• Meat is a nutrient-dense, protein-rich food that is extremely perishable and has a short shelf life if not preserved.
• The shelf life and maintenance of meat quality are affected by a number of interrelated elements, including keeping temperature, which can result in degraded meat quality characteristics.
• The most significant factor affecting the keeping quality of meat is spoilage due to microbial development.
• In the majority of developing nations, including Nigeria, fresh meat constitutes a considerable share of meat consumption.
• It is either either cooked or transformed into another form to prevent associated deterioration. The primary cause of such deterioration has been related to the absence of adequate storage facilities and the favourable ambient temperature that typically occur in tropical poor countries.

Contamination of Meats and Meat Products – Contamination source and causes

Meat spoilage can occur through natural processes as well as microbial contamination. Let’s explore the factors responsible for microbial contamination and the events that take place during the rigor mortis phase after animal slaughter:

1. Factors responsible for microbial contamination:
   • Bacterial flora of the animal: The presence of bacteria on the animal’s skin, in the gastrointestinal tract, and other body parts can contribute to meat contamination.
   • Knives, utensils, hands, and clothing of workers: Poor hygiene practices during meat processing can introduce bacteria from the external environment.
   • Pre-slaughter and post-slaughter handling: Inadequate handling of livestock before slaughter and improper handling of meat after slaughter can lead to microbial contamination.
   • Temperature controls: Inconsistent temperature controls during slaughter, processing, and distribution can promote bacterial growth on meat.
   • Packaging: The type of packaging used for meat products can affect the risk of contamination if it does not provide adequate protection.
2. Events during rigor mortis: Rigor mortis refers to the stiffening of muscles that occurs after animal slaughter. Several events take place during this phase:
   • Cessation of respiration: ATP synthesis stops due to the lack of oxygen, leading to a reduction in energy production.
   • Stiffening of muscles: The absence of ATP causes muscles to become rigid.
   • Reduction in oxidation-reduction potential: The lack of oxygen reduces the potential for oxidative reactions.
   • Development of rancidity: The loss of vitamins and antioxidants during rigor mortis can contribute to the development of rancidity in meat.
   • Glycolysis and pH reduction: Glycogen in the muscles is converted to lactic acid through glycolysis, resulting in a decrease in pH.
   • Susceptibility to microorganisms: The cessation of the reticuloendothelial system makes meat more susceptible to microbial contamination.
   • Cooling and solidification: The animal’s body temperature decreases, causing fat to solidify.
   • Accumulation of metabolites: Various metabolites accumulate during rigor mortis, which can contribute to protein denaturation.

These events and factors emphasize the importance of proper hygiene practices, temperature control, and handling procedures during meat processing. Implementing good manufacturing practices, maintaining appropriate temperature conditions, and ensuring proper packaging can help minimize the risk of microbial contamination and maintain the quality and safety of meats and meat products.

Preservation of Meats and Meat Products

• As with most perishable commodities, meats are often preserved using a variety of preservative techniques.
• The fact that most meats are excellent culture media—high in moisture, nearly neutral in pH, and rich in nutrients—combined with the possibility of organisms in the lymph nodes, bones, and muscle, and the near-inevitability of contamination with spoilage organisms, makes the preservation of meats more challenging than that of most other foods.
• Before being treated for preservation, meat may experience undesired changes in look and flavour and may facilitate the growth of microbes if it is not rapidly cooled after slaughter.
• There may be some increase in the number of microbes after long-term storage at cold temperatures.

1. Asepsis 

• Asepsis, or keeping microbes as far away from meats as possible during slaughtering and handling, makes all methods of preservation simpler.
• Storage period under chilling circumstances can be extended, the risk of ageing for tenderization decreases, curing and smoking procedures become more reliable, and heating operations are more effective.
• Asepsis begins with the maximal avoidance of contamination from the animal’s exterior. The animal should be sprayed with water before to slaughter to remove as much nasty dirt as possible from the hair and hide, and a foot bath can be used to clean the hooves.
• Nevertheless, during skinning, the animal’s hide and hair are significant sources of contamination for the surfaces of the carcass.
• The knife used to bleed animals after slaughter may transfer bacteria into the still-circulating bloodstream as well as organisms when entering the hide.
• During scalding, organisms may be added to the hide and lungs of pigs. There is contamination not just from the hide during the skinning process, but also from the employees’ knives and clothing.
• During evisceration, contamination might come from the animal’s gut, the air, the water used to wash and rinse the corpse, the towels and brushes used on the carcass, the numerous knives, saws, etc., and the employees’ hands and clothing.
• Some creatures may originate from carcass-touched walls or floor splashes or mists. Meat in the chill room is susceptible to infection from the air, the walls, and the personnel. Particularly noteworthy as a source of mould spores is the typically scattered sawdust on the floor.
• Knives, saws, conveyors, tables, air, water, and employees also contribute to contamination during cutting and trimming.
• The fact that the microbes supplied from these sources typically include almost all species involved in the deterioration of meats in significant numbers highlights the significance of aseptic procedures.
• Once meat is contaminated with bacteria, it is difficult to remove them. Surfaces may be washed to remove coarse soil, however the wash water may include organisms.
• Utilizing hot water or pressurised sanitizer sprays is an excellent method for reducing the overall amount of bacteria on the carcass’s surface and possibly extending its refrigerated shelf life.
• Large portions of meat, especially “hang” or aged meat, may have mouldy or otherwise deteriorated surfaces clipped off, but this is not an efficient method of preservation.
• Films used to wrap meat inhibit the growth of microorganisms and prevent their entry. The permeability of these coatings to water, oxygen, and carbon dioxide varies greatly.
• It has been reported that less permeable films have a lower storage life for meats. Fresh meats retain their red colour more effectively in an oxygen-permeable film that is not evacuated.
• With an oxygen-impermeable membrane, more carbon dioxide from bacteria is retained, resulting in a less vibrant colour while lactic acid bacteria, Lactobacillaceae, and Brochothrix thermosphacta are favoured.
• Ideally, cured meats are vacuum-sealed in an oxygen-impermeable membrane. Evacuation helps inhibit the growth of aerobes, particularly moulds, slows the rate of growth of staphylococci, and promotes the growth of lactics, but it does not appear to promote the growth of Clostridium botulinum any more than simple overwrapping does.

