Future trends

It is clear that the conventional thermal methods can lead to desirable destruction of microbial pathogens and spoilage organisms and endogenous, desirable changes such as protein coagulation, starch swelling, textural softening and formation of aroma components. However, undesirable changes also occur, such as loss of vitamins and minerals, formation of thermal reaction components of biopolymers, and in minimal processing terms, loss of fresh appearance, flavor and texture. The classical approach to overcome or at least minimize these undesirable quality changes in thermal processing is the HTST (high temperature short time) or the UHT (ultra high temperature) concept. These are based on the fact that normally the inactivation of microorganisms has more temperature sensitivity than that of quality factors. High temperature will give rapid inactivation of microorganisms and enzymes, which is aimed for in pasteurization or sterilization, and short times will give less undesired quality changes. Unfortunately, the HTST and UHT concepts are severely limited for solid foods. This is because parts of the food in contact with the hot surfaces will be overheated during the time needed for the heat to transfer to the interior or coldest spot of the food. The surface overheating will give quality losses that in severe cases will counterbalance the advantages of the HTST or UHT concept. Thus, as alternative methods, novel thermal processing techniques, such as ohmic heating, high frequency heating and microwave heating, and non-thermal processing techniques, such as high hydrostatic pressure processing and pulsed electricity method, have been receiving more and more interest from both food scientists and industries.

10.7.1 Novel thermal processing techniques

Ohmic heating

Ohmic heating, also called electric resistance heating, is a direct heating method in which the food itself is a conductor of electricity, taken from the mains that are 50 Hz in Europe and 60 Hz in the USA. The food may also be immersed in a conducting liquid, normally a weak salt solution of similar conductivity to the food. Heating is accomplished according to Ohm's law, where the conductivity, or the inverse, the resistivity, of the food will determine the current that will flow between the ground and the electrode. Normally, high voltages up to 5000 V are applied. The conductivity of foods increases considerably with increasing temperature. To reach high temperatures it is therefore necessary to increase the voltage current or to use longer distances between the electrodes and ground. The best known electric resistance heating system is the APV ohmic heating column, where electrodes immersed into the food are transported in a vertical concentric tube. The ohmic system of APV has been installed for pasteurization and sterilization of a number of food products with excellent resulting quality. The majority of these installations are found in Japan for the production of fruit products (Tempest, 1996). Other industrial cooking operations for electric resistance heating involve rapid cooking of potatoes and vegetables for blanching in the industry and for preparing foods in institutional kitchens. One of the major problems with these applications is ensuring that the electrode materials are inert and do not release metal ions into the conducting solutions and eventually into foods.

High frequency heating

High frequency heating is done in the megahertz region of the electromagnetic spectrum. Frequencies of 13.56 and 27.12MHz are set aside for industrial heating applications. Foods are heated by transmitting electromagnetic energy through the food placed between an electrode and the ground. The high frequency energy used will allow transfer of energy over air gaps and through non-conducting packaging materials. To achieve sufficiently rapid heating in foods, high electric field intensities are needed.

High frequency heating is accomplished by a combination of dipole heating, when the water dipole tries to align itself with the alternating electric field, and electric resistance heating from the movement of the dissolved ions of the foods. In the lower temperature range, including temperatures below the freezing point of foods, dielectric heating is important, whereas for elevated temperatures, electric conductivity heating dominates. The conductivity losses or the dielectric loss factor increases with increasing temperature, which may lead to problems of runaway heating when already hot parts of the food will absorb a majority of the supplied energy. The dielectric properties of foods are reasonably abundant in the low temperature range, but few data are available in temperatures above normal room temperature.

The largest application in the food industry for high frequency heating is in the finish drying or post-baking of biscuits and other cereal products. Another application is in drying products such as expanded cereals and potato strips. Previously, defrosting of frozen food using high frequency was a major application, but problems of uniformity with foods of mixed composition limited the actual use. The interest in high frequency defrosting has increased again in the last few years.

Microwave heating

Microwaves used in the food industry for heating are of the ISM (industrial, scientific and medical) frequencies 2450 MHz or 900 MHz, corresponding to 12 or 34 cm in wavelength. In this frequency range, the dielectric heating mechanism dominates up to moderate temperatures. Polar molecules, dominated by water, try to align themselves to the rapidly changing direction of the electric field. This alignment requires energy that is taken from the electric field. When the field changes direction, the molecule 'relaxes' and the energy previously absorbed is dissipated to the surroundings, that is, directly inside the food. This means that the water content of the food is an important factor for the microwave heating performance of foods. The penetration ability of the microwaves in foods is limited. For normal 'wet' foods the penetration depth from one side is about 12 cm at 2450 MHz. At higher temperatures, the electric resistance heating from the dissolved ions will also play a role in the heating mechanisms, normally further reducing the penetration depth of the microwave energy. The limited penetration depth of microwaves implies that the distribution of energy within the food can vary. The control of the heating uniformity of microwave heating is difficult, as the objects to be heated are of the same size as the wavelength in the material. Difficulties in controlling heating uniformity must be seen as the major limitation for industrial application of microwave heating. Thus, an important requirement of microwave equipment and microwave energy application in the food industry is the ability to control the heating uniformity properly (Ohlsson, 1983).

