Preservation of Meat by Drying, Smoking, Chilling, Salting, Irradiation, Chemicals etc.

(Part 6, Meat Science, LPT)

Preservation of Meat and Poultry : Drying, Salting, Curing, Smoking, Chilling, Freezing, Thermal Processing, Canning, Irradiation and Chemicals

Introduction : 

You know that meat is highly perishable because 

• It has a nearly neutral pH
• It contains high moisture and 
• It is rich in nutrients that help the growth of microbes. So, we should adopt proper preservation methods, otherwise, microbes will grow and deteriorate the meat.

You also know that the contamination of meat with spoilage organisms is unavoidable. After contamination, microbial activity, enzymatic activity, chemical reactions continue, and physical changes occur. So, at first, we should avoid contamination as much as possible because, once the meat is contaminated with microorganisms, their removal is difficult. For this reason, we apply preservation techniques.

There are 3 principles of preservation, namely, 

• application of low temperature,
• application of high temperature, 
• removal of water from meat (moisture control), and 
• In a few instances, direct microbial inhibition is explored.

Depending upon these principles, various methods of preservations are employed, namely, dehydration, smoking, chilling, freezing, curing, canning (thermal processing), irradiation etc. and we will be discussing all these methods.

Dehydration or Drying : 

If we remove water from the meat, what will happen? The concentration of water-soluble nutrients will increase and these will be unavailable to the microbes. How much water is available to the microbes is expressed as water activity (aW). Dehydration lowers the water activity and prevents the growth of spoilage microbes. There are a few methods of dehydration, for example,

1. Sun drying of meat chunks- this is a very age-old practice, but rehydration of such meat during consumption is very limited. 
2. Next method is the Mechanical drying process- it is the passage of hot air with controlled humidity, but here also the same problem of rehydration happens. 
3. Nowadays, freeze-drying of meat is applied which has better reconstitution properties, better nutritive quality. In this process, water from food is removed by sublimation from a frozen state to a vapor state by keeping it under vacuum and by applying a low heat treatment. Meat is first frozen at -40°C, then it is dried under vacuum for 9 to 12 hours at low temperature in plate heat exchangers at 1 to 1.5 mm pressure of mercury. Thus, ice crystals are sublimated to water vapor and there is no rise of temperature. Initially, free and immobilized water of meat (freezable, 90 % of total moisture) is removed. Afterward, high temperature is applied during secondary drying when the remaining 10% of bound water is removed. These are then vacuum packed and these have very good storage stability.

Smoking :

 Smoke contains a large number of wood degradation products, such as aldehydes, ketones, organic acids, phenols, etc. and these exert a bacteriostatic effect and also impart a characteristic smoky flavor. Moreover, during smoking, there is surface dehydration along with lowering of surface PH. Smoke constituents have antioxidant properties also. Smoke is generated in smokehouses where sawdust or hardwood are subjected to combustion at a temperature of about 300°C. This high temperature is desirable to minimize the production of carcinogenic compounds. In this way, numerous organic compounds and their condensation derivatives are produced, for example, aldehydes, and phenols condense to form resins which constitute 50% of smoke components and contribute most of the color of smoked meat products. Phenols act mainly as bacteriostatic and formaldehyde as the chief bactericidal compound. Nowadays, many liquid smoke preparations are commercially available which are produced from hardwood and polycyclic hydrocarbons are removed by filtration. Liquid smoke is applied to the product surface before cooking to impart a smoky flavor which is liked by the consumers.

Curing :

Preservation by salting is known as curing which is an old practice, you know. Nowadays, mild curing of meat products is practiced mainly for specific flavor and color development whereas, the preservative effects of curing ingredients are an added advantage.

 Sodium chloride, sodium nitrate, sodium nitrite, and sugar are the main curing ingredients. Sodium chloride exerts its preservative action by dehydration and it alters osmotic pressure that inhibits the growth of spoilage bacteria. Chloride ions in the salt act directly on the microbes. Moreover, common salt slows down the action of proteolytic enzymes in the meat, interacts with fatty acids to enhance the flavor of the cured products, and also contributes to the tenderness of the product.

 Sodium nitrate and nitrite serve to stabilize the attractive cured meat color and impart characteristics of cured meat flavor. You should know the color reactions of cured meats. First, nitrate-reducing organisms convert nitrate into nitrite which in absence of light and air is converted to nitric oxide (NO). This NO is combined with myoglobin to form nitric oxide metmyoglobin (NOMMb) and this under favorable condition is converted to nitric oxide myoglobin (NOMb) which imparts cured meat color but this pink color is unstable. So, Nitric oxide myoglobin when exposed to heat or smoke is converted to nitroso hemochromogen which is the stable pink pigment. Permissible limits of nitrates and nitrites are 500 and 200 ppm respectively. These act as preservatives by inhibiting the growth of a number of bacteria especially Clostridium botulinum. They also retard the development of rancidity. The cured flavor is developed due to a reaction between fatty acids and sodium nitrite and forms benzonitrile and phenyl acetonitrile. But this nitrite is also responsible for the formation of nitrosamine which is supposed to be carcinogenic.

