Microbial spoilage of meat and meat products

Introduction

• Meat and its product are highly nutritious food that is widely consumed by people all over the world.
• Meat can be obtained from various birds (chicken, turkey, ducks, etc.) or mammals (pork, mutton, buffalo, sheep), and after slaughtering, carcasses and primary cuts are processed to raw or processed food products.
• It is a nutritious, protein-rich food that is highly perishable and has a short shelf life.
• The biological and chemical nature of meat leads to its deterioration from the time of slaughter until consumption. 
• Meat and its products such as ham, sausages, cooked meat, dry meats, smoked meats, vacuum-packed meat, minced meat, etc. are all susceptible to microbial spoilage.

Contamination source and causes

Meat spoilage can be caused by natural processes, such as lipid oxidation or autolytic enzymatic that occurs in the muscle after slaughtering. Several factors are responsible for microbial contamination of meat such as:

1. Bacterial flora of animal.
2. Knives, utensils, hands, and clothing of the workers.
3. Pre-slaughter handling of livestock and post-slaughter handling of meat.
4. handling during slaughtering, evisceration, and processing
5. temperature controls during slaughtering,
6. processing and distribution
7. type of packaging used
8. Handling and storage 

A series of the event takes place during rigor mortis after the slaughter of the animal such as:

1. Respiration ceases, which stops ATP synthesis.
2. The lack of ATP causes stiffening of muscle
3. Reduction of oxidation-reduction potential due to lack of oxygen
4. The loss of vitamins and antioxidants causes the development of rancidity.
5. Glycolysis begins in which most glycogen is converted to lactic acid that reduces pH
6. The ending of a reticuloendothelial system leads to the susceptibility of meat to microorganisms.
7. Nervous and hormonal regulations cease, thereby causing the temperature of the animal to fall and fat to solidify.
8. Various metabolites accumulate that also aid protein denaturation.

Spoilage of fresh meat

• Fresh meat is subjected to spoilage by its enzymes and microbial action.
• The autolysis changes cause proteolytic action on muscle and connective tissue and hydrolysis of fats.
• The survival and growth of microorganisms are influenced by the composition of the atmosphere surrounding the meat.
• Fresh meat contains nutrients such as sugars, amino acids, vitamins, cofactors, etc and it had pH (5.5-5.9) and Aw (0.85) values that influence the growth of microorganisms.
• The most common bacteria isolated from fresh meat are bacteria of the genera Acinetobacter, Pseudomonas, Brochothrix thermosphacta, Flavobacterium, Psychrobacter, Moraxella, Staphylococci, Micrococci, lactic acid bacteria (LAB), and various genera of the Enterobacteriaceae.
• The microbial pathogens found in fresh meat are Salmonella, Campylobacter, E.coli, Listeria monocytogenes.

There are two types of spoilage of meat:

1. Spoilage under aerobic condition
2. Spoilage under anaerobic condition

The kind of defects caused by microorganisms on fresh meat

Spoilage of meat

• Microbial growth, oxidation, and enzymatic autolysis are the three basic mechanisms responsible for the spoilage of meat.
• The nutrient composition, high water content, and moderate pH of meat make it an excellent medium for microbial growth.
• The normal flora of an animal’s lymph nodes contaminating meat is Staphylococcus, Streptococcus, Clostridium, and Salmonella.
• Meat may contain different bacteria that include species of Acinetobacter, Aeromonas, Alcaligenes, Alteromonas, Brochothrix, Carnobacterium, Escherichia, Enterobacter, Enterococcus, Flavobacterium, Lactobacillus, Leuconostoc, Micrococcus, Proteus, Pseudomonas, Sarcina, Serratia, and Streptococcus.
• Pathogenic microbial species contaminating meat are Salmonella enteric strains, Yersinia enterocolitica, Campylobacter jejuni, Aeromonas hydrophila, Listeria monocytogenes, and Escherichia coli. 
• Mold species found in meat include Cladosporium, Sporotrichum, Geotrichum, Penicillium, and Mucor while yeasts species include Candida spp., Cryptococcus spp., and Rhodotorula spp.
• The main defects observed in meat are off-odor, off-flavor, discoloration, and gas production.

Some defects caused by microorganism in different kinds of meat are:

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 is the meats in which are preserved by aging, drying, canning, brining, or smoking for enhancement of flavor and to extends its shelf life.
• Some examples of cured meats are sausage, bacon, salami, ham, canned meat, dry spicy meat, meat pickles, kebab, meatballs, meat patty, etc.
• The cured meat has a long shelf-life compared to fresh and raw meat however they are not immune to spoilage.
• The bacterial spoilage causing organisms in processed and cured meats are lactic acid bacteria (such as Lactobacillus sake, Lactobacillus curvatus, Leuconostoc gelidium, Leuconostoc carnosum, Leuconostoc mesenteroides), Acinetobacter, Bacillus, Micrococcus, Serratia, and Staphylococcus.
• The spoilage causing  mold found in cured meat includes Aspergillus, Penicillium, Rhizopus, and Thamnidium
• The spoilage causing yeast found in cured meat includes Candida, Debaryomyces, Torula, Torulopsis, and Trichosporon.
• Other spoilage causing microorganism found in cured meats includes Clostridium spp, Hafnia spp, Weisella spp, Shewanella spp, Pseudomonas spp, Enterococcus spp, etc. 
• The pathogenic microorganisms found in cured meat include Escherichia coli, Salmonella, Staphylococcus aureus, Listeria monocytogenes, Clostridium botulinum, and Toxoplasma gondii.
• Microbial growth in cured meat can result in slime formation, structural components degradation, decrease in water holding capacity, off odors, and texture and changes in appearances.

Some defect in different cured meat products caused by microorganism are:

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