Factors related to the medical professional: lack of time in medical consultations, poor doctor-patient communication, uninformative explanation

In the quest for better quality shelf stable, low-acid foods, a number of emerging technologies have been considered (Food and Drug Administration 2000).Food engineering will continue to evolve, but more and more food engineering research will be shifted to nontraditional processing and nonthermal processing, such as microwave and radio frequency processing, ohmic heating, high-pressure process-ing, pulsed electric fields, and so on. Ohmic heating had a lot of potential in 1989–91 and went through some testing, but except for a liquid egg processor, nobody is using it for particulates (Mermelstein 2001). Pulsed electric fields (PEFs) are considered a form of pasteurization, suitable for high-acid foods such as fruit juices (Clark 2002a). High-pressure processing extends shelf life, but the product still re-quires refrigeration, since the pressure does not in-active spores (Clark 2002b). To achieve shelf stable foods, high-pressure processing must be combined with a mild heat treatment. Although alternative processes have been developed over the years, ther-mally processed food products maintain a clear

4 Microwave and Radio Frequency Applications 89

dominance in the marketplace, primarily as a result of the wealth of theoretical and empirical knowledge that has been developed regarding thermal inactiva-tion of pathogenic microorganisms and their spores (Mermelstein 2001). Microwave sterilization is a nontraditional, but solely thermal, process and so can be regarded by technologists and regulators as another terminal thermal sterilization technique.

Microwave heating offers numerous advantages in productivity over conventional heating methods such as hot air, steam, and so on. These advantages include high speed, selective energy absorption, ex-cellent energy penetration, instantaneous electronic control, high efficiency and speed, and environmen-tally clean processing (Cober Electronics, Inc.

2003). Unfortunately, although for the last 35 years expectations have been high that radio frequency and microwave processing of foods might find a niche in the industry, there has been only modest growth in sales of microwave processing equipment over this period. Currently, both microwave and radio frequency are laboratory or pilot scale, and there are no known large microwave systems operat-ing in the food industry, except for bacon precook-ing or temperprecook-ing (Schiffmann 2001). But it remains a very exciting processing tool, unmatched by any other technology if attention is paid to its selection.

The following sections examine a number of mi-crowave food processes that are interesting from an academic point of view.

DRYING ANDDEHYDRATION

Microwave drying is more rapid, more uniform, and more energy efficient than conventional hot air dry-ing, and sometimes it results in improved product quality. But it is highly unlikely that an economic advantage will be demonstrated if only bulk water removal by microwave heating, such as occurs in the constant-rate region is desired (Buffler 1993). Dur-ing the fallDur-ing-rate period, because of low thermal conductivity and the evaporative cooling effect, high product temperatures are not easily obtained using convective drying. Surface hardening and thermal gradients again provide further resistance to mois-ture transfer. Actually, it has been suggested that mi-crowave energy should be applied in the falling-rate period or at low moisture content for finish drying (Funebo and Ohlsson 1998, Kostaropoulos and Saravacos 1995, Maskan 1999, Prabhanjan et al.

1995). Correspondingly, sensory and nutritional damage caused by long drying times or high surface

temperatures can be prevented. It is important to un-derstand the dielectric properties of the material as a function of moisture content during microwave dry-ing. The ability of dielectric heating to selectively heat areas with higher dielectric loss factors and the potential for automatic moisture leveling afford a major advantage even for drying of these types of materials (Buffler 1993).

Because internal microwave heating facilitates a more predominant vapor migration from the interior of the material than occurs during conventional dry-ing, microwave dried products have been reported to show a higher porosity because of the puffing effect caused by internal vapor generation (Fu 1996, Tong et al. 1990, Torringa et al. 1996). Similar results are also found for pasta drying (Buffler 1993). Micro-wave drying produces a slightly puffed, porous dle that rehydrates in half the time required for noo-dles dried by conventional methods (MicroGas Corporation 2003). Using miniature fiber-optic tem-perature and pressure probes, Tong and others (1990) investigated temperature and pressure distribution during microwave heating in a dough system with porosity ranging from 0.01 to 0.7. Pressure build-up to approximately 14 kPa occurred during the initial stages of the heating process when the initial poros-ity was less than 0.15 and disappeared when the pres-sure exceeded the rupture strength of the dough.

