Types of thermal process

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Thermal processing of fruits and vegetables can be achieved by a variety of techniques using hot water or steam (cooking, blanching, pasteurization, sterilization, evaporation and extrusion), hot air (drying) and irradiated energy (microwave, infrared radiation and ionising radiation), which are described below.

10.2.1 Cooking

Cooking is a heat-processing technique, the primary objective of which is to improve the palatability of the food. It can be considered to encompass several operations that are commonly carried out in the household: boiling, baking, broiling, roasting, frying and stewing, all of which differ in the method of application of heat. Boiling and stewing are done by placing the product in boiling water (or steam). Baking, broiling and roasting require dry heat and these processes are carried out in hot air ovens to improve and alter the eating quality of foods.

Cooking can be considered to be a preservation technique because many cooked foods can be stored longer under proper refrigerated conditions than their uncooked counterparts, if recontamination can be minimized. Cooking results in the destruction or reduction of microbial load and inactivation of undesirable enzymes, two important requirements of most preservation techniques. It can also inactivate toxins occurring naturally or through microbial contamination (in fresh or processed foods), improve digestibility and alter color, flavor and texture to suit the consumer's need. Again, while imparting these desirable effects, cooking will also result in loss of certain heat-labile nutrients.

10.2.2 Blanching

Blanching is a mild heat treatment used to inactivate the oxidative enzymes in fruits and vegetables prior to further processing (canning, freezing and dehydration), which otherwise will result in undesirable changes in color, flavor and nutritive value of the product during handling and storage. Apart from enzyme inactivation, blanching also serves several additional functions: it removes the tissue gases (to achieve a better vacuum in cans, reduce the strain on can closures during processing and to create reduced oxygen levels in the can), increases the bulk temperature of the tissue, cleanses the tissue, wilts the tissue to facilitate in packing and, in some instances, assists in improving (fixing) the color of green vegetables.

Of the oxidative enzyme systems, the enzyme peroxidase is considered to be the most heat resistant; therefore, peroxidase inactivation has been traditionally used as an index of blanching adequacy. Steam and hot water blanching are the two most commonly used blanching techniques. These processes are simple and inexpensive but are also energy intensive, result in considerable leaching of soluble components (which occurs both during heating and cooling) and produce large quantities of effluent. The merits and disadvantages of these techniques that are discussed below were summarized by Fellows (2000).

Conventional water blanching has lower capital cost and better energy efficiency than steam blanching but results in larger losses of water-soluble components, including vitamins, minerals and sugars. It also results in larger volumes of effluents and risk contamination by thermophilic bacteria. With steam blanching it is possible to reduce significantly the effluent volume as well as leaching losses if air cooling is adopted instead of water. However, uneven blanching can result if the food is blanched in multilayer piles. The individual quick blanching (IQB) technique (Lazar et al., 1971) is an innovation based on a two stage heat-hold principle and has been shown to improve the nutrient retention significantly. Research and engineering efforts led to the development of improved blanching equipment that makes use of steam (saturated or superheated) and recirculating hot water to improve nutrient retention, reduce leaching losses and improve energy efficiency (Cumming et al., 1984). Other non-conventional blanching procedures use moisturized hot gas, microwave or ohmic heating techniques generally together with air cooling to minimize leaching. The blanching time (10 s to 15min) usually depends on the type and size of the fruit or vegetable, the type (water, steam, hot gas or microwave) and temperature of the heating medium, as well as the method of heating.

10.2.3 Pasteurization

Pasteurization is also a mild heat treatment performed on foods to destroy vegetative microorganisms (especially pathogens) and inactivate the enzymes. Because the process is not severe enough to kill the spore formers, pasteurized foods must be stored under conditions of refrigeration to minimize microbial spoilage. Also, because only mild heat treatment is involved, the sensory characteristics and nutritive value of the food are minimally affected. The severity of the heat treatment and the length of storage depends on the nature of the product, pH conditions, the resistance of the test microorganism or enzyme, the sensitivity of the product and the type of application of the heat (Fellows, 2000; Holdsworth, 1997).

10.2.4 Sterilization

Sterilization involves a more severe heat treatment aimed at destroying the pathogens and spoilage-causing microorganisms in a food that is packaged in a hermetically sealed environment to prevent recontamination. The process takes into account the heat resistance of the spore formers in addition to their growth sensitivity to oxygen, pH and temperature. The presence of vacuum in cans prevents the growth of most aerobic microorganisms and if the storage temperature is kept below 25°C, the heat-resistant thermophiles pose little or no problem. From the public health perspective the most important microorganism in low acid (pH > 4.5) foods is C. botulinum, a heat-resistant, spore-forming anaerobic pathogen that, if it survives processing, can potentially grow and produce the deadly botulism toxin in foods. Because C. botulinum and most spore formers do not grow at pH < 4.5 (acid and medium-acid foods), the thermal processing criterion for these foods is the destruction of heat-resistant vegetative microorganisms or enzymes.

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