Technology eyed as an alternate sterilization method for produce

Ready-to-eat convenience products, like grab-and-go salad mixtures and fresh-cut vegetables and fruit, are a growing market. Fresh products, though, are susceptible to microbial spoiling. Pathogenic bacteria also pose a potential risk to consumers.

SAFEFRESHThe SAFEFRESH project is looking at innovative ways to detect and curb those risks. It is a partnership between the Fraunhofer Institute for Process Engineering and Packaging IVV, the Leibniz Institute for Agricultural Engineering Potsdam-Bornim and the Leibniz Institute for Plasma Science and Technology, among others. One of the innovations they have looked at is pulsed light technology.

The fresh-cut challenge

Leafy greens and fresh-cut salads are a very sensitive product group. They are highly perishable and usually only have a short shelf life. This is because they are living tissue that is actively respiring. Along the supply chain – from the field to further processing – there are numerous potential sources of contamination where bacteria and microbes can colonize.

“The microbes that come in contact with the product’s surface may be able to attach to very complex plant structures, like stomata and cuticula,” said Bernd Kramer, a scientist at Fraunhofer Institute for Process Engineering and Packaging. Kramer spoke on pulsed light technology at this year’s Anuga FoodTec fair in Cologne, Germany. “And some may even be able to form biofilms on the products.”

During the cleaning process, it can be difficult to remove those microbes again. Washing processes are not efficient enough, he said, and chlorine is unwanted because of its harmful byproducts. Finally, these products are usually eaten raw, so the bacteria and microbes cannot be removed by way of heating.

To help mitigate those risks, the SAFEFRESH project was established. Its aim is to improve the safety of fresh-cut produce through careful analysis of microbial contamination and colonization patterns.In order to rapidly detect microbial contaminations, the project utilizes a monitoring system known as MALDI-TOF-MS. 

What is pulsed light technology?

Pulsed light technology is the application of very intense and short light pulses. These light pulses are generated in xenon tubes and xenon flash lamps where a high voltage is discharged, creating very high-intensity light pulses. Flash lamps have a spectrum of about 200 to 1,100 nanometers, and high UV content (UVA, UVB and UVC). They also produce visible light and near infrared radiation.

The light pulses cause the photochemical effects necessary for the breakdown of microbes, and they also produce photo-thermal effects, which have an affect on micro-biostructure. “So we can say that it is actually a multi-target process,” said Kramer.

“We have no chemicals that are applied, so we don’t have any problems with residues that would have to be removed otherwise,” he said.

Pulsed light has been shown to be more efficient for surface sterilization than continuous UV, and FDA has already approved it for food application.

“However, there still are no industrial applications so far, at least to my knowledge,” Kramer said. “And it’s still under research.”

Pulsed light technology doesn’t just kill bacteria. It can kill various microorganisms, including yeasts, molds and bacterial endospores. It also works on materials with different geometries.

Testing pulsed light

Using endive salad pieces and mung bean sprouts, pulsed light technology was tested against two strains: E. coli DSM 498 and Listeria innocua DSM 20649. Samples were inoculated with test bacteria using a special dipping method. The initial load was ~10 (to the power of 5) cfu/g. “This was done in order to simulate potential contaminations with pathogenic bacteria,” Kramer said.

The researchers then performed their treatment of pulsed light using a three-lamp reflector. Samples were treated from both sides at a distance of about 10 centimeters. They then determined the surviving cell fraction on the product by traditional plate methods, and investigated the treatment’s effect on the overall quality of the product, looking at color measurements and its effect on produce respiration.

The first samples tested were relatively fresh, with four days until the eat-by date. Both the endive salad and mung bean sprouts were found to contain microbiologicals, including pseudomonads, enterobacteria, lactic acid bacteria and yeast and molds, although microbiological activity was higher in the sprouts.

In the inactivation trials, starting with the endive salad, the researchers found that irrespective of the total viable counts of the native microflora, inactivation occurred at about 2 log10 CFU/g. “Nevertheless, we have about a two-log reduction, which is actually 99 percent of the present microflora and this is actually a very positive result,” Kramer said.

After assessing inactivation, the researchers went on to assess quality, measuring for discoloration in the endive salad. “We saw that after the treatment, pronounced browning was indicated,” he said.

In fact, the more they increased light, the more the browning occurred. So the researchers decided to test optical filters to see if that would minimize discoloration. Using filters UG11 (275-375 nm) and 400S (>400 nm) and UG5, they discovered that they could reduce browning, and in some cases, completely prevent it.

Discoloration isn’t the only issue the researchers encountered. They also tested the product in storage to be sure that the technology didn’t shorten its lifespan. Product was placed in measuring cells in dark storage at a temperature of 5˚ C. Results show that pulsed light treatments without filters increase respiration rates, while the use of optical filters decreases this effect, leaving it suitable for up to six days of storage.

Next, the researchers tested the mung bean sprouts using the same process. Using pulsed light technology, they recorded a maximum inactivation of about 1.8 log10 CFU/g, effectively reducing the microbiological loads. When testing for color changes, they found that the pulsed light treatment had a positive effect on color and texture. Respiration measurements were similar. Mung beans were stored in measuring cells in dark storage at a temperature of 5˚ C. Whereas the reference samples showed increased respiration and microbial growth after just five days, pulsed light had no effect on respiration rates.

“We did not see any negative affects on the sprouts,” Kramer said. “In contrast, we saw that the respiration was higher with the reference samples after five days of storage. This is most likely due to the high microbial growth of the reference sample.”

In storage tests, it was also shown that L. innocua concentrations remained low. There was about one log difference between the treated and untreated samples of the total aerobic cell count.


Pulsed light is an efficient method for fast decontamination of surfaces. In cases where product is sensitive to pulsed light, optical filters enable more gentle treatment and can even prevent browning and respiration rate. In some cases, it can even improve the product’s appearance, as it did in the case of the mung bean sprouts. Unfortunately, pulsed light technology has limited effect on bulk goods, but works well in industrial sorter systems for singularize products, like leafy greens.

— By Melanie Epp, contributing writer

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