Nonthermal processing is a value-added technique with diverse benefits, including increased shelf life and improved taste through the preservation of amino acids. One major use for nonthermal processing is as an alternative sterilization method for processors that want to maintain a product’s taste and appearance without sacrificing food safety. Manufacturers seeking microbiological sterilization through nonthermal means may choose from a number of methods including high-pressure, ultraviolet, irradiation, pulsed light and ultrasonic processing depending on sterilization requirements, product type, line configuration and other considerations.

For instance, high-intensity pulsed light is most effective as a treatment for product surfaces and packaging, as it only marginally penetrates food products. High-pressure processing (HPP), meanwhile, is a suitable replacement for heat sterilization for many products with high levels of water activity.

High-pressure processing

One of HPP’s most important benefits over other nonthermal sterilization techniques is its ability to act instantaneously and simultaneously in a food product regardless of size and shape. Additionally, the ability to treat products post packaging lowers the risk of decontamination, says Jaime Nicolás-Correa, global commercial manager for HPP equipment supplier Hiperbaric. Nicolás-Correa is quick to point out ancillary benefits to HPP that go beyond food safety, saying, “The protection of the brand is a key factor, but extending shelf life and creating cleaner label products also have a significant effect on ROI.”

According to Dr. Mukund V. Karwe, chair of Rutgers University’s food science department, treating orange juice at 483 MPa for 60 seconds achieved a seven-log reduction of pathogens including E. coli and Salmonella. Ready-to-eat meats processed at 600 MPa, meanwhile, achieved a three- to four-log reduction of L. monocytogenes.1

HPP-amenable products span a broad range in the packaged, refrigerated category: RTE meats, raw ground beef and poultry; fresh fruits, juices and smoothies; soups, wet salads, yogurts and sauces; and seafood and shellfish.  

“One segment that is not very promising is baked goods,” says Amy Lawless, managing director of Stay Fresh Foods, an HPP toll processor based in Meriden, CT. “With their high air content, they don’t stand up to pressure very well—just think of what happens to a loaf of bread at the bottom of your grocery cart. But most food products that can be subjected to traditional heat sterilization also can be treated with HPP.”

When adding HPP as a production step, processors must typically reformulate their products. This reformulation results in the removal of chemicals and preservatives, producing a clean label that appeals to consumers.

Processors must also ensure the use of modified air packaging and HPP in conjunction won’t damage the packaging. “The main problem is that, under pressure, we compress all the gas inside the package,” says Nicolás-Correa. “If the package is not very flexible, and requires too much gas, it will be deformed under pressure and have a hard time returning to its original form. Some of these challenges can be overcome by reducing the amount of gas. Round corners and more flexible materials also help the package return to its original form.”

The cost of installation is another major consideration for processors. Consequently, many of them, especially those developing new products, have opted to access HPP through a tolling company such as Stay Fresh Foods, which allows them to realize the benefits of the technology without making a long-term financial investment.

“A tolling company focuses 100 percent on the HPP process, so customers can concentrate on what they do best,” says Lawless. “We just installed our second Avure 350L HPP system to support growing volume and add redundancy to our facility. HPP plays a vital role in our customers’ production processes, and we’re committed to providing the services they need to integrate this step, including assistance on product testing, packaging selection, shelf-life studies and formulation changes.”

Ultrasonic treatment

While HPP is the most well-established nonthermal processing technique in the US, other emerging sterilization technologies are giving processors more options. Suitable for liquid, slurry or paste products, ultrasonic treatment is a sterilization method that uses alternating high-frequency electric currents, amplified and applied via an ultrasonic probe, to produce cavitation and shear forces. When sonicating liquids at high intensities and amplitudes, the sound waves that propagate into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates dependent on the frequency.

Ultrasound systems offer the advantage that only one part—the sonotrode, a titanium rod that transmits the ultrasound vibrations into the process medium—requires regular replacement, which is easily and inexpensively accomplished.

“Per kilowatt hour of ultrasonication, we calculate cost of approximately $0.50 based on an operation of 8,700 hours per year,” says Kathrin Hielscher, head of project development for Hielscher Ultrasonics GmbH. “This includes the equipment and energy costs, maintenance and depreciation over three years.”

