Lecture Microbiology and Deterioration of Meat PDF

Microbiology and Deterioration of Meat  The muscle of healthy animals is considered to be free from bacteria or other contaminants. Natural barriers such as skin and walls of gastrointestinal and respiratory tracts prevent bacterial access. The immune system is responsible destroying those b...

Microbiology and Deterioration of Meat  The muscle of healthy animals is considered to be free from bacteria or other contaminants. Natural barriers such as skin and walls of gastrointestinal and respiratory tracts prevent bacterial access. The immune system is responsible destroying those bacteria that may penetrate the barriers.  Contamination of carcass with bacteria from animal and surroundings begins during slaughter process. Later on bacteria on the surface are transferred to cuts and products during fabrication and processing. Thus, control measures must be applied at slaughter and continue through fabrication, distribution and preparation of products for consumption. Common control measures include sanitary operating procedures that minimise contamination and low temperature storage to minimise bacterial growth. Controlling contamination at farm level Efforts to control bacterial contamination are extending back to the farm or feedlot where vaccinations may be used to control specific pathogens. Improvements in housing conditions may be used to reduce the presence of manure on the animal`s skin. Withdrawal of feed several hours before slaughter is one practice commonly used to lower the contents in the gastrointestinal tract and thus reduce the likelihood of contamination during evisceration. Contamination at carcass surfaces Most of the bacteria that populate surfaces of carcasses and fresh meat cuts are spoilage bacteria. These bacteria may cause objectionable odours, flavours, and discolouration and lead to disposal of the product Meat products may also be populated by pathogenic bacteria that can cause illness in people consuming those products. In fresh meat products, pathogenic bacteria are typically greatly outnumbered by spoilage bacteria.  However, certain types of pathogens can cause serious illness when only a few of the bacteria are present. Bacteria on meat products are detected and enumerated using various procedures most of which require many hours or days before results are available. Effective sanitation and refrigeration reduce total bacterial numbers and delay spoilage but cannot assure the absence of pathogens. Control of contamination at cooking stage Thermal destruction of bacteria during cooking is an effective way to assure that pathogens do not infect consumers. Ready –to-eat products such as luncheon meats and hot dogs are cooked during manufacture, then packaged and distributed to consumers. Meat processors take great care to assure that ready –to-eat products are properly cooked and that they are not re-contaminated after cooking. It is imperative that pathogens are excluded because these products may be consumed without further heating.  For most fresh meat products cooking is done at the point of consumption-home or restaurant. Consumer and food handler education is required to assure adequate cooking and avoid cross contamination during preparation. Sources of microbial contamination Initial contamination of meat results from introduction of microorganisms into the vascular system by captive bolt stunning. The other initial route of introducing microbes is when non-sterile knives are used for exsanguination (sticking or bleeding). Since blood continues to circulate for short time, microorganisms may be disseminated throughout animal body.  Subsequent contamination occurs by transfer of microorganisms to meat surfaces in almost every operation performed during slaughtering, cutting, processing, storage and distribution of meat. Carcasses may be contaminated by contact with hides, feet, manure, and viscera especially the oesophagus, rectum and other openings resulting from punctures during slaughter.  Other potential sources of microbial contamination include equipment, clothing, and hands of personnel as well as air, water, walls and doors. Thus, total microbial load and presence of specific pathogens are important factors in determining shelf life and safety of all meat products. Factors influencing microbial activity in meat Microbial survival and growth in meat products depends on a variety of factors in the meat and the surrounding environment. Among factors these include: moisture content, pH, oxidation-reduction potential, nutrients available and presence or absence of inhibitory substances. Additionally, factors such as temperature, oxygen, and physical form of meat affect growth rates of microorganisms. Temperature Each microorganism has an optimum, as well as a minimum and maximum temperature for growth The temperature at which meat is stored influences the kind, rate and extent of microbial growth. Temperature change of only a few degrees may favour the growth of entirely different organisms and result in different types of spoilage.  