Volume expansion was observed up to the point where the dough sample ruptured, producing visible cracks in the structure. So microwaves produce a pressure gradient that pumps out the moisture. This property can be used to advantage to speed up the drying process. The results might be positive or neg-ative to the dried product. If the rupture strength of the sample is smaller than the pressure build-up, the solid matrix might be damaged, and visible cracks in the structure would be seen. In an experiment on mi-crowave finish drying of starch pearls, significant visible cracks developed on the outside at microwave powers greater than 200 W, which created unaccept-able product (Fu et al. 2003e). Alternatively, if the pressure build-up does not exceed the rupture strength of the structure, the result may be an en-hanced porous structure of the samples. So, it is a difficult task to reduce drying time and increase qual-ity at the same time. Careful studies need to be done to establish the correct amount of microwave energy to be used in the process.

Nonuniformities in the microwave electric field and associated heating patterns can lead to high tem-peratures in various previously dried regions,

caus-ing product degradation (Lu et al. 1999). To achieve improvement, fluidized bed dryers or spouted bed dryers can be used to average the uneven electric field (Feng and Tang 1998, Kudra 1989). The com-bination of microwave and vacuum drying (Boehm et al. 2002, Durance et al. 2001, Gunasekaran 1999, Langer 2000, Sunderland 1980, Whalen 1992) or freeze-drying (Barrett et al. 1997; Litvin et al. 1998;

Ma and Peltre 1975a,b; Wang and Shi 1999) also has potential. The vacuum process opens the cell struc-tures (puffing) due to fast evaporation, resulting in an open pore structure. Reduced drying time is the primary advantage of using microwaves in the freeze-drying process, but no commercial industrial application can be found, due to high costs and a small market for freeze-dried food products.

Pasta and potato chips have been dried success-fully. Freeze-drying and vacuum drying, in conjunc-tion with microwave energy, have also shown prom-ise, and although the process is interesting from an academic point of view, it does not meet economic criteria. A new technology from Battelle Ingenieur-technik GmbH of Germany for drying fruits and vegetables has been developed, wherein air belt dry-ing is followed by microwave-vacuum puffdry-ing, then further air belt drying or vacuum drying, before sort-ing and packagsort-ing. Effects of this procedure on physicochemical properties, sensory properties, and the ultrastructure of fruits and vegetables are consid-ered together with the avoidance of microwave hot spots and other products that would be suitable for processing by this method (Langer 2000, Räuber 1998). Recently, a relatively new and successful combination of microwave energy and frying is used to produce fried goods, such as chips, noodles, and chickens, with 60% reduced time, 50% reduced fat content and 33–60% energy saving (FIRDI 2003).

PASTEURIZATION ANDSTERILIZATION Pasteurization inactivates pathogenic vegetative cells of bacteria, yeast, or molds. Pasteurized prod-ucts generally have to be refrigerated. Sterilization processes are designed to inactivate microorganisms or their spores. Thermal sterilization is usually done at temperatures in excess of 100°C, which means they are usually done under pressure. Industrial mi-crowave pasteurization and sterilization systems have been reported on and off for over 30 years.

Studies with implications for commercial pasteur-ization and sterilpasteur-ization have also appeared for many years (Burfoot et al. 1988,1996; Cassanovas et al.

1994; Hamid et al. 1969; Knutson et al. 1988; Kudra et al. 1991; Proctor and Goldblith 1951; Villamiel et al. 1997; Zhang et al. 2001). Early operational sys-tems include batch processing of yogurt in cups (Anonymous 1980) and continuous processing of milk (Sale 1976). A very significant body of knowl-edge has been developed related to these processes.

As of this writing, two commercial systems world-wide can be found that currently perform microwave pasteurization and/or sterilization of foods (Akiy-ama 2000, Tops 2000). As a specific example, Tops Foods (Belgium) (Tops 2000) produced over 13 mil-lion ready-to-eat meals in 1998 and installed a newly designed system in 1999. Although continu-ous microwave heating in a tube flow arrangement has been studied at the research level, no commer-cial system is known to exist for food processing.

Microwave pasteurization can reduce the come-up time, which can be shortened to a small fraction of the time used in the conventional process. After pasteurization, the microwave-heated meals pass into a nonmicrowave hot air tunnel for the hold time period, and then to the cooler. With microwaves it is difficult to hold a constant temperature, and they should not be used at this stage. Especially in Eur-ope, food pasteurization by microwave processing has been successfully accomplished for decades.