2. Use of Heat 

• Meat canning is a highly specialised process because the procedure varies greatly depending on the meat product to be preserved.
• The majority of meat products are low-acid meals that are suitable for bacterial growth. In meat soups, the rate of heat penetration is quite high, whereas in tightly packed meats and pastes, it is extremely slow.
• Chemicals added to meats, such as spices, salt, or nitrates and nitrites during curing, also alter the heat processing, typically enhancing its efficacy.
• Nitrates in meat aid in the destruction of anaerobic bacteria spores by heat and prevent the germination of any surviving spores.
• Based on the type of heat processing employed, commercially canned meats can be split into two groups:
  • Meats that are heated in an attempt to render the contents of a can sterile or at least “commercially sterile,” such as canned meats for retail shelf storage.
  • Meats that have been heated sufficiently to eliminate a portion of spoilage organisms, but must be refrigerated to prevent spoilage. This is how canned hams and luncheon meat loaves are handled.
• Group 1 canned meats are referred to as shelf-stable, while group 2 canned meats are referred to as non-shelf-stable or “keep refrigerated.” The microbiological stability of the canned cured meats is due to the heat treatment and the various curing salts.
• The processing temperature for shelf-stable canned cured meats is 98 degrees Celsius, and the container size is typically under 1 pound.
• The non-shelf-stable cured meats are packaged in canisters weighing up to 22 pounds and heated to approximately 65 degrees Celsius.
• Other methods exist for applying heat to meat products than canning. It has been suggested that meat surfaces be treated with hot water to extend their shelf life, despite the fact that this may reduce nutrients and harm colour.
• Using steam or hot water to cook hot dogs in the packing factory reduces the quantity of bacteria and aids in preservation. The application of heat during the smoking of meats and meat products reduces bacteria populations.
• Precooking or tenderising hams reduces the amount of microorganisms, but does not disinfect them. These products must be refrigerated because they are perishable and may support the growth of food-poisoning organisms if kept at room temperature.
• Similar considerations apply to cooked sausages with spices, such as frankfurters and liver sausage, which must also be refrigerated.
• Cooking meats for direct consumption drastically reduces their microbial content, hence extending their shelf life. Precooked frozen meats should have only a small number of live bacteria.

3. Use of Low Temperatures 

Low temperatures are used to preserve more meat than any other method, and chilling is used far more than freezing.

a. Chilling

• Modern packing-house techniques entail fast cooling meat to temperatures close to freezing and storing it at temperatures just above freezing.
• Less potential will exist for the growth of mesophilic microorganisms the quicker and quicker this cooling occurs.
• The principles involved in chilling storag are applicable to other foods as well. 1.4 to 2.2 C are acceptable storage temperatures, with the lower temps being recommended.
• Depending on the amount of microorganisms, the temperature, and the relative humidity, the chilling storage limit for beef is around 30 days; for pig, lamb, and mutton, the restriction is 1 to 2 weeks; and for veal, the limit is even shorter.
• Uncooked sausage, including uncured pork sausage in bulk or in links, must be maintained by refrigeration. With a rise in storage temperature, relative humidity often decreases.
• Meats can be stored for longer periods of time in an atmosphere containing extra carbon dioxide or ozone, or the temperature and relative humidity can be increased without reducing the storage period.
• Although substantial experimental work has been conducted on the gas storage of meats, this method has not been widely adopted. Successful use has been made of ships equipped for the storage of meat in a carbon dioxide-controlled atmosphere.
• Increasing levels of carbon dioxide in the environment hinder the growth of microbes, but also accelerate the creation of metmyoglobin and, thus, the loss of “bloom” or natural colour. According to reports, this type of gas storage has increased the shelf life of meat.
• There is no consensus among experts regarding the ideal concentration of carbon dioxide, with recommendations ranging from 10 to 30 percent for the majority of meats to 100 percent for bacon.
• The presence of 2.5 to 3 ppm ozone in the environment can further lengthen the period of storage. There have been reports of up to 60 days of mold- and slime-free storage at 2.2 degrees Celsius and 92% humidity.
• Fats may acquire an oxidised or tallow-like flavour if ozone, an active oxidising agent, is present. It has been discovered that, while the amounts of ozone cited will hinder the growth of germs, far greater concentrations are required to stop the growth of microorganisms that has already begun.
• The psychrotrophic bacteria, especially those of the genus Pseudomonas, are the most problematic in the chilling storage of meats. However, yeasts and moulds, as well as bacteria of the genera Acinetobacter, Moraxella, Alcaligenes, Micrococcus, Lactobacillus, Streptococcus, Leuconostoc, Pediococcus, Flavobacterium, and Proteus, can grow in meats at low temperatures.

b. Freezing 

• The majority of meat sold in retail stores has not been frozen; nonetheless, freezing is frequently employed to preserve meats during shipment over great distances or storage until times of shortage, and significant quantities of meat are now frozen in home freezers.
• Large portions of meat, such as halves and quarters, are quickfrozen, whereas hamburger and smaller, finer cuts may be quickfrozen in wrapped packets.
• As the storage temperature drops from 12.2 to 28.9 C, the preservation of frozen meats becomes increasingly successful.
• Meats intended for freezing are susceptible to the same hazards of contamination and growth of microbes as meats intended for other uses. The freezing process kills around half of the bacteria, and their numbers decline slowly throughout storage.
• The low-temperature bacteria that grow on meat during chilling, including species of Pseudomonas, Acinetobacter, Moraxella, Alcaligenes, Micrococcus, Lactobacillus, Flavobacterium, and Proteus, can continue growth during slow thawing.
• Following the instructions, packaged quick-frozen meats thaw too quickly for significant microbial development.