These characteristics provide the basis for use of temperature as a means of controlling microbial activity. The optimum temperature for growth of most microorganisms range from 15 to 400C. However, some organisms grow well at refrigeration temperatures. Some even grow well at sub-zero temperatures and others grow at temperatures exceeding 1000C.  Microorganisms that have their optimum growth at temperatures lower than 200C are called psychrophiles. Those that have growth optima at temperatures higher than 450C are called thermophiles. Microorganisms with growth optima between the psychrophiles and thermophiles are called mesophiles.  Bacteria, molds, and yeasts each have some genera that are thermophiles, mesophiles and psychrophiles. However, molds are generally most psychrophilic, followed by yeasts and bacteria in that order. Molds and yeasts are the least thermophilic. Although bacteria, molds and yeasts may all be present in meat at normal refrigeration temperatures of -10C to 30C, mold and yeast growth is more likely to occur than bacterial growth. Moisture Water is an absolute requirement for microbial growth. The availability of water determines which types of microorganisms can grow. Water activity (Aw) is the term used to describe the availability of water for bacterial growth. Water activity may be reduced by removal of water or by addition of solutes (salt) which make the water unavailable to microorganisms.  Bacteria generally require highest Aw of.90. Most yeasts thrive in the range of.87 to.92 while molds grow at Aw of.84 or less. Water for microbial growth is in excellent supply in meat animal carcasses and fresh meat products which typically have Aw in excess of.98.  Reduced relative humidity in carcass coolers leads to surface drying and reduced Aw so that surface bacteria are inhibited. Fat covered surfaces of the carcass are especially affected since limited moisture is available in the fat. The application of a water shower in the carcass cooler keeps the Aw of the surface high. Oxygen Availability of oxygen is important because it determines the type of microorganisms that are active. Some microorganisms have an absolute requirement for oxygen. Others grow only in the complete absence of oxygen, and still others grow either with oxygen (aerobic organisms) or without oxygen (anaerobic organisms).  All molds that grow in meat are aerobic and yeasts also grow best when anaerobic conditions prevail. On the other hand, bacteria found in meat may be aerobic, anaerobic, microaerophilic or facultative organisms. Microbial growth occurring in meat surfaces is largely that of aerobes with some facultative organisms.  The interior portions of meat support growth of anaerobic, microaerophilic and facultative bacteria. Some processes such as grinding lead to incorporation of oxygen into the meat while vacuum packaging involves removal of oxygen. Long term changes in microbial populations may result from such procedures. Acidity Microorganisms have a pH range for optimum growth near neutrality (pH 7.0). Molds have widest range of pH tolerance (pH 2.0-8.0), although their growth is generally favoured by acid pH. They (molds) can thrive in media that are too acid for either bacteria or yeasts.  Although yeasts also can grow in acid environments, they grow best in intermediate acid (pH 4.0-4.5). Bacterial growth is generally favoured by near-neutral pH. In meat with low pH values (pH 5.2 or lower), microbial growth is markedly reduced from that at normal pH. Meat with high ultimate pH is generally very susceptible to microbial growth, even under excellent sanitary conditions. Redox potential Oxidation-reduction potential of meat indicates its oxidizing and reducing power. Some microorganisms require reduced conditions and others oxidized conditions to attain optimal growth. Aerobic microorganisms are strongly favoured by high oxidation-reduction potential (oxidizing activity).  Low potential (reducing activity) largely favours growth of anaerobic organisms. Facultative microorganisms are able to grow under either condition. Microorganisms are capable of altering the oxidation-reduction potential of meat to the extent that activity of other organisms is affected. For instance, aerobes may decrease oxidation-reduction to such a low level that their own growth is inhibited while growth of anaerobes is promoted.  