Food is considered contaminated when unwanted microorganisms are present. Most of the time, the contamination is natural, but sometimes it can be artificial too.
NATURAL CONTAMINATION occurs when microorganisms attach themselves to foods while the foods are in growing stages.
ARTIFICIAL CONTAMINATION occurs when the food is handled or processed such as when fecal bacteria enter food through improper handling procedures.
CAUSES OF FOOD SPOILAGE:

1. Growth and Activity of microorganisms – Bacteria, yeasts and molds are microorganisms that cause food spoilage. They produce various enzymes that decompose various constituents of food.
2. Enzyme activity – Action of enzymes start the decomposition of various food components after death of plants and animals.
3. Chemical reactions – These are the reactions that are not catalyzed by any enzyme. E.g. Oxidation of fats
4. Vermin – It includes weevils, ants, rats, mice, birds, larval stage of some insects. Vermin are important due to asthetic aspect of their presence, possible transmission of pathogenic agents and consumption of food.
5. Physical changes – These include changes caused by freezing, burning, drying, pressure etc.
   SOURCES OF FOOD CONTAMINATION. PHYSICAL SPOILAGE is due to physical damage to food during harvesting, processing or distribution. The damage increases the chance of chemical or microbial spoilage and contamination because the protective outer layer of food is broken and microorganisms can enter through it. CHEMICAL SPOILAGE in food are responsible for changes in the color and flavor of foods during processing and storage. After harvesting, chemical changes begin automatically within foods and lead to deterioration. Every living organism uses specialized proteins called enzymes to drive the chemical reactions in its cell. After death, enzymes play an important role in the decomposition of living tissues in a process called as autolysis (self-destruction) or ENZYMATIC SPOILAGE. MICROBIAL SPOILAGE is due to bacteria, yeasts or molds. They produce various enzymes that decompose various constituents of food. • Besides natural microorganisms, foods can be contaminated with different types of microbes coming from outside sources such as air, soil, sewage water, humans, food ingredients, equipments, packages, insects, etc.

The primary sources of microorganisms in food may include –

1. Soil and Water – Soil grows agricultural produce and raise animals and birds which might contain several microorganisms. Also, these microbes can multiply in soil and their numbers can be even very high as expected. Fecal materials may also contaminate soils which can act as a source of microorganisms. Sewage water can also contaminate crops with variety of microorganisms when sewage water is used as a fertilizer. So, Sewage must be always treated before using as a fertilizer.
2. Plants and plant products – The inside tissue of food from plant sources are essentially sterile except for few porous vegetables such as radish, onion and cabbage. Also it has been observed that some plants produce natural metabolites that can limit the presence of microorganisms in those particular foods. Fruits and vegetables contain a variety of microorganisms on their surface and their presence and number depends on various factors such as disease of the plant, storage, etc.
3. Food utensils – Many different microorganisms can contaminate food utensils from which they can transmit to human body and make them ill if pathogenic. Proper cleansing and sanitization of food utensils is required before serving food in them.
4. Food handlers – Food handler is a person who touches or handles food. The microorganism may be transmitted from his hand to the food and may be harmful for the person consuming that particular food. The microbes can come from animals or from the environment.
5. Animal hides and skins – Food animals and birds normally carry various indigenous microorganisms some of which are pathogens and are responsible for food-borne diseases in humans. The number of these microorganisms is less than10/g.
6. Air and dust – Microorganisms may be present in dust and moisture droplets in the air. The microorganisms which are present in air may be transient or variable depending on the environment. Some pathogenic microorganisms may cause air-borne diseases.