4. Use of Irradiation 

• In conjunction with chilling storage, UV irradiation has been utilised to increase the shelf life. Utilized mostly on large, hung cuts of beef in plant storage rooms.
• The rays reduce the amount of microorganisms in the air and inhibit or kill them on the meat surfaces directly contacted by the rays.
• For microbes to be harmed, they must be on the surface and unprotected by greasy or opaque substances. In order to prevent the surface growth of microorganisms, particularly moulds, irradiation has been utilised in the quick ageing of meats “hang” at higher-than-usual cooling temperatures.
• The meat is tenderised by its own proteolytic enzymes during the ageing or hanging process, which is utilised to make steaks and other elegant cuts.
• Ordinarily, ageing takes many weeks at 2.2 to 3.3 degrees Celsius, 80 to 90 percent relative humidity, and 10 to 30 feet per minute of airflow; however, when exposed to ultraviolet light, the time is shortened to two to three days at 18 degrees Celsius and 85 to 90 percent relative humidity.
• Some oxidation, encouraged by UV light, and hydrolysis of lipids may occur throughout the ageing process.
• Meat irradiation using gamma radiation is still experimental and limited. When sterilisation is accomplished, doses of 20 to 70 KiloGrays are used, and undesired colour and flavour changes may occur.

5. Preservation by Drying 

• Drying meats for preservation has been a centuries-old process. Sun-dried beef strips, or jerky, was a staple diet of American pioneers.
• Some types of sausage are maintained by their dryness primarily. In dried beef, created mostly from cured, smoked beef hams, microbe development may occur before to processing and in the “pickle” during curing, but the smoking and drying processes minimise the quantity of organisms.
• Organisms may infect the dried ham during storage and during slicing and packaging of the slices. Some types of meat products, such as dry sausages, dry salamis, and dry cervelats, are kept primarily by their low moisture content.
• A protective outer layer of dryness on the casing of any sausage. Typically, salting and smoking are included in the older ways of drying meats.
• During World War II, freshly cooked beef and pig were dehydrated using heat. A second method for drying pork involves a brief nitrate-nitrite cure and the addition of lecithin as a stabiliser and antioxidant.
• Vacuum-drying, tray-drying, or other drying processes may be used. The finished product does not require refrigeration. Fresh meats are less successful when freeze-dried than processed meats such as beef patties, meatballs, and stew.
• The process is becoming so cost-effective that manufacture for retail sale is now feasible for niche consumers, such as campers, backpackers, and hikers.
• Meat intended for dehydration should be of acceptable bacteriological quality, with no significant microbial growth or unpleasant flavour.

6. Use of Preservatives 

Meat storage under a controlled atmosphere containing additional carbon dioxide or ozone has been explored. This antiquated method of food preservation typically results in a subpar product. Typically, salting is used in conjunction with curing and smoking to achieve maximum effectiveness.

a. Curing 

• The curing of meats is limited to beef and pork, either ground meat or specific portions such as the hams, brisket, and leg muscles of cattle and the butts, jowls, sides, loins, and bellies of hogs.
• Originally, curing was used to preserve meats by salting without refrigeration; however, most cured meats today have other ingredients added, are refrigerated, and many are smoked and hence partially dried.
• The permissible curing chemicals are sodium chloride, sugar, sodium nitrite, and vinegar; however, only the first three are typically employed.
• These are the roles of the ingredients: Sodium chloride, also known as table salt, is largely employed as a preservative and taste ingredient.
• The cover pickle, which is used to submerge the meat, may have approximately 15 percent salt, but the pumping pickle, which is injected into the flesh, has approximately 24 percent salt.
• Its principal function is to reduce the aw. Sugar contributes flavour and provides energy for nitrate-reducing bacteria in the curing solution or pickle.
• Sucrose is typically used, although glucose may be replaced if a short fermentation period is employed, or sugar may not be added.
• Indirectly, sodium nitrate acts as a colour fixative and is bacteriostatic in acid solution, particularly against anaerobes. During the lengthy curing process, it has also functioned as a reservoir from which nitrite can be produced via bacterial reduction.
• Nitric oxide, which is the true colour fixative and has a bacteriostatic effect in acid solution, is derived from sodium nitrite.
• Therefore, the majority of the preservation action of the curing agents is attributable to the sodium chloride, with some bacteriostatic effect from the nitrite and a little effect from the nitrate. The combination of salts, sugar, and meat protein reduces the aw value of cured meats, such as hams, to between 0.95 and 0.97.
• Low curing temperature and smoke are further preservatives. Additionally, nitrate affects the colour of meats. For instance, the purplish-red hue of meats is a result of blood haemoglobin and muscle myoglobin; oxygenation of these substances generates oxyhemoglobin and oxymyoglobin, which are vivid red.
• Myoglobin and haemoglobin are converted into red nitric oxide myoglobin and nitrosohemoglobin under acidic and reducing conditions in the presence of nitrite.
• The acid condition is generated by the meat itself, the reduced condition by the bacteria, and the nitric oxide for the reaction is generated by nitrite reduction.
• Four techniques exist for putting curing chemicals into meat:
  • The dry cure, which involves rubbing dry substances into the meat, as in belly bacon curing.
  • The pickling cure, in which the meats are submerged in an ingredient solution.
  • As with pork hams, the injection cure involves injecting a concentrated solution of the components into the arteries and veins of the meat via an artery or into the muscle tissue in various areas of the meat.
  • The direct-addition method, in which curing ingredients are put directly to finely ground meats, such as sausage, for preservation purposes.
• The curing temperature, especially when using a pickling solution, is typically between 2.2 and 3.3 degrees Celsius, and the curing period varies depending on the methods employed and the meats to be cured.
• The traditional ways of pickling need many months, whereas the current “rapid cure” approach, in which the pickling solution is poured into the flesh, significantly reduces this period.
• After curing, the majority of meats are smoked to aid in their preservation; however, some meats, such as corned beef, are not smoked and must be refrigerated.
• During the curing process, certain forms of sausage, including Thuringer, cervelat, Lebanon bologna, salamis, and dry and semidry summer sausages, experience an acid fermentation, preferably of the mixed lactic acid variety.
• This not only prevents undesired fermentations but also imparts the proper sour taste. The utilisation of pure cultures, such as Pediococcus cerevisiae *, has increased control over the desired fermentation and lowered fermentation time.
• The table provides a summary of the numerous bacteria identified in fermented and cured meats.
• When preserving items such as pickled pigs’ feet, pickled beef with spices, and souses, vinegar is added to the pickling solution.
• The feet of pigs are cured in a solution of salt, sodium nitrate, and sodium nitrite, boiled, and then preserved in a salt-and-vinegar brine.
• The vegetables are then placed into jars or other containers, coated with a fresh salt-vinegar brine, and sealed. Unless the acidity is excessively low, the product will not deteriorate.

