acid is the more useful. It can be combined with sodium lactate to buffer the pH since it is the undissociated lactic acid molecule that is effective, rather than the lactate ion. Trisodium phosphate works by dislodging bacteria from the skin, so allowing them to be washed away.
Its effectiveness is attributable to its high alkalinity (pH = 12).
Physical decontamination methods comprise ultraviolet light, ionizing radiation such as gamma and X-rays, and ultrasound.
Ultraviolet light has very poor penetration and parts of the carcass may
‘shadow’ other areas. Ionizing radiations can be very effective, particu- larly against pathogens, but suffer from consumer resistance to the idea of irradiated food. There is also the danger that their effectiveness might lead to less care being taken in preventing contamination in the first place. Ultrasound may be less useful for carcasses than for equip- ment such as poultry shackles. An excellent review of decontamina- tion procedures that have been used, or proposed, for beef carcasses is that of Dorsa (1997).
resistant than the vegetative cells. For example, some spores may resist 100°C for several hours. Of the food poisoning bacteria, Clostridium forms spores but Salmonellaand Staphylococcusdo not. The spores of Clostridiumare not destroyed by normal cooking methods. In the UK, the Food Safety (Temperature Control) Regulations 1995 (SI 1995 No 1763) specify that, in general, food likely to support the growth of pathogenic bacteria must be stored below 8°C. If the food is to be served cold it can be kept at a higher temperature but only for a maximum of 4 h. Food to be served hot must be kept at 63°C or above, and for a maximum of 2 h.
Pathogenic bacteria may cause disease through infection, such as Salmonella and Yersinia, or through producing toxins, such as Clostridium and Staphylococcus,or may be both infectious and produce toxins, such as Streptococcus. The importance of toxin formation is that the toxin can be present even after the bacteria have been killed.
Also, the effects of the toxin may occur very rapidly after ingestion since bacterial proliferation need not take place. Clostridium perfringens produces a toxin that irritates the gut wall causing diarrhoea. The bacterium is a common cause of food poisoning. Typically this might be through consumption of meat dishes prepared on one day but eaten the next. Cooking does not kill the bacterial spores. Reheating the food on the second day makes the spores germinate and grow. The related species Clostridium botulinum causes very severe, often fatal, food poisoning (botulism). In this case the toxin (a neurotoxin) is extremely poisonous, only a small quantity leading to severe illness. Because it is a strict anaerobe, multiplying only in the absence of oxygen, it can grow in tins of meat if these have not been sterilized adequately.
The germination of spores and growth of Clostridium is inhibited by the nitrite used in the production of cured meat. A concentration of nitrite of at least 120–200 mg kg"1 seems to be necessary – much higher than the concentrations needed to develop the characteristic colour and flavour of cured products. However, when meat is heated in the presence of nitrite to about 70°C or more, a chemical agent is Table 9.3. Food poisoning bacteria and source of contamination.
Bacterium Source
Salmonellaspp. Gut of animals
Staphylococcus aureus Skin, nose, cuts in man and animals Clostridium perfringens Gut of animals
Clostridium botulinum Soil
Campylobacter jejuni Gut of animals Listeria monocytogenes Gut of animals Escherichia coliO157 Gut of animals Yersinia enterocolytica Gut of animals
formed, the ‘Perigo factor’, named after its discoverer. Fortunately, this agent is very much more (#10) inhibitory to Clostridial growth than nitrite alone. Information on methods to eliminate or counteract pathogens in meat is given in Smulders (1997).
Spoilage microbes
Microbes that cause spoilage can be bacteria, yeasts or other fungi (moulds). The bacteria can be those that thrive only in the presence of oxygen or those that grow under conditions where oxygen is absent.