Generally, the oxidation-reduction potential falls following exsanguination and subsequent periods during conversion of muscle to meat. In post-mortem muscle, reduced conditions normally prevail. Oxygen penetration is markedly inhibited, and many reducing groups available.  The oxidation-reduction potential is highest at meat surfaces because of oxygen from the air and lowest in interior portions. Comminution of meat increases its oxidation-reduction potential markedly by incorporating more oxygen. Other nutrients Aerobes have other nutrient requirements in addition to water and oxygen. Most microorganisms need external sources of nitrogen, energy, minerals and B vitamins to support growth. They obtain nitrogen from amino acids and other non-protein nitrogen sources but some use peptides and proteins.  Pseudomonas and other organisms common on fresh meat use carbohydrate for energy until supply becomes limited. They rely more heavily on amino acids for energy and release increasing amounts of sulphur containing compounds. These compounds (hydrogen sulphide, methyl-sulphide and di-methyl- sulphide) contribute to putrid sulphury odour of spoiled meat.  Minerals are needed by all microorganisms, but their requirements for vitamins and other growth factors vary. Meat has an abundance of each of these nutrients; consequently, it is an excellent medium for microbial growth. Spoilage bacteria The surfaces of carcasses usually have high proportions of bacteria of faecal and soil origin following slaughter. These bacteria may be pathogenic but the overwhelming majority are non-pathogenic. These are commonly referred as spoilage bacteria since their growth and metabolic products eventually lead to objectionable appearance and odour.  Growth of psychrophilic bacteria is favoured as the carcass temperature declines. Pseudomonas, Moraxella, Psychrobacter and Acinetobacter are the predominant organisms populating the aerobic surface of refrigerated meat and poultry carcasses and cuts. Other bacteria that may be present in smaller numbers include Aeromonas, Alteromonas, Shewanella, Micrococcus, Lactobacillus, Streptococcus, Leuconostoc, Pediococcus, Flavobacterium and Proteus.  As storage conditions change the predominant species may change also. For example, addition of salt, especially in fermented meat products, inhibits gram-negative spoilage bacteria such as Pseudomonas while more salt tolerant lactic acid bacteria thrive and produce lactic acid. Freezing kills many bacteria on meat and the number of viable organisms continues to decrease during subsequent freezer storage. However, many bacteria survive and resume growth upon thawing. Pathogenic bacteria Development of gastrointestinal disturbances following ingestion of food can result from any of several causes. Food-poisoning is most appropriately defined as an illness caused by ingestion of toxins. Foo infection is defined as ingestion of pathogenic organisms (disease- producing) that grow and cause illness in the host. Toxins are produced by bacteria and fungi (molds and yeasts).  Toxins produced by fungi are referred as mycotoxins. They are common in mold-infested grains and legumes, so mycotoxins often constitute a health problem for livestock. Meat (especially cured and smoked meat, aged, dry sausages) frequently contains mold and yeasts associated with mycotoxins.  Meat from animals that have consumed mycotoxins in feeds may contain toxin residues. Illnesses following ingestion of foods containing toxins are usually of bacterial origin. These bacterial toxins are relatively odourless and tasteless and readily consumed by unsuspecting victims. Table 1: Characteristics of some common Food poisonings and infections Illness Causative Symptoms Av. time Foods Preventive agent b4 onset involved measures Botulim (food Toxins by Impaired 12-48 hours Canned meat, Proper poisoning) Clostridium swallowing, seafoods, canning, botulinum speaking, smoked, smoking, dizziness, processed fish cooking, respiration refrigeration Staphylococcal Enterotoxin Nausea, 3-6 hours Dairy Pasteurization (food by vomiting, products, of susceptible poisoning) staphylococc abdominal cooked ham, foods, proper us aureus cramps tongue, refrigeration poultry and sanitation Salmonellosis Ingestion of Nausea, 6-24 hours Insufficient Cleanliness, (food infection) salmonella vomiting, cooking or sanitation of species that diarrhoea, fever warming over handlers and can grow in and abdominal meat, poultry, equipment, human GIT pain. eggs, dairy pasteurisation products. , proper refrigeration. Campylobact Campylobac Headache, 1-7 days Insufficiently Cleanliness er (Food ter jejani fever, cooked and infection) and abdominal poultry and sanitation of campylobac pains and meat handlers ter coli diarrhoea products, and unpasteuris equipment, ed milk and pasteurisati dairy on, proper cooking. Listeriosis Listeria Flu-like Up to 8 weeks Refrigerated Avoid (Food monocytogen