Meat and its products are highly nutritious foods that are widely consumed across the world.
Meat can be sourced from different birds such as chicken, turkey, and ducks, as well as from mammals like pork, mutton, buffalo, and sheep.
After slaughtering, the carcasses and primary cuts of these animals are processed further into raw or processed food products.
Meat is known for being a highly nutritious and protein-rich food, but it is also very perishable and has a limited shelf life.
The biological and chemical composition of meat makes it prone to deterioration, starting from the moment of slaughter and continuing until it is consumed.
A wide range of meat and meat products—including ham, sausages, cooked meats, dry meats, smoked meats, vacuum-packed meats, and minced meat—are all vulnerable to microbial spoilage.
Contamination source and causes
Meat spoilage can occur due to natural processes such as lipid oxidation or autolytic enzymatic activity that takes place in the muscle after slaughtering.
Several factors are responsible for microbial contamination of meat, including:
The natural bacterial flora of the animal.
Knives, utensils, hands, and clothing of workers.
Pre-slaughter handling of livestock and post-slaughter handling of meat.
Handling during slaughtering, evisceration, and processing.
Inadequate temperature controls during slaughtering.
Conditions during processing and distribution.
The type of packaging used.
Improper handling and storage.
After the slaughter of an animal, a series of events occur during rigor mortis, such as:
Cessation of respiration, which halts ATP synthesis.
Lack of ATP leading to stiffening of muscles.
Reduction of oxidation-reduction potential due to the absence of oxygen.
Loss of vitamins and antioxidants, resulting in rancidity development.
Glycolysis, where most glycogen is converted into lactic acid, lowering the pH.
The ending of the reticuloendothelial system, which makes meat more susceptible to microbial attack.
Cessation of nervous and hormonal regulation, causing a drop in animal body temperature and solidification of fat.
Accumulation of various metabolites that promote protein denaturation.
Spoilage of fresh meat
Fresh meat is highly prone to spoilage due to the combined effects of its own enzymes and microbial activity.
Autolysis in fresh meat causes:
Proteolytic action on muscle and connective tissues.
Hydrolysis of fats.
The survival and growth of microorganisms in meat are strongly influenced by the composition of the atmosphere surrounding it.
Fresh meat contains a wide variety of nutrients, including sugars, amino acids, vitamins, and cofactors.
Its pH range (5.5–5.9) and water activity (Aw ≈ 0.85) play a key role in influencing microbial growth.
Common bacteria isolated from fresh meat include:
Acinetobacter
Pseudomonas
Brochothrix thermosphacta
Flavobacterium
Psychrobacter
Moraxella
Staphylococci
Micrococci
Lactic acid bacteria (LAB)
Various members of the Enterobacteriaceae family.
Major microbial pathogens associated with fresh meat are:
Salmonella
Campylobacter
Escherichia coli (E. coli)
Listeria monocytogenes
There are two main types of meat spoilage:
Spoilage under aerobic conditions
Spoilage under anaerobic conditions
Microorganisms cause different defects in fresh meat depending on oxygen availability:
Under aerobic conditions:
Surface slime → caused by Pseudomonas, Moraxella, Streptococcus, Bacillus, Micrococcus.
Red color “bloom” (due to oxidizing compound production) → caused by Lactobacillus, Leuconostoc.
Oxidative rancidity → caused by Pseudomonas spp., Achromobacter.
Red spot → caused by Serratia marcescens.
Blue color → caused by Pseudomonas syncyanea.
Greenish-blue or brownish-black spots → caused by Chromobacterium lividum.
Stickiness, whiskers, and green patches → caused by molds.
Under anaerobic conditions:
Putrefaction → caused by Clostridium spp., Alcaligenes, Proteus.
Souring → caused by Lactic acid bacteria (LAB).
Spoilage of meat
Meat spoilage occurs mainly due to microbial growth, oxidation, and enzymatic autolysis.
The nutrient-rich composition, high water content, and moderate pH of meat make it highly suitable for microbial growth.
The normal flora of an animal’s lymph nodes, including Staphylococcus, Streptococcus, Clostridium, and Salmonella, can contaminate meat.
Meat may harbor several bacterial species such as Acinetobacter, Aeromonas, Alcaligenes, Alteromonas, Brochothrix, Carnobacterium, Escherichia, Enterobacter, Enterococcus, Flavobacterium, Lactobacillus, Leuconostoc, Micrococcus, Proteus, Pseudomonas, Sarcina, Serratia, and Streptococcus.
Pathogenic bacteria that contaminate meat include Salmonella enterica strains, Yersinia enterocolitica, Campylobacter jejuni, Aeromonas hydrophila, Listeria monocytogenes, and Escherichia coli.
Molds such as Cladosporium, Sporotrichum, Geotrichum, Penicillium, and Mucor are also associated with meat spoilage.
Yeasts including Candida spp., Cryptococcus spp., and Rhodotorula spp. are found in meat as well.
The most common defects of meat spoilage are the development of off-odor, off-flavor, discoloration, and gas production.
In vacuum-packed meat, spoilage can result in sulfide odor caused by Clostridium spp. and Hafnia spp., H₂S greening caused by Shewanella spp., and blown pack defects caused by Clostridium spp. and lactic acid bacteria.
In fresh meat, putrefaction may occur due to Alcaligenes, Clostridium, Chromobacterium, Proteus vulgaris, and Pseudomonas fluorescens, while souring may be caused by Chromobacterium and Pseudomonas.
In cured meat, spoilage may lead to moldy odor caused by Aspergillus, Penicillium, and Rhizopus, greening caused by Pediococcus and Streptococcus, souring due to Micrococcus and Pseudomonas, and sliminess resulting from Leuconostoc.
