Food Biochemistry and Food Processing (2 edition)

BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come

38 Thermal Processing Principles 743

Novel Thermal Food Processing Technologies

Microwave and Radio Frequency Heating

Microwave (MW; 300–300,000 MHz) and Radio Frequency

(RF) waves (0.003–300 MHz) are a part of the electromag-

netic spectrum. MW and RF energy generates heat in dielectric

materials such as foods through dipole rotation and/or ionic po-

larization. MW ovens are now common household appliances.

Popular industrial applications of MW heating in food process-

ing operations include tempering meat or fish blocks and pre-

cooking bacon or meat patties, while RF heating is commonly

used in finishing drying of freshly baked products. Such applica-

tions shorten processing times, reduce floor space, and improve

product qualities compared to conventional methods.

Extensive research has been carried out over the past 50 years

on MW and RF energy in pasteurization, sterilization, drying,

rapid extraction, enhanced reaction kinetics, selective heating,

disinfestations, etc., but with limited applications. Technologi-

cal challenges remain and further research is needed for those

applications. MW/RF sterilization applications demand more

thorough and systematic studies as compared to other applica-

tions. These studies will have far reaching impacts to the food

industry and research communities.

Several commercial 2450 MHz MW sterilization systems pro-

duce shelf-stable packaged foods in Europe (e.g., Tops Foods,

Olen, Belgium) and Japan (Otsuka Chemical Co., Osaka), but

these systems are designed with multi-mode MW cavities. Gen-

erally, MW/RF heating is a promising alternative to conventional

methods of heat processing as it is regarded as a volumetric form

of heating in which heat is generated within the product, which

reduces cooking times and could potentially lead to a more uni-

form heating. Reduction in processing time and uniform heating

results in getting high-quality product in terms of its nutrient

content, desired flavor, texture, color, and taste.

Ohmic Heating

Ohmic heating (OH) is based on the passage of alternating elec-

trical current through a food product that serves as an electrical

resistance. Because of the current passing through the food sam-

ple and its resistance to the flowing current, relatively rapid

heating occurs. OH has good energy efficiency since almost all

of the electrical power supplied is transformed into heat. Many

factors affect the heating rate of foods undergoing OH: electri-

cal conductivities of fluid and particles, the product formulation,

specific heat, particle size, shape, and concentration as well as

particle orientation in the electric field.

As compared to conventional heating technologies, internal

heat generation by OH eliminates the problems associated with

the heat conduction in food materials and then prevents the

problems associated with overcooking. Aseptic processing has

been used commercially for long time for liquid foods, but for

products containing particulates, the use of conventional heat-

transfer techniques leads to overprocessing of the liquid phase to

ensure that the center of a particulate is sterilized. This can result

in destruction of flavors and nutrients, and mechanical damage to

the particulate. However, OH-treated product is clearly superior

in quality than those processed by conventional technologies.

This is mainly due to its ability to heat materials rapidly and

uniformly, leading to a less-aggressive thermal treatment. Hence,

OH can be considered as a HTST aseptic process (Castro et al.

2004).

Novel Nonthermal Processing Technologies

High-Pressure Processing

High-pressure Processing (HPP) is an innovative technological

concept that has a great potential for extending the shelf life

of foods with minimal or no heat treatment. It is a process

aimed at controlling deteriorative changes such as microbial

and enzymatic activity without subjecting the product to drastic

thermal processing and mass (drying) transfer techniques such

that the original quality is retained.

The application of hydrostatic pressure to food results in the

instantaneous and uniform transmission of the pressure through-

out the product independent of the product volume. The treat-

ment is unique in that the effects neither follow a concentration

gradient nor change as a function of time. A significant advan-

tage is the possibility of operation at low or ambient temperatures

so that the food is essentially raw. Gelation, gelatinization, and

texture modification can be achieved without the application of

heat. Apparently, HPP is a physical treatment and is not expected

to cause extensive chemical changes in food system. Once the de-

sired pressure is reached, the pressure can be maintained without

the need for further energy input. Liquid foods can be pumped

to treatment pressures, held, and then decompressed aseptically

for filling as with other aseptic processes.

HPP is a novel technique for processing of foods and has

attracted considerable attention in recent years. It has been com-

mercialized for a variety of acid and acidified food products. For

low-acid foods, it has been used only as a temporary measure

of extending the shelf life under refrigerated storage conditions.

It has also been used for several other purposes, including con-

trol of some pathogens and viruses, for inducting functional

changes, as well as improving nutritional and sensory quality of

foods. HPP has been used mainly for refrigerated and high-acid

foods. Pasteurization by HPP can be carried out at pressures

in the range of 400–600 MPa at relatively moderate tempera-

tures (20–50◦C). Under these conditions, HPP can be effective

in inactivating most vegetative pathogens and spoilage microor-

ganisms, but the quality factors remain unaffected.

HPP for producing shelf-stable low-acid foods is still a topic

of considerable controversy. It has the potential to produce better

quality foods than possible from the use of processing novelties

such as MW, RF, or OH techniques in combination with aseptic

processing. This is because HPP allows the product temperature

to increase very rapidly (due to adiabatic heating) from around

90–100◦C to the sterilization zone (120–130◦C; in about 2 min-

utes) and bring it back almost instantaneously by depressuriza-

tion. The process can be formulated either as a pressure-assisted

thermal processing. A better understanding of the inactivation

kinetics of pathogens or their surrogates under pressure-assisted