MICROBIOLOGY OF MEAT PROCESSING
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 microorganisms. Microbial numbers detected in aseptically sampled tissues are usually less than 10 cfu/kg.
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 encountered on on hides from intensively reared cattle or from those transported or held in crowded conditions.
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 microorganisms, 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, althogh bactericidal washing treatments such as hot water (80 ?C), chlorinated water (50 mg/l) or diluted lactic acid (1-2 %) have been shown to reduce the surface microflora by amounts varying between about 1 and 3.5 log cycles. Surface numbers of bacteria at
242the end of dressing will tipically be of the order of 10-10 cfu/cm.
Spoilage of fresh meat
Before chilling, over the temperature of 25 ?C bacterial
spoilage occures, which begins with the growth of aerotolerant anaerobs (streptococci). As a result of the growth and the biochemical reactions of the muscle tissue the redox-potential decreases.
At a properly low redox-potential begins the growth of anaerob microoganisms, typically Clostridium perfringens. During its
78multiplication (10-10 cfu/g) the further decreasing in redox-potential renders the environment favorable for the obligate anaerobes such
as Cl. oedematiens, Cl. bifermentans, Cl. histolyticum, Cl. sporogenes.
In the case of slow precooling. Near the bones clostridia and bacilli.
Surface microflora: Pseudomonads and Enterobacteria.
LOW-TEMPERATURE STORAGE - CHILLING AND FREEZING
Using low temperatures to preserve food was only practicable where ice was naturally available.
In the 11th Century BC the Chinese had developed ice houses as a means of storing ice through summer months.
1626 FRANCIS BACON the English natural philosopher stopped his coach in Highgate in order to fill a chicken carcass with snow to confirm that it delayed putrefaction. Outcome: As a result of his exertions in the snow, it is claimed Bacon (65 years old) caught a cold which led to his death shortly after.
From the 17th Century ice houses in Europe and North
By the 19th Century the cutting and transporting of natural ice had become a substantial industry in areas blessed with a freezing climate.
In the 1830s Mechanical methods of refrigeration and ice making were first patented. (Based on the cooling of produced by the vaporization of refrigerant liquids, originally ether but later liquid ammonia.)
In 1872 at the Melbourne Exhibition, Joseph Harrison exhibited an "ice house" which capt beef and mutton carcasses in good condition long enough for some of it to be eaten at a public luncheon the following year.
thBy the end of the 19 Century techniques had been refined to the
extent that shipping chilled and frozen meat from North and South America and Australia to Europe was large and profitable enterprise.
Chilled foods are those foods stored at temperature near, but above their freezing point, typically 0-5 ?C. This commodity area has shown a massive increase in recent years as traditional chilled products such as fresh meat and fish and dairy products have been joined by a huge variety of new products including complete meals, prepared and delicatessen salads, dairy desserts and many others.
Effect of chill storage on microbial activity
The rates of most chemical reactions are temperature dependent; as the temperature is lowered so the rate decreases. Since food spoilage is usually a result of chemical reactions mediated by microbial and endogenous enzymes, the useful life of many foods can be increased by storage at low temperatures.
Effect on microbes
; The lag period and generation time become longer
; The growth rate is decreasing
; Below the minimum temperature of growth, death of microbes
; Quick cooling results in a “cold shock”
These phenomena are caused by the membrane transformation during cooling.
The ability of organisms to grow at low temperatures appears to be particularly associated with the composition and architecture of the plasma membrane. As the temperature is lowered, the plasma membrane undergoes a phase transition from a liquid crystalline state to a rigid gel in which solute transport is severely limited. The temperature of this transition is lower in psychrotrophs and psychrophiles largely as a result of higher levels of unsaturated and short chain fatty acids in their membrane lipids. If some organisms are allowed to adapt to growth at lower temperatures they increase the proportion of these components in their membranes.
There seems to be no taxonomic restriction on psychrotrophic organisms which can be found in the yeasts, moulds, Gram-negative and Gram-positive bacteria. One feature they share is that in addition to their ability to grow at low temperature, they are inactivated at moderate temperatures. A number of reason for this marked heat
sensitivy have been put forward including the possibility of excessive membrane fluidity at higher temperatures. Low thermal stability of key enzymes and other functional proteins appears to be important factor, although thermostable extracellular lipases and proteases produced by psychrotrophic pseudomonads can be problem in the dairy industry.
Though mesophiles cannot grow at chill temperatures, they are not necessarily killed. Chilling will produce a phenomenon known as cold shock Which causes death and injury in proportion of the population but its effect are not predictable in the same way as heat processing. The extent of cold shock depends on a number of factors such as the organism (Gram-negatives appear more susceptible than Gram-positives,) its phase of growth (exponential-phase cells are more susceptible than stationary phase cells), the temperature differential and the rate of cooling (in both cases the larger it is, the greater the damage), and the growth medium (cells grown in complex media are more resistant).
The principal mechanism of cold shock appears to be damage the membranes caused by phase changes in the membrane lipids which create hydrophilic pores through which cytoplasmic contents can leak out. An increase in single strand breaks in DNA has also been noted as well as the synthesis of specific cold shock proteins.
A typical cold shock is demonstrated by figures 1 and 2.
Effect of traditional cooling on microbial contamination of pig
Effect of quick precooling (2.5 h at -7 ?C) on microbial contamination
of pig carcasses halves.
Since chilling is not a bactericidal process, the use of good microbiological quality raw materials and hygienic handling are key requirement for the production of safe chill foods. Mesophiles that survive cooling, albeit in an injured state, can persist in the food for extended periods and may recover and resume growth should conditions later become favourable. Thus chilling will prevent an increase in the risk from mesophile pathogens, but will not assure its elimination. There are however pathogens that will continue to grow at some chill temperatures (Aeromonas hydrophila, Listeria
monocytogenes, Yersinia enterocolitica) and the key role of chilling in
the modern food industry has focused particular attention on these.
Chill storage can change both the nature of spoilage and the rate at which it occurs. There may be qualitative changes in spoilage characteristics as low temperatures exert a selective effect preventing the growth of mesophiles and leading to a microflora dominated by psychrotrophs. This can be seen in the case of raw milk which in the days of milk churns and roadside collection had a spoilage microflora comprised largely of mesophilic lactococci which would sour the milk. Nowadays the milk is chilled almost immediately it leaves the cow so that Psychrotrophic Gram-negative rods predominate and produce an entirely different type of spoilage. Low temperatures can also cause physiological changes in
microorganisms that modify or exacerbate spoilage characteristics. Two such examples are the increased production of phenazine and carotenoid pigments in some organisms at low temperatures and the stimulation of extracellular polysaccharide production in Leuconostoc