Growth of Microorganisms in Meat | Microbiology

In this article we will discuss about the growth of microorganisms in meat.

Due to high moisture, rich in nitrogenous food, plentified supply of minerals and accessory growth factors usually has some fermentable carbohydrate (glycogen) and a favourable pH allows most of the microbes to grow.

The following factors influence the growth of microorganism:

(a) The kind and amount of contamination with microorganisms and the spread of these organisms in the meat,

(b) Physical properties of meat,

(c) Chemical properties of meat,

(d) Availability of oxygen, and

(e) Temperature.

Fish and other Sea Food:

Various bacteria involved in the spoilage are part of the natural flora of the external slime of fishes and their intestinal contents. At higher temperature Micrococcus and Bacillus species involved in fish spoilage while at ordinary temperature, species of Escherichia, Proteus, Serratia, Sarcina and Clostridium are found.

Seafood includes fresh, frozen, dried, pickled and salted fish, as well as various shellfish. Fresh water fishes also get contamination from the surrounding microflora present in the water in which they live. The main genera that covers outer surfaces are: Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, Corynehacterium, Sarcina, Serratia, Vibrio, Salmonella and Bacillus species.

The presence of bacteria in the water content depends upon the temperature of water. Both psychrophiles and mesophiles are present in the water. Fresh water fish carry fresh water bacteria.

Boats, boxes, bins, fish ponds and fish houses including fisherman soon become heavily contami­nated with these bacteria and transfer them during cleaning. Most of the fishes pass large amount of water through their bodies pick up soil and water microorganism in this way, including pathogens if they are present.

The numbers of microorganisms on the skin of fish can be influenced by the method of catching. The other sources of contamination includes potatoes, spices and flavours used in fish cake. The microbial contents of fresh fish vary in the microbial fish products.

(i) Microbiology of Fish Brines:

The temperature of brine and salt concentration influences the bacterial activity. The kind and amount of bacteria also vary which result to fish spoilage. Contamination comes from the fish, which ordinarily introduces species of Pseudomonas, Alcali­genes and Flavobacterium. The continuous use of fish brine may contribute additional pathogens from successive lots of fish and because of salt tolerant bacteria (micrococci, etc.).

(ii) Spoilage:

Autolysis, oxidation or bacterial activity spoil fish like meat. The fish flesh is autolysed more quickly due to the presence of fish enzymes and because of the less acid reaction of fish flesh that favours microbial growth.

Many of the fish oils seem to be more susceptible to oxida­tive deterioration than most animal fats. The lower the pH of fish flesh, the slower in general bacterial decomposi­tion. Lowering the pH of fish results from the conversion of muscle glycogen to lactic acid.

(iii) Factors Influencing Kind and Rate of Spoilage

(a) The kind of fish:

The various kinds of fish differ considerably in their perishability. Certain fatty fish deteriorate rapidly because of oxidation of unsaturated fats of their oils.

(b) The condition of fish when caught:

Fish full of food when caught, are more readily perishable than those with an empty intestinal tract.

(c) The kind and extent of contamination of the fish flesh with bacteria:

These may come from mud, water handlers, exterior slime, and the intestinal content of the fish and are supposed to enter the gill of the fish, from which they pass through the vascular system and thus invade the flesh, or to penetrate the intestinal tract and thus enter the body cavity.

In general, the greater the load of bacteria, the more rapid the spoilage. The contamination may take place in the net, in the fishing boat, on the docks, or later, in the plants. This process is accelerated by the digestive enzymes attacking and perforating the gut wall and viscera, which in themselves have a high rate of autolysis.

(d) Temperature:

Bacterial growth is delayed at lower temperature. Cooling or chilling should be as rapid as possible 0 to -1°C. The high temperature reduce the life of fish. The prompt and rapid freezing of the fish is more effective in preservation.

(e) Use of an Antibiotic Ice or Dip:

Some antibiotics are recommended to avoid spoilage.

(iv) Characteristics of Spoilage:

The bright colours of fish fade and dirty, yellow-brown discolouration appear. The slime on the skin of fish increases, especially on the flaps and gills. The eyes gradually sink, and; oh shrinkage the pupil becomes cloudy and the cornea opaque.

Flesh becomes soft and juice erodes when squeezed. The discolouration takes place towards tail due to oxidation of haemoglobin and the different types of odours evolved during spoilage and cooking will bring out these odours more strongly.

(v) Control of Spoiling Microorganisms:

Keeping microorganisms away from meats is called Asepsis. This process begins with avoidance from contamination. Before slaughtering, the animal should be carefully washed with water so as to remove dust from hair and hoof.

The knife may also introduce microorganism in the circulating blood. During evisceration contamination may come from the internal body parts like intestine, the air, the water, cloths, and brushes used on the carcass. Some organism may come from the surface soil and from workers.

