Thermal Principles and Kinetics

Thermal processing of food materials is one of the most widely used methods of food preservation. Foods may be thermally processed using numerous heating systems such as retorts (batch or continuous), direct heating systems (steam injection or steam infusion), indirect heating systems (tubular heat exchangers, shell and tube heat exchangers, plate heat exchangers, scraped surface heat exchangers), volumetric heating systems (microwave or ohmic heating), and combinations of these. The choice of the heating system is based on several factors, including the characteristics of the product (pH, water activity, composition, etc.), properties of the product (density, viscosity, specific heat, thermal conductivity, thermal diffusivity, electrical conductivity for ohmic heating, and dielectric properties for microwave heating), quality of the product, need for refrigeration, need or acceptability of moisture addition/removal, and cost. The extent of thermal treatment required for a food product depends on whether it is an acid product, an acidified product, or a low-acid product. An acid food product is one with a natural pH of less than 4.6. Acid food products include apple juice, orange juice, ketchup, etc. An acidified food product is one with an equilibrium pH of less than 4.6 and a water activity (aw) greater than 0.85. Examples of acidified foods include peppers treated in an acid brine, pickled foods (excluding foods pickled by fermentation), etc. Acidified food products are typically treated at 90–95 C for a period of 30–90 sec to inactivate yeasts, molds, and bacteria (usually Lactobacillus species). A low-acid food product is any food other than alcoholic beverages, with a natural equilibrium pH greater than 4.6 and a water activity greater than 0.85. These food products include butter, cheese, fresh eggs, pears, papaya, and raisins (Skudder 1993). Low-acid food products are capable of sustaining the growth of Clostridium botulinum spores. Clostridium botulinum is an anaerobic, gram-positive, heat-resistant spore-forming bacterium that produces a potent neurotoxin. Food-borne botulism is a severe type of food poisoning caused by the ingestion of foods containing the potent neurotoxin formed during the growth of Clostridium botulinum. The spores of Clostridium botulinum must be destroyed or effectively inhibited to prevent germination and subsequent production of the deadly toxin, which causes botulism. Low-acid food products come under the regulatory authority of either the Food and Drug Administration (FDA) or the United States Department of Agriculture (USDA), depending on the proportion of meat or poultry in the food product. The FDA regulates most food products, except those containing more than 3% raw or 2% cooked meat or poultry ingredients, which fall under the jurisdiction of the USDA. The general thermal process requirements of both regulatory agencies are similar and they are compiled in Code of Federal Regulations 21 CFR Parts 108 (emergency permit control), 113 (low-acid canned foods), and 114 (acidified foods) for the FDA, and 9 CFR Parts 308, 318 (meat products), 320, 327, and 381 (poultry products) for the USDA. The FDA requires registration of the processing facility (form 2541) and a detailed process filing (form 2541a for acidified and low-acid canned foods and form 2541c for low-acid aseptically processed foods) (Chandarana 1992).