Meat: Meaning and Composition | Microbiology

In this article we will discuss about:- 1. Meaning of Meat 2. Structure and Composition of Meat 3. The Microbiology of Primary Processing 4. Spoilage of Fresh Meat.

Meaning of Meat:

Originally meat was a term used to describe any solid food, but has now come to be applied almost solely to animal flesh. As such, it has played a significant role in the human diet since the days of hunting and gathering, and animals (sheep) were first domesticated at the beginning of the Neolithic revolution around 9000 BC.

Though abjured by some on moral or religious grounds, meat eating remains widely popular today. In the main, this is due to its desirable texture and flavour characteristics, although meat protein does also have a high biological value.

Meat consumption is often something of a status symbol and is generally far greater in wealthy societies. This is because large-scale meat production is a relatively inefficient means of obtaining protein.

It requires agriculture to produce a surplus of plant proteins which can be fed to animals: with modern production techniques, it takes two kilos of grain to obtain 1 kilo of chicken, four for 1 kilo of pork and eight for 1 kilo of beef.

Though numerous species are used as a source of meat around the world, ranging from flying foxes to frogs and from kangaroos to crocodiles, the meat animals of principal importance in economic terms are cattle, pigs, sheep, goats and poultry.

Structure and Composition of Meat:

Edible animal flesh comprises principally the muscular tissues but also includes organs such as the heart, liver, and kidneys. Most microbiological studies on meat have been conducted with muscular tissues and it is on these that the information presented here is based.

Though in many respects the microbiology will be broadly similar for other tissues, it should be remembered that differences may arise from particular aspects of their composition and microflora.

Structurally muscle is made up of muscle fibres; long, thin, multinucleate cells bound together in bundles by connective tissue. Each muscle fibre is surrounded by a cell membrane, the sarcolemma, within which are contained the myofibrils, complexes of the two major muscle proteins, myosin and actin, surrounded by the sarcoplasm.

The approximate chemical composition of typical adult mammalian muscle after rigor mortis is presented in Table 5.5. Its high water activity and abundant nutrients make meat an excellent medium to support microbial growth.

Though many of the micro-organisms that grow on meat are proteolytic, they grow initially at the expense of the most readily utilized substrates—the water soluble pool of carbohydrates and non-protein nitrogen. Extensive proteolysis only occurs in the later stages of decomposition when the meat is usually already well spoiled from a sensory point of view.

The carbohydrate content of muscle has a particularly important bearing on its microbiology. Glycogen is a polymer of glucose held in the liver and muscles as an energy store for the body. During life, oxygen is supplied to muscle cells in the animal by the circulatory system and glycogen can be broken down to provide energy by the glycolytic and respiratory pathways to yield carbon dioxide and water.

After death the supply of oxygen to the muscles is cut off, the redox potential falls and respiration ceases, but the glycolytic breakdown of glycogen continues leading to an accumulation of lactic acid and a decrease in muscle pH. Provided sufficient glycogen is present, this process will continue until the glycolytic enzymes are inactivated by the low pH developed.

In a typical mammalian muscle the pH will drop from an initial value of around 7 to 5.4-5.5 with the accumulation of about 1% lactic acid. Where there is a limited supply of glycogen in the muscle, acidification will continue only until the glycogen runs out and the muscle will have a higher ultimate pH.

This can happen if the muscle has been exercised before slaughter but can also result from stress or exposure to cold. When the ultimate pH is above 6.2, it gives rise to dark cutting meat a condition also known as dry, firm, dark (DFD) condition. Because the pH is relatively high, the meat proteins are above their isoelectric point and will retain much of the moisture present.

The fibres will be tightly packed together giving the meat a dry, firm texture and impeding oxygen transfer. This, coupled with the higher residual activity of cytochrome enzymes, will mean that the meat has the dark colour of myoglobin rather than the bright red oxymyoglobin colour. The higher pH will also mean that microbial growth is faster so spoilage will occur sooner.

Another meat defect associated with post mortem changes in muscle carbohydrates is known as pale, soft, exudative (PSE) condition. This occurs mainly in pigs and has no microbiological implications but does give rise to lower processing yields, increased cooking losses and reduced juiciness.

The PSE condition results when normal non-exercised muscle is stimulated just before slaughter leading to a rapid post mortem fall in pH while the muscle is still relatively warm. This denatures sarcoplasmic proteins, moisture is expelled from the tissues which assume a pale colour due to the open muscle texture and the oxidation of myoglobin to met-myoglobin.

The Microbiology of Primary Processing of Meat:

The tissues of a healthy animal are protected against infection by a combination of physical barriers and the activity of the immune system.

