CHAPTER 1
CHILLED AND FROZEN RAW MEAT, POULTRY AND THEIR PRODUCTS
Rhea Fernandes
Leatherhead Food Research
Randalls Road
Leatherhead
Surrey
KT22 7RY
United Kingdom
1.1 Definitions
Carcass refers to the body of any slaughtered animal or bird, often, but not always, after bleeding and dressing.
Chilled meat is meat that has been cooled to, and maintained at or below 7 °C but not below -2 °C during storage, transportation and sale.
Dressing is the progressive separation of a slaughtered animal or bird into a carcass (or sides of a carcass), offals and inedible by-products. Dressed carcasses (or sides) can be skin-on (e.g. chicken, pork) or skin-off (e.g. lamb, beef).
Evisceration is the removal of the viscera from a carcass.
Fresh meat is meat that has not been treated in any way other than refrigeration, with or without preservative packaging, to maintain its fitness for human consumption.
Frozen meat is meat that has been cooled to, and maintained at or below 2 °C (normally below -12 °C) during storage, transportation and sale.
Initial microflora is the association of microorganisms present on an eviscerated carcass after skin removal (if appropriate) but prior to washing, grading, chilling, and further processing.
Meat is the edible part (musculature and edible offal) of an animal or bird slaughtered for human consumption.
Microbial contamination refers to microorganisms directly or indirectly transferred onto a carcass or edible offal; hence, contaminating microflora means those microorganisms present as a consequence of such transmission.
Modified-atmosphere packaging (MAP) refers to packaging systems in which the natural gaseous environment around the product is intentionally altered, and then gradually changes as a consequence of the interaction between product, packaging and the external environment.
Controlled-atmosphere packaging (CAP) refers to packaging systems in which the altered internal gaseous environment is maintained at a specified composition regardless of product respiration, temperature or other environmental changes.
Offal is all the edible and inedible parts of a slaughtered animal or bird other than the carcass.
Poultry refers to domesticated birds slaughtered for human consumption; hence, poultry meat means all edible parts of any domesticated bird slaughtered for human consumption.
Preservative packaging is a general term describing any packaging system that prevents or restricts the growth of spoilage microorganisms.
Raw meat refers to meat that has not been cooked but excludes meat treated with curing salts and/or subjected to fermentation; hence, raw meat products refers to uncured non-fermented whole tissue or comminuted meat products intended for sale in the raw state.
Spoilage describes changes that render meat objectionable to consumers; hence, spoilage microflora describes an association of microorganisms that through its development on meat renders that meat objectionable to consumers.
Spoilage potential is a measure of the propensity of microorganisms to render meat objectionable to consumers through the production of offensive metabolic by-products.
Slaughter is the killing of an animal or bird for human consumption generally, but not necessarily performed within a premise (abattoir) that is approved and registered for that purpose.
Viscera mean the organs of the thoracic and abdominal cavities, and include the kidneys.
1.2 Initial Microflora
The initial microflora associated with skin-off meat processing is composed of microorganisms transferred onto the naked carcass during slaughter and dressing. With skin-on meat processing, the initial microflora will also contain microorganisms resident on the skin of the live animal that are not removed during dressing.
Since slaughter and dressing procedures differ not only between meat animal species but also for a given species, the term initial microflora must be precisely defined. The association of microorganisms present on dressed eviscerated carcasses before washing was selected for this chapter as being the initial microflora, to allow the effects of processing variables such as washing and chilling to be considered more fully in the next section, 1.3 Processing and its Effects on the Microflora.
The major sources of the initial microflora found on carcasses are the slaughter animals themselves, the process workers, and the processing environment. The animal sources of contaminating microorganisms include external body surfaces (skin, hide, fleece, feathers, feet and hooves), and the gastrointestinal and respiratory tracts. Contact, either direct or indirect (e.g. via a process worker's hands or implements), between the carcass and external body surface of a slaughtered animal will result in a heterogeneous mixture of microorganisms derived from the animal's pre-slaughter environment, including those of faecal, soil, water and feed origin, being transferred to the carcass. Consequently, the microbiological quality of a dressed carcass is determined by a complex interaction between the microflora carried by the live animal and the hygienic efficiency of the slaughter and dressing process. Despite species and processing differences, the mainly Gram-positive-dominated initial microflora found on dressed meat and poultry carcasses are remarkably similar (1, 2). Unless subsequent processing includes antimicrobial treatments, microbial numbers increase progressively with further processing, at first because of additional contamination associated with handling, and later because of microbial growth.
