According to the Oxford English Dictionary to spoil is to ‘deprive of good or effective qualities’. When a food is spoiled its characteristics are changed so that it is no longer acceptable. Such changes may not always be microbiological in origin; a product may become unacceptable as a result of insect damage, drying out, discolouration, staling or rancidity for instance, but by and large most food spoilage is a result of microbial activity.
Spoilage is also a subjective quality; what is spoiled for one person may be perfectly acceptable to another. The perception of spoilage is subject to a number of influences, for example foods acceptable in some cultures are unacceptable in others. Some products such as well matured cheeses and game birds that have been hung for extended periods are esteemed by some and highly objectionable to others.
Affluence is another contributory factor, many are not in the position to be able to discard food due to some slight sensory defect, in the 18th and 19th Century navy, sailors often preferred to eat in dark corners so that they could not see the weevils and maggots in their food.
A general feature of microbial spoilage is its relatively sudden onset – it does not appear to develop gradually, day by day a little worse, but more often as an unexpected and unpleasant revelation. This is a reflection of the exponential nature of microbial growth and its consequence that microbial metabolism can also proceed at an exponentially increasing rate.
If a microbial product associated with spoilage, for example an off odour, has a certain detection threshold, the level will be well below this threshold for most of the product’s acceptable shelf-life. Once reached however, it will be rapidly passed so that a comparatively short time after, levels will be well in excess of the threshold and the product will be profoundly spoiled (Figure 5.1).