Industrial applications of microwave heating are found for most heat treatment operations in the food processing industries. For many years the largest application has been defrosting or thawing of frozen foods, such as blocks of meat, prior to further processing. Often meat is only partially defrosted (tempered) before it can be further processed. Another large application area is for pasteurization, and now also sterilization, of packaged foods. Primarily ready-made foods are processed. The objective of these operations is to pasteurize the food to temperatures in the range of 75-80°C, in order to prolong the shelf-life to about 3 to 4 weeks. Sterilization using microwaves has been investigated for many years, but commercial introduction has only come in the last few years in Europe and Japan. Microwave pasteurization and sterilization promise to give very quick heat processing, which should lead to small quality changes caused by thermal treatment, according to the HTST principle. However, very high requirements of heating uniformity must be met in order to fulfill these quality advantages (Ohlsson, 1991).

Pasteurization by microwave heating can also be done for pumpable foods. Microwaves are directed to the tube where the food is transported and heating is accomplished directly across the tube cross section. Again, uniformity of heating must be ensured, requiring selection of the correct dimensions of the tube diameter and the proper design of the applicators (Ohlsson, 1990). The destruction kinetics of some microorganisms such as Saccharomyces cerevisiae, Lactobacillus plantarum and Escherichia coli, as well as inactivation of enzymes under continuous microwave heating, have been reported (Tajchakavit and Ramaswamy, 1997; Tajchakavit et al., 1999; Koutchma and Ramaswamy, 2000).

Further application of microwave heating is for drying in combination with conventional hot-air drying. Often microwaves are primarily used for moving water from the wet interior of solid food pieces to the surface, relying on the preferential heating of water by microwaves. Applications can be found for pasta, vegetables and various cereal products, where puffing by rapid expansion of the interior of the food matrix can also be accomplished using microwave energy (Tempest, 1996).

10.7.2 Non-thermal processing techniques

New non-thermal processes, such as pulsed electric field (PEF) and high pressure (HP) preservation, have been applied to a variety of prototype food products. These processes are best categorized as pasteurization processes because they are not completely effective in reducing the activity of bacterial spores. Treated and properly packaged foods may have extended refrigerated shelf-life or may be shelf-stable if natural or added acids are present to control spore outgrowth.

High pressure processing (HPP) is gaining in popularity within the food industry because of its capacity to inactivate pathogenic microorganisms with minimal heat treatment, resulting in the almost complete retention of nutritional and sensory characteristics of fresh food without sacrificing shelf-life. Other advantages of HPP over traditional thermal processing include: reduced process times, minimal heat damage problems, retention of freshness, flavor, texture and color, no vitamin C loss, no undesirable changes in food during pressure-shift freezing caused by reduced crystal size and multiple ice phase forms and minimal undesirable functionality alterations.

Changes that may be made improve functional properties of food constituents resulting in value-added products. Minimization of damage during pressure-shift freezing and thawing using HPP, non-thermally induced enzyme inactivation and desirable changes in starch-gelatinization properties are some other examples of potential benefits of HPP. However, spore inactivation is a major challenge for HPP. Methods used to achieve full inactivation of spores using HPP are yet to be developed. In thermal processing, D (time required in minutes to reduce the microbial population 10-fold), z (temperature in °C yielding a 10-fold change in D) and F0 (the integrated lethal value from all heat received by a treated food with a reference temperature of 121.1°C, assuming a z-value of 10°C) values are standard processing parameters; however, there is a need to develop and standardize HPP process parameters with respect to microbial inactivation, because none exists. This is essential for the commercial success of this technology.

Use of pulsed electric fields (PEF) for inactivation of microorganisms is another promising non-thermal processing method. Inactivation of microorganisms exposed to high-voltage PEF is related to the electromechanical instability of the cell membrane. Electric field strength and treatment time are the two most important factors involved in PEF processing. Encouraging results have been reported at the laboratory level, but scaling up to the industrial level escalates the cost of the command charging power supply and of the high-speed electrical switch. A successful continuous PEF processing system for industrial applications has yet to be designed. The high initial cost of setting up the PEF processing system is the major obstacle confronting those who would encourage the system's industrial application. Innovative developments in high-voltage pulse technology will reduce the cost of pulse generation and will make PEF processing competitive with thermal processing methods (Jeyamkondan et al., 1999).

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