Microbiology of Meat-Curing Brines 

• The microorganisms in curing brines and on meats immersed in them vary according on the original state of the meat and the curing process applied.
• The microbial composition of the salt appears to be of little importance, except on salted meat, which, following removal from brine or dry salting, can produce red surface colonies of halophilic bacteria similar to those carried by the salt.
• In modern American short techniques of curing meats, such as ham, bacteria in the brine appear to have minimal effect on the changes that occur in the meat, as they do not reach high concentrations and are largely eliminated by the subsequent smoking.
• Except for the surface, where mirococci and yeasts may form, these brines are dominated by lactic acid bacteria.
• The predominant lactic organisms are lactobacilli and pediococci. In the traditional long cure, bacteria, particularly micrococci, were responsible for converting nitrate to nitrite and fixing the meat’s red colour.
• Immersion in somewhat concentrated brines and prolonged usage of these brines are typical of bacon-curing techniques used in other cultures.
• In addition to micrococci, the brines appear to accumulate a unique blend of cocci, gram-positive and gram-negative rods that, for the most part, form minute colonies on agar plates.
• They are halophilic to halotolerant and convert nitrates to nitrites. The growth of salt-tolerant, nitrate-reducing psychrotrophs is permitted when pig bellies are treated with a dry curing mixture and compacted in boxes.
• Some beef-curing brines have been reported to contain micrococci, lactobacilli, streptococci, Achromobacter*, vibrios, and maybe pediococci, as well as minor amounts of other bacteria.
• Micrococcus species are active in many pickling solutions, especially those with a high salt concentration that are used to cure British and Canadian bacon.

b. Smoking 

• In Chapter 9, the use of wood smoke as a preservative was examined. It was noted that smoking has two primary functions: imparting desired flavours and aiding in preservation.
• It has been shown that the preservation ingredients applied to the meat, along with the action of the heat during smoking, have a germicidal effect, and that the drying of the meat, along with chemicals from the smoke, inhibits the growth of microorganisms during storage.
• Older methods of curing and smoking, in which high salt concentrations were employed in curing and higher drying and integration of preservation compounds were achieved in smoking, produced hams, dried beef, etc. that could be stored at room temperature.
• However, many of the more recent techniques produce perishable goods that must be refrigerated. High-moisture, precooked or tenderised hams and sausages are examples.

c. Spices 

• Spices and condiments added to meat products such as meat loaves and sausages are not present in sufficient concentrations to be preservatives, but they may enhance the action of other preservatives.
• Undoubtedly, the maintaining quality of bologna, Polish, frankfurter, and other sausages is due to the combined effects of seasoning, curing, smoking (drying), cooking, and refrigeration.

d. Antibiotics 

• Although the sole allowed use of antibiotics in flesh foods in the United States is in fish (chlortetracycline at 7 ppm in ice), tests have shown that antibiotics can be used successfully to extend the shelf life of meats at freezing or higher temperatures.
• The most often prescribed antibiotics are chlortetracycline, oxytetracycline, nisin, and chloramphenicol.
• Meats may be treated with antibiotics in numerous ways:
  1. The animal may be fed the antibiotic for an extended period of time.
  2. Before slaughter, it may be fed more intensively for a brief period of time.
  3. It may be instilled into the carcass or portions of the corpse.
  4. It may be applied on the surface of meat or combined with meat that has been ground.
• Feeding an antibiotic causes a selection of microorganisms in the animal’s intestinal tract, thus reducing the amount of spoilage bacteria there and, consequently, the number of germs that are likely to reach the meat from that source during slaughtering and dressing.
• It has been suggested that pre-slaughter antibiotic injections could be used to prolong the time carcasses spend at ambient temperatures before they reach the refrigerator, or to hold beef briefly at temperatures that promote tenderization of specialty cuts and lengthen the shelf life of meats held at chilling temperatures.
• Infusing an antibiotic directly into the corpse after slaughter or into specific body parts might serve similar reasons.
• Meats’ shelf-life can be prolonged by dipping them in an antibiotic solution or by incorporating antibiotics into ground beef.

Spoilage of Meats and Meat Products

• Raw meat is susceptible to alteration by its own enzymes and microbial action, and its fat may undergo chemical oxidation.
• A significant level of autolysis is desirable in the hanging or ageing of beef and game, but not in the majority of other raw meats.
• Autolytic alterations include some proteolytic action on muscle and connective tissues as well as a minor hydrolysis of lipids.
• Excessive autolysis has been referred to as “souring,” an imprecise phrase that refers to a range of types of food spoilage and, in fact, practically any type that emits a sour odour.
• It is difficult to identify sourness generated by autolysis from faults caused by microbial action, notably simple proteolysis.
• However, this first breakdown of proteins by meat enzymes unquestionably helps microbes begin to proliferate in meat by supplying the simpler nitrogen molecules required by many germs that cannot attack entire native proteins. 

General Principles Underlying Meat Spoilage

During slaughter, dressing, and slicing, bacteria originate mostly from the exterior of the animal and its intestinal tract, but additional microbes are introduced by blades, cloths, air, personnel, carts, boxes, and equipment in general. It can be anticipated that, under normal settings, the vast majority of potential spoilage organisms are present and will be able to proliferate if favourable conditions exist.

a. Invasion of Tissues by Microorganisms 

Upon death, contaminating bacteria invade the tissues of the animal. The following elements contribute to this invasion:

1. The animal’s intestinal load: The bigger the load, the greater the tissue invasion. Therefore, a 24-hour fast prior to slaughter has been recommended.
2. The animal’s physiological status just prior to slaughter: If the animal is excited, feverish, or exhausted, bacteria are more likely to enter the tissues, bleeding is more likely to be incomplete, thereby promoting the spread of bacteria, and chemical changes may occur more readily in the tissue, such as those due to better bacterial growth due to a higher pH, earlier release of juices from the meat fibres, and faster denaturation of proteins. Due to the consumption of glycogen during exhaustion, the pH will not drop from 7.2 to 5.7 as it normally would.
3. The killing and bleeding method: The better and more hygienic the bleeding, the longer the meat will stay fresh. Little is known about the influence of humane methods of killing on the keeping quality of meat, however it has been stated that pork and bacon from animals stunned with electricity experienced greater greening than those killed using carbon dioxide.
4. The rate of cooling: Rapid cooling will minimise the rate at which germs invade tissues. Microorganisms are transmitted by the blood and lymph vessels, as well as the interstices of connective tissue, and by grinding in the case of pulverised meat.

b. Growth of Microorganisms in Meat 

Meat is a suitable culture medium for numerous organisms due to its high moisture content, plenty of nitrogenous meals of varying degrees of complexity, and abundance of nutrients and growth agents. In addition, it typically contains fermentable carbohydrates (glycogen) and a pH that is beneficial for the majority of microbes. The elements that determine the proliferation of microorganisms and, consequently, the type of spoiling are as follows: These factors include the following:

1. The kind and amount of contamination with microorganisms and the spread of these organisms in the meat

• For example, meat with a high percentage of psychrotrophic organisms in its infecting flora would spoil at cooling temperatures more quickly than meat with a low percentage of these organisms.

