Current recommendations for handling all meat products are to keep them clean, cold and covered in order to maintain quality and protect against food poisoning and disease. Generally contamination occurs when the product comes into contact with dirty hands, clothing, equipment or facilities. If the product is kept clean there will be little or no contamination by microorganisms whether bacteria, yeasts, moulds, viruses or protozoa or by helminths and poisonous chemicals.

EFFECT OF BACTERIAL GROWTH ON THE SHELF-LIFE OF MEAT AND MEAT PRODUCTS

The total viable count of bacteria (TVC) expressed as organisms/cm2 or as organisms/g on fresh meat or a meat product sets a limit to its shelf-life. Meat will “spoil” with TVC at 106/cm2 because of off-odours. Slime and discoloration appear at 108/cm2. The main factors determining the time taken for the TVC to reach these levels are the initial count due to contamination during slaughtering and processing, further contamination during storage, temperature, pH and relative humidity. An example of how the level of contamination affects shelf-life is shown in Table 1.

After cleanliness, keeping meat products cold is the second most important requirement in order to achieve a desirable shelf-life. Microorganisms rapidly proliferate at elevated temperatures and slime development is a definite visual sign of microbial growth. The importance of temperature in the control of microbial growth is shown in Table 2.

Bacteria relevant to meat, meat products and other food are divided into three groups according to the temperature range within which they can grow: mesophiles 10–45°C, psychrophiles 0–28°C and psychrotrophs 10– 45°C, or slow growth at 0–10°C. Mesophiles will not grow below 10°C but psychrotrophs, of which Pseudomonas are the more important, will grow down to 0°C. The nearer to 0°C the storage temperature the slower the growth of the spoilage bacteria and the longer the shelf-life (Fig. 9).

TABLE 1
Effect of initial contamination on the storage life of lean beef

Initial bacterial count
(org./cm2)		 Days at 0°C
before slime development
100 000  8
  10 000 10
    1 000 13
       100 15
         10 18

TABLE 2
Relationship between storage temperature and slime development

Storage temperature (°C) Days before slime develops
  0 10
  1   7
  3   4
  5   3
10   2
16   1

Under ideal conditions bacteria double in number every 20 minutes. A single bacterium multiplies to over one million in less than seven hours:

Time Number of bacteria
12.00 1
12.20 2
12.40 4
13.00 8
14.00 64
15.00 512
16.00 4 096
17.00 32 768
18.00 262 144
18.40 1 048 576

Some bacteria cause product spoilage, others cause food poisoning. The former limit product shelf-life but the latter cause illness. Almost all foodpoisoning bacteria are mesophiles so refrigeration below 10°C offers good protection. Many mesophiles cause spoilage, but since meat is refrigerated most spoilage is due to psychrophiles. Storing meat at temperatures close to 0°C will inhibit the growth of pyschrotrophs. Shelf-life will be extended by avoiding contamination through good hygiene practices.

Aerobic bacteria have an absolute requirement for oxygen which limits their growth to the meat surface. Anaerobic bacteria grow within the meat as they need the absence of oxygen. Facultative anaerobes can grow slowly within oxygen but grow better in its presence. Food-poisoning bacteria are anaerobes and facultative anaerobes. The most important spoilage bacteria (Pseudomonas spp.) are aerobic.

9. Different types of bacteria can grow within different temperature ranges

Water is required by micro-organisms so reducing the water available below the optimum level will prolong shelf-life. If meat is stored at a relative humidity (RH) below 95 percent, moisture will be lost from the surface. Since most spoilage bacteria, being aerobic, can grow only on the surface, this surface drying will extend the shelf-life. Moulds (fungi) are able to grow in drier conditions than bacteria so that desiccation has a selective effect on microbial growth.

Meat pH is the level of acidity in meat. Stored sugars are broken down to lactic acid. In living muscle it is near 7.0 (above this is alkaline, below is acid). It falls to 5.4–5.6 within 24 hours. High final-pH values result when animals are exhausted at slaughter, for instance because of fighting in lairage or transport. Spoilage bacteria multiply rapidly on high-pH meat and shorten the shelf-life. Exhausted animals should be rested before slaughter.

A high TVC resulting from severe contamination during slaughter or processing will shorten the shelf-life even in ideal conditions. It also indicates poor hygiene so that contamination with food-poisoning bacteria is likely.

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