Consumption of fresh foods is usually preferred to processed and/or preserved food because of a possible decrease in nutritional value. However, most animal products have to be processed and/or preserved by one means or the other before they are consumed so that they can be eatable or palatable and safe.
Another reason for processing and/or preservation is to make long storage possible or storage of excess food. It is difficult to effectively separate processing from preservation and even storage because processing usually leads to preservation, and storage is a form of preservation.
This article will explain different meat processing and preservation/storage methods such as salting, curing, cooking, cooling, freezing, packaging, etc. Knowledge of these methods is important in order to be able to use meat more efficiently.
Why Preserve or Process Meat?
The main reason for preserving meat is to prevent spoilage. Traditional meat processing involved sun drying, then drying over a burning wood fire, and salt was added later in order to preserve meat for future use.
Advances in preservation technology especially refrigeration and packaging provided opportunities for many new processed products. The reasons for processing modern meat products are:
Development of unique flavors and forms of products;
Provision of a variety of products;
Development of new products.
As we have learned spoilage of meat or flesh from animals starts soon after they are killed. Fish will spoil quicker than meat from terrestrial animals, and organ meats
(such as liver) spoil quicker than these. Spoilage is the deterioration of food, which makes it taste bad and/or makes it a carrier of disease organisms. Typical spoilage in flesh can be caused by the following:
Microbiological due to bacteria on the surface of flesh or from within the body or intestines. It can also be from external contamination due to unhygienic post-slaughter handling
Autolytic is due to internal enzymes, which break down different compounds that affect smell, taste, and texture. For example, cathepsins break down rigor in the flesh
Rancidity due to oxidation of the fat in fatty fish and meat exposed to air for a long time. This causes an unpleasant smell or taste
The application of proper processing techniques such as heating and fermentation can remove these negative effects.
Meat Processing and Storage
All processed meat products can be grouped into comminuted and non-comminuted products.
Comminution involves sub-dividing or cutting raw meat so that the product will consist of small meat pieces, chunks, or slices. Such products include sausages, hamburgers, ground or minced meat, corned beef, etc.
Comminution is important because it results in improved uniformity of products due to more uniform particle size and distribution of ingredients. In addition, it leads to an increase in tenderness as the meat is sub-divided into smaller particles.
Non-comminuted products are prepared from whole or intact cuts of meat with or without bone. All processing techniques can be applied to both comminuted and non-comminuted products.
Such processing techniques include curing, cooking, drying, smoking, fermentation, etc.
Curing is the application of salt, color-fixing ingredients, and seasonings to meat in order to impart unique properties to the end products.
The traditional objective of curing was to preserve the meat with a very high concentration of salt but a high salt concentration oxidizes myoglobin giving the meat an unattractive greyish or greyish-brown color.
Therefore, color-fixing ingredients were applied to give a bright reddish-pink color such as sodium nitrite and potassium or sodium nitrate.
Sodium nitrite reacts with myoglobin to form a heat-stable red color and a small amount of nitrite is needed for this purpose. The level of nitrite is about 150 mg/kg in meat products.
To reduce the risk of overdosing nitrite, it is made available in a homogenous mixture with common salt in the proportion of 0.5% nitrite plus 99.5% sodium chloride. The mixture is called nitrite curing salt, and it is applied to meat products at a dosage level of 1.5 – 3% to achieve a salty flavor and a reddish color.
High concentrations of nitrite are toxic and could be carcinogenic (cause cancer) if used in meat products that are heated to a very high temperature. Nitrite is preferred to nitrate because of the longer time it takes before the red color is produced. Some seasonings used in curing include spices, herbs, vegetables, and sweeteners.
The incorporation of cure ingredients must be uniform throughout the entire product. Cure ingredients are incorporated according to the following methods:
Dry Cure: The cure ingredients are rubbed in dry form over the surface of meat cuts and then placed on top of the other in a curing container or box or tank.
The curing mix gradually permeates the meat, which can take very long from several days to several weeks. This method is usually used for meat products that are also fermented called cured raw fermented products.
Wet Cure or Pickling: The meat cuts are immersed in a cure solution, which completely covers all the pieces. A room temperature of 8 to 10oC is recommended and curing takes several days to two weeks for equal penetration.
