Novel Applications of Sometimes-Novel Processing Technologies
By Huub Lelieveld
For several decades, there has been an enormous amount of R&D, mainly research, on promising “novel” technologies. Most of these technologies had been designed to preserve food while retaining more of its desired properties, such as nutritional content (e.g., vitamins, antioxidants) and flavor. These properties are significantly affected by heat (thermal preservation treatments, such as pasteurization and sterilization). The search, therefore, was for nonthermal processing technologies that would inactivate microbes with less energy than that needed for their thermal inactivation. The methods receiving most of the attention are high-pressure and pulsed electric field processing (HPP and PEF, respectively). Both technologies can be very effective on a small (laboratory and pilot plant) scale, but have inherent limitations for applications on the large scale often needed in the industry.
The limitation of HPP is the maximum pressure that can be attained. The forces on the walls of the machinery are proportional to their surface areas, with the consequence that pressures that can be fairly easily reached on a laboratory scale may be impossible on a production scale. Therefore, the size of industrial machinery for HPP is limited; thus, its applications are also limited. Because the inactivation of vegetative microorganisms can be sufficiently achieved by lower pressures, there are more possibilities to use HPP for products that would otherwise be pasteurized. HPP-“pasteurized” products have been on the market in Japan, the U.S. and Europe for several decades. For sterilization, the pressures required would be so high that large-scale application becomes impossible. This is true if the aim is “nonthermal;” if the aim becomes “better than thermal,” then a combination of pressure with heat comes into the picture, and products can be sterilized with a lower total heat input than would be needed otherwise, and the resulting product may be of significantly better quality. This process is known as “pressure-assisted thermal sterilization” (PATS). PATS-preserved products are probably not yet on the market for cost reasons, but there may be niche products that could justify the required investment. For treatment of pumpable foods, large-scale applications may develop by using continuous high-pressure processes. However, although semicontinuous, high-pressure process equipment is available and fully continuous HPP of pumpable products has been described, there seem to be no industrial applications of continuous HPP. The fully continuous process is based on the property of liquids becoming more viscous with increasing pressure. At 1,200 MPa, the viscosity of ethanol is 10 times higher than at atmospheric pressure (0.1 MPa). The viscosity of eugenol, an extract of olive oil, is 10,000,000 times higher at 1,200 MPa than at atmospheric pressure.
Pulsed Electric Field
In the case of PEF, its limitation is the strength of the electric field required to destroy the microbial cell membrane, which is on the order of 1 V per micron. In the laboratory, when working with electrode distances of about 1 mm, meaning that a voltage of 1 kV is required, this is no problem. Even working with a distance of 10 mm in a pilot plant, a 10-kV requirement is doable. For practical applications, higher voltages are needed or special electrode arrangements must be devised, making the technology more complex. Nevertheless, in principle, the limitations are more in terms of the cost of the electronics than of the technique. High-voltage components are expensive, although they are becoming less so with time. The major limitation for the application of PEF is the fact that it destroys cell membranes of vegetative cells but not microbial spores; hence, the process may be used to replace pasteurization but not sterilization. Also with PEF, the addition of heat enhances the effect, but not to the extent that spores are affected. PEF is particularly suited for fruit juices, because their acidity does not allow microbial spores to grow and only vegetative microbes must be inactivated. It is relatively impossible to distinguish between fresh and PEF-treated fruit juices. Today, PEF-treated fruit juices with a shelf life of 3 weeks are available in many European countries.
Microwave, Ohmic and Radiofrequency Heating
The problem with the use of heat to ensure the correct treatment of the entire product is heat transfer, which causes part of the product to overheat. Although hardly a novel approach, techniques that generate heat inside the product are slowly finding applications. These techniques include microwave, ohmic and radiofrequency heating. Although these methods can heat very fast, cooling down remains dependent on heat transfer and quality gain is limited.
Light and Cold Plasma
Other technologies aimed at preservation of food that have received significant attention are based on the use of light or plasma. Pulsed light is claimed to be very effective, but reports have been contradictory, with some claims that it works effectively and others that it is no more effective than the same dose of UV light in the same total time, pulsed or not. Nevertheless, some applications are slowly emerging that may prove useful in making some products safer more efficiently than with UV. For the food industry, a more novel technology is photosensitization, which has been extensively used for a long time in medical applications to destroy cancer cells (Figure 1). There is no reason why the same technology cannot be used to destroy microbial cells. Indeed, in the past decade, much research has been done to attach edible photoactive compounds, such as chlorin e6, a degradation product of chlorophyll that can be made from, for example, live Chlorella, to microbial cells in food and then expose the food to normal, visible light. The compounds selectively absorb the light and, in the presence of oxygen, cause locally cytotoxic reactions and thereby selectively harm the microbial cell, leaving the product unaffected. Similar techniques, called photocatalyzation (using titanium oxide) and photoactivation (using zinc oxide) may also become industrially relevant. The oxides may be attached to or incorporated into, for example, packing materials for food contact applications. A limitation to the use of light is the shadow effect, such that any microbe in the shadow is not harmed, and technologies therefore must ensure that all product or material to be freed of microbes be exposed to the light, UV or otherwise. Cold plasma, the fourth state of matter (in addition to gas, liquids and solids), also destroys microbes, including microbial spores. Although plasma is visible due to the presence of photons, it is not a light treatment. Plasma contains a variety of charged particles. In the presence of oxygen, plasma produces ozone that in turn oxidizes components of microbial cells, including spores, viruses and parasites. The advantage of plasma in comparison to light is that it does not suffer from shadow effects. Plasma goes everywhere and, if properly applied, reaches all surfaces. Cold plasma is easy to produce and may find useful applications to destroy microbes on surfaces of food and packaging materials.
