Bacteria are single-celled organisms. They are present in air, water and on most solid materials. Bacterial cells are very small and can only be seen with the aid of a microscope. Some bacteria are useful in milk processing, causing milk to sour naturally, leading to products such as irgo. However, milk can also carry pathogenic bacteria such as Salmonella and Tuberculosis, and can thus transmit disease.

Other bacteria can cause spoilage of the milk, and spoilage and poor yields of products. In addition to being a nutritious food for humans, milk provides a favourable environment for the growth of microorganisms. Yeasts, moulds and a broad spectrum of bacteria can grow in milk, particularly at temperatures above 16°C.

Microbes can enter milk via the cow, air, feedstuffs and the milker. Once microorganisms get into the milk their numbers increase rapidly. It is more effective to exclude micro-organisms than to try to control microbial growth once they have entered the milk.

Pasteurisation is the most common process used to destroy bacteria in milk. In pasteurisation, the milk is heated to a temperature sufficient to kill pathogenic bacteria, but well below its boiling point. This also kills many non-pathogenic organisms and thereby extends the storage stability of the milk. In the end you get milk that is nutritious.

The carbon cycle is a complex cyclical process through which all of the carbon atoms in existence rotate.
Under the right conditions carbon-based fossil fuels, coal, oil and gas are formed. These are mined from the ground. Carbon is released into the atmosphere: when fossil fuels are burned, by respiration of decomposers and by respiration of living organisms. Photosynthetic microbes and green plants take in carbon during photosynthesis and produce glucose. The gas oxygen is released as a waste product. Dead organisms and waste products decay bringing the carbon into the ground.Over millions of years and under the right conditions some decomposed remains will be turned into fossil fuels such as coal and gas. Without bacteria that take part in the carbon cycle, life on earth would cease to exist.

Nitrogen is required by all living organisms for the synthesis of proteins, nucleic acids and other nitrogen containing compounds. The Earth’s atmosphere contains almost 80 % nitrogen gas. It cannot be used in this form by most living organisms until it has been fixed, that is reduced (combined with hydrogen), to ammonia.
The nitrogen cycle is a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is a continuous cycle that is maintained by the decomposers and nitrogen bacteria.

Plants absorb the nitrate ions by diffusion and active transport. The plants need the nitrogen for the synthesis of proteins and other compounds.
The nitrogen compounds are passed through the food chain as other organisms feed on the plants and each other. Waste products and dead organisms which contain nitrogen compounds are added to the soil. The decomposers - certain soil bacteria and fungi (ammonifying bacteria) break down proteins in dead organisms and animal wastes, releasing ammonium ions.
Nitrification is a two-step process that is carried out by the nitrifying bacteria. Ammonia or ammonium ions are oxidized first to nitrites and then to nitrates. Nitrates are reduced to nitrogen gas by denitrifying bacteria.
Nitrogen fixation - this is the first step in the synthesis of virtually all nitrogenous compounds.

Atmospheric fixation - this occurs spontaneously due to lightning. Only a small amount is fixed this way.
Industrial fixation - the