Nonthermal processing is a large umbrella covering several technologies that may be based on different principles but have common aims and attributes, including minimal processing, improved safety, and environmentally friendly applications. Many nonthermal processing technologies have emerged as sound possibilities to process the foods of the future. One of the most successful technologies at the industrial level is pulsed electric fields (PEF) processing, which is a method that uses short bursts, or pulses, of high voltage electric fields to achieve microbial inactivation or modification of food structures.
The possibility of accessing the cytoplasm membrane of cells through electroporation is the reason that PEF can be considered a key enabling technology with applications beyond food processing like medicine, biotechnology, waste management, controlled release, and pharmacy. In addition, the technology’s ability to inactivate microorganisms at lower temperatures than those used in thermal pasteurization is attractive to the food industry because it prevents the negative impact of heat on the sensorial and nutritional characteristics of foods. There is no question that PEF processing satisfies to a certain extent current consumer demand for minimally processed, wholesome, and nutritious foods.
PEF also can serve as an effective pre-treatment to enhance mass transfer of targeted substances required to cross cytoplasmic membranes of plant or animal tissue cells, like in juice extraction. In addition, PEF can be used in some food processing operations such as dehydration or infusion of compounds into the cells, bringing attractive results. Furthermore, electroporation may change the food structure integrity of animal or plant tissues resulting in enhanced efficiency in several food processes such as cutting and peeling.
PEF is gaining significant attention and recognition because it encompasses a broad range of preservation applications, including pasteurization of juices and other pumpable foods, intermediate processing of potato strips and chips and other vegetables, and wine making, to name a few. One common element among these applications is the generation of pores, either in the microbial cell membranes or in the tissue of solid and semisolid foods. The mechanism behind the formation of these pores is so-called electroporation, which is due to a voltage difference between two electrodes placed at a given distance. The ratio between these two magnitudes is known as electric field.
In the case of PEF, the electric field is pulsed with a given frequency, duration, and length. Depending on the application and considering the microbial cell size and the media (mainly, its electrical conductivity), these variables will be set in a given range for food processing applications. The minimum electric field for microbial inactivation is 10 kV/cm, whereas for the formation of pores in solids, less than 5kV/cm will be sufficient. The duration of the pulses is in the 1–100 µs range, although nanoseconds are under exploration. Total specific energy in commercial applications ranges from 10 to 100 kJ/kg. Today, commercially available PEF industrial-sized units can process foods and beverages from 1 to 100 ton/hr, depending on the application.
PEF offers several benefits to food and beverage companies. First, PEF is an excellent example of a “green technology.” It is a sound alternative to some conventional thermal treatments that can contribute to a food company’s sustainability goals. For example, the short duration of the overall process application enables the food processor to realize important energy savings. Another advantage is the technology’s ability to enhance the quality attributes of the treated foods with minimal processing. For example, the quality of some PEF processed foods are considered “like fresh” because they have similar sensorial characteristics of their unprocessed counterparts. It’s not surprising that these PEF processed foods are known as “fresh-like.” And, using PEF in conjunction with essential oils, bacteriocins like nisin, and natural antimicrobials–either in sequence or simultaneously– can produce synergistic effects that result in high quality foods at lower costs.
PEF technology has a considerable number of applications in food processing and related areas, including olive oil extraction; microalgae component extraction; drying enhancement in osmotic dehydration, freeze- and air drying; tenderizing and ageing of meat products; pigment extraction from plant matrices; and improved tomato peeling. Today, there are several industrial applications of PEF that are becoming very appealing for use– and in some cases, a must–in modern processing lines.
Potato strips and chips. One of the most successful current applications of PEF is in the potato processing industry to manufacture strips that eventually end up as French fries. The PEF-based process includes a stage prior to cutting in which the tubers are immersed in water while treated with pulsed electric fields. The new potato process includes the following stages: washing, peeling, PEF treatment, cutting, blanching, drying, and frying. The maximum throughput is in the range of 100 ton/hr.
The PEF treatment softens the potato tissue by opening pores, which reduces the resistance to cutting and facilitates oil absorption prior to frying. The process results in 10% less oil uptake and it has been reported that there is a 90% energy reduction overall when this new process is utilized. Additional advantages over traditional processing of potato chips and strips include the elimination of the preheating stage, which increases production yield by 1.5%. At the same time, better quality products are attained since PEF-treated strips are more flexible and longer with improved color, and product issues like shattering, twisting, non-shear cutting, and starch losses are minimized. PEF also significantly reduces the amount of water utilized in the entire process, as well as reduces potato blanching, drying, and frying times. There is no question that this novel approach to processing potatoes contributes greatly to sustainability by significantly reducing the carbon footprint of the whole operation.
