Food spoilage may be defined as undesirable changes taking place in food which makes food unfit/unacceptable for human consumption.

Food spoilage affects the way foods look e.g. the colour of food, the way foods smell, and the texture and consistency of foods. Food spoilage can also lower the nutritional value of food and make food unfit for consumption due to the creation of toxins in food. It is estimated that around a third of manufactured foods is lost to food spoilage. Most households are also affected by food spoilage e.g. the development of sour milk, stale bread, mouldy cheese and soft fruit.

Different foods are subject to different types and methods of food spoilage e.g. bread becomes spoiled due to the presence of fungal spores in the grains used to produce bread. Milk and dairy products sour due to effect of bacteria (lactobacillus and streptococcal bacteria) which survive the pasteurisation of milk, while meat becomes spoiled due to being stored at incorrect temperatures or being contaminated by bacteria on cutting boards and other equipment.  Foods can be categorised according to the rate of food spoilage e.g. eggs, fruit and vegetables, milk and meat are categorised as being highly perishable, while potatoes and nuts are semi perishable and dry beans, flour and rice are stable foods (non-perishable).

Food spoilage can be due to insect damage, physical damage such as from bruising fruit and vegetables or damage caused by freezing foods. Food spoilage also results from the growth and multiplication of microorganisms and from the action of enzymes.

Food Microbiology
Food microbiology is the study of the microorganisms which inhabit food. These microorganisms can have beneficial or detrimental effects on food. The beneficial effects of microorganisms have been utilised in the food industry where they help provide taste and texture to food, including the use of microorganisms to produce bread, cheese, vinegar, beer and wine. A great example is the specially cultured moulds that are used in the production of specialty cheese – forming the coloured veins in blue and gorgonzola cheese, and the edible rinds on brie and camembert cheese.

On a negative side microorganisms can cause food borne illnesses and food spoilage. The main types of microorganisms present in food are bacteria and fungi. Moulds are the most common microorganisms which cause food spoilage. Moulds are microscopic fungi which consist of filaments of cells which join together forming a network within foods. Moulds develop most rapidly in damp humid conditions.  Moulds are generally not harmful to the consumer although a small portion of moulds are capable of producing toxins which are hazardous to health. Moulds can, however, affect the sight and texture of foods causing a fluffy growth on food which is typically coloured white, grey, yellow or blue depending on the type of mould.

Yeasts are another type of fungi which cause foods to spoil. This time food spoilage is caused by the ability of yeast to act on foods such as fruit juices and syrups and cause fermentation. Foods that are most effected by yeast fermentation include fruit juices, honey, jams and jellies.  Bacteria also cause food to spoil e.g. lactobacillus and lactococcus bacteria cause milk to sour, while some bacteria are also pathogenic causing disease

Microbes and the causes of food spoilage

We have noted some of the microbes which cause food spoilage. Food spoilage from these microorganisms is dependent on the ability of these microbes to grow and multiply. This is in turn affected by a range of factors such as water, pH (acidity) , oxygen and temperature as well the physical structure of the food itself. Foods that spoil most rapidly are those that are moist, of neutral pH, are not refrigerated, and ground or sliced. In contrast dry, acidic and refrigerated foods are more resistant to spoilage. 
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