symptoms, ready to eat recontaminati infection) es diarrhoea, foods, on after heat fever, luncheon processing, abortion. meat, hot effective dogs, milk, sanitation. cheese, melons, strawberries Haemorrhagic Escherichia Flu-like 2-5 days Under-cooked Cook ground colitis coli symptoms, ground beef, beef to 710C fever, bloody contaminated and avoid diarrhoea water recontaminati on. Preventing microbial contamination Bacterial contamination occurs as plant workers and machinery repeatedly touch contaminated surfaces and carcass. Design of operating procedures, especially for hide removal and evisceration can greatly reduce the problem. Frequent washing and sanitising of hands, tools, and machinery is required in order to bacteria before they reach exposed surface of the carcasses. Corrective procedures for removing bacteria Trimming is the first response when visible contamination is present on a carcass in form of faecal material or other contaminants. The presence of visible stains or residue is a good indication that bacteria are present. Trimming is preferrable to washing since a water spray may spread contamination to other areas. Some parts of carcass which are prone frequent contamination may be routinely subjected to steam/vaccum process even when no visible contamination present. Deterioration of Meat Deteriorative changes caused by microbes Definitive determination of when meat is spoiled is not possible since that which one individual considers spoiled might well be considered edible by another. The usual characterisation of spoiled meat or any food, is the point at which it becomes unfit for human consumption. Spoilage is equated with decomposition and putrefaction resulting from microorganisms.  There is seldom any doubt about fitness of meat for consumption when it shows evidence of such spoilage. But presence of pathogens such as Listeria or Salmonella may render a product unfit for consumption without decomposition or putrefaction. Deteriorative chemical changes Endogenous enzymes are responsible for degradation of complex molecules as numbers of microorganisms and their activity increase. Endogenous enzymes hydrolyse complex molecules into simpler compounds that are used as nutrient and energy sources for microbial growth and activity. End products of microbial action vary with microbial species and available nutrients.  Fresh meat normally has enough carbohydrate to temporarily support aerobic bacteria that produce CO2 and water or anaerobic bacteria that produce lactic acid. When carbohydrate is depleted the microbes depend more on protein for energy. Under aerobic conditions, products of protein hydrolysis are simple peptides and amino acids. Deteriorative physical changes Microbial spoilage usually results in obvious physical changes in meat and these changes may appear in colour, odour, flavour, tenderness and processing properties. Meat spoilage is classified as being either aerobic or anaerobic depending on conditions under which it occurs as well as principal organisms causing spoilage :bacteria, molds and yeasts.  Aerobic spoilage by bacteria and yeasts results in slime formation, undesirable odours and flavours (taints), and colour changes. Some species of bacteria cause greening in sausage. Pigmented bacteria and yeasts cause various other surface colourations. Hydrolysis of lipids is enhanced by lipolytic bacteria and yeasts with development of oxidative rancidity and undesirable odours and flavours.  Aerobic spoilage by molds results in sticky meat surfaces. The formation of whiskers by molds is common and long-aged meat always exhibits some mold growth. Aerobic spoilage is essentially limited to meat surfaces where oxygen is readily available.  Anaerobic and microaerophilic spoilage occurs in vacuum packaged products, in sealed containers or sometimes deep inside whole muscle cuts where oxygen is either absent or present in limited quantities. This type of spoilage is caused by facultative and anaerobic bacteria and usually described by terms such as taint, souring and putrefaction.  Taint is a non-specific term used to describe off odours and flavours. Souring (development of a sour odour or flavour) results mainly from accumulation of organic acids during bacterial enzymatic degradation of complex molecules. Occasionally, souring is accompanied by production of various gases. Deteriorative changes by insects Insects rarely constitute a problem in meat because sanitation and insect control measures are sufficiently strict. Insect infestation reduces product acceptability and it’s a health hazard due to disease transmission potential. Among commonly found insects in these products are butterflies, leaping larvae that feed on muscle and soft tissues. In addition to structural damage, discolouration will occur.