of fruits, particularly grapes. The nectar of flowers and the exuding sap of trees and plants may contain large numbers of yeasts, which are carried to distant places by wind and insects.

The yeast cells which are always present in the atmosphere may contaminate food and cause its spoilage. They produce pigments and undesirable chemical products during their metabolism. Yeasts may cause spoilage of fruit juices, syrups, molasses, honey, jellies and other foods, converting their sugar into alcohol and carbon dioxide. Controlled action of yeast is now used to advantage in food industry.


Yeast grows on substances rich in sugar and carbohydrates. It ferments the carbohydrates and converts them into alcohol and carbondioxide. This property is now widely used in the modern food industry for the preparation of wines and other alcoholic beverages. Grapes, sugar cane juice, beetroot, rice, barley and potatoes are some of the foods used for making wine and beer.


Yeast is used as a leavening agent in bread making. It gives the required flavour and sponginess to the bread, Yeast is a rich source of vitamin B. Yeast tablets are of great therapeutic value at low cost.


It is noticed that foods belonging to the perishable category get spoilt faster as compared to those belonging to the semi-perishable or non-perishable categories. The obvious difference in the three catego­ries is their water-content. Bacteria are more aquatic than terrestrial and in the presence of a high percentage of moisture they thrive. Although most bacteria thrive at a higher concentration of moisture, they require very little water to keep alive. Thus drying of foods as a means of preservation is based upon the fact that moisture is needed to sustain microbial activity. Drying will not kill all bacteria. Upon hydrating certain dehydrated foods might undergo spoilage.


Enzymes are organic catalysts, produced by living cells. The life of every cell of plant or animal tissue depends upon the chemical reactions activated by these organic catalysts. Minute amounts of enzymes can catalyst substrate one million times heavier than the enzyme, without the enzyme itself being used up in the reaction. Chemically, enzymes are proteins in nature and hence may be denatured by heat.

Food undergoes changes during storage. These changes may be produced by the enzymes of the food itself or by the enzymes formed by the micro-organisms which contaminate the food. For example, enzymes within a raw fruit help it to ripen.


At the same time, the enzymes of contaminating yeasts might produce germination in a bruised spot, resulting in the spoilage of the fruit. Similarly, the enzymes in meat are responsible for some of the increased tenderness that occurs in it during storage. Enzymes from other sources can be added to meat to increase its tenderness.

The optimum temperature at which most enzymes act rapidly is about 37° C, but they are rendered inactive by heating.


Worms; bugs, weevils, fruit-flies, moths and other insects cause extensive damage to food and man. They strike in two ways:

(i) Direct Damage

They may render food grains, pulses and other food products unfit for human consumption by eating the kernel, contaminating the food and destroying the structures and containers.

(ii) Indirect Damage


Insects can assist in the dissemination of moulds and micro-organisms, including harmful germs and bacteria. They may also transmit parasites to man, bigger animals and birds.