Chapter 15 - Low-Acid and Acidified Low-Acid Foods in Hermetically Sealed Containers (Canned Foods)
15.3 Background

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In order to understand why certain procedures have been developed for the canning of foodstuffs, it is necessary to have some knowledge of the spoilage organisms themselves, in particular bacteria, yeasts and moulds. These organisms, by nature of their small size, are referred to as microorganisms.

Microorganisms are ubiquitous, occurring wherever conditions are favourable to them. It must not be assumed that all microorganisms are harmful. In fact the vast majority do not fall into this category and most are essential or actively beneficial to life in general. Those microorganisms which are capable of causing disease are referred to as pathogens.

15.3.1 Important General Characteristics of Microorganisms

Many microorganisms are capable of extremely rapid multiplication under favourable conditions. For example, some species of bacteria can pass through a generation within 20 minutes under optimal conditions. Since each generation potentially doubles the number of bacteria, it can be seen that enormous numbers of bacteria can be attained within a period of a few hours.

When conditions for growth or survival become unfavourable, many microorganisms have the ability to develop resistant structures known as spores, which can, if necessary, remain dormant for prolonged periods of time (several years in some instances) until conditions become favourable to support the growth of the species. At this time, the spore reverts to the growth or vegetative form.

Certain microorganisms in the vegetative form have the ability to produce toxins and these toxins may be extremely harmful to man. If such microorganisms are present in large numbers in foodstuffs, not only the microorganisms themselves, but also the toxins produced by them, make the foodstuffs unsafe for human consumption.

15.3.2 Environmental Characteristics Affecting Growth

15.3.2.1 Nutritional Requirements

Unless the microorganisms possess chlorophyll and can therefore synthesize their food requirements from water and carbon dioxide, they are dependent on an external source of food such as carbohydrates, fats and oils. In addition, they have specific requirements for minerals and vitamins. Because of the wide range of requirements by different microorganisms, any foodstuff is capable of supporting growth of one kind or another.

15.3.2.2 Moisture Requirements

Microorganisms, particularly bacteria, require water for growth. The water which is present in cells of plants or animals and hence, in a foodstuff, is largely bound within the cells and as such, is not available to microorganisms. However, free or available water is in and around tissues in a foodstuff, and it is this water on which the growth of microorganisms depends. The amount of free or available water in a foodstuff is often expressed in terms of the water activity (aw) value. If methods are employed to reduce the aw of a foodstuff, it will render it less suitable for bacterial growth. It should be noted that freezing is an effective method for preserving foodstuffs, because the tissue water is converted into ice and as such, is no longer available to microorganisms.

15.3.2.3 Environmental pH

Most foods are acidic, but to different degrees. The pH value is used to represent the degree of acidity or alkalinity of a substance. The pH scale runs from 0 to 14, with 7 being the neutral point, and any value below 7 is acid.

In general, bacteria are less tolerant than yeasts or moulds, and not only is their growth affected by the pH, but also their rates of survival during storage, heating, drying and other forms of processing.

15.3.2.4 Temperature Requirements

Each microorganism has an optimum temperature range for growth and on the basis of the temperature requirements, are classified into four groups:

Psychrophilic 14-20°C
Mesophyllic 30-37°C
Facultative thermophilic 38-46°C
Obligatory thermophilic 50-66°C

15.3.2.5 Oxygen Requirements

All microorganisms require oxygen to carry on their metabolic process. Free oxygen exists in the air, and those microorganisms which can grow in the presence of this free oxygen are described as aerobes.

However, some microorganisms exist in the absence of atmospheric oxygen and are described as anaerobes.

Most bacteria fall into a category known as facultative anaerobes, which can tolerate to some degree either the presence or absence of atmospheric oxygen. In sealed containers, anaerobic conditions exist and bacterial decomposition of the foodstuff tends to be of a putrefactive nature, sometimes leading to the formation of foul gases.

15.3.3 Clostridium botulinum

Of all the microorganisms concerned with the spoilage of food, there is one species of bacteria with which we are concerned above all others. This is Clostridium botulinum, an anaerobe, spore former and toxin producer. This bacterium produces a toxin which is the strongest biological toxin affecting man and animals. It has been calculated that 1 gram of toxin, properly diluted, could kill more than 500 million people. The food poisoning syndrome associated with the bacterium is known as botulism.

15.3.4 Low Acid Foods

Low acid foods are defined as those having a pH of 4.6 or more. Included in this category are meat products and vegetable products. The reason for selecting this value is that Clostridium botulinum will not grow at a pH of 4.8 or less. (A safety factor of 0.2 is allowed). This bacterium also requires an aw of 0.93 or greater for growth. In this instance a safety factor of 0.08 is allowed, giving a critical aw of 0.85. The definition of a low acid food is often modified to include this factor and reads "a food, other than alcoholic beverage, where any component of that food has a pH value greater than 4.6 and a water activity (aw) greater than 0.85".

A low acid food, when canned, must be subject to a thermal process incorporating a sufficiently high temperature maintained for a long enough time to ensure the destruction of Clostridium botulinum and its toxins.

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