Heat Processing

Meat products, after being restructured from raw materials, are stuffed into casings, bags, or molds and are subjected to heat processing—except for the fresh-product category. The basic functions of smoking and heating processes are to inhibit bacterial growth, to provide desirable color, and to impart specific flavor and texture. Heat processing in the meat industry covers smoking, drying, cooking, cooling, and freezing (19). The smokehouse is the major heat-processing equipment. When meat products are subjected to heat processing in a smokehouse, not only does simultaneous heat and mass transfer occur, but also microbial and biochemical reactions are induced. The theoretical approach to process and engineering design is limited. According to material handling criteria, there are two types of smokehouses available. One is a batch-type smokehouse and the other is a continuous smokehouse. Batch-type smokehouses provide a cold-water shower for precooling. The final chilling is conducted at the separate blast-air chilling or brine chilling room. For a continuous smokehouse, the product is loaded on a conveyor and automatically carried through smoking, drying, cooking, and chilling processes. Products, after leaving the continuous oven, are ready for peeling, slicing, and packaging. Detailed heat-processing flowcharts for both batch and continuous smokehouses are shown in Figure 2.

Smoking

The smoking of meat is an ancient practice to preserve meat products in addition to developing color and flavor. The role of smoke as a meat preservative has declined owing to the availability of refrigeration. Today, the smoking process is used to enhance flavor and surface color. Several hundred chemical compound have been isolated from wood smoke. The most important compounds are acids, phenols, and carbonyls. Phenols and carbonyls are major contributors of smoke flavor and color, respectively. The acidic substances accelerate the curing reaction and contribute to the pink color of cured meats. For processing convenience, natural smoke is, for some uses, condensed to liquid form. It can be added directly to a product or applied on a product surface by dipping, spraying, atomizing, or regenerating.

Natural Smoke Generation

Natural smoke is produced from thermal decomposition of wood and is known as pyrolysis. The major constituents of wood are cellulose, hemicellulose, and liqnin. During thermal decomposition, a temperature gradient exists between the outer and inner core of wood chip or sawdust. The outer surface is being oxidized, and the inner surface is being dehydrated before it can be oxidized. During the dehydration process, the outer surface is at about water boiling temperature. Carbon monoxide, carbon dioxide, and some volatile short-change organic acids are released during the dehydration process. When the internal moisture is dried out, the temperature rapidly rises to 300-400°C (570-750°F). As long as the internal temperature reaches this range, the decomposition occurs and smoke is given off. The hemicellulose fraction is the first to undergo degradation. Cellulose and liqnin degradation follow accordingly.

Under normal smoking conditions, the smoking temperature ranges from 100 to 400°C (212-750°F) or higher. This results in the generation of more than 400 smoke compounds. Smoke composition can vary substantially with smoke generation temperature as well as with different varieties of wood. The effective smoke compounds— phenols, acids, and carbonyls—change with generation temperature, as shown in Table 1 (20); wood used here is maple sawdust.

Thermal decomposition of wood is induced by high temperature. Heat sources can be hot plate, self-burning, or friction. Sawdust is normally used in a smoke generator with hot plates. Sawdust is loaded in a hopper, and the feeding rate is controlled by the rotation speed of the feed plate. Under the feed plate there are three hot plates with electric heaters under each plate. On top of each hot plate there is a wiper to sweep sawdust across the hot-plate sur-

Air intake

Heat processing flow chart |To atmosphere

Humidification

Air intake

Heat processing flow chart |To atmosphere

Humidification

Product out

Smokemaker

Product out

Smokemaker

Air intake

Batch type smokehouse jToatmosphere Air ¡ntake

To atmosphere

Cooling source

Air intake

Batch type smokehouse jToatmosphere Air ¡ntake

To atmosphere

Cooling source

Humidification

Product in

Brine

Humidification

Product in o o o M C

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