Light-Emitting Diodes in Horticulture
Not all novel technologies are aimed at preservation. The development of LEDs (light-emitting diodes) has led to the growing of food on a large scale in relatively small spaces, using several layers of crops under specific wavelengths of light. The technology allows the growth of fruits and vegetables day and night, summer and winter, with low-energy usage. The technology makes it possible to grow fruits and vegetables in winter—and less necessary to fly them in from the other, warmer side of the globe.
Another application of a not-so-novel-anymore technology is cooking with PEF. Relatively mild PEF treatments had been shown to separate cells in animal and plant tissues. Because PEF also perforates cell membranes, in 2008, the author had the idea that PEF should also be suitable to make tough meat tender. This was followed up initially by small-scale (~1 g) but successful experiments by Van Oord and Lelieveld that showed that in a few seconds—with an actual treatment time of just a few milliseconds—stew meat became tender. Similarly, potatoes were ready to eat in a short time. Van Oord decided to patent the application and to scale up the equipment, resulting in several prototypes that were made available to a few restaurants, among which was a Michelin three-star restaurant. The owner and chef of the latter declared that the meat prepared with the PEF equipment was better than could be achieved in a traditional way. This resulted in several innovation awards in the past few years, and ultimately in commercial equipment. Investigations by Mastwijk showed that for the cooking of potatoes, the total amount of energy needed is less than 20 percent of what is needed for traditional cooking. Because tenderness is achieved in milliseconds, stew meat requires less than about 5 percent of the energy, mainly to heat the meat to serving temperature.
Attempts to make good quality meat replacers have not been very successful because of the difficulty in creating fibers from the proteins that can be used to make a product with a bite similar to that of real meat. One method that has been tested is electrospinning, a technique that allows the production of very long fibrils a few hundred nanometers in diameter. Spinning can be successful if the proteins are mixed with gelatin. In the laboratory, successful electrospinning was possible with milk and soy proteins mixed with gelatin. The spun proteins seem quite suitable as the basis of meat-like products and may help reduce the demand for meat, because rapid increase in the demand for meat will soon make it impossible to produce enough meat by animal farming.
Printing of Food Products
Three-dimensional printing is now also done with food, and the costs of such printers are decreasing rapidly. Hence, it is likely that food printing will increasingly be used. Initially it will be used for niche applications, but because the process can be fast, it may well be that products will be in the shops within just 1 or 2 years. Birthday cakes and cookies will be printed in the shop while you wait. An interesting possibility offered by food printing is that single items can be products meeting specific demands, such as for hospital patients needing specific food, which can be easily mixed with medicines. In the Netherlands, food printing can be seen at work, and more websites are showing that this technology is rapidly becoming popular.
Although research on using ultrasound for processing started more than half a century ago, industrial applications still are few, but research shows that the technology has potential in the food industry. Ultrasound can have any frequency above 20 kHz, the maximum frequency that humans can hear. Ultrasound’s effect is caused by the absorption of sonic energy. In a homogeneous liquid, all energy will be absorbed and may eventually heat the liquid. If the liquid is not homogeneous, most energy is absorbed by parts with a resonance frequency close to the frequency of the ultrasound. If the ultrasound frequency matches the resonance frequency of a material, the energy absorption is maximal. The energy absorbed may have various effects, such as enhancing chemical reactions. Ultrasound may also change the properties of materials locally, whereby the resonance frequency changes and eventually matches the ultrasound frequency. The effect can be impressive, for example, causing cavitation, when vapor bubbles are formed that continue to absorb energy until the bubbles explode, suddenly transferring all of the accumulated energy to the immediate environment and causing a shock wave that may exert a mechanical effect that may change the properties of the liquid or affect solid materials in the environment, like soil on a surface. The first practical applications indeed are in cleaning, in particular to remove tenacious soil such as biofilms. In the past decade or so, however, much research has been done with food products that showed that ultrasound may potentially be applied to food processing, such as emulsification, drying, extraction, homogenization and inactivation of microbes.
Most of the food processing technologies discussed above are not novel, but many have innovative applications that may enhance product quality and safety in the years to come. With the disadvantages of thermal technologies, there is an imperative to develop applications that can both ensure microbial inactivation as well as be amenable to large-scale industrial processes, increasing their effectiveness and utility for the food industry.
Huub Lelieveld is president of the Global Harmonization Initiative, a member of the Executive Committee and past president of the European Federation of Food Science and Technology. He is a member of the Editorial Advisory Board of Food Safety Magazine.
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Categories: Process Control: Packaging, Processing Technologies