The major advantage of the microwave process is that the product may be pasteurized within a pack-age. A wrapped product goes through the line con-tinually, package by package, pallet by pallet. Shelf life can be extended from days to over a month with-out preservatives. For example, due to higher mois-ture content, the usability of untreated toast bread is quite short—approximately six days. Distinctive pasteurization effects can be achieved by fast micro-wave heating (< 35 seconds) and a 15-minute pause at a temperature higher than 50°C (ROMill^®). The condition of durability can be optimally fulfilled, even from the microbiological point of view, at an output temperature of 77°C after only 20 seconds of exposure from the initial temperature of 22°C, which is considerably faster than any other method of heating. If slow cooling follows, the tests of shelf life show a usability time of longer than 45 days.

For commercial sterilization, temperatures in the product may be 121–129°C (250–265°F), with hold times of 20–40 minutes. Come-up time may be sig-nificantly reduced by use of microwaves, and re-duced come-up time would provide greater product quality since quality attributes normally have an ac-tivation energy much lower (10–40 kcal/mol) than

4 Microwave and Radio Frequency Applications 91

that of microbial spores (50–95 kcal/mol). Micro-wave sterilization is more flexible than ohmic heat-ing and aseptic processheat-ing. Liquid, semisolid, and solid prepackaged food products can also be steril-ized. CAPPS and Industrial Microwave Systems manufacture a flow-through cylindrical microwave reactor to eliminate the heat-up time of thermal processing. In the cylindrical reactor, microwaves are focused to provide uniform exposure of product to energy within the reactor cavity. The uniform en-ergy exposure region of the reactor is approxi-mately 1.5 inches in diameter and 6 inches long.

This reactor also allows for integration with exist-ing continuous processexist-ing lines (Mermelstein 2001). In Europe, microwave-sterilized foods, pri-marily pasta dishes such as lasagna and ravioli, are on many grocery shelves, with no reported difficul-ties. Safety regulations are less stringent in Europe.

For example, in one implementation (Tops 2000) the process design consists of microwave tunnels with several launchers for each different type of product (ready meals). Microwave-transparent and heat-resistant trays with shapes adapted for mi-crowave heating are used. Exact positioning of the package is made within the tunnel, and the package receives a precalculated, spatially varying mi-crowave power profile optimized for that package.

The process consists of heating, holding, and cool-ing in pressurized tunnels. The entire operation is highly automated.

Use of microwaves for food sterilization has not been approved by the Food and Drug Administration in the United States. There are several practical con-cerns and problems that must be addressed before microwave sterilization can be applied at the indus-trial level. The main issue has been the regulation of process parameters so that commercial sterility can be achieved. For conventional retort processes, mon-itoring the time-temperature history at the cold point with a thermocouple thermometer is reasonably easy and accurate for determining microbial lethal-ity through mathematical calculations. But, deter-mining the microbial lethality for a microwave ster-ilization process is not straightforward. The cold point during microwave sterilization is not always located on the central axis. The difficulty of provid-ing a uniformly heated product makes it extremely time consuming and costly to adjust the microwave pattern to produce the quality advantage that is the-oretically possible with the use of microwaves. Each product could require custom adjustment. The pres-ence of uneven heating (hot and cold spots) makes it

very difficult to ensure that all portions of a meal have reached a kill temperature. Microbiological safety is the major reason for the slow acceptance of microwave sterilization. In addition, the technical ability to accurately measure the temperature distri-bution throughout an entire microwave-sterilized product has not been demonstrated. From the engi-neering point of view, no computer simulation mod-els are available for investigating the feasibility of microwave sterilization. These computer simulation models are not only required by the Food and Drug Administration for regulating and approving mi-crowave sterilization processes, but also are in high demand by the food industry for performing cost/benefit analyses. Without reliable inputs of di-electric properties, thermophysical properties, and boundary conditions, a computer model is com-pletely useless. Unfortunately, literature values on these properties are only available at room tempera-ture to 60°C and are not readily available for sterili-zation temperatures.

TEMPERING ANDTHAWING

Thawing and tempering of biological products used to be a slow process. For many production proc-esses, incoming raw material is frozen in thick blocks and stored at 23 to 10°C until ready to use. The first operation on this material usually is to dice, slice, or separate individual sections into smaller pieces. This mechanical operation requires that the blocks be “tempered” from their solid frozen state to a point just below freezing (7 to

1°C), at which point cutting or separation can be done without damage to the product. Thawing and tempering of frozen food materials is an important part of some food processes, especially in the meat industry and in food service. Reduced thawing time results in a decrease in product quality, such as more drip loss and surface drying, as well as increased risk of microbial growth.

