Pulsed light applications in food processing

Fundamentals and technical aspects of pulsed light applications

Pulsed light, which is sometimes known as pulsed ultraviolet light (PUV) or pulsed white light (PWL), is a versatile emerging technology that has been proposed as a feasible technique for different operations of interest in the food industry, from liquid food pasteurisation to food enhancement (Figure 1, page 56). This food technology involves the application of high intensity and short duration pulses (100-400 microseconds) of a continuous broad spectrum light in which around 40% of the light emitted corresponds to the UV region. These pulses have high peak energy, producing a light intensity per time unit approximately 20,000 times greater than sunlight at sea level.

Although the usefulness of continuous wave UV (CW UV) in food processing was identified in the 1920s, the study of PL began in the 1970s. In the first experiments, PL showed higher penetrating capability and energetic efficiency than UV continuous light radiation, since peak power provided using pulses is superior to that obtained using continuous UV of equivalent energy input1. Moreover, PL may reduce temperature build up if short pulse durations and/or suitable heat dissipation periods between pulses are used. Due to both these aspects, PL has been proposed as a more efficient technology than CW UV.

A PL system is basically composed of a pulse generator (comprising a high voltage DC power supply, capacitor and pulse controlling system) and a treatment chamber (including flash lamp, lamp cooling system and reflectors) (Figure 2). The capacitor stores the high voltage electric power provided by the power supply. A pulse controlling system provokes the suitable discharge of the capacitor electric power in the inert-gas flash lamp. Here the electric energy is transformed in radiant energy that is finally received by the target material located in the treatment chamber. Normally the light source is a Xenon lamp, which emits from deep UV-C (200nm) to near-infrared (1,100nm) light, with a high amount of UV radiation in the 200-300nm range (Figure 3)². Reflectors are used to maximise the radiant energy received by the target material. Finally, in order to avoid lamp overheating, the treatment chamber also includes an air or water cooling system.

The treatment dose per pulse is quantified by its fluence, which represents the total radiant energy received by the sample per unit area, typically expressed in J/cm². The total fluence (fluence multiplied by the number of pulses) is normally used to characterize and compare the different treatments³. The distance between food and the radiant source, and also the presence and disposition of reflectors to optimise the radiant energy received by the sample, are key aspects that determine the applied fluence. Therefore, in order to maximise it and, as a consequence, the efficiency of the process, the optimisation of the treatment chamber is necessary dependent on the desired food characteristics and the application (Figure 4).

Microbial inactivation on food, packaging materials and food contact surfaces

The use of PL for microbial inactivation has been heavily studied. Fluences that range from 0.01-50J/cm² are capable of inactivating pathogens and spoilage microorganisms, including viruses, molds and bacteria (vegetative and spore forms), using treatment times measured in seconds and without a detrimental increase in product temperature. Thus, if the treatment is correctly tuned, sensorial and nutritional properties are expected to undergo minimal or no changes⁴. The efficiency of microbial inactivation depends on microbial characteristics (for example, spores are more resistant than vegetative forms), processing factors (treatment chamber configuration, pulse frequency) and product characteristics (transmittance, thickness and roughness).

One of the main disadvantages of the technology is its low penetration power. For that reason, PL technology has been initially proposed for surface decontamination of solid food, unpackaged or packaged in ‘UV transparent’ materials. In these applications, food surface topography is the key aspect to take into account. If the treated surface has a high roughness or a pored structure, shadow effect occurs and microorganisms located in these areas can survive the treatment. However, if treatment is well optimised, PL technology is a good choice for surface decontamination of different kinds of solid foods such as fruit, vegetables, bakery products, meat products, fish and egg shells, obtaining reductions of up to five log cycles using fluences up to 30J/cm²⁵.

The use of PL has been also proposed for water treatment and liquid food cold pasteurisation, using continuous flow reactors⁶. In these kinds of food, the transmittance, especially UV transmittance, is the critical characteristic. Consequently, PL has been suggested for high transmittance foods like clarified juices. For example, it has been reported that a PL treatment of 12J/cm² can achieve E. coli O157:H7 inactivation levels of five log cycles in apple juice⁷. The treatment of liquid foods with poor transmittance, like milk, liquid egg or smoothie, is now being investigated and some promising results have been obtained. In this case, the design of the reactor plays a key role since it is necessary to assure that all volume of product receives the minimal fluence to reach the inactivation objective.

Finally, PL can be also used to inactivate microorganisms on the surface of food packaging materials and also on food contact surfaces in the industry (such as conveyor belts or cutting devices). In this case, the use of pulsed light could avoid the need for preservatives or chemical agents. Chemical surface agents, such as hydrogen peroxide, propylene oxide, or peracetic acid may form residue compounds and/or require prolonged times to attain satisfactory reduction levels. PL has the advantages of not leaving undesirable residues and short duration of treatment³.

Food enhancement: Post-harvest photostimulation

PL technology is also a useful technology for fruit and vegetable enhancement in molecules of interest, such as some human health-related compounds like vitamin D₂, or pigments where production is light mediated. Although there is great interest and potential in this application, due to its novelty published results are currently scarce. In contrast, post-harvest photostimulation using CW UV is well known. Yet again, the use of PL is more energy efficient than the use of CW UV, reducing the exposure time of fruit and vegetables from 5-20 minutes to 1-90 seconds. However, optimisation in the function of the product is always needed, since large exposures to PL can cause discolouration and oxidation phenomena⁸.

