Canning of meat products and hermetical closure

from http://www.fao.org/DOCREP/004/T0279E/T0279E06.htm

Container. The container protects the canned product from spoilage by recontamination with micro-organisms, therefore reliable containers and properly adjusted closing machines are essential to prevent access of spoilage organisms during the cooling operation and during the shelf-life of the product. Before filing, containers have to be rinsed or otherwise cleaned from dust or other impurities (Fig. 197).

Tin-plate cans are most commonly used, such as the soldered side-seam three-piece can, consisting of can body, can end and lid. The cemented sideseam and welded side-seam cans constitute the majority of cans produced, and remain one of the most reliable and acceptable packages ever conceived. The benefits of using three-piece cans include no size limitations, a wide range of plate thicknesses and tempers for body and ends, abuse resistance, strong end profiles and long shelf-life. Two-piece cans are becoming popular for some applications. Can end and body is drawn in a single piece from flat sheet stock. Only shallow cans (height less than half the diameter) can be fabricated this way. Enamelled cans are more suitable for canned meat products because interior corrosion of cans can be avoided (Figs. 196, 197 and 198).

Increasingly more attention is being given to aluminium for manufacturing cans and other containers for canned meat products, where special opening features are desired. Important advantages of aluminium cans are that they are lead-free and do not rust. Most often two-piece aluminium cans are used in meat canning (Fig. 196). Aluminium for can ends and bodies is, without exception, coated with enamel on both sides. It is widely used in flexible and semi-rigid containers as a protective packaging for a large number of meat products. As a result of extensive development, the use of flexible, laminated pouches and formed aluminium containers for shelf-stable sterilized products is a commercial reality.

The retortable pouch, a thin rectangular package used for heat-sterilized meat products, offers potential improvements in convenience and quality because of its shape and composition. During heat processing, its shape and size allows rapid heat transfer to destroy micro-organisms at the innermost part of the pouch without excessively overheating the product near the pouch wall. The contents are likewise more rapidly cooled at the end of sterilization. A suitable laminate used to make retortable pouches consists of three foils (polyester, aluminium and polypropylene film or resin). These containers are heat-sealed.

Glass containers. Glass is an inert container, although damage to the meat product may result from exposure to light. Glass jars are sealed with caps, twist-off lids and many other systems (crowns, side seal closures, rolled-on closure, screw caps or lug caps) and are used for frankfurters in brine and similar sausages and to some extent also for liver sausage mixtures or sausage mixtures with high-collagen content.

Retortable synthetic casings. Recently a new kind of casing has been used for sausage filling, especially if it is necessary to obtain a longer shelf-life for sausages, sterilized in a retort. This kind of container is sealed with clips made of aluminium.

199. Manual closing machine	200. Principles of can-seaming operation (a, b) and cross-section of correct can closure by double seam (c). (a) initial stage of curling; (b) fully developed curling; (c) seam tightening. 1 seaming chuck, 2 seaming roll, 3 tin wall, 4 tin end, 5 seaming roll, 6 lining compound

Smaller size containers are more suitable for meat canning because heat penetration of the meat is by conduction, so if larger containers are used extremely severe heat treatments will be necessary for sterilization. Such severe heat treatment will result in a much lower quality of canned product which will be extremely overcooked. That is why larger pieces, such as hams, shoulders, etc. are pasteurized at temperatures of about 80°C.

Hermetic closure. Tin-plate cans and aluminium cans are closed by machines which form a double seam in three stages: pressing the cover (lid) to the can body, forming the first operation roll seam, and forming the second-operation roll seam (Figs 199 and 200).

201. Autoclave (retort). 1 body, 2 lid, 3 counterbalance, 4 nuts, 5 heater, 6 vent, 7 relief valve, 8 pressure gauge, 9 water supply valve, 10 steam supply valve	202. Autoclave (retort) for small-scale production; retort lid opened (above left), basket for loading cans (above right)

Good double seams are absolutely essential to prevent losses due to spoilage. The food processor is responsible for the quality of seams produced in the plant and must provide adequately trained personnel to perform seam evaluations and make closing machine adjustments. Making good seams requires careful closing machine maintenance, frequent, regularly scheduled seam evaluations and immediate correction of seam conditions which are outside established tolerances. Hermetic closure of cans under vacuum is highly recommended, particularly for countries with a hot climate.

Autoclave (retort)

Batch retorts are heated with steam or water under pressure (Figs 201 and 202). Saturated air-free steam is an excellent heating medium for processing canned meat products. Hot water under pressure has many of the advantages of saturated steam as a heating medium but it must be circulated among containers and continually reheated.

The canning operation must be performed only by fully trained personnel. Operation of batch retorts heated with steam is performed in the following steps:

• load the cans in trays or baskets and close and tighten the retort lid;
• open the drain, vent condensate valves and the high-and low-pressure steam supplies;
• remove the air by a flow of steam during venting;
• close drain and vent valves;
• bring retort to operating temperature and pressure;
• regulate steam supply by temperature control;
• time the process from when the retort reaches the specified temperature;
• turn off the steam at the end of the process;
• start cooling by flooding the retort with cold water for about ten minutes (smaller cans up to 500 grams capacity), or 20 minutes (larger cans up to 1 500 grams);
• continue cooling until the average temperature of the product is 40°C when the process is completed;
• open the lid of the retort, remove baskets with canned products and let them dry.

Venting is one of the most important operations in processing canned meat products in steam, because air remaining among cans in an inadequately vented retort forms an insulating layer and will reduce the efficiency of the process.

In general, the same principles apply to processing in water. Some containers such as glass jars and flexible synthetic pouches are usually retorted in hot water in vertical retorts under superimposed air pressure.

Chlorination of cooling water is recommended (0.5 ppm free chlorine should be the lowest amount at the point of discharge).

Frequent faults committed during canning

Insufficient sterilization results in spoilage of canned product and is identified by can swell (hard, soft and springer swell) or by flat sour spoilage.

203. Insufficient can closure (droop, lip and improper first roll seam caused by damaged can are shown)	205. Products in opened cans, liver paste (left) and luncheon meat (right)
	204. Can deformations (critical, because double seam (1 and 3) or side seam (2) is damaged)

Overprocessing results in too soft a consistency and a burnt taste.

Deformation of cans and semi-rigid containers, breakage of glass jars and bursting of casings are the results of a careless, cooling operation, especially improper superimposed pressure (Fig. 204).

Leaking cans are the result of improper closing machine adjustment and deformation of cans prior to closing or after being processed, especially near the double seam (Fig. 203).

Shelf-life

Fully preserved canned meat products keep up to four years, but in the tropics it is only about one year.

Canned pasteurized products should be clearly labelled as perishable product (keep under refrigeration or store at/below 5°C) and must not be confused with fully sterilized products which do not require refrigerated storage.

Most canned meat products are eaten after emptying the can (Fig. 205), without reheating, stews and frankfurters being the exceptions.