Food preservation processes in a food processing context

Food processing serves a number of purposes. First of all it is key to the transformation of perishable foodstuffs into microbiologically stable and safe food products as has been discussed above. While this is clearly a prerequisite, processed foods are sold, particularly in the developed world, first and foremost on the basis of their organoleptic (i.e., taste and flavour) characteristics and their nutritional value; all well perceivable consumer benefits.

The former notions stress that a preservation method of choice always has to balance the quality against the stability constraints of a given product. To give an example, pasteurised milk tastes better and has a higher nutritional value than sterilised milk. However, since bacterial spore formers of the genus Bacillus show a higher than negligible risk of being present in raw milk and can survive preservation methods, pasteurised milk may be spoiled by germinating Bacillus spores (Zwietering et al., 1996; Brown, 2000). Vegetative B. cereus cells actively grow in foods and may produce toxin (Agata et al., 2002). The risk of having spores of Bacilli spoiling pasteurised milk by day seven after collecting the milk has been calculated and thus the time limit for consumption of pasteurised (often referred to as fresh) milk was set (see for further background reading Zwietering et al., 1996). On the other hand, sterilised milk has, even at elevated (ambient) temperatures, a very long shelf life.

Table 23.2 Need for preservation and the techniques available for this

Preservation targets

Preservation techniques

Micro-organisms

Drying

Insects

Heating

Enzymes

Cooling, freezing

Chemicals

Fermentation

Damage

Chemical preservation

Organoleptics

Mechanical (High pressure processing, US)

Freshness,

Physical means (PEF; irradiation, UV-light)

fit for consumption

Milk is one example of a simple straightforward liquid food. Nonetheless there are already locally major differences in preferences for either pasteurised or sterilised milk. The situation is often even more complex in the manufacturing of other processed foods such as for example mildly acid sauces. Often, a high-temperature processing step (typically 96-98 0C for up to ten minutes) combined with the mildly acidic conditions are sufficient to prevent outgrowth of pathogenic bacteria and common spoilage bacteria of the genus Bacillus (Ramaswamy et al., 1997). However, consumers in the developed world more and more prefer less acid-processed food products, for instance, sauces containing fresh-like vegetable particles (recently discussed in Lado and Yousef, 2002). As a consequence, there has been a tremendous drive for research on new preservation systems based on a combination of physical treatments with the action of a natural preserving compound, often a regular food ingredient.

In summary, there always were and there still are many reasons why food needs to be preserved well. Preservation techniques available for practical use are, however, unfortunately still relatively limited. Table 23.2 summarises the reasons why we need to preserve foods and the currently available techniques for this. The next section will discuss these techniques in some detail and indicate some new developments.

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