In this article we will discuss about the microbial contamination of meats.
The important contamination comes from external source during bleeding, handling and processing. During bleeding, skinning, and cutting, the main sources of microorganisms are the body parts of the animal and the intestinal tract.
The contaminating bacteria on the knife soon will be found in meat in various parts of the carcass, carried by blood and lymph. The exterior of the animal harbors large numbers of microorganisms from soil, water, feed and manure, as well as its natural surface flora and the intestinal contents. Knives, cloths, air and hands, clothing of the workers can serve as an intermediate source of contaminants.
During handling contamination may come from carts, boxes or other containers, from other contaminated meat from air and from personnel. Sometimes, it comes during refrigeration. The psychrotrophic bacteria may also contaminate meat. The various equipment’s, grinders, sausage stuffers, casing, and ingredients in special products e.g. fillers may add organisms on surfaces touching the meats.
Molds of many genera may reach the surfaces of meats and grow there. The species of Cladosporium, Sporotrichum, Geotrichum, Thamnidium, Mucor, Penicillium, Altemaria, and Monilia often contaminate meat.
Some of the important bacteria are Pseudomonas, Alcaligenes, Micrococcus, Streptococcus, Sarcina, Leuconostoc, Lactobacillus, Proteus, Flavobacterium, Bacillus, Clostridium, Escherichia, Salmonella and Streptomyces involved in meat spoilage.
Many of these can grow at chilling temperature. Sometimes, human pathogens can also act on meat contamination. In the retail market and in the home additional contamination usually takes place. The refrigerator containers serve as sources of spoilage.
(i) Microbiology of Meat-Curing Brines (Saline Water):
The brines usually contain lactic acid bacteria except at the surface, where micrococci and yeasts may develop. The lactis are chiefly lactobacilh and pediococci. Presence of micrococci imparts red colour in meat.
Besides micrococci, brines contain a special mixture of cocci and Gram-positive rods that form tiny colonies on agar medium. They are halo tolerant to halophilic and reduce nitrates to nitrites. Some beef curing brines have been found to contain micrococci, lactobacilli, streptococci, Achromobacter, vibrios, and perhaps pediococci in some small numbers.
(ii) General Types of Meat Spoilage:
On the basis of conditions (aerobic or anaerobic) whether they are caused by bacteria, yeasts or molds spoilage is divided into two categories.
(iii) Spoilage of Meats under Aerobic Conditions:
(a) Surface Slime:
There are different categories of spoilage. Some are given below; It is caused by species of Pseudomonas, Alcaligenes, Streptococcus, Leuconostoc, Bacillus and Micrococcus. Some species of Lactobacillus can produce slime.
(b) Changes in Colour of Meat Pigments:
The red colour of meat called its “bloom”, may be changed to shades of green, brown or grey as the result of the production of oxidizing compounds e.g. peroxides or of hydrogen sulfide by bacteria. Lactobacillus and Leuconostoc are reported to cause the greening of sausage.
(c) Changes in Fats:
The oxidation of unsaturated fats in meats takes place chemically in air and may be catalysed by light and copper. Lipolytic bacteria may cause some lipolysis and also may accelerate the oxidation of the fats.
Butter fat becomes tallow on oxidation and rancid on hydrolysis; but most animal fats develop oxidative rancidity when oxidized, with off-odour due to aldehydes and acids. Rancidity may be caused by lipolytic species of Pseudomonas and Achromobacter or by yeasts.
(d) Phosphorescence:
Phosphorescent or luminous bacteria grow on the surface of the meat e.g. Photo-bacterium sp.
(e) Change in Surface Colour:
The surface of the meat appears red due to Serratia marcescens, blue due to Pseudomonas syncyanea and yellow because of Micrococcus spp. and Flavobacterium sp. It becomes greenish blue to brownish black due to Chromo bacterium species.
(f) Off-odours and Off Tastes:
“Taints” or undesirable odours and tastes, appear in the meat due to growth of bacteria. “Souring” cold storage flavour or taints is an indefinite term for a stale flavour. Actinomycetes may be responsible for a musty or earthy flavour. Aerobic growth of molds may cause meat to become sticky, whisker, black spot, white spot, green patches, and decomposition of fats with off-odours and off taste.
(iv) Spoilage under Anaerobic Conditions:
Facultative and anaerobic bacteria are able to grow within the meat under anaerobic conditions and cause spoilage.
Following are the major spoilage:
(a) Souring:
This could be caused by formic, acetic, butyric, propionic and higher fatty acids, or other organic acids such as acetic or succinic acid. It is due to action of meat’s own enzyme during aging or ripening. The anaerobic production of fatty acids or lactic acid by bacterial action, and proteolysis without putrefaction may cause souring.
(b) Putrefaction:
Anaerobic decomposition of protein results in the production of foul smelling compounds. Pseudomonas and Alcaligenes putrefy meat. Hydrogen and carbon dioxide formation by Clostridia make meat putrefy.
(c) Taint:
Usually, it imparts bad odour and bad taste. Besides air, temperature has important influences on the spoilage of meat. Many bacteria and molds produce slimes, discolouration, and spots of growth on the surface and may cause souring such as Pseudomonas, Lactobacillus, Alacaligenes, Leuconostoc, Streptococcus and Flavobacterium species.