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Factors that Affect the Quality of Milk
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Many food products present a favourable physical environment for the multiplication of micro-organisms. For example, milk is a highly nutritious substance, and thus provides a favourable environment for the growth of micro-organisms especially at temperatures above 16°C.
Microorganisms are living organisms that are too small to be seen with the naked eye. These organisms are found everywhere: in the air, water and the ground. Many micro-organisms cause disease in people and animals while others are beneficial and even necessary for human and animal life.
“Micro-organism” is the term applied to all small living organisms. Three main types of organisms found in milk and milk products are:
- Bacteria
- Yeasts and
- Moulds
In dairying, some micro-organisms are harmful as they may spoil milk and milk products. Some are pathogenic in that they can be harmful to the health of humans. Bacteria are the most numerous of the milk organisms. Even if production is done aseptically from the animal, contamination can occur thereafter. Bacteria can enter the milk through:
- The animal
- From the air
- From animal feeds
- From the milker
- From the milk container
Once micro-organisms have entered the milk they grow quickly and once they are established they are difficult to remove.
Milk drawn aseptically from a healthy udder contains less than 1000 bacteria/ml. There is considerable variation in the number of organisms found in milk at different stages of milking. During the milk harvesting process, bacteria from various sources may enter the milk. During storage, even under refrigeration, these bacteria will multiply further.
Click here to view a video that explains how dairy cows are milked.
Cooling of the milk prevents the growth of most types of bacteria to a considerable degree. If milk has to be stored for 12 - 18 hours only, cooling down to 8°C is enough. If milk must be stored for one day or longer, cooling down to 4°C is necessary. Microorganisms can be killed by pasteurization. This is done by heating the milk to 72°C for 15 seconds.