2. The physical properties of the meat

• The amount of exposed flesh surface has a significant effect on the rate of spoiling, as this is typically where the highest number of microorganisms and oxygen are available for aerobic organisms.
• However, fat itself is susceptible to spoiling, which is mostly enzymatic and chemical in nature.
• As a result of this and the fact that it releases moisture and spreads bacteria throughout the flesh, meat grinding significantly increases the surface area and fosters microbial development.
• The skin of meat serves to protect the meat within, despite the fact that bacteria can develop on it.

3. Chemical properties of the meat

• It has been noted that meat is an excellent medium for the growth of germs. Moisture content is crucial for determining whether organisms can develop and what types can grow, particularly at the surface, where drying may occur.
• Thus, the surface can be so dry that no growth can occur, a little damp to permit mould growth, still damper to promote yeast growth, and highly damp to encourage bacterial growth.
• In this sense, the relative humidity of the storage atmosphere is crucial. Food is abundant for microorganisms, but the low concentration or absence of fermentable carbohydrates and the high protein content tend to benefit nonfermenting organisms, which may exploit proteins and their breakdown products for nitrogen, carbon, and energy.
• Depending on the quantity of glycogen present at the time of slaughter and subsequent modifications, the pH of raw meat can range from approximately 5.7 to over 7.2.
• A higher pH value encourages microbial development, whereas a lower pH value typically retards it and may be selective for certain organisms, such as yeasts.

4. Availability of oxygen 

• Molds, yeasts, and aerobic bacteria flourish in an aerobic environment on the surface of meat.
• Within the solid pieces of meat, conditions are anaerobic and tend to remain so because the O-R potential is strongly poised at a low level. However, oxygen will diffuse slowly into ground meat and slowly raise the O-R potential, unless the casing or packaging material is oxygen-impermeable.
• True putrefaction thrives in anaerobic environments.

5. Temperature

• Meat should be stored at temperatures just above freezing, where only bacteria that thrive at low temperatures can grow.
• Molds, yeasts, and psychrotrophic bacteria grow slowly and generate problems that will be explored in greater detail later.
• At these temperatures, true putrefaction is uncommon, although it is probable at room temperature. As with most foods, temperature is the most influential factor in determining the types of organisms that will grow and the forms of spoiling that will result.
• At chilly temperatures, for instance, psychrophiles are favoured and proteolysis is probable, as a result of a dominant bacterial species, followed by secondary species’ use of peptides and amino acids.
• At normal air temperatures, mesophiles, such as coliform bacteria and Bacillus and Clostridium species, would proliferate, producing moderate amounts of acid from the few carbohydrates present.

General Types of Spoilage of Meats 

The primary varieties of meat spoilage can be categorised according to whether they occur under aerobic or anaerobic circumstances and are caused by bacteria, yeasts, or moulds.

A. Spoilage under Aerobic Conditions 

Bacteria may cause the following in aerobic conditions:

1. Surface slime

• Species of Pseudomonas, Acinetobacter, Moraxella, Alcaligenes, Streptococcus, Leuconostoc, Bacillus, and Micrococcus may create surface slime. Some Lactobacillus species can create slime.
• Temperature and the availability of moisture affect the kind of microorganisms that cause slime on surfaces.
• At chilly temperatures, high humidity favours the Pseudomonas-Alcaligenes group; with less humidity, like on frankfurters, micrococci and yeasts thrive; and with even less humidity, moulds may develop.
• At room temperature and higher, micrococci* and other mesophiles compete favourably with pseudomonads and related bacteria.
• The amount of bacteria that must be present in meats and other protein-rich foods before an off-odor or slime can be detected. There must be measurements in the millions per square centimetre or gramme.

2. Changes in color of meat pigments

• As a result of the creation of oxidising substances, such as peroxides or hydrogen sulphide, by bacteria, the red colour of meat, referred to as its “bloom,” might change to colours of green, brown, or grey.
• According to reports, heterofermentative Lactobacillus and Leuconostoc species are responsible for the greening of sausage.

3. Changes in fats

• The chemical oxidation of unsaturated fats in meats occurs in air and can be catalysed by light and copper.
• Lipolytic bacteria are capable of causing lipolysis and accelerating fat oxidation. Some fats, such as butterfat, become tallowy upon oxidation and rancid upon hydrolysis, but the vast majority of animal fats develop oxidative rancidity upon oxidation, with off-odors caused by aldehydes and acids.
• Flavor is imparted by the liberated fatty acids during hydrolysis. Yeasts or lipolytic organisms like Pseudomonas and Achromobacter* can produce rancidity in lipids.

4. Phosphorescence

• This somewhat rare abnormality is produced by the growth of phosphorescent or fluorescent bacteria, such as Photobacterium spp., on the surface of the meat.

5. Various surface colors due to pigmented bacteria

• Therefore, “red spot” may be caused by Serratia marcescens or other red-pigmented bacteria.
• Pseudomonas syncyanea* can provide a surface a blue hue. Yellow discolorations are typically caused by Micrococcus* or Flavobacterium species with yellow pigments.
• The bacteria Chromobacterium lividum and others cause greenish-blue to brownish-black stains on beef that has been kept. Yellow-pigmented cocci and rods are the cause of the purple “stamping-ink” staining of surface fat.
• When the fat becomes rancid and peroxides develop, the yellow colour changes to a bluish-purple hue and then to a greenish hue.

