Course:FNH200/Lessons/Lesson 05/Page 05.5
5.5 Factors Affecting Microbial Growth, Enzyme Activity and Chemical Reactions in Foods
Temperature, moisture and oxygen as well as light may have profound influences on microbial growth, enzyme activity and chemical reactions in foods.
- Rates of reactions generally increase as the temperature increases until an optimum is achieved, after which further increases in temperature cause the rates of reaction to decrease because of inactivation of microorganisms or enzymes or because of inhibitory effects on chemical reactions.
- Excessive heat also denatures proteins, breaks emulsions, removes moisture from foods (drying out), and destroys vitamins.
- Cold temperatures can also deteriorate food. A well known example is "chill injury", the change in texture and discolouration of fruits and vegetables when they are exposed to freezing temperatures.
- Freezing temperatures can deteriorate liquid foods such as milk, causing emulsions to break and fat to separate, and denaturing protein causing it to curdle or coagulate. "Freezer burn" due to loss of moisture can occur in solid and liquid foods.
Gain or Loss of Moisture:
- Water loss during storage (e.g. wilting of lettuce in the refrigerator), or water uptake (e.g. by dehydrated foods) can lead to deterioration.
- Retrogradation of starch, resulting in staling of bread, is caused by packing of linear starch molecules leading to the exclusion of water that was previously absorbed during gelatinization. The bread becomes tough and develops a dry texture.
- Changes in water activity (free versus bound water) can influence chemical and enzymatic reactions and microbial growth.
- Oxygen is an important factor in food quality, since many oxidative reactions lead to deterioration in the quality of food and, in some cases, to losses in nutritive value. Oxidative deterioration is often accelerated by light.
- For example, deterioration frequently occurs because of oxidation of the fats in food products. The development of rancidity in breakfast cereals, vegetable oils and oil-based products, and in deep-fried foods is due to reaction of oxygen with fats, particularly those with high unsaturated fatty acid content. This type of rancidity is known as oxidative rancidity. This is in contrast to rancidity induced in foods upon the release of free fatty acids by very high temperatures or by the action of lipase enzymes, either endogenous or produced by spoilage-causing microorganisms. The latter rancidity is known as hydrolytic or lipolytic rancidity.
- Oxidation of vitamins and colour pigments can lead to the deterioration of nutritive quality and aesthetic appeal of foods.
- Physical abuse causes tissue disruption and release of enzymes into tissues which can lead to changes such as enzymatic browning mentioned earlier.
- Furthermore, improper packaging, for example, can cause crushing and tissue damage, making foods such as fruits and vegetables particularly susceptible to microbial invasion as well as enzymatic and chemical reactions.
- For the majority of foods, quality will decrease with time.
- Food preservation, packaging and storage practices are aimed to maintain this quality for as long as possible (shelf life); however, eventually the quality of any food will decrease with time.