Injection Cure: This is an alternative and quicker way than those above to accelerate the diffusion of the cure solution by pumping it into the meat tissue.
Injection of the cure solution can be done manually by using simple pumping devices such as a syringe (small or large) or mechanically by semi-automatic multi-needle injectors, which inject the solution into many parts of the meat at the same time.
2. Heat Treatment or Cooking
Heat treatment of processed meat products serves two purposes:
Enhancement of desirable texture, flavor, and color (increase palatability);
Reduction of microbial content to achieve preservation and food safety.
High temperature induces coagulation and denaturation of meat proteins and the structural and chemical changes in fats and carbohydrates, which make the meat tastier and tenderer.
The numbers of microorganisms increase steadily through slaughtering, then meat cutting and initial processing. Therefore, heat treatment is important for microbial control to reduce or eliminate contaminating microorganisms.
There are two types of heat treatment, which are:
Pasteurization (or cooking): Heat treatment at temperatures below 100oC, mostly between 60-85oC.
Sterilization: Heat treatment at temperatures above 100oC.
There are three principal cooking methods, which are dry-heat, moist heat, and microwave.
Dry heat occurs when meat is surrounded by hot air such as found in boiling (meat placed in the oven); Pan frying (meat browned on both sides in the pan); Stir frying (small meat pieces constantly stirred in a large bowl-like frying pan called wok); Deep fat frying (meat completely immersed in fat); Roasting (meat placed on a grill).
Moist-heat occurs when meat is surrounded by hot liquid such as found in braising (meat cooked with water and ingredients such as milk or vegetables); Stewing (cooking in liquid with small meat pieces); Simmering (cooking in liquid with large meat pieces under low temperature and a long time).
Microwave cooking occurs in a microwave machine where electromagnetic waves cause the molecules in meat to vibrate against each other to create friction and heat from within the meat pieces.
Cooking here is more rapid than you find in the other two methods and temperature and cooking time can be pre-set.
Read Also: The Different Properties of Fresh Meat
Drying is the lowering of water activity in meat and meat products. Water activity is the measure of free unbound water available for microbial growth.
The growth of microorganisms is halted when the available free water is lower than that required for the particular species to grow.
Meat drying could be done to preserve or extend storage life (shelf life) or could just be part of the steps in the manufacturing of specific meat products. The manufacture of fermented meat products is an example where drying is an important step in the process.
The drying of such products is mostly done in climate-controlled chambers where temperature and humidity can be controlled. Drying could also be done in ovens at temperatures between 70-80oC.
However, drying of meat can be done under natural conditions in the air or sun. This is a popular method in many developing countries because of its cheapness and effectiveness.
Pieces of meat are cut into specific uniform shapes (long strips and flat pieces) that permit the gradual and equal drying of whole batches of meat. It is predominantly carried out for meat preservation.
Meats suitable for sun drying include lean meat (meat with little or no fat) from beef cattle, goat, and camel but not pork (pig meat) or such that have fat.
Continuous evaporation of water from within the meat tissues brought to the surface results in meat pieces becoming smaller, thinner, slightly wrinkled, darker, and harder in texture. However, the protein content remains unchanged.
Meat strips with strings or hooks attached are usually suspended to dry in the sun on long sticks similar to hanging clothes on wires to dry; and flat pieces of meat are placed in drying trays exposed to the sun.
Some of the disadvantages of sun drying are limitations to the dry season, and exposure to contamination from dirt, wind, rain, insects, rodents, and birds. Microbial contamination is also possible.
An example of a sun-dried meat product is kilishi, where thinly sliced flat meat pieces are semi-dried in the sun and then spiced or dry-cured before finally drying over the fire.
However, there is an improvement over this method, which is called solar drying. Solar drying involves the use of a solar dryer, which is equipment constructed to collect solar energy through the aid of materials that can store it (solar collectors such as a black surface).
The heat absorbed by the collector heats the air around it, which is passed or moved into a chamber to dry meat pieces arranged there. This system avoids most of the disadvantages of sun drying.
Smoking is the process of allowing smoke generated from natural woods to act on the surface of meat or meat products. Smoke is produced by the thermal decomposition of wood into molecular weight products, a process called pyrolysis.