Wine. The wine making industry is another food sector enjoying the benefits of PEF application, including significant energy savings, reduction of CO2 emissions, and reduced water consumption, resulting in a more sustainable approach to obtaining high quality wines in comparison to traditional methods. This application is based on the PEF treatment’s ability to permeabilize the grape skins and microbial cells of spoilage microorganisms (See Figure 1). The extraction of polyphenol compounds from the skins is faster and it is possible to reduce the maceration time in red grapes by half. In addition, PEF treatment has a positive impact on some quality attributes such as color, which are enhanced without compromising sensorial attributes. Permeabilization of the grape skins by PEF has been approved by the International Organization of Wine as a valid method in wine making.
Inactivation of spoilage microorganisms might result in eliminating or reducing the usage of sulfites for wine stabilization without altering sensorial attributes or physicochemical characteristics, maintaining, or improving the overall quality of the wine. Several studies covering the inactivation by PEF of spoilage microorganisms in wine manufacturing have shown that this technology holds promise as a replacement for sulfites. Some of the microorganisms under consideration have been yeast from alcoholic fermentation, lactic acid bacteria, acetic acid bacteria, and yeast from the genus Brettanomyces. In particular, Brettanomyces/Dekkera was studied because it is present in the aging barrels and its inactivation is important to avoid wine spoilage at this stage of the process.
It has been reported by several authors that aging in bottles or oak barrels for wines from different grapes manufactured either by conventional technologies or PEF are very similar, adding to the credibility of this novel approach. In addition, PEF treatment significantly reduces aging time for wine lees, the leftover yeast particles from autolysis used to add beneficial textures and flavors to white or sparkling wines.
Pumpable foods. The pasteurization of pumpable foods has long been one of the most used and successful applications of PEF technology. In general, the overall quality of the finished food products is better than those thermally treated, and the processing time is significantly shorter. Other advantages over conventional thermal treatments include significantly less energy consumption, longer shelf life, better color, better nutrient retention of bioactive compounds and vitamins, and better fresh-like characteristics due to low temperature processing.
A typical PEF system used to pasteurize liquid and semi-liquid foods utilizes as its main component a treatment chamber housing the electrodes where the food receives the electric pulses. Other relevant components include a high-voltage pulse generator, pumps, heaters, coolers, and an aseptic filler. Untreated fluid food is placed in a tank and pumped through a heater, to the chamber, and through the cooler. The treated product may go to an aseptic filler or may be recirculated to receive additional pulses.
The main industrial applications of these PEF systems are fruit and vegetable juices (either high-acid or low-acid), fruit purees, smoothies, soups, milk products, and liquid eggs. Among the extensive list of PEF-processed juices currently on the market are orange, pineapple, apple, strawberry, pear, broccoli, tomato, carrot juice, dragon fruit, and watermelon.
In the early 1990s, the driving force behind further development of PEF technology was to attain food products of high quality to achieve safer, lower-cost “fresh-like” foods and beverages with minimal processing. This trend evolved into other applications, including the recovery of added-value compounds from microbial cells for applications in the food, cosmetic, pharmaceutical, and biofuel industries, improvements in extraction yields, and the reduction of costs and environmental impacts associated with manufacturing systems. As the food industry adopts pulsed electric fields and other nonthermal processing technologies, it is critical to incorporate associated basic principles and applications into food science and engineering curriculum so that the next generation of industry leaders will champion their implementation.
There is no question that PEF has evolved quite rapidly in the past 30 years to make it a feasible and sound reality at the industrial level. This has been possible thanks to the concerted efforts of many research centers around the world; the cash and in-kind support received from public and private agencies; champions in the food industry who believed in this technology; regulatory agencies that are facilitating adoption of new technologies, professional societies, and publishers. Along the way, PEF has emerged as a technology that also contributes to the food industry’s sustainability initiatives, offering a viable way to process quality foods while reducing the carbon footprint at the manufacturing level. We should consider PEF a technology of the future even though today it is assuredly a nice reality.
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Editor’s note: This article is authored by members of IFT’s Nonthermal Processing Division, who are leading scientists and researchers advancing PEF and other nonthermal processing technologies for use in industrial food processing. Click here to learn more about the NPD.