6. Off-odors and off-tastes

• “Taints,” or unpleasant aromas and flavours, that develop on the surface of meat as a result of bacterial development are frequently detectable before other indicators of deterioration.
• Almost any fault that imparts a sour aroma, such as those caused by volatile acids, such as formic, acetic, butyric, and propionic, or even yeast growth, is referred to as “sour.”
• A “cold-storage flavour” or taint is an imprecise phrase for a rancid taste. Actinomycetes could be the cause of a musty or earthy flavour.
• Under aerobic conditions, yeasts can develop on the surface of meats, causing sliminess, lipolysis, off-odors and flavours, and white, cream, pink, or brown discoloratins due to pigments in the yeasts.

Aerobic growth of molds

Aerobic mould growth may result in the following:

1. Stickiness: Initial mould growth causes the surface of the meat to become tacky to the touch.
2. Whiskers: When meat is held at temperatures close to freezing, mycelial development may occur without sporulation to a limited extent. This type of white, fuzzy growth can be generated by a variety of fungi, including Thamnidium chaetocladioides, or T. elegans; Mucor mucedo, M. lusitanicus, or M. racemosus, Rhizopus, and other species. A controlled cultivation of a specific strain of Thamnidium has been suggested for the enhancement of beef flavour during ageing.
3. Black spot: This is typically caused by Cladosporium herbarum, but other dark-pigmented fungi may also be responsible.
4. White spot: Sporotrichum carnis is the most common cause of white spot, however any mould with moist, yeast-like colonies, such as Geotrichum, can also cause white spot.
5. Green patches: Green patches are typically caused by the green spores of Penicillium species such as P. expansum, P. asperulum, and P. oxalicum.
6. Decomposition of fats: Numerous moulds include lipases, which result in the breakdown of fats. Molds also contribute to the oxidation of lipids.
7. Off-odors and off-tastes: Molds impart a musty quality to meat in their surroundings. Occasionally, the defect is given a name that indicates its cause, such as “thamnidium taint.”

Spots of surface deterioration caused by yeasts and mould are typically very confined and can be removed without harming the remainder of the meat. The depth of the defect will be determined by the time allotted for the diffusion of decomposition products into the meat and the rate of that diffusion. Significant surface bacterial growth may result in relatively deep penetration. Also, facultative bacteria may grow slowly inwards.

B. Spoilage under Anaerobic Conditions

Under anaerobic conditions, facultative and anaerobic bacteria can develop within the flesh and cause deterioration. The nomenclature used to describe this deterioration is imprecise. The labels “souring,” “putrefaction,” and “taint” are the most common, yet they appear to have varied meanings for different people.

1. Souring

• The phrase suggests a sour aroma and possibly taste. This could be the result of formic, acetic, butyric, propionic, and higher fatty acids, as well as lactic or succinic acid.
• Souring can be caused by (a) the action of the meat’s own enzymes during ageing or ripening, (b) the anaerobic production of fatty acids or lactic acid by bacterial action, or (c) proteolysis without putrefaction, caused by facultative or anaerobic bacteria and sometimes referred to as “stinking sour fermentation.”
• The activity of “butyric” Clostridium species and coliform bacteria on carbohydrates results in acid and gas production.
• Typically, lactic acid bacteria thrive on vacuum-packed meats, especially those packaged in airtight containers.

2. Putrefaction

• True putrefaction is the anaerobic degradation of proteins that results in the generation of odorous chemicals such as hydrogen sulphide, mercaptans, indole, skatole, ammonia, and amines.
• It is typically caused by Clostridium species, however facultative bacteria can also produce or aid in putrefaction, as indicated by the extensive list of species with the specific names putrefaciens, putrificum, putida*, etc., primarily in the genera Pseudomonas and Alcáligens. Additionally, some Proteus species are putrefactive.
• The confusion in the use of the term “putrefaction” stems from the fact that any sort of spoilage with foul aromas, whether caused by the anaerobic decomposition of protein or the breakdown of other molecules, even nonnitrogenous ones, can be incorrectly referred to as putrefaction.
• Thus, the putrid scents of trimethylamine in fish and isovaleric acid in butter are characterised. Clostridial putrefaction is accompanied by the production of hydrogen and carbon dioxide.

3. Taint

• “Taint” is an even less precise term for any off-taste or off-odor. The word “bone taint” refers to either fermentation or putrefaction adjacent to the bones, particularly in hams.
• Typically, it refers to putrefaction. Both air and temperature have a significant impact on the sort of meat deterioration that might be anticipated. Molds, yeasts, and bacteria that can develop at low temperatures are the only microorganisms that can thrive when meat is stored at temperatures below 0 degrees Celsius, as suggested.
• These include several of the types that induce sliminess, staining, and growth patches on the surface, as well as many that can cause sourness, including Pseudomonas, Acinetobacter Moraxella, Alcaligenes, Lactobacillus, Leuconostoc, Streptococcus, and Flavobacterium species.
• The majority of real putrefiers, such as those in the genus Clostridium, require temperatures higher than that of a refrigerator.

Spoilage of Different Types of Meats 

Meats that have undergone curing, smoking, drying, or canning typically endure alterations to their microbial ecology that promote types of deterioration not seen in fresh meats.

A. Spoilage of Fresh Meats 

• The deterioration of fresh meats was discussed in the topic of general types of spoiling that came before.
• Normally, Pseudomonas, Acinetobacter, and Moraxella organisms decompose fresh meats after prolonged refrigeration.
• Even at refrigerator temperatures, lactic acid bacteria, primarily of the species Lactobacillus, Leuconostoc, Streptococcus, Brevibacterium, and Pediococcus, are present in the majority of fresh and cured meats.
• In general, their modest growth does not diminish the quality of the meat; in fact, the lactic fermentation is promoted in specific forms of sausage, such as salami, Lebanon, and Thuringer. However, lactic acid bacteria may be responsible for three types of spoilage: (1) development of slime on the surface or within the product, especially in the presence of sucrose; (2) production of a green discoloration; and (3) sourness when excessive levels of lactic and other acids are formed.