Other reactions take place with decomposition such as condensation, polymerization, and oxidation to produce more than 1000 compounds from smoke. The most important of these products are phenols, aldehydes, organic acids, carbonyls, hydrocarbons, and alcohols.
Freshly produced smoke has the following effects on meat:
Preservation: The preservative effect comes from the drying action of smoke with the destruction and/or inhibition of microbial growth. In addition, anti-oxidants (phenols and aldehydes) from smoke delay the rancidity of fat.
Most of the smoke components are present or deposited on the meat surface and do not penetrate more than a few milliliters into the meat.
Colour Production: This is caused by aldehydes, phenols, and carbonyls, and the development of Maillard reaction products. Maillard’s reaction is also browning or caramelization.
Aromatization: This is caused by phenols, aldehydes, carbonyls, and volatile organic acids that give a peculiar smell and taste or flavor to smoked meat.
However, the smoke has been found to contain polycyclic hydrocarbons (benzopyrene) that have been implicated to be carcinogenic. One way to remove hydrocarbons is the production of liquid smoke.
Liquid smoke is obtained from the condensation of wood smoke. This allows for fractional distillation to enable the separation and removal of smoke components such as polycyclic hydrocarbons.
The liquid smoke can be painted on the surface of the products or mixed into the ingredients of the products such as in some sausages.
There are two basic smoking techniques, cold smoking, and hot smoking. Cold smoking is the traditional way of smoking meat products and was primarily used for meat preservation.
The optimal temperature of the smoke in cold smoking is 15-18oC and is a long process that may take several days. Hot smoking is carried out at temperatures greater than 60-80oC and this enables rapid color and flavor development. Hot smoking ranges from about ten minutes to one hour.
Smoking is usually done in smoke chambers where pieces of meat or meat products are placed for application of smoke generated within (traditional method) or outside modern method) the chamber.
Smoke generators outside the modern smoke chambers generate smoke by burning sawdust slowly by direct fire or by very hot steam or by friction on a log of wood by a fast rotating steel drum. The separate smoke generator allows for the removal of unwanted smoke components such as benzopyrene.
5. Cooling or Refrigeration
Refrigeration is a preservative and a storage method for meat and meat products. The most common method for prolonging the shelf life of meat is refrigeration, which involves the storage of meat at temperatures between -2 and 5oC.
Refrigeration should begin immediately after slaughtering and continue through subsequent processing to consumption.
The internal temperature of the carcass, which is between 30 – 39oC after slaughter must be reduced rapidly to about 5oC at the thickest portion of meat in order to slow down the degeneration of the meat.
Beef, pork, and lamb carcasses are usually cooled or chilled at temperature ranges of between -4 to 0oC, while fish and poultry carcasses are cooled by immersion in ice water.
The rate of cooling a carcass depends on the size of the carcass, the amount of external or internal fat, and the temperature of the cooling environment. The rate of cooling will be slow if the carcass is large, the fat is much, and the temperature of the cooling environment is high.
Meat or meat products should only be stored under refrigeration for a short time because degenerative changes continue to occur and accelerate with time.
The major factors that influence the storage life of meat under refrigeration are the initial microbial load, temperature and humidity conditions during storage, the presence or absence of protective covering or packaging, the species of animal, and the type of product stored (raw or processed).
The storage life of meat will be shorter if the initial microbial load is high, humidity is low and temperature high, covering is absent, meat is from fish or poultry, and the meat is raw.
Fresh meat purchased for consumption should not be stored under refrigeration for more than four days before consumption unless frozen.
Freezing is an excellent method for preserving and storing meat. It results in less undesirable changes in qualitative and sensory (flavor, taste, juiciness, and texture) properties of meat.
Very low temperature retards microbial growth, and enzymatic or chemical changes thereby retaining nutritive value. Freezing also prevents access by microbes to water available for microbial growth.
However, freezing can cause damage by discoloration (freezer burn), formation of ice crystals in meat, and the precipitation of cell constituents depending on whether freezing is done at a fast rate or at a slow rate. Freezing at a fast rate is more desirable than at a low rate.
There are different methods for freezing meat and meat products, which are:
Still air: Air is the heat transfer medium and freezing depends on convection with meat freezing very slowly. The home freezer unit and the refrigerator freezer operate on this principle. Temperature ranges from about -10 to -30oC.