Pathogenic microorganisms, particularly those of animal origin, can also be expected to be found in the saprophyte-dominated initial microflora; except in unusual and highly undesirable circumstances, their numbers are generally low. With the exception of Staphylococcus aureus and Yersinia enterocolitica, most of the major food poisoning organisms (Salmonella spp., Clostridium perfringens, pathogenic Escherichia coli, and Campylobacter jejuni) are associated with direct or indirect faecal contamination of carcasses. Other pathogens such as Listeria monocytogenes may be animal-associated, but can also be found in the processing environment.
The significance of a particular microorganism being present in the initial microflora is questionable because of the adventitious nature of their occurrence, unless that microorganism poses a health hazard, is likely to contribute to product spoilage, or has specific meat plant or process associations. A compilation of the bacterial genera most frequently reported on raw meat and poultry is presented in Table 1.I.
The majority of the bacteria present in the initial microflora are mesophilic and therefore will not generally contribute to spoilage as they are unable to grow at refrigeration temperatures. Consequently, most microorganisms derived from the intestinal tract will not contribute to spoilage although they may represent a safety hazard. A small but variable proportion of the initial microflora will, however, be psychrotrophic and capable of growth that will eventually result in spoilage of the refrigerated product. Those genera that are most frequently represented in aerobic and anaerobic spoilage microflora are identified in Table 1.I. In respect to spoilage, yeasts and moulds (Table 1.II) do not play an important role except under conditions that preclude the growth of spoilage bacteria. In the initial microflora on beef, yeasts and moulds accounted for only 2.6% of those able to grow at 20 °C but the yeasts accounted for 35% of those able to grow at -1 °C (3). In Saudi Arabia, yeasts and moulds accounted for 2% of the microorganisms on broiler chicken carcasses after evisceration and air chilling that were able to grow at between 25 and 35 °C (4).
1.2.1 Skin-off processing
Included in this grouping are cattle, sheep and all other major food animals including emu and ostrich that are usually skinned (in the case of the two avian species after manual plucking), as part of the normal carcass dressing procedure. For some markets, notably Japan, pig carcasses are skinned rather than processed skin-on. With skin-off processing of healthy slaughter stock, the tissues that are destined to become meat are generally sterile in the intact animal, and their contamination with microorganisms after slaughter is an undesirable but unavoidable consequence of the process by which live animals are converted into meat for human consumption.
Typically, Gram-positive cocci dominate the initial microflora of skinned carcasses. These microflora also frequently contain Acinetobacter, Aeromonas, coryneforms, Enterobacteriaceae, Flavobacterium, Moraxella, and Pseudomonas in addition to Micrococcus and Staphylococcus spp. (1). Irregular occurrences, often in low numbers, of lactic acid bacteria (LAB), Brochothrix thermosphacta,Bacillus spp., yeasts and moulds are not infrequently reported. The proportion of psychrotrophs present in such an initial microflora shows both latitudinal and seasonal variation, ranging in temperate latitudes from approximately 1% in summer to 10% in winter (5). Psychrotrophic strains are often virtually absent from the initial microflora of stock slaughtered in tropical regions. The presence of psychrotrophs is of concern because not only can they directly potentiate product spoilage but also they may pose a secondary contamination hazard should they become established in a chilled processing environment through inadequate or ineffective sanitation.
1.2.2 Skin-on processing
This group includes most pig and poultry processing. With skin-on processing the initial microflora is made up of skin microorganisms surviving the dressing process and those introduced during slaughter and dressing. With both pig and poultry processing, carcasses are subjected to a scalding treatment to loosen hair or feathers prior to their mechanical removal.
Scalding and dehairing of pig carcasses removes approximately 95% of the microorganisms initially present on the skin. The remaining population is comparable to that typically found on skinned carcasses. Subsequent singeing operations reduce that residual skin population to levels as low as 100 bacteria/cm2. From this minimum population the microflora increases through contamination during further processing to reach levels between 103 and 104 bacteria/cm2 at chiller entry (6). This initial microflora contains a small proportion of thermoduric cocci and spore formers that survive scalding and singeing. The major part of the microflora is, however, made up of a heterogeneous mixture of microorganisms introduced from the processing environment after singeing and also from the animal during evisceration.
Scalding of poultry results in the removal of many microorganisms from the feathers. The subsequent defeathering process, however, causes considerable spread of mainly skin microorganisms between carcasses and into the processing environment. Skin-associated microorganisms include Acinetobacter, Corynebacterium, Flavobacterium, Micrococcus, Moraxella, Pseudomonas, Staphylococcus and yeasts. Despite the violent action of the plucking machines, extrusion of faeces and subsequent dispersion of enteric bacteria are not reported to be a major problem (7). However, Salmonella spp. and C. jejuni present on the feathers and feet of birds are spread during plucking. Further contamination with these and other enteric microorganisms during high speed automatic evisceration also contributes to the not infrequent presence of these microorganisms on dressed poultry carcasses. Contamination of carcasses with psychrotrophic microorganisms, particularly Pseudomonas spp. that grow on the wet product contact surfaces within the processing plant, remains an important process hygiene problem that impinges significantly on the composition of the initial microflora (4). With the addition of enteric bacteria and Gram-negative psychrotrophs to the skin-derived microorganisms, the qualitative if not quantitative similarity of the initial microfloras on plucked poultry and skinned carcasses is unmistakable. Unfortunately for the poultry processor, that population tends to be significantly larger than that typically found on beef or lamb carcasses and usually shows a predominance of psychrotrophs. The consequences of this quantitative difference are profound with respect to spoilage and not insignificant with respect to product safety.
