1. Introduction

Canning is basicaly a heat processing operation where heat flows from a hot body to a cold body.Among the various methods of extending meat shelf life, canning has the adventage of keeping as much as possible of the original chemical, physical, and sensory characteristics of meat. In addition, canning allows storing and transporting meat, a highly perishable material, in experiments where no other preservation method is succesful. Canning is especially appropriate for meats marketed to tropical conditions, where temperature and humidity are high.

The aim of canning is to destroy microbial populations and/or enzymes responsible for meat deteriorations.In 1810, Peter Duran and Thomas Saddinton in England first commercially applied canning technology to extend meat shelf life by placing it into sealed containers and heating them thoruoghly. However, earlier efforts to preserve food materials by this method were made by Nicholas Appert in France in 1790.

2. Microbial Destruction by Heating

In activation of pathogens and spoilage microorganism by heat is calculated from the point of view of shelf life extension as well as by alteration of sensory characteristics.

Depending of microbial species, vegetative cells of bacteria and fungi can be destroyed when heated at 60˚ C to 90˚ C. Inactivation of bacterial spores require temperature between 115˚C and 121˚ C. In activation of meat enzymes occurs in most cases at 60˚ C to 75˚ C

The process also ensures sanitary conditions, destroying this respect to two bacteriological problems to be solved by canning are as follow:

1. Elimination of vegetative cells and spores that can grow and produce toxins

2. Elimination or inhibition of the development of spoilage microorganisms

During heat treatment, the main problems related to microbial populations are as follow:

1. All microorganisms fisible to grow and produce toxins must be eliminated. Canned meat must be free of Clostridium botulinum, which is most dangerous and produce a fairly heat-stable toxin

2. Spoilage microorganisms must be reduced to a safe limit. Any canned food is sterile if it is free of spoilage microorganisms such as Cl. perfingens. Spore forming thermophiles such as Cl. sporogenes must be considered only when storage temperatures are high.

Heat treatment severe enough destroy Cl. botulinum or Cl. perfingens gives as a result a stable food without the need for applying further preservation methods. However, because severe heating can alter sensory characteristics, it is necessary to achieve a compromise between preservation and alteration of sensory attributes. It is important to note that heat treatment can also improve sensory characteristics of meat, such as texture and flavor.

Thermal resistance of microorganisms increases because of the following factors:

1. pH and water

2. Fat, carbohydrate, protein, and salt content

3. Presence of other microbial inhibitors

Meat, in general, has low acidity (>4.5), with the exception of fermented meats. In this case, preservation achieved by acid production of lactic acid bacteria. Acids of chemical origin also acts as antimicrobial agents. In this case meats require mild heat treatment. In low acid or ph neutral foods, such as most canned meats, stronger heat treatments needed because potential pathogens or spoilage microorganisms can grow.

3. Commercial Heat Treatments

Vegetative cells are destroyed at temperatures slightly higher than their maximum growth temperature, whereas spores can survive at much higher temperatures.

Basically there are four heating process applied to food materiels, based on temperature increase.

1. Blanching

2. Cooking

3. Pasteurization

4. Sterilization

3.1. Blanching

Blanching is applied to inactivate enzymes in products receiving further heat treatments. For meats, volume reduction usually occurs. It is also used to eliminate gas from the tissues or simply to provide an initial cleaning of the food material. Blanching temperatures around 65˚ C.

3.2. Cooking

Cooking is applied to improve sensory characteristics of tho food material, although it also destroys a number of microorganisms and inactivates some enzymes. It takes place at around 85˚ C.

3.3. Pasteurization

Pasteurization destroys pathogenic vegetative cells, but certain heat resistant microorganismms and spores can survive. Pasteurization temperatures are 140 C to 150˚ C for 1 to 45 seconds, or 70˚ C to 73˚ C for 15 to 20 seconds.

3.4. Sterilization

Sterilization destroy vegetative cells as well as spores; the shelf life of sterilized foods is considerably extended even without the application of additional preservation methods. Time-temperature relationship of the sterilization process depends on thermal resistance of a given microorganism, taken as spoilage indicator.

4. Meat Canning Process

Meat canning consist of several steps, although the basic principle is heat treatment of a sealed container. Once the meat and other ingredients are prepared, canning basically includes three main operations:

1. Can filling

2. Exhausting and closing

3. Sterilization treatment

Can Filling

When a solid material canned together with a liquid, heat penetration is affected by the solid-liquid ratio as well as the solid distribution within the can. Solid material packed loosely is heated faster than closely packed material.

In general, 30% of the can volume must be liquid (brine or sauce) in order to have a good heat transfer. Liquid is always filled after solids. In order to calculate heat tranfer, headspace must be always taken into consideration.

