When animal or vegetable material is removed from its natural source of energy and nutrient supply, chemical changes begin to occur which lead to deterioration in its structure. These changes can be slowed using MAP techniques. For example, unsaturated fats and oils tend to combine with oxygen in the atmosphere. In some fatty foods, this oxidation can lead to rancidity development a process that can be slowed with good effect if the food is packaged in a low oxygen atmosphere.

Four types of micro-organisms that can be controlled by MAP

Bacteria, yeasts and moulds have different respiratory and metabolic needs and can be grouped into four types, according to their oxygen requirement for metabolic and growth processes.

Aerobic Microbes - require oxygen or air for respiration and growth, e.g. Pseudomonas species, some Bacillus species, Acinetobacter/Moraxella species, Micrococcus species, film yeasts and moulds. Therefore some control of these organisms can he obtained by excluding oxygen from a pack.

Anaerobic Microbes - do not require oxygen or air to grow and many are inhibited or killed by the presence of small quantities of oxygen, e.g. Clostridium species.

Microaerophilic Microbes - require low levels of oxygen for optimum growth. Some also require increased levels of carbon dioxide for optimum growth, e.g. Campylobacter and Lactobacillus species.

Facultative Anaerobic Microbes - can respire and grow with and without the presence of air or oxygen, e.g. Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Brochothrix species, Salmonella species, Vibrio species, fermentative yeasts and some Bacillus species.

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