Offensive putrefaction is generally associated with the growth of bacteria growing in the absence of oxygen and producing indole, methylamine and hydrogen sulphide from decomposition of proteins and amino acids. Sour odours are produced by the decomposition of sugars. An excellent account of the bacteria found on meat and of the effects of different storage procedures on their growth and control is given in Dainty and Mackey (1992). Whitfield (1998) describes the types of taints produced by microbial spoilage. Methods for the measurement of microbial contamination of meat are described by Fung (1994).
Bacteria are classed as Gram-negative or Gram-positive, based on their reaction with various dyes. Gram-positive bacteria retain the stain, crystal violet, while Gram-negatives do not. Examples of Gram- negative spoilage bacteria often found on carcasses are Pseudomonas, Acinetobacter and Psychrobacter, as well as Salmonella and Campylobacter. Examples of Gram-positive bacteria are Micrococcus, Bacillusand Brochothrix.
Pseudomonasis one of the commonest and most important spoilage bacteria found on both red meat and poultry. Pseudomonads may form up to 90% of the flora on the surface of carcasses stored in chill rooms because many species will still grow at refrigerated temperatures. They are a large and varied group of rod-shaped bacteria, often motile with one or more flagella, and found ubiquitously. Many species cause plant and animal diseases. Pseudomonascan metabolize glucose to gluconate and 2-oxo-gluconate. Unlike other bacteria it can also break these compounds down further, giving it a competitive advantage. Where the pH of the meat is high, as in DFD (dark, firm, dry) meat, Brochothrix thermosphacter, which grows best at pH values above 6.5, may be important, especially at temperatures above 5°C. Under anaerobic conditions, and where the carbon dioxide concentration increases to perhaps 20%, for example in vacuum packages, the normal aerobic flora is suppressed and lactic acid-producing bacteria, such as Lactobacillus, are favoured. These tolerate high carbon dioxide levels.
Fungi (moulds) are considerably less important than bacteria as spoilage organisms. They can cause surface stickiness, or ‘whiskers’ –
the hyphae that form the threadlike vegetative parts of the fungus.
Fungi will grow where too little water is available for the proliferation of bacteria. They may therefore be a problem on frozen meat where the storage temperature is too high. Bacon can be prone to spoilage by moulds because of its low water activity and high fat content.
The growth of bacteria
Four phases are recognized in the growth of bacterial colonies. In the initial lag phasethe bacteria adjust to the environment. There follows an exponential phasewhere the numbers of bacteria multiply rapidly.
Then comes a stationary phase when the rates of growth and multi- plication are balanced by the number of bacterial cells dying. Finally, in the reduction phase there is a progressively greater death of cells because the substrate is depleted. As we have already seen for Clostridium, under unfavourable conditions some bacteria can produce spores. These are often very resistant to, for example, high temperatures or dry conditions. Bacteria grow by each cell dividing into two daughter cells. In exponential growth, the number of cells doubles at progressive equal time intervals. Because bacterial growth is exponential it is therefore often defined in terms of the time needed for the doubling of cell numbers. Under optimal conditions for growth the doubling time can be as short as 20 min. After 1 h, eight daughter cells will have been produced and after 4 h more than 8000. Unless limited, for example by exhaustion of nutrients, by 6 h the number will have increased to more than a quarter of a million. Because of the very large numbers involved in describing bacterial populations they are often expressed using a logarithmic scale.
On the surface of carefully handled carcasses the numbers of bacteria (the total viable count) may be up to 103to 104organisms cm"2. With poor hygiene this could rise to more than 106organisms cm"2and off-odours often start to develop at approximately 107–108 organisms
cm"2. Slime may form if adjacent bacterial colonies coalesce.
Factors affecting bacterial growth
An overview of the growth requirements of microbes in relation to their growth on food, and the preservation of food, is given in Boddy and Wimpenny (1992). Important factors are temperature, oxygen availability, pH, redox potential, competition with other bacteria and moisture availability of the substrate. The availability of water is more correctly described in terms of water activity (aw). The water activity of a solution is the ratio of its vapour pressure to that of pure water at the