1. Fresh Beef 

Fresh beef experiences the following colour changes:

• The action of oxygen and microbes on haemoglobin and myoglobin, the red pigments in the blood and muscles, respectively, causes a loss of colour and the development of reddish-brown methemo-globin and metmyoglobin as well as greenish-gray-brown oxidation pigments.
• Pigmented microorganisms cause white, green, yellow, and greenish-blue to brown-black patches and purple discolorations.
• Phosphorescence.
• Spots caused by several bacteria, yeasts, and fungi. Beef is also susceptible to surface sliminess caused by bacteria or yeasts, stickiness caused by moulds, whiskers caused by mycelial development of moulds, and bacterial putrefaction and sourness. At 10 C or below, pseudomonads predominate in beef, but at 15 C or above, micrococci* and pseudomonads grow in roughly equal numbers.

2. Hamburger 

• At temperatures close to freezing, hamburger that has been stored at room temperature has a stale, unpleasant stench.
• Low-temperature sourness is mostly caused by Pseudomonas, Acinetobacter, and Moraxella species, with assistance from lactic acid bacteria.
• In some samples, Alcaligenes, Micrococcus*, and Flavobacterium species may thrive. Numerous types of bacteria have been identified in hamburger stored at elevated temperatures, but no differentiation has been made between mere presence and development.
• Bacillus, Clostridium, Escherichia, Enterobacter, Proteus, Pseudomonas, Alcaligenes, Lactobacillus, Leuconostoc, Streptococcus, Micrococcus, and Sarcina are among the recorded genera of bacteria, whereas Penicillium and Mucor are among the documented genera of moulds. Several yeasts have also been discovered.

3. Fresh Pork Sausage 

• The primary ingredient in fresh sausage is ground fresh pork, to which salt and spices are added. It may be packaged in natural or artificial casings or sold in bulk.
• Pork sausage is a perishable item that requires refrigeration and may only be stored for a brief period of time before spoiling.
• Souring, the most prevalent type of spoilage at refrigerator temperatures of 0 to 11 C, has been attributed to the growth and acid generation of lactobacilli and leuconostocs; however, Microbacterium and Micrococcus* species can grow at higher storage temperatures.
• Long-term storage of encased pork sausages, particularly those of the “little-pig” variety, may result in the production of slime on the exterior casing or variably coloured patches owing to mould growth.
• Thus, it has been discovered that Alternaria causes microscopic dark patches on chilled linkages.

B. Spoilage of Cured Meats 

• The majority of the cured meats are pork, although some beef pieces may also be cured. The inhibiting impact of nitrite on anaerobes has already been mentioned.
• It is believed that sodium nitrite favours lactic acid bacteria in fermented sausages such as Thuringer and Essex.
• Meats preserved using curing salts are more conducive to the growth of gram-positive bacteria, yeasts, and moulds than to the gram-negative bacteria that often cause meats to decay.
• In addition, they lessen the amount of thermal processing required to make stable heated meat items, such as pig luncheon meats.
• Certain meats, such as bulk chipped beef, are preserved by their high salt chloride concentration. The amount of microorganisms on the piece of meat to be cured as well as any deterioration will affect the success of the curing process.
• Thus, unwanted changes in the pigments of the meat will result in a discoloured cured product, the onset of spoiling will diminish the product’s look and flavour, and excessive populations of spoilage bacteria may interfere with the cure.

1. Dried Beef or Beef Hams 

• Beef hams are rendered spongy by Bacillus species, sour by numerous bacteria, red by Halobacterium salinarium or a red Bacillus species, and blue by Pseudomonas syncyanea*, Penicillium spinulosum (purple), and species of Rhodotorula yeasts.
• Water content and relative humidity are the most important factors. As the relative humidity rises and the product absorbs more water, the shelf life decreases.
• A denitrifying aerobic bacteria like Pseudomonas fluorescens is responsible for the presence of gas in chipped dry beef.
• The gases are nitrogen oxides. In the jars, Bacillus species have been observed to create carbon dioxide.

2. Sausage 

• Spoilage bacteria may grow on the exterior of the casing, between the casing and the meat, or within enclosed sausages. Only if adequate moisture is present can organisms grow on the exterior of the casings.
• If moisture is present, micrococci* and yeasts can develop a slimy film, as is frequently the case with frankfurters that have gotten moist after being removed from the refrigerator and exposed to higher temperatures.
• Molds may develop fuzz and discolouration if there is less moisture present. Carbon dioxide, mostly produced by heterofermentative lactic acid bacteria, may cause packages of wieners or breakfast sausages sealed in gas-tight flexible film to expand.
• The accumulation of moisture between the casing and the meat during cooking is conducive to growth if the casing is permeable to water; when using two casings, the inner casing can be wetted before the outer casing is attached, trapping water between them.
• The slime on the surface of the meat or between the casings is created primarily by micrococci* that produce acid. The inner casing’s permeability to soluble nutrients promotes bacterial growth.
• It has been claimed that many types of bacteria can grow within sausages during long periods of refrigeration or at temperatures exceeding 10.5 degrees Celsius. It is possible for acid-forming micrococci* such as Micrococcus candidus* to grow in liverwurst and bologna, and Bacillus species have been discovered growing in liverwurst.
• In addition to leuconostocs and lactobacilli, psychrotrophic leuconostocs and lactobacilli can proliferate and generate a souring that is undesirable in most sausages but desired in certain, such as Lebanon, Thuringer, and Essex sausages.
• The transformation of the red colour of sausage into a chalky grey hue has been attributed to air, light, and bacteria.
• Chill rings have been attributed to oxidation, bacterial formation of organic acids or reducing chemicals, high water content, and undercooking.
• Greening may manifest as a green ring at the casing, a green core, or a green surface.
• According to Niven, greening is most likely caused by the generation of peroxides, such as hydrogen peroxide, by heterofermentative species of Lactobacillus and Leuconostoc or other catalase-negative bacteria (1961).
• Jensen (1954) suggested that hydrogen sulphide may potentially be a factor. A slightly acidic pH and the presence of minute amounts of oxygen promote greening.
• The green ring beneath the surface of large sausages or the green core of small sausages develops 12 to 36 hours after processing, even when refrigerated; it is visible as soon as the sausage is cut and is typically not accompanied by surface slime.
• Before smoking and cooking, bacterial growth and the generation of heat-stable peroxide have occurred, and the peroxide continues to generate greening after processing.
• Green cores in large sausage, such as large bologna, typically develop after four or more days of storage and within one to twelve hours of slicing, as a result of poor processing and inadequate refrigeration.
• Greening of a cut surface implies contamination with and growth of salt-tolerant, peroxide-forming bacteria (likely lactobacilli) that are capable of growing at low temperatures. Frequently, surface sliminess accompanies the greening.
• The fault can be transmitted from one sausage to another. It has been found that nitrate-reducing bacteria produce nitric oxide gas in sausages.
• Carbon dioxide may collect as a result of the action of heterofermentative lactics and induce swelling if the casing or packaging material does not allow the passage of carbon dioxide.
• This can also occur in packaged slices, cured meats, sandwich spreads, and similar items in plastic casings or packaging.

