Heat Treatment

Populations of all life forms show characteristic death rates when exposed to elevated temperatures, pressures, and concentrations of certain chemicals. Enzymes show similar inactivation kinetics. The benefits of heat preservation, for example, the high inactivation rates of microbes and deteriorative enzymes, considerably outweigh the

Table 5. Typical Temperatures, Pressures, and pH Values in Food-Processing Operations

Temperature

Example of use

Limits of applications

Cryogenic freezing—liquid nitrogen Cryogenic freezing—solid carbon dioxide ( - 78.5°C) Cryogenic freezing—air, plate and aqueous base freezants (-40 to — 5°C) Refrigerated storage (0-10°C) Room temperature storage, air

(20-40°C) Water, air, atmospheric steam (50-100°C)

Oil, steam, infrared radiation

(180°C) Various infrared radiation sources (180°C)

Heat-transfer media

Rapid freezing to minimize moisture loss

Rapid freezing

Commercial freezing; parasite and insect destruction; storage; freeze drying

Commercial refrigerated storage

Canned and dry food storage

Pasteurization of milk and eggs; blanching; sterilization of acid foods (pH 4.5); air drying; cooking;

Thermal sterilization of nonacid foods; destruction of antinutritive factors

Frying, roasting, baking, generation of browning reaction products

Surface heating for flash drying, peeling

Cost, stress cracking during freezing Cost

Foods not suitable for freezing

Microbial growth possible

Suitable only for preserved and packaged foods

Excessive times will cause poor color, flavor, and structure

Rapid thermal degradation of nutrients, pigments, structure, and flavors Short duration surface treatments

Charring, pyrolysis without precision control

101 kPa 0.1-1 MPa

pH 12

33% sodium hydroxide

Pressurea

Freeze (hying

Hypobaric storage, deaeration, vacuum concentration, vacuum cooling Most processing operations Steam sterilization, over pressure for glass and pouch packs; carbonated and aerosol packaged food Extruders, hydraulic pressing, homogenization, microbial and enzyme inactivation at 1 GPa (10,000 atm) and higher

Hydrogen ion concentration

Acid hydrolysis

Lemons, limes, vinegar, organic acids Fruits, acidified foods Normal pH of most foods

Solubilization of certain proteins for extraction, alkali process cocoa Limed corn

Peeling of fruits and vegetables

Cost Cost

Cost

Cost, specialized products

Must be neutralized Taste

Noncompatability of foods; protein denaturation

Must neutralize to normal pH for consumption

Color, flavor, nutrient loss Surface treatment only, must neutralize to normal pH for consumation

"To convert kPa to mmHg, multiply by 7.5; to convert MPa to atm, divide by 0.101.

drawbacks, for example, heat-induced losses of desirable food nutrient, structures, colors, and flavors.

The heat treatment needed to inactivate microbes or enzymes can be calculated from the energy for activation and the known rate of inactivation at a given temperature, usually 121°C. These values depend on the food system and its pH, water activity, and chemical profile. Typical activation energies and inactivation rates at 121°C for heat-resistant microbes are 209-335 kJ/mol (50-80 kcal/mol) and 0.1 to 10 decimal reductions/min. Commercial heat preservation operations assume a starting concentration of C. botulinum spores (mixed varieties) of 10 to 12/g and an inactivation rate of 5 to 10 decimal reductions/min at 121°C.

Preservation heating times at any temperature can be determined by integrating the lethal temperature-time effects at the slowest heating point in the package. Thus if the geometry of the container and the thermophysical properties of the food are known, it is possible to calculate the heating time necessary to ensure a safe level of microbes or residual enzyme activity in the food and the residual concentration of desired nutrients. In general, higher life forms (eg, insect eggs, mites, etc) are killed by even mild heat treatments. Computer programs are available for determining safe heat-preservation operations when given information about the product, including its initial temperature, container characteristics, fill, and heat-exchange system (pure steam, steam-air, water, etc).

The hydrogen ion is extremely toxic to most microbes. Foods having a pH below 4.5 can be commercially sterilized by heating to 100°C with a limited holding period.

Aseptic preservation involves performing separate heat-preservation operations on the food and the containers prior to assembly. Liquid products can be heated and cooled rapidly under optimum conditions of heat transfer in specialized heat exchangers. The sterile product is packaged under sterile filling conditions into sterile unit retail packages, drums, or bulk-storage tanks.

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