1.3 Processing and its Effects on the Microflora
Process flow diagrams for generic meat animal and poultry processing are presented in Figures 1.1 and 1.2 respectively. It must be appreciated that individual slaughter and dressing processes will differ in the sequence of some operations and that further processing of raw meats into raw meat products has been represented as a simplified series of generalised operations. Notwithstanding obvious differences in abattoir technology, slaughter, dressing and further processing are, irrespective of meat species, sufficiently similar functionally to reasonably allow generic consideration of their microbiological implications.
In the conversion of live slaughter stock into either raw meat or raw meat products, there are no operations that kill or otherwise eliminate microorganisms (i.e. no Hazard Analysis and Critical Control Point (HACCP), critical control points (CCP's), see chapter 8 on HACCP), However, there are many steps in which the introduction, spread or proliferation of microbial contaminants can be minimised (i.e. see CCP's in the chapter on HACCP). Current emphasis on pathogen reduction may see antimicrobial interventions such as decontamination sprays or irradiation treatments mandated into meat processing systems. Despite the introduction of such innovations, microbial status of the animal at the time of slaughter, care and standards of hygiene, and sanitation used during slaughtering and carcass dressing will remain as it is, on the removal of the skin (skinned carcasses) and alimentary tract with minimal transfer of microbial contamination to the carcass surface, and, in the case of skin-on carcasses, on a minimal persistence of the normal skin microflora.
1.3.1 Stock presentation
Approximately 30% of the microflora present on a given area of cattle hide is transferred to a corresponding area of carcass surface following direct hide/carcass contact (3). Consequently, the number of microflora present on hide, hooves, feet, hair and feathers is a major factor determining the microbiological status of slaughter stock. However, the microflora of the intestinal tract cannot be ignored. The latter can contribute significantly to the former when animals and birds become soiled with their own faeces or that of others as a result of poor animal husbandry, transportation, or meat plant holding practices. Therefore, the microbiological status of stock can be expected to show a positive correlation with their visual cleanliness.
Cage rearing of chickens, for example, a husbandry practice that separates the birds from their droppings, results in lower microbial loads and the complete absence of faecal coliforms on live birds compared with floor-reared birds (8). A general trend that woolly and dirty lambs produce carcasses with increased levels of contamination after pelt removal has been observed (9). Similarly, one of the explanations proposed by Empey and Scott in 1939 (3) to explain the more than 100-fold difference between maximum (3.1 × 107 bacteria/cm2) and minimum (1.0 × 105 bacteria/cm2) counts on beef hides was the extent to which the hides were contaminated by soil. In the light of these observations, common logic would suggest that even with animals that are dressed skin-off, pre-slaughter cleaning of stock should significantly reduce carcass contamination, since for those animals the major source of carcass contamination is direct or indirect hide/carcass contact.
Transportation of slaughter stock over long distances not only provides the opportunity for faecal contamination but also, through stress and starvation, potentiates the excretion of pathogenic organisms such as Salmonella spp. by symptomless carriers (10). The resulting cross contamination via the faecal oral route can quickly spread the pathogen through entire truck loads of some classes of susceptible stock such as young calves, pigs, old ewes, and dairy cows. Some approaches being used to control this problem are the pre-transportation use of competitive exclusion strategy, probiotics, or prebiotics (11, 12) The overall goal of any of the strategies used is to promote the growth of groups of beneficial bacteria that are competitive with, or antagonistic to, pathogenic species. In the competitive exclusion strategy, a non-pathogenic bacterial culture (composed of one or more strains or species of bacteria, but derived from the animal of interest) is added to the intestinal tract; probiotic preparations consist of individual species or mixtures of LAB or yeast without being species specific or even being of animal origin; prebiotics are organic compounds that are unavailable to, or indigestible by the host animal, but are available to a specific proportion of the microbial population (11, 12). The so-called "Nurmi effect", which has proven efficacious in reducing salmonellosis in young poultry and pigs, employs the oral administration of viable beneficial gut microorganisms including Lactobacillus, Bifidobacterium and Propionibacterium strains. These organisms, through mechanisms including competitive exclusion, maintain the balance of the gut microflora during times of stress, thereby preventing gastric disturbances and colonisation by pathogens.