3. Bacon 

• Since the portions of the pig used to make bacon and the curing techniques vary from region to region, so do the types of spoilage and the organisms involved.
• The American method utilises relatively unchanged bellies that are reportedly relatively clear of moulds and yeasts and low in germs as they emerge from the smokehouse.
• Streptococcus faecalis is frequently present due to its tolerance to salt and capacity to develop at low temperatures. Bacon’s surface flora may also include micrococci* and staphylococci.
• Molds are the most prevalent spoiling organisms on cured bacon, particularly on sliced, packed (air-permeable) bacon when stored in the home refrigerator.
• In late summer and early fall, Aspergillus, Alternaria, Monilia, Oidium, Fusarium, Mucor, Rhizopus, Botrytis, and Penicillium cause the most problems. There are few microbiological issues with dry-salt bellies and Oxford-style bellies. Typically, rancidity is the result of chemical changes.
• Bacteria that produce oxidising and lipolytic byproducts can degrade sliced bacon over protracted storage, although chemical oxidation can also occur.
• Oxidizing and sulfide-forming bacteria may also be responsible for providing a bad colour in the meat portion of bacon, but excessive nitrite concentrations are usually to blame, and chromogenic bacteria may generate discoloured patches.
• Proteolytic bacteria have been attributed to a yellowish brown colour indicative of tyrosine present. Gum production by any of a vast variety of kinds of bacteria and yeasts gives pickles and bellies their now-rare gumminess.
• In Canada, a comprehensive investigation of the bacteriology of Wiltshire bacon has been conducted. The flanks of the pig are cured in a highly concentrated brine for a short period of time (6 to 8 days) at a low temperature (3.3 to 4.5 C), allowing only psychoactive, salt-tolerant bacteria to proliferate.
• There is minimal growth in the pickle during the curing process, but there are significant increases in bacteria on the sides of the meat, sometimes enough to cause the surface to become slimy.
• When the number exceeds 71.5 million per square centimetre, visible growth or slime typically occurs. Other organisms, unable to thrive in the cold brine, may grow before to brining or during storage of the pickled sides following baling.
• Unopened, packaged (air-impermeable) sliced bacon is damaged primarily by lactobacilli, but micrococci* and faecal streptococci may grow, particularly if the wrapper is moderately oxygen permeable.
• Molds can deteriorate opened bacon. As a method for lowering nitrite levels in bacon, the Wisconsin method has been proposed. Bacon cooked with 40 or 80 ppm sodium nitrite, 0.7% sucrose, and Lactobacillus plantarum was more antibotulinal than bacon prepared with 120 ppm sodium nitrite alone.

4. Ham 

• As applied to the spoilage of hams, the term “souring” encompasses all significant types of spoilage, ranging from relatively odourless proteolysis to putrefaction with its extremely offensive odours of mercaptans, hydrogen sulphide, amines, indole, etc., and may be caused by a wide variety of psychrotrophic, salt-tolerant bacteria.
• Jensen (1954) mentioned a variety of genera whose species may produce sourness: Alcaligenes, Bacillus, Pseudomonas, Lactobacillus, Proteus, Serratia, Bacterium*, Micrococcus, Clostridium, and others, in addition to several nameless, hydrogen sulfide-producing streptobacilli that cause flesh-souring of ham.
• According to their position, souring is classed as shank or tibial marrow, body or flesh, aitchbone, stifle joint, body-bone or femur marrow, and butt. “Puffers,” or gassy hams, are not encountered commercially but can occur when improper curing techniques are employed.
• The usual, or quick-cure, method of curing hams, in which the curing solution is pumped through the veins of the ham, has significantly reduced the incidence of sourness.
• The reduction of bacterial contamination and growth by proper slaughter and bleeding of pigs, adequate refrigeration, sealing of the marrows by sawing in the right places, prompt handling, use of a bacteriologically acceptable pickling solution, and good overall sanitation has assisted in reducing the amount of souring.
• Hams that have been tenderised are actually precooked and somewhat cured. These hams are perishable and should be refrigerated and protected from contamination during storage to prevent microbial decomposition.
• It has been observed that Escherichia coli, Proteus spp., and food-poisoning staphylococci (Staphylococcus aureus) are among the prevalent meat-spoilage germs that can contaminate improperly handled tenderised hams.

5. Refrigerated Packaged Meats 

• The oxygen and carbon dioxide permeability of packaging films favours aerobic bacteria such as Pseudomonas, Acinetobacter, and Moraxella and their generation of off-flavors, slime, and even putrefaction.
• This deterioration is comparable to that of the unwrapped meat. Lactic acid bacteria thrive on films with weak gas permeability, especially when paired with vacuum packaging.
• Over time, these bacteria generate sourness, slime, and unusual flavours.

6. Curing Solutions or Pickles 

• Possible spoilage of the pickling or curing solution for ham and other cured meats when sugar is present and the pH is considerably above 6.0. Vibrio, Alcaligenes, or Spirillum are typically responsible for the putrescence of multiuse brines.
• Slime can be caused by Leuconostoc or Micrococcus lipolyticus*. Lactobacillus and Micrococcus can generate sourness.
• The cloudiness of vinegar surrounding pickled pigs’ feet or sausages is created primarily by lactic acid bacteria from the meats, although yeasts may also be implicated.
• The black stains on pickled pigs’ feet may be formed by bacteria that produce hydrogen sulphide, while the gas in vacuum-packed pickles may be produced by heterofermentative lactic acid bacteria or yeasts.

FAQ

References

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• Olaoye, Olusegun & Ntuen, Iniobong. (2011). Spoilage and preservation of meat: a general appraisal and potential of lactic acid bacteria as biological preservatives. 
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