The main types of micro-organism found in foods are:
Moulds Moulds are the most common type of food spoilage organism. They are microscopic fungi and consist of filaments of cells joined together to form a network. They develop rapidly in humid conditions. They are nearly always undesirable (except in blue cheeses).
Yeasts Yeasts are another group of fungi. They can cause spoilage in some foods, although they are important to the food industry. Yeasts can ferment sugars to make ethanol and carbon dioxide. This is used to make alcoholic drinks and make bread rise.
Bacteria Two common types of bacterial cell are cocci (spherical) and bacilli (rod shaped). Undesirable bacteria can be divided into two groups: spoilage and pathogens. Spoilage bacteria make food unpalatable and reduce the nutritional value e.g. lactobacillus and lactococcus, cause souring. Pathogens cause disease. Some pathogens cause illness by growing in the body and others form toxins when growing on the food and the toxins cause the harm.
Bacterial Growth Micro-organisms reproduce by cell division. As the number of mature cells increases the rate of growth also increases. It takes about 20minutes for a cell to reach maturity. Under favourable conditions it takes about 7 hours before the population reaches 10 6 per gram of food. The growth of organisms can be represented by a bacterial growth curve. time Lg no. of cells Lag phase Exponential phase Stationary phase death phase
Bacterial Growth Food borne micro-organisms can be divided into 3 classes according to the optimum temperature at which they grow. Thermophilic (spore forming) grow best between 55-65 C, mesophilic (salmonella) between 20-40 C and psychrophilic (water bourne) between 10-15 C. They might also be classed according to their need for oxygen (aerobic & anaerobic). Rate of growth also depends on humidity and pH.
Food Poisoning Salmonella – to be a health risk the organism must be consumed live and in high numbers. The travel to the small intestine, invade cells, grow and release toxins. Refrigeration below 4 C prevents growth and heating at 60 C for 2 mins destroys it. Campylobacter – it cause disease in a similar way to salmonella but the effects are more acute, severe abdominal cramps and explosive diarrhoea. Rarely fatal. Under cooked chicken and unpasteurized milk can cause this. Listeria – only affects very young children, the elderly, pregnant women and people with certain diseases. Can result in serious meningitis or in pregnant women abortion of the baby. Mortality rate is about 30%. Soft cheeses, raw meat, pate, poultry and milk may carry the organism. Heating at 70 C for 2 mins destroys it.
Food Poisoning Staphylococcus aureus – it is carried on the skin and in the nose of most animals. It produces a toxin in the food which causes vomiting and diarrhoea. The bacteria is heat sensitive but the toxin is heat stable. Illness often results from cold food which has been left to stand in a warm room e.g. buffet food Botulinum (botulism) – it produces toxin which can be inactivated by boiling at 100 C for a few mins but the spores may take up to 6 hours to be destroyed. Mortality rate is about 50%. Most incidents are associated with canned or bottled vegetables, fruits and fish. Low pH foods don’t sustain the bacteria.