Exhausting and Closing

Exhausting efficiency depends largely on the headspace volume. Approximately 0.5% of the total can volume must be left for headspace. Foods easily react with oxygen; color, flavor, and wholesomeness may change because of reactions with the air components.

Air evacuation from the headspace, as well as from the bulk of food, is necessary to achieve good heat penetration and the desired sterilization temperature. When large meat pieces are canned, exhausting during filing and closing is enough for a complete air evacuation. Conversly in meat batter air is incorporated during almost all processing opetarions if these are not carried out in vacuum.

Air evacuation from the headspace reduces the risk of causing a rise of internal pressure during heat treatment, resulting in can blowing or deformation. Air exhaustion also rduces the risk of promoting growth of aerobes, particularly if the product is only pasteurized.

Exhausting can be achieved by heating, by mechanical operations, and by vapor injection. Heating of water promotes an increase in its vapor pressure, and the air in the container is replaced by the water vapor, the cans are then immediately closed. When the cans are cooled down, vacuum is produced due to condensation of the water vapor. Heating at 75˚ C to 95˚ C is applied just before filling and closing.Alternatively, cans be conveyed on a belt into a exhausting chamber or tunnel in which the cansa re heated at 85˚ C to 95˚ C, removing 90% or more of the air in the head space.

4.3. Sterilization

Thermal treatments include two cycles:

1. heating

2. cooling

Whereas heating is involved in microbial and enzyme inactivation, cooling is applied for several reasons, such as easy handling and reduction of sensory characteristic deteriorations.

5. Alterations In canned Meat

Canned meat can undergo different type of alterations; most of them affect the quality of food. Besides alterations before heat treatment, in-line alterations could be caused by microbial activity, chemical reactions of the meat with the container, and physical alterations

5.1. Alteration Before Heat Treatment

These alterations occur when there is a delay in can filling and closing, or prior to heat processing, causing microbial growth to take place. It is recommended that processing must not be delayed more than 20 minutes after closing the cans.Further heat treatment may sterilized the product but gas or other microbial metabolities remain in the can.

5.2. Microbial Alterations

Alterations due to microorganisms are the result of insufficient thermal treatment.

As a consequence, certain microorganisms survivce, some of them producin gas, resulting in blown cans. Other surviving microorganisms, such as lactic acid bacteria, generate acid without gas.

When this alteration occurs, the first step to be taken is identfy the responsible microorganism; in most cases they are sporulated bacteria. Process conditions and microbial quality of raw materials must be checked, as well as sanitation of the equipment, water supply, and so forth. Insufficient thermal treatment can also be a result of can disribution in the retort, causing a delay in convection.

When cooling after heating is insufficient, themophiles can grow. As a general rule canned meat products cooled at 35˚ C. At this temperature the can outer surface rapidly colls down. However, if cans are stored in large blocks, further cooling is slow, allowing thermophiles to grow. Microbial contaminations also occurs through seals if cooling water lacks adequate sanitary quality. Recontamination after heat treatment is one of the most common problems, and cause of can blowing.

5.3. Chemical Alterations

Blowing can also occur as a consequence of reactions of meat components with the packaging material, producing hydrogen or stannus sulfide. This happens during corrosion if the can coating present failures or if the tin layer is not thick enough.

This reaction produces blue-black spots in the food. Tin cans reacts without producing any visible alterations, although flavor can be drastically altered, producing an astringent, metalic taste.

Meat compounds can also undergo chemical changes promoted by heat treatment, such as maillard reactions. This seldom occurs in meats.

5.4. Physical Alterations

Physical alterations occur as the result of mishandling of the sterilization equipment, such as a fast pressure increase in retorts, insufficient vacuum, or excessive can fill. These alterations include deformation of the cans during inadequate use of the retort.

If pressure reduction is carried out at a higher rate, higher pressure builds up inside, can producing tension and distortions that appear as blowing.

When a can blows in the bottom end and end can be slightly pressed back to its original shape, the cause is insufficient exhaustion. Presence of air in the can promotes an excessive internal pressure during heating as aresult of gas expansion. Excessive filling can also cause can deformation during heat processing due to expansion of the food. This mainly occurs in products cooked inside the can, such as luncheon meat.

6. Conclusion

Meat is a highly perishable material so many preservation techniques are developped. The main objective of heat treatment are enzyme inactivation, and destruction of pathogens and spoilage microorganisms. Meat canning is the most effective technique to preserve characteristic of meat due to developping technology, other methods generally traditional methods and they cause alterations on texture and sensory characteristics of meat.

7. References

1. Meat Science and Applications, Y.H. Hui, Wai-Kit Nip , Robert Rogers

2. The Science of Meat and Meat Products, James F. Price, Bernard S. Schweigert

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