In modified atmosphere packaged meats, souring and off-odor may be caused by Leuconostoc and Lactobacillus, while Brochothrix thermosphacta is also involved in souring.
In refrigerated packaged meat, spoilage may lead to off-flavors, slime formation, and putrefaction due to Pseudomonas, Acinetobacter, and Moraxella, while sour taste, slime, and flavor changes are often caused by lactic acid bacteria.
Spoilage of refrigerated meat
When fresh meat is refrigerated at 4 ± 1°C, they remain in good condition for 5-7 days
Refrigerated temperature favors the growth of psychrophilic organisms in due course of time.
The contaminations occur during slicing and serving operations, from hands, slicing machines, and other equipment.
Inadequate hygiene can lead to meat contamination by spoilage and pathogenic microorganisms
The important bacterial genera associated with spoilage of refrigerated meat are Acinetobacter, Moraxella, Pseudomonas, Aeromonas, Alcaligenes, and Micrococcus.
The mold genera associated with spoilage of refrigerated meat are Alternaria, Cladosporium, Geotrichum, Mucor, Monilia, Penicillium, Sporotrichum, and Thamnidium; and yeast genera associated with spoilage of refrigerated meat are Candida, Torulopsis, Debaryomyces, and Rhodotorula.
Generally, Brochothrix thermosphacta and lactic acid bacteria are the bacteria that cause spoilage of refrigerated meat.
Pathogenic microorganisms found in refrigerated meats include C. botulinum type E, Yersinia enterocolitica, enteropathogenic Escherichia coli, Listeria monocytogenes, and Aeromonas hydrophila as they are capable of growing at temperatures below 5°C.
Spoilage of cured meat
Cured meat refers to meat preserved by aging, drying, canning, brining, or smoking in order to enhance flavor and extend shelf life.
Examples of cured meats include sausage, bacon, salami, ham, canned meat, dry spicy meat, meat pickles, kebabs, meatballs, and meat patties.
Although cured meat has a longer shelf life compared to fresh and raw meat, it is still not completely immune to spoilage.
Bacterial spoilage organisms in cured and processed meats include lactic acid bacteria (Lactobacillus sake, Lactobacillus curvatus, Leuconostoc gelidium, Leuconostoc carnosum, Leuconostoc mesenteroides), as well as Acinetobacter, Bacillus, Micrococcus, Serratia, and Staphylococcus.
Molds associated with spoilage of cured meat include Aspergillus, Penicillium, Rhizopus, and Thamnidium.
Yeasts involved in cured meat spoilage include Candida, Debaryomyces, Torula, Torulopsis, and Trichosporon.
Other spoilage-causing microorganisms include Clostridium spp., Hafnia spp., Weisella spp., Shewanella spp., Pseudomonas spp., and Enterococcus spp.
Pathogenic microorganisms found in cured meats include Escherichia coli, Salmonella, Staphylococcus aureus, Listeria monocytogenes, Clostridium botulinum, and Toxoplasma gondii.
Microbial growth in cured meat may result in slime formation, degradation of structural components, reduced water holding capacity, off-odors, changes in texture, and alterations in appearance.
Some defects in different cured meat products caused by microorganisms include:
In dried-fermented sausage:
Small dark spots caused by B. thermosphacta, Lactobacillus, Leuconostoc, Microbacterium, Micrococcus, and Alternaria.
CO₂ production caused by heterofermentative lactic acid bacteria.
Nitric oxide gas production caused by nitrate-reducing bacteria.
Greening with hydrogen peroxide caused by heterofermentative Lactobacillus fructivorans, L. jensenii, L. viridescens, Leuconostoc, Enterococcus faecium, Enterococcus faecalis, and Pediococcus.
Surface slime formation caused by Micrococcus and yeasts.
Fuzziness and discoloration caused by Penicillium verrucosum and Aspergillus glaucus.
In cured meats in general:
H₂S production caused by Vibrio and Enterobacteriaceae.
In bacon:
Cabbage-like odor caused by Providencia.
White, gray, or grayish-green spots caused by Aspergillus, Alternaria, Fusarium, Mucor, and Rhizopus.
In ham:
Putrefaction caused by Enterobacteriaceae and Proteus.
Souring caused by Lactic acid bacteria, Enterococcus, Micrococcus, Bacillus, and Clostridium.
Bone taint caused by Serratia spp., Clostridium spp., and Proteus spp.
Red spot caused by Halobacterium salinarum.
Blue spot caused by Pseudomonas syncyanea, Penicillium spinulosum, and Rhodotorula spp.
Dark spot caused by Alternaria.
In sausages:
Souring and off-odors caused by lactic acid bacteria.
Surface slime caused by Bacillus, Lactobacillus, and Leuconostoc.
Greening caused by Enterococcus, Lactobacillus, Leuconostoc, and Pediococcus.
In dried meats:
Surface slime caused by Micrococcus.
Souring caused by Halobacterium salinarum.
Off-odors caused by Flavobacterium.
Blue color caused by Pseudomonas syncyanea and Penicillium spinulosum.
Red color caused by Bacillus spp.
In smoked products:
Off-odors caused by Micrococcus, yeasts, and molds.
Souring caused by Lactobacillus plantarum, Lactobacillus mesenteroides, and Clostridium spp.
In pickled meat:
Putrefaction caused by Vibrio, Alcaligenes, and Spirillum.
Souring caused by Lactobacillus and Micrococcus.
Slime formation caused by Leuconostoc.
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