Consequently, internal organs and muscles from a freshly slaughtered carcass should be relatively free from micro-organisms. Microbial numbers detected in aseptically sampled tissues are usually less than 10 cfu/kg^-1 although there is evidence that numbers can increase under conditions of stress and they will of course be higher if the animal is suffering from an infection.

Since some animal diseases can be transmitted to humans, meat for human consumption should be produced only from healthy animals. Visual inspection before and after slaughter to identify and exclude unfit meat is the general rule, although it will only detect conditions which give some macroscopic pathological sign.

The most heavily colonized areas of the animal that may contaminate meat are the skin (fleece) and gastrointestinal tract. Numbers and types of organisms carried at these sites will reflect both the animal’s indigenous microflora and its environment.

The animal hide, for example, will carry a mixed microbial population of micrococci, staphylococci, pseudomonads, yeasts and moulds as well as organisms derived from sources such as soil or faeces. Organisms of faecal origin are more likely to be encountered on hides from intensively reared cattle or from those transported or held in crowded conditions.

The various processing steps in slaughter and butchering. With reasonable standards of hygienic operations, contamination of meat carcasses from processing equipment, knives and process workers is less important than contamination from the animals themselves.

The greatest opportunity for this occurs during dressing, the stages during which the head, feet, hides, excess fat, viscera and offal are separated from the bones and muscular tissues.

Skinning can spread contamination from the hide to the freshly exposed surface of the carcass through direct contact and via the skinning knife or handling. Washing the animal prior to slaughter can reduce microbial numbers on the hide but control is most effectively exercised by skillful and hygienic removal of the hide.

The viscera contain large numbers of micro-organisms, including potential pathogens, and great care must be taken to ensure the carcass is not contaminated with visceral contents either as a result of puncture or leakage from the anus or oesophagus during removal.

After dressing, carcasses are washed to remove visible contamination. This will have only a minor effect on the surface microflora, although bactericidal washing treatments such as hot water (80°C), chlorinated water (50 mg 1^-1) or dilute lactic acid (1-2%) have been shown to reduce the surface microflora by amounts varying between about 1 and 3.5 log cycles.

After dressing the carcass is cooled to chill temperatures during which cold shock may cause some reduction in numbers. At chill temperatures, microbial growth among the survivors is restricted to those psychrotrophs present and these can be further inhibited by the partial surface drying that takes place.

Surface numbers of bacteria at the end of dressing will typically be of the order of 10²-10⁴ cfu cm^-2. Counts are generally higher in sheep carcasses than beef and higher still in pigs which are processed differently, the skin not being removed from the carcass but scalded and de-haired.

Psychrotrophic organisms form only a small percentage of the initial microflora but come to predominate subsequently as the meat is held constantly at chill temperatures.

An increase in microbial numbers is seen during cutting and boning, but this is due less to microbial growth, since the operation is usually completed within a few hours at temperatures below 10 °C, than to the spreading of contamination to freshly exposed meat surfaces by equipment such as knives, saws and cutting tables.

The primary processing of poultry differs from red meat in a number of respects that have microbiological implications. First among these is the sheer scale of modern poultry operations where processing plants can have production rates up to 12000 birds per hour.

This leaves little opportunity for effectively sanitizing equipment and exacerbates problems associated with some of the procedures and equipment used which favour the spread of micro-organisms between carcasses.

During transport to the plant contamination can be spread between birds by faeces and feathers and from inadequately cleaned transport cages. Once at the plant, birds are hung by their feet on lines, electrically stunned and killed by cutting the carotid artery. The close proximity of the birds and the flapping wings further contribute to the spread of contamination.

This is followed by scalding where the birds are immersed in hot water at about 50 °C to facilitate subsequent removal of the feathers. Each bird contributes large numbers of micro-organisms to the scald water and these will be spread between birds.

This can be reduced to some extent by using a counter-current flow of birds and water so that the birds leaving the scalder are in contact with the cleanest water. Higher scald water temperatures will eliminate most vegetative bacteria but cause an unacceptable loosening of the skin cuticle.

After scalding, birds are mechanically de-feathered by a system of rotating rubber fingers. A number of studies have demonstrated how these can pass organisms, for example Salmonella, from one carcass to others following it and when the fingers become worn or damaged they are liable to microbial colonization. As the poultry carcass is not skinned, skin-associated organisms will not be removed.

The intestinal tract of poultry will contain high numbers of organisms including pathogens such as Salmonella and Campylobacter. Poultry evisceration therefore poses similar microbiological hazards to those with other animals but the size and structure of the carcass make it a much more difficult operation to execute hygienically.

To allow high processing rates, poultry evisceration is usually automated but this too leads to a high incidence of carcass contamination with gut contents. Since the carcasses are not split like those of sheep and cattle, effective washing of the gut cavity after evisceration is more difficult.

