Spoilage usually starts with aerobes and facultative anaerobes. Then, as the oxygen is used up, obligate anaerobes take over. Moulds and yeasts tend to cause spoilage when the conditions do not favour bacterial growth. Signs of spoilage include discolouration, off odours and taste, slime, rancidity, gas production and changes to the taste and texture. Cans and pack may ‘blow’. Unlike spoilage bacteria, pathogenic bacteria do not normally affect taste, smell or the appearance of food. They are therefore difficult to detect.
How quickly food spoils depends on the condition of the food, the PH, water availability, temperature, oxygen, the presence of inhibitory substances (preservatives) and the type and number of spoilage organisms present.
Insects, vermin and parasites can also spoil food. So can some chemicals which, even at very low levels, can cause unacceptable taint. Contamination and the action of oxygen (oxidation) can cause deterioration. Even excessive cold can cause damage by extracting water from the food, a fault known as freezer burn.
Foods which are most prone to spoilage are known as ‘perishable’. These include meat, poultry, fish, dairy products, fruit and vegetables. Foods such as sugar, flour and dried fruit, are unlikely to be affected by spoilage unless they are handled badly, for example, by storing under damp conditions. These are often described as stable or ‘non-perishable’. These include meat, poultry, fish, dairy products, fruit and vegetables. Foods such as sugar,, flour and dried fruits, are unlikely to be affected by spoilage unless they are handled badly, for example, by storing under damp conditions. These are often described as stable or ‘non-perishables’.
ADD PHOTO OF FISH OR DAIRY PRODUCTS
The presence of mould usually results in food having a musky odour and flavour and, although usually considered harmless, increasing concern is being expressed about poisonous fungal toxins known as mycotoxins. These are usually associated with grain and nuts.
The acidity of fruit ensures that most primary spoilage is caused by moulds and yeasts which are able to multiply at higher acidity than bacteria. Thus, vegetables stored in vinegar, such as beetroot, may be attacked by yeasts. Yeast spoilage of food can often be detected by the alcoholic taste and smell and the presence of bubbles in liquid. Moulds are responsible for most of the spoilage of baked products especially bread and pies. As mould spores are destroyed by normal cooking temperatures, the spoilage usually arises from airborne spores and contact with contaminated surfaces after cooking.
Rope in bread and other bakery products is caused by a spore-forming bacteria in the flour. Affected bread becomes yellow and brown and develops a fruity, sickly smell and soft sticky texture. Chemical preservatives are used to prevent rope.
Staleness of bread usually develops with prolonged holding, due to physical changed in the carbohydrates. Refrigeration increases the rate if staling; however, staling does not occur during frozen storage at -18°C.
Rancidity is the term used to describe the breakdown of fats or fatty substances. It occurs when the fats are broken down into free fatty acids by naturally occurring enzymes, known as lipases. These can also be produced by micro-organisms. Heating may destroy lipase-producing bacteria but not any lipase already formed and Rancidity may still occur.
Rancidity may also occur as a result of the interaction between fats and oxygen often in the presence of copper or iron contamination. The prolonged cold storage fatty fish, bacon and pork results in rancidity unless vacuum packed.