3.2.4) have attracted some attention in this respect. Attention has also
been paid to the bacteriocins produced by food-grade micro-organisms
such as the lactic acid bacteria. Nisin (see Section 9.4.1) is an already well
established example and its use can be extended by expedients such as
inclusion of whey fermented by a nisin-producing strain ofLactococcus
lactisas an ingredient in formulated products like prepared sauces.
4.6 Modification of Atmosphere
At the start of the 19th century it was believed that contact with air
caused putrefaction and that food preservation techniques worked by
excluding air. We have already seen (Section 4.1) how this misapprehen-
sion applied in the early days of canning and it was thought that drying
operated in a similar way, expelling air from the interior of food. Some
preservation techniques, such as covering a product with melted fat and
allowing it to set, did in fact rely on the exclusion of air but it is only in
the last 30 years or so that shelf-life extension techniques based on
changing the gaseous environment of a food have really come to be
widely used.
Modified atmospheres exert their effect principally through the inhi-
bition of fast-growing aerobes that would otherwise quickly spoil per-
ishable products. Obligate and facultative anaerobes such as clostridia
and the Enterobacteriaceae are less affected. Thus keeping quality is
improved but there is generally little effect on pathogens, if present, and
the technique is invariably applied in conjunction with refrigerated
storage.
In practice three different procedures are used to modify the atmos-
phere surrounding a product: vacuum packing, modified-atmosphere
packing or gas flushing, and controlled atmospheres. Here we will
discuss some of their important characteristics although other aspects
will be dealt with under specific commodities in Chapter 5.
An essential feature of all three techniques is that the product is
packed in a material which helps exclude atmospheric oxygen and retain
moisture. This requires that it should have good barrier properties
towards oxygen and water and be easily sealed. The packaging materials
used are usually plastic laminates in which the innermost layer is a plastic
such as polyethylene which has good heat sealing properties. Mechanical
closures on packs are far less effective as they often leave channels
through which high rates of gas exchange can occur. Overlying the layer
of polythene is usually another layer with much better gas barrier
properties. No plastics are completely impermeable to gases, although
the extent of gas transmission across a plastic film will depend on the the
type of plastic, its temperature, the film thickness and the partial pressure
difference across the film. In some cases, it can also be affected by factors
108 The Microbiology of Food Preservation