Frozen foods can be considered a mixture con-taining two components: (1) a fixed structure of ice and biological material surrounded by a monomol-ecular layer of strongly bound water and (2) liquid water saturated with dissolved salts. The dielectric activity of this mixture is much higher than that of pure ice, but much lower than that of the same ma-terial at temperatures above 0°C. The loss factor (ε) of water is approximately 12, while that of ice is approximately 0.003. The penetration depth in water (1.4 cm) is much lower than in ice (1160 cm)

(von Hippel 1954).If the thickness value is much greater than the penetration depth, the temperature profile will be similar to that observed for a “semi-infinite” body. That is, the temperature will de-crease exponentially from the surface in accordance with Lambert’s law. Surface layers thus absorb more energy and heat up a little faster than the in-side of the product. But for thickness values smaller than a certain value, resonance cannot be avoided, and the inside of the slab can be heated directly at a high intensity, resulting in quick thawing. As the loss factor increases with the temperature, the sur-face heats up faster and faster, and the penetration depth continually decreases. Spots of free water and spots that have reached temperature > 0 °C absorb more energy than ice crystals, which leads to fur-ther acceleration of heating. Microwave energy penetrates a food material and produces heat inter-nally. The main advantage of microwave energy consists in speed: tempering by microwaves takes minutes instead of hours or even dozens of hours.

For example, a 20 cm thick piece of beef, frozen to

16°C, thaws in more than 10 hours at the sur-rounding temperature of +4 °C. On the other hand, the whole cycle of microwave tempering following slicing, modification, and repeated freezing takes only 30 minutes (ROMill^®2003). In another exam-ple, from Microdry Corporation, cartons of frozen food in solid blocks weighing up to 100 pounds are raised in temperature to just below freezing using conventional tempering (Microdry, Inc. 2003).

Most plants dunk the blocks into warm water.

Others use hot air. Many use floor tempering alone, without any heat aid, which may take 48–72 hours.

By contrast, microwave tempering is applied on a moving belt to food still in cartons and generally takes less than five minutes. Thus, without doubt, this is a major successful application of microwave heating in industry. There are at least 400 tempering systems operating in the United States alone. Food is heated to just under freezing temperatures, allow-ing easy choppallow-ing, cuttallow-ing, processallow-ing, and so on.

In the United Kingdom there are several large sys-tems, up to 200 kW, utilized for tempering frozen beef, as well as butter. The lower frequencies, for example, the 915 MHz band, are used to advantage for microwave thawing and tempering of larger blocks of food. As a general rule, microwave energy at 915 MHz has three times the penetration depth of 2450 MHz, thus allowing for greater bed depths and processing of larger product geometries. For exam-ple, when tempering 18 cm thick blocks at 915

MHz, the temperature gradient is half that of the gradient for 2450 MHz (ROMill^®2003). 915 MHz tempering systems, batch and continuous, are sold worldwide.

Although microwaves have been successfully ap-plied to tempering frozen products, microwave thawing remains a major problem. A main difficulty is formation of large temperature gradients (run-away heating) within the product. The preferential absorption of microwaves by liquid water over ice is a major cause for runaway heating. Maximum ho-mogeneity is achieved with temperatures slightly above zero. After that the nonhomogeneity rises again. Therefore, it is advantageous to reduce the thawing process to plain tempering, that is, to stop the heating at temperatures of 5 to 2°C. Another reason to prefer tempering is the progress of energy consumption as a function of temperature. With most biological materials and water, energy con-sumption starts to rise sharply at temperatures above

5°C; the less fat the product contains, the higher the microwave absorption. Since thawed material has a much higher dielectric loss, microwave pene-tration depth at the surface is significantly reduced, in effect developing a “shield.” Surface cooling helps reduce the gradient in a frozen food, thus en-abling the microwave power to remain on longer, further decreasing the thawing time. Temperature uniformity during microwave thawing can be im-proved when appropriate sample thickness,

5°C; the less fat the product contains, the higher the microwave absorption. Since thawed material has a much higher dielectric loss, microwave pene-tration depth at the surface is significantly reduced, in effect developing a “shield.” Surface cooling helps reduce the gradient in a frozen food, thus en-abling the microwave power to remain on longer, further decreasing the thawing time. Temperature uniformity during microwave thawing can be im-proved when appropriate sample thickness,