Foods, by their very nature, need to be nutritious and metabolizable and it should be expected that they will offer suitable substrates for the growth and metabolism of micro-organisms. Before dealing with the details of the factors influencing this microbial activity, and their significance in the safe handling of foods, it is useful to examine the possible sources of micro-organisms in order to understand the ecology of contamination.
DIVERSITY OF HABITAT
Viable micro-organisms may be found in a very wide range of habitats, from the coldest of brine ponds in the frozen wastes of Polar Regions, to the almost boiling water of hot springs. Indeed, it is now realized that actively growing bacteria may occur at temperatures in excess of 100 1C in the thermal volcanic vents, at the bottom of the deeper parts of the oceans, where boiling is prevented by the very high hydrostatic pressure (see Section 3.2.5).
Micro-organisms may occur in the acidic wastes draining away from mine workings or the alkaline waters of soda lakes. They can be isolated from the black anaerobic silts of estuarine muds or the purest waters of biologically unproductive, or oligotrophic, lakes. In all these, and many other, habitats microbes play an important part in the recycling of organic and inorganic materials through their roles in the carbon, nitrogen and sulfur cycles (Figure 2.1).
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They thus play an important part in the maintenance of the stability of the biosphere. The surfaces of vegetable structures such as leaves, flowers, fruits and especially the roots, as well as the surfaces and the guts of animals all have a rich micro flora of bacteria, yeasts and filamentous fungi. This natural, or normal flora may affect the original quality of the raw ingredients used in the manufacture of foods, the kinds of contamination which may occur during processing, and the possibility of food spoilage or food associated illness
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