Plate freezer: Metal is the heat transfer medium and freezing depends on conduction, which is slightly faster than still air. Products are placed in direct contact with the freezer metallic plate or shelves. Temperature also ranges from about -10 to -30oC.
Blast freezer: Air is the medium of heat transfer but the heat is transferred at a higher velocity on account of the fans installed for rapid air movement. Temperature also ranges from about -10 to -40oC and air velocity from 30 to 1070 meters/minute.
Liquid immersion and liquid sprays: These are used mostly for packaged products, especially poultry and fish. The freezing rate is rapid and comparable to blast freezing.
The products are wrapped in plastic and immersed in the freezing liquid or the cold liquid is sprayed over them. The liquid must be non-toxic, cheap, have low viscosity, low freezing point, and high heat conductivity. Examples are sodium chloride brine (salt solution), glycerol, and glycol.
Cryogenic freezing: This is the most rapid method of freezing. It involves extremely low temperatures where the freezing agent such as liquid or gaseous nitrogen or liquid, gaseous or solid carbon dioxide is applied. They can be applied as liquid immersion or spray.
Before meat under freezer storage can be used it has to be changed from the frozen state to the former state before freezing. This process is called thawing. The quality of meat after thawing will depend on conditions under frozen storage, thawing method, freezing rate (slow or fast), and condition of meat before freezing. There are five ways of thawing frozen meat products:
Cold air: meat put into a refrigerator or cooler
Warm air: meat left under ambient temperature
Water: meat put in water or under running water
Cooking: meat cooked directly as soon as taken from the freezer
Microwave: meat thawed according to the microwave thawing option
The cold air option is recommended as the best thawing method if the products are not going to be cooked or microwaved directly from the frozen state.
Packaging refers to the material that holds or surrounds meat and meat products in order to prevent them from direct handling and enables compactness in usage. The basic functions of packaging are the following:
Protection of products from undesirable impacts on quality regarding microbiological and physic-chemical damage or alterations.
In this perspective, products are protected from secondary contamination during processing, storage, and distribution due to dirt, microorganisms, parasites (e.g. insects), toxic substances, and influences affecting color and taste and loss or uptake of moisture.
Presentation of products to consumers in the most attractive manner.
In spite of the fact that packaging can prevent secondary contamination, the further growth of microorganisms, which are already present in meat, cannot be prevented except when combined with other measures such as heating or sterilization, freezing, etc.
There are many materials suitable for meat packaging but the choice of a specific type will depend on whether the meat product is fresh or processed.
For example, the packaging requirements for frozen meat include low moisture vapor transmission (to prevent dehydration), pliability, strength, and grease resistance from fat.
Moreover, packaging material used for fresh meat display must allow ample amounts of oxygen to pass through in order for oxymyoglobin to be stable and maintain reddish color. However, cured meat products need to be protected from light and oxygen so that the color imparted by cure ingredients will not fade.
Different types of packaging materials available include paper, aluminum foil, glass, films, or foils manufactured from polyethylene, polypropylene, polyesters, nylon, polystyrene, and polyvinyl chloride (PVC).
Combinations of these materials are called laminates, which possess a wide variety of functional properties depending on the components. Some basic attributes required for the films and/or foils are they must:
Have mechanical strength;
Be odorless and hygienic;
Resistant to hot/cold temperatures;
Resistant to oil and fat;
Have good barrier properties against gases;
Be heat sealable;
Have low cost.
More advanced forms of packaging include vacuum packaging and modified atmosphere packaging (MAP). Vacuum packaging involves the removal of air so that air is not in contact with the product.
This will be suitable for cured products that require very little oxygen. Modified atmosphere packaging involves the removal of air in order to be replaced by carbon dioxide and nitrogen.
Nitrogen prevents rancidity and carbon dioxide prevents the growth of bacteria and molds in processed products. For fresh meat, a combination of oxygen and carbon dioxide is used to replace air.
After the removal and replacement of air, the packaging material is heat-sealed to maintain the new atmosphere.
In conclusion, meat is one of the most important foods to man and this attribute can only be maintained if its wholesomeness (good quality) can be guaranteed. Continuous development and application of processes that ensure the quality of meat and meat products have a significant bearing on their use as food.
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