Ultrasonic systems also offer the benefit of flexibility. “Smaller quantities can be ultrasonically processed in batch mode, while high-volume streams are fed through an inline system,” says Hielscher. “Various designs of flow cell reactors allow retrofitting into the production line. Since sonication is an innovative food processing technology, most customers choose an installation on site [rather than using a toll processor] to save their process knowledge.”

According to FDA, the efficiency of ultrasound in lysing microbial cells approaches 100 percent in laboratory settings using small-scale, temperature-controlled batch operation where the sonicator is immersed in a small volume of cells. However, in an industrial food production setting, less control of critical factors would prevent near 100 percent efficiency. Instead, FDA says, “a combination of ultrasound with other preservation processes (for example, heat and mild pressure) appears to have the greatest potential for industrial applications.”

Pulsed light

Pulsed light is another nonthermal sterilization technique with some inherent advantages and limitations. Pulsed light treatment involves applying a series of very short, high-power pulses of broad spectrum light into foods to kill pathogenic and spoilage microorganisms, including bacteria, yeasts, molds and viruses.

Light pulses are produced by electromagnetic energy accumulated in a capacitor during fractions of a second and then released in the form of light within nanoseconds to milliseconds, resulting in an amplification of power with minimum energy consumption. In food processing applications, light pulses are typically emitted at the rate of one to 20 flashes per second and an energy density of about 0.01 to 50 J/cm2 at the treated surface. Pulsed light technology generates a light spectrum similar to that of sunlight, but causes disinfection because the intensity of the light is between 20,000 and 90,000 times higher than that of sunlight at the earth’s surface.

The use of pulsed light for the decontamination of food or food contact surfaces has been approved by FDA, provided the treatment uses a xenon lamp with the emission of wavelengths between 200 and 1000 nm. In addition, the pulse width cannot exceed two milliseconds; the cumulative level of the treatment cannot not exceed 12 per centimeter.

However, pulsed light’s main drawback is its limited penetration depth. “Since the effectiveness of pulsed light is strongly influenced by the interaction of the substrate with the incident light, the treatment is most effective on smooth, non-reflecting surfaces or in liquids that are free of suspended particulates,” writes Carmen Moraru of Cornell University’s department of food science. “In surface treatments, rough surfaces hinder inactivation due to cell hiding, while for very smooth surfaces, surface reflectivity and cell clumping caused by hydrophobic effects are also limiting the degree of microbial reduction.”2

However, pulsed light does offer some definite benefits for packaging sterilization. For instance, Claranor, a pulsed light equipment supplier based in France, uses pulsed light in the sterilization of caps and closures, as well as trays and cups.

“We can treat either flat caps or complex-shaped caps with great success,” says Morgane Busnel, marketing manager at the firm. “For example, pulsed light treatment is especially well suited for sport cap treatment because it reaches the whole internal surface of the cap without the liquid trapping associated with chemical treatment. Cups and trays are also very good candidates for pulsed light treatment, since their shapes enable very good decontamination results for molds and bacteria. We have sold cup sterilization equipment mainly for yogurt cup decontamination, but also for margarine trays.”

Claranor’s system is composed of an optical cabinet, which delivers the flashes of light, and an electronic bay placed beside the line. For cap sterilization equipment, the optical cabinet is integrated just above the capper on the cap chute; for cup sterilization equipment, the cabinet is integrated just before the filling step. (For effective decontamination with pulsed light, a system should be integrated as close as possible to the subsequent process step to minimize the recontamination hazard.)

“For new lines, the integration is easy as it is scheduled from the beginning of the line design. It is often done directly by the equipment supplier,” says Busnel. “When retrofitting lines, we have to adapt the equipment to the existing line, but even if the optical cabinet is very compact, it can be a challenge in very tight areas.” In addition to the space issue, pulsed light equipment must be designed to ensure operators are shielded from potentially dangerous high-intensity light emissions.

Claranor says pulsed light systems are a good choice for dairy beverages and juices or smoothies that require closure decontamination to increase shelf life and prevent mold growth. L’Armoricaine Laitière, a French dairy cooperative founded in 1950, integrated a single pulsed light lamp into a Serac packaging system for a yogurt beverage and achieved a 3.6-log reduction of Aspergillus niger. The pulsed light system also compares favorably to irradiation and chemical treatments according to estimates from Claranor and L’Armoricaine Laitière, costing just €.04 to operate per 1,000 closures compared to €1.60 for irradiation and €.27 for chemical treatment.