Once meat is contaminated with microbes their removal is difficult. The use of hot water or sanitizer sprays under pressure is a procedure of decreasing the bacterial number. Moldy or spoiled portion of large piece of meat may be trimmed off.

Meats have been reported to have a shorter storage life in films with less permeable to water. Cured meats are packed in an oxygen- tight film with evacuation. It checks the growth of aerobes especially molds, reduces the rate of growth of staphylococci.

(a) Use of Heat:

Canning of meat differs from product to product to be preserved. Most meat products are low-acid foods and are good culture media. Rates of heat penetration range from fairly effective in meat soups to very slow in tightly packed meats or in paste. Various additives such as spices, chemical salts and flavour also affect the heat processing. The process becomes more effective.

On the basis of heat processing, canned meat can be divided into two groups:

(a) Meat that are processed in one attempt (shelf-stable canned meat), and

(b) Meat that are heated enough to kill part of the spoilage organisms but must be kept refrigerated to prevent spoilage. This type of meat is known as non-shelf stable. The shelf-stable meat is processed at 98°C and the size of container is usually less than a kilogram. Cured meat temperature for processing should be 65°C and the container used during packing is of up to 22 kg.

Hot water treatment is also a method to remove the microbes from meat surfaces. But this may lessen nutrients and can damage colour. Heat applied during the smoking of meat and meat products helps in reducing microbes.

The cooking of meats for direct consumption greatly reduces the microbial content and hence lengthen the keeping time. Precooked frozen meat should contain few viable microbes. More meat is preserved by the use of low temperature either by chilling or freezing. Chilling is more common.

Meat can be preserved promptly and rapidly to temperature near freezing and chilling at only slightly above the freezing point. Meat may be held in chilling storage for a limited time with little change from their original constitution. Enzymatic and microbial changes in the foods are not prevented but are slowed down considerably. Cooler temperature prevents growth but slow metabolic activity may continue.

The storage time can be prolonged in an atmosphere containing added CO₂ and O₃. Ships equipped for storage of meat in a controlled atmosphere of CO₂ have been employed successfully. Increasing amounts of CO₂ inhibit microbial growth but also enhance the formation of met-myoglobin resulting into loss of colour. Storage time can be increased by the pressure of 2.5 to 3 ppm of ozone in the atmosphere.

Ozone is an active oxidizing agent, that may give an oxidized or tallowy flavour to fats. Few bacteria, molds and yeasts can grow in meats at low temperature are known as psychrotrophic bacteria (Sta­phylococcus, Alcaligenes, Micrococcus, Leuconostoc, Flavobacterium and Proteus).

(b) Freezing or Frozen Storage:

Under the usual conditions of storage of frozen foods microbial growth is prevented entirely and action of food enzymes is greatly retarded. The lower the storage temperature the slower will be any chemical or enzymatic reaction. The preservation of frozen meats is increasingly effective as the storage temperature drops from -12.2°C to -28.9°C.

The freezing process kills the bacteria. The rate of freezing of meat and other food items depends upon a number of factors, such as the temperature, circulation of air, or refrigerant, size and shape of package, kind of food etc. Sharp freezing refers to freezing in air with only natural air circulation or at best with electric fans. The freezing temperature is usually -23.3°C or lower but may vary from -15 to -29°C, and may take from 3 to 72 hours.

This is called slow freezing. Quick freezing is accomplished by one of the three methods:

(a) Direct immersion in a refrigerant,

(b) Indirect contact where the meat is in contact with the passage through which the refrigerant at -17.8 to -45.6°C flows, and

(c) Air-blast freezing where frigid air at -17.8 to -34.4°C is blown across the materials being frozen. Certain items now are being frozen into liquid nitrogen.

The advantages of quick freezing are:

(a) Smaller ice crystals formation and less destruction of intact cells of the food,

(b) A shorter period of sodification and therefore, less time for diffusion of soluble materials and for separation of ice,

(c) More prompt preservation of microbial growth, and

(d) More rapid enzyme action.

(c) Irradiation:

The ultraviolet rays serve to reduce number of microorganisms in the air and to inhibit or kill them on the surfaces of the meat reached directly by the rays. Irradiation also is used in the rapid aging of meats at higher than the usual chilling temperature to reduce the growth of microorganisms, especially molds, on the surface. Some oxidation, favoured by UV rays, and hydrolysis of fats may take place during aging.

(d) By Drying:

Some types of sausages are preserved primarily by their dryness. In dried beef, made mostly from cured, smoked beef hams, growth of microorganism may take place before processing and may develop in the “pickle” during curing, but numbers of organism are reduced by the smoking and drying process.

Organisms may contaminate the dried ham during storage and the slices during cutting and packing. Salting and smoking are usually employed during meat drying.

Another method of drying pork involves a short addition of lecithin as an antioxidant and stabilizer. Drying may be in vacuum in trays, or by other methods. The meat for drying should be of good bacteriological quality.