Poultry to be frozen is usually chilled in water and this offers a further opportunity for cross contamination. This is controlled by chlorination of the cooling water, use of a counter-current flow as in scalding, and a sufficient flow rate of water to avoid the build up of contamination.

Spoilage of Fresh Meat:

Aerobic storage of chilled red meats, either unwrapped or covered with an oxygen permeable film, produces a high redox potential at the meat surface suitable for the growth of psychrotrophic aerobes. Non-fermentative Gram-negative rods grow most rapidly under these conditions and come to dominate the spoilage microflora that develops.

Taxonomic description of these organisms has been somewhat unsettled over the years with some being described as Moraxella and Moraxella-like. Such terms have now been largely abandoned in favour of a consensus that has emerged from numerical taxonomy studies.

In this, the principal genera are described as Pseudomonas, Acinetobacter and Psychrobacter with Pseudomonas species such as P.fragi, P. lundensis and P. fluorescens generally predominating. A dichotomous key describing the differential characteristics of these organisms and some of the names used previously to describe them is presented as Figure 5.4.

Other organisms are usually only a minor component of the spoilage microflora, but include psychrotrophic Enterobacteriaceae such as Serratia liquefaciens and Enterobacter agglomerans, lactic acid bacteria and the Gram-positive Brochothrix thermosphacta.

The first indication of spoilage in fresh meat is the production of off odours which become apparent when microbial numbers reach around 10⁷ cfu cm^-2.

At this point it is believed that the micro-organisms switch from the diminishing levels of glucose in the meat to amino acids as a substrate for growth. In meat with lower levels of residual glucose this stage is reached earlier (10⁶ cfu cm^-2) and this accounts for the earlier onset of spoilage in high pH meat.

Bacterial metabolism produces a complex mixture of volatile esters, alcohols, ketones and sulfur-containing compounds which collectively comprise the off odours detected. Such mixtures can be analysed by a combination of gas chromatography and mass spectrometry and the origin of different compounds can be established by pure culture studies.

These have confirmed the predominant role of pseudomonads in spoilage of aerobically stored chilled meat. P. fragi is the major producer of ethyl esters which contribute the sweet, fruity note of the odour. The putrid, sulfury component comes from sulfur compounds such as methane thiol, dimethyl-sulfide and di-methyl-disulfide which is also produced by pseudomonads.

In the later stages of spoilage an increase in the meat pH is seen as ammonia and a number of amines are elaborated. Some of these have names highly evocative of decay and corruption such as putrescine and cadaverine but in fact do not contribute to off odour.

When microbial numbers reach levels of around 10⁸ cfu cm^-2, a further indication of spoilage becomes apparent in the form of a visible surface slime on the meat.

Vacuum and modified-atmosphere packing of meat changes the meat microflora and consequently the time-course and character of spoilage. In vacuum packs the accumulation of CO₂ and the absence of oxygen restrict the growth of pseudomonads giving rise to a microflora dominated by Gram-positives, particularly lactic acid bacteria of the genera Lactobacillus, Carno-bacterium and Leuconostoc.

Spoilage of vacuum packed meat is characterized by the development of sour acid odours which are far less objectionable than the odour associated with aerobically stored meat. The micro-organisms reach their maximum population of around 10⁷ cfu cm^-2 after about a weeks storage but the souring develops only slowly thereafter.

Organic acids may contribute to this odour, although the levels produced are generally well below the levels of endogenous lactate already present. Some work has suggested that methane thiol and dimethyl sulfide may contribute to the sour odour.

The extension of shelf-life produced by vacuum packing is not seen with high pH (> 6.0) meat. In this situation Shewanella putrefaciens, which cannot grow in normal pH meat, and psychrotrophic Enterobacteriaceae can grow and these produce high levels of hydrogen sulfide giving the meat an objectionable odour.

In modified-atmospheres containing elevated levels of both CO₂ and O₂ growth of pseudomonads is restricted by the CO₂ while the high levels of O₂ maintain the bright red colour of oxygenated myoglobin in the meat. Here the microflora depends on the type of meat, its storage temperature, and whether it was vacuum packed or aerobically stored previously.

In general terms though, the microflora and spoilage tend to follow a similar pattern to that of vacuum packed meat. Heterofermentative lactic acid bacteria can be more numerous due to the stimulatory effect of oxygen on their growth and, under some circumstances, Brochothrix thermosphacta, Enterobacter­iaceae and pseudomonads can be more important.

Meat can be processed in a number of different ways which affect its characteristics, shelf-life and microbiology. The variety of these is illustrated by Figure 5.5, they are treated in greater generic technologies.