Irradiation

Irradiation treatment exposes food products to a controlled amount of radiant energy in the form of gamma rays, electron beams or X-ray waves to kill harmful bacteria such as E. coli O157:H7, Campylobacter, Listeria and Salmonella. The technology can also control insects and parasites, reduce spoilage and inhibit ripening and sprouting.

Irradiation works by passing energy waves through food or beverage products to generate reactive ions, free radicals and excited molecules. These agitated particles chemically attack essential biomolecules including the DNA and RNA, membrane lipids, proteins and carbohydrates of bacteria, as well as other pathogens and pests, causing their death or preventing them from reproducing. Accordingly, irradiation is best suited to eradicate food safety problems that contain more nucleic acid: parasites and insect pests, bacteria and bacterial spores, in that order.

For some perspective on the relative efficacy of irradiation treatment on each, consider that parasites and insect pests require only a decimal reduction value (the dose of radiation that causes a 10-fold reduction in a given microorganism) of .1 kGy (kiloGray) or less. Bacteria’s smaller DNA has a decimal reduction value ranging from .3 to .7, while bacterial spores’ values are around 2.8 kGy, according to the Grocery Manufacturers Association.3 Viruses, the smallest pathogens with nucleic acid, can have decimal reduction values of 10 kGy or higher.

Studies show irradiation at approved levels kills 99.9 percent of common foodborne organisms, including Salmonella, Campylobacter jejuni, E. coli O157:H7, Listeria monocytogenes, Vibrio, Toxoplasma gondii and Trichinella spiralis. Treating meat and poultry products with irradiation achieves a reduction of bacterial load comparable to heat pasteurization, although irradiation is not effective against microbial toxins and mycotoxins.

Because irradiation can be used to treat packaged foods, reducing the risk of cross-contamination, regulatory agencies view the technique as an effective critical control point for HACCP and HARPC. Compliance can easily be monitored by measuring the absorbed radiation dosage.

One barrier to the marketing and sale of irradiated foods is an incorrect belief on the part of some consumers that irradiated foods are radioactive. But because irradiation energy simply passes through food products, and the acceptable levels of irradiation are not enough to cause changes at the atomic level, there is no possibility of irradiated foods retaining radioactivity after treatment.

Another concern, this one on the processing side, is the potential exposure of factory workers to cobalt 60, a source of gamma rays for food radiation. All facilities using radiation sources are regulated by the Nuclear Regulatory Commission; facilities that use electron beam or X-ray sources are regulated by FDA’s medical X-ray imaging arm.

“While several accidents have injured or killed workers worldwide over the past 30 years due to radiation exposure, all of these accidents occurred because safety systems and control procedures had been bypassed,” says GMA. “Furthermore, in North America, in over four decades of transporting the types of radioactive isotopes used for irradiation, there has never been an accident resulting in the escape of radioactive materials into the environment.”

With the variety of nonthermal processing techniques now available on the market, processors can weigh product, packaging and line configuration considerations to find the option that works best for them.

For more information:

Amy Lawless, Stay Fresh Foods, amy@stayfreshfoods.com, 855- 477-8655, www.stayfreshfoods.com

Jaime Nicolás-Correa, Hiperbaric, jaime.nc@hiperbaric.com, 305-639 9770, www.hiperbaric.com

Morgane Busnel, Claranor, mbusnel@claranor.com, 33(0)4 86 40 84 65, www.claranor.com

Kathrin Hielscher, Hielscher Ultrasonics GmbH, kathrin@hielscher.com, 49 (0) 3328 437 428, www.hielscher.com

References:

^₁ “Non-thermal food processing,” Dr. Mukund V. Karwe, Rutgers University food science department, (http://foodsci.rutgers.edu/103/CFPA%20POWERPOINT/Non-thermal%20Food%20Processing%202008%20KarwePDF.pdf)

^₂ “Pulsed Light Treatment to Enhance Food Safety,” Carmen Moraru, Cornell University department of food science (http://fshn.illinois.edu/food_processing_forum/presentations/c5_Moraru_abstract.pdf)

^₃  “Food Irradiation: A Guide for Consumers, Policymakers and the Media,” Grocery Manufacturers Association, (http://www.gmaonline.org/downloads/research-and-reports/SPP_Irradiation5.pdf)