 Sugar counteracts the harsh hardening effect of salt and adds to the flavor development and also as an energy source for nitrate-reducing bacteria in the curing solution or pickle. Sucrose or dextrose is mainly used for this purpose. Traditionally curing of meat is limited to pork (ham and bacon) and beef.

 There are several methods for curing: dry cure (where dry ingredients are rubbed to meat), pickle cure (meat cuts are immersed in ingredient solution/pickle, injection cure (concentrated solution of the ingredients is pumped into the meat through an artery or injected by needles in the muscular pork) or may me direct addition method (where curing agents are added directly to finely ground meat. Please remember, the temperature of the curing room is maintained at 3±1°C and the curing process is allowed for 3 to days depending on the strength of the curing pickle. 

Chilling/Refrigeration : 

This is the most widely used method for short term storage of meat. Chilling or refrigeration actually slows down the microbial growth and enzymatic as well as chemical reactions. Here, the temperature is maintained at 2 to 5°C and begins with the chilling of animal carcasses and continues through the entire channel of holding, cutting, transit, retail display, and even in the consumer household before ultimate use. The relative is generally kept at 90 percent in order to check excessive shrinkage due to loss of moisture. Carcasses are first held in chill coolers at 15°C to remove their body heat and then passed on to holding coolers at 5°C. For the convenience of meat plant workers, the temperature in cutting and packing halls generally exceeds 5°C and so utmost care should be taken during the handling of meat to check further microbial contamination. It is important to maintain proper spacing between carcasses so as to allow thorough air circulation.

 Fresh meat is maintained in good condition for a period of 5 to 7 days at a refrigerated temperature of 4±1°C although it depends on species of origin, initial microbial load, packaging, and temperature along with humidity during storage. Processed meat products are less perishable as compared to fresh meat and depend on the processing steps followed in each case. 

Freezing : 

When we want to preserve meat for a longer duration, we opt for the freezing method. Perhaps you know, from our last class that chilling or refrigeration delays microbial growth but here in freezing, it stops the microbial growth, retards the action of enzymes and also retains most of the nutritive value of meat during storage except a very little loss of nutrients in the drip during the thawing process. Since drip is not possible in cooked meat products, proper freezing conditions retain most of the nutritional and sensory properties of meat products. 

Before freezing, we wrap fresh meat in suitable packaging film to avoid freezer burn. This abnormal condition occurs due to progressive surface dehydration by sublimation of ice crystals and results in the increase of concentration of meat pigments on the surface. This discoloration in frozen meat is an irreversible condition and on cooking, freezer burn meat is quite tough and lacks juiciness. 

The quality of frozen meat is also influenced by freezing rate. In slow freezing, extracellular water freezes more quickly due to low solute concentration as compared to intracellular water. Thus, there is the formation of large extracellular ice crystals which may cause mechanical damage to the muscular tissue, giving it a distorted appearance in the frozen state. Contrary to this, in fast freezing, numerous small ice crystals are formed uniformly throughout the meat tissue. Thus, the problem of muscle fiber shrinkage and distorted appearance is not there. Besides, drip losses during thawing are considerably low as intracellular water freezes within the muscle fiber itself. Numerous small ice crystals on the surface of fast-frozen meat also impart a desired lighter color as compared to slowly frozen meat. 

Various types of freezers are employed to freeze meat and meat products. In plate type freezers, meat is placed in trays that remain in direct contact with metal freezer plates. A temperature of -10 ℃ or so is achieved. Blast type freezers are used in large meat plants. Such freezers render fast freezing of meat products due to rapid air movement. A temperature range of -10 ℃ to -30 ℃ is generally achieved.

 The quality of meat and meat products can be preserved for months together during frozen storage at -10 ℃. However, a storage temperature of -18 ℃ is recommended because at this level almost all water in meat is frozen and minor temperature fluctuations can be taken care of. At -18 ℃, storage life of buffalo meat, beef, mutton and chevon is approximately 6 months, while that of pork and poultry is less (4 months) because of associated unsaturated fat, prone to rancidity development. Storage life of cured and salted meat products is still limited (2 months) as salt is a pro-oxidant. However, at -10 ℃, the storage life of these meats is reduced to half or even less. Thawing of meat should be done within the package itself preferably in a refrigerator so as to minimize the drip losses. However, if thawing is to be accomplished at a short notice, warm air or lukewarm water may be used. Refreezing a thawed meat is not suggested in tropical countries and repeated freezing should not be practiced. 

At times, freezing and thawing of young chicken may pose the problem of bone darkening due to leaching of hemoglobin from the marrow of porous bones to the adjoining muscle tissue. This tissue appears grey or black after cooking, although other sensory attributes are not affected.

Thermal processing :

 Unlike refrigeration of meat that slows or stops microbial growth, thermal processing as a preservative method is employed to kill the spoilage microorganisms. Two temperature regimes namely, pasteurization and sterilization are generally used. 

Pasteurization refers to moderate beating in the temperature range of 58 °C to 75℃ whereby most of the microorganisms present including Trichinae occasionally found in pork are killed. Incidentally, this is also the cooking temperature range of most processed meats. This heat treatment significantly extends the shelf life of meat, although such products also need to be stored under refrigeration. 

Sterilization refers to severe heating at temperatures above 100°C whereby all spoilage microorganisms in meat are killed or their microbial cells are damaged beyond repair. This heat treatment renders the meat products commercially sterile because some bacterial spores may still survive. Such meat products have a recommended shelf-life of two years in cans and one year in retort pouches at ambient temperature in the tropics. However, exposure of meat to high temperatures imparts sulfhydryl flavor in cans and modifies texture also.

 Various meat products differ in water content, amount of fat, and consistency. These factors have a definite bearing on the thermal processing schedule. For example, moist heat is much more effective in killing microorganisms and spores as compared to dry heat, so a meat product with higher moisture content will require comparatively less heat for sterilization. 

Canning : 

It is a process of preservation achieved by thermal sterilization of a product held in hermetically sealed containers. Canning preserves the sensory attributes such as appearance, flavour and texture of the meat products to a large extent. Besides, canned meat products have a shelf-life of at least 2 years at ambient temperature. Conventional canning is done in the following steps:

1.  Preparation of meat and gravy: Carcass is deboned and 4 cm meat chunks are prepared. Meat gravy is prepared using condiments, tomatoes, dry spices and salt, etc. 
2.  Precooking: Meat and gravy, both are precooked at 70℃ for 15 minutes. It causes the inevitable shrinkage of meat chunks and reduces the initial microbial load. 
3. Filling: Filling in cans may be done manually or mechanically leaving proper headspace as per Bureau of Indian Standards (BIS) specifications. Half of the gravy is filled first followed by meat chunks and finally the rest of the gravy. Special care is taken to avoid trapping of air during this operation. 
4.  Exhausting: It refers to the removal of air from the container before it is closed. It is necessary to minimize the strain on the can seams due to expansion of air during heat processing. Mechanical exhausting may vacuum seal the cans. 
5. Seaming: This is usually done by a double seamer machine. 
6.  Retorting or thermal Processing: The product is subjected to high temperature under pressure for sufficient duration to achieve commercial sterility. 
7.  Cooling: Retorting is followed by very fast cooling upto 30 to 40 ℃ to give a shock to the thermophilic bacteria. 
8.  Storage: Cans should be stored in a cool and dry place preferably at a temperature of about 20℃. 

Irradiation :

Radiation is the emission and propagation of energy in the material medium. Electromagnetic radiations are in the form of continuous waves. These are capable of ionizing molecules in their path. These radiations can destroy the microorganisms by fragmenting their DNA molecules and causing ionization of inherent water within microorganisms. Since microbial destruction of foods takes place without significantly raising the temperature, food irradiation is many times referred to as cold sterilization. Among radiations, alpha and beta rays are charged particles and have limited use in food irradiation. However, lambda rays are electronic waves of short wavelength and not the charged particles. These are easily obtained from isotopes like 60Co and 137Cs and have excellent penetration power. Gamma radiations produce desired effect only during food irradiation and have no effect after removal of source. These are widely used in food preservation. A dose of 50 to 100K rad (radurization) can enhance the shelf-life of fresh meat cuts and poultry products by 19 days whereas a dose of 4 to 5 Mrad (rad appertization) can sterilize pork, poultry and fish. Among the non-ionizing radiations, ultraviolet radiations of 2650 A are most bactericidal in nature, but due to poor penetration power, these are used only for surface sterilization of meats. It may be mentioned that certain chemicals like ascorbates have been found to increase the sensitivity of the microorganisms to radiation.

 

Chemical : 

In addition to the above mentioned preservation techniques, there are many chemicals which prevent microbial growth in foods and act as preservatives. Several organic acids have been generally recognized as safe (GRAS) for use as chemical preservatives. Citric acid, propionic acid, benzoic acid, sorbic acid and their salts are effective mold inhibitors. Acetic acid and lactic acid prevent bacterial growth, whereas sorbate and acetate are capable of arresting the growth of yeast in foods. 

 It may be noted that modern meat food processors do not rely on any single preservative factor or technique. They employ a combination of preservative factors (hurdles) in a balanced manner to derive maximum benefit. Technologists have exploited the hurdle concept in the development and keeping quality enhancement of intermediate moisture and shelf stable food products.

Hope the note can help to find the Answers related to the Preservation of Meat. Click on the links to know about the Previous Meat Science (LPT) notes of  Indian Meat Industry, Structure and Composition of Muscle, Conversion of Muscle to Meat,  Nutritive value of Meat and Properties of Fresh Meat. For any query knock us. Thank You.