In 2009, PL technology was patented to enhance vitamin D₂ content in whole or sliced mushrooms⁹. Dietary intake of vitamin D₂ enriched mushroom could help to improve the vitamin D status of individuals and according to the authors, PL treated fresh mushrooms contained up to 1,800% of the recommended daily value of vitamin D in one serving.

Additionally, insufficient sunlight can cause low anthocyanin and polyphenol concentrations in certain fruits and vegetables, meaning poor colouration and reduced antioxidant properties. These facts clearly affect its quality and its marketability. Post-harvest photostimulation by PL could be a solution for these kinds of products, increasing quality and added value. For example, the exposure of poorly coloured figs to PL for just 90 seconds and subsequent storage in the dark at 20ºC increases the concentration of anthocyanins in fruit skin twenty-fold compared to untreated controls⁸.

Improvement of functional properties

The use of PL as an efficient non-thermal technology for improving the functional properties of biomolecules, especially proteins, has been recently proposed by AZTI-Tecnalia (Spain)¹⁰. Many properties can potentially be improved: thickening, dispersing, emulsifying, foaming or gelling properties, hydration, glass transition, solubility, water retention capacity, oil retention capacity, film formation properties, bulking agent properties, surface properties (wettability, adhesion), etc. For example, the PL treatment of β-lactoglobulina solutions improves their foaming properties¹¹ and mechanisms of action beyond the application are now being studied. It has been published that changes to protein properties are essentially based on the PL-mediated modification of its conformation (secondary and tertiary structure), implicating a non-thermal partial denaturation¹¹.

The implementation of this technology in some food industry sectors could be worthwhile for the production of high added value ingredients. Furthermore, a PL treatment could potentially be an efficient non-thermal process for microbial inactivation while at the same time improving, for example, the foaming properties of solutions with high protein content such as whey. In any case, as a novel application, deep study of its potential uses has to be accomplished.

Industrial applications

Although research into PL technology began in the 1970s, the industrial exploitation was limited for several decades due in part to a lack of reliable and affordable industrial size equipment, but also due to the necessity of equipment optimisation in function of application and food characteristics. One of the early key steps in the industrialisation of the technology was achieved in 1988 by PurePulse Technologies Inc., who developed a microbial decontamination process called PureBright®. Later on, the USA’s Food and Drug Administration (FDA) approved the use of PL treatments up to 12J/cm² for decontamination of food or food contact surfaces in 1996¹².

The application that has clearly gained the industry’s attention is packaging decontamination. For example, the French company Claranor has commercialised specific solutions for caps, cups, and trays, which are especially difficult to decontaminate using conventional technologies. The first machine for cup sterilisation was installed in 2006 at Nestlé Waters France. Since then, an increasing number of companies have integrated Claranor technology into their facilities, with 100 machines commissioned by 2013¹³.

Since 2000, several industrial experiments in food preservation have been carried out in North America and Europe, mostly confidential. As a result, some PL treated products have now reached the market. Some of the most recent are PL pasteurised coffee and cooking syrups commercialised by Taylerson’s Malmesbury Syrups (UK). The PL process, developed by MicroTek Process (UK), allows the original sensory properties of these products, especially flavours, to be maintained, with the added benefit of energy savings in respect to conventional heat treatments¹⁴.

Due to its novelty, the use of PL for food enhancement and for improving functional properties has not received the same industrial attention as microbial inactivation. However, the first industrial application for food enhancement was carried out in 2009 by Xenon Corporation (USA) and the Dole Company (USA), the world’s largest producer and marketer of fresh fruit and vegetables. This company has commercialised Portobello mushrooms enriched in Vitamin D by means of PL treatment¹⁵. The improvement of functional properties by means of PL has also been recently patented¹⁰ and the first industrial application is now anticipated.

Conclusions

PL is a versatile food technology that brings novel and exciting opportunities to the food industry. As things stand, the main limitations for PL industrial developments have been largely solved and the first industrial uses have now appeared in the market. These successful cases have revealed that treatment optimisation in function of the application and the characteristics of food or material is absolutely key. Based on these new technological developments and current figures, a progressive growth of PL industrialisation is expected in the near future.

References

1. Dunn, JE et al. (1989) Methods for preservation of foodstuffs. US patent number 4871559 A.
2. Panico, LR (2002). Instantaneous surface sanitation with pulsed UV. Hygienic Coatings Global Conference, 8-9 July, Brussels, Belgium.
3. Moraru, CI, Uesugi, AR (2010). Pulsed-light treatment. Principles and applications. In: TN Koutchma, LJ Forney and CI Moraru (Eds), Ultraviolet light in Food Technology. Principles and Applications, pp. 235-265, CRC Press: Boca Ratón.
4. Oms-Oliu, G, Martín-Belloso, O, Soliva-Fortuny, R (2010). Pulsed light treatments for food preservation. Food and Bioprocess Technology 3, 13-23.
5. Lasagabaster, A (2009). Factors determining the efficacy of pulsed light technology for inactivation of foodborne microorganisms. PhD Thesis. Basque Country University.
6. Artiguez, ML, Martínez-de Marañón, I (2015). Improved process for decontamination of whey by a continuous flow-through pulsed light system. Food Control 47, 599-605.
7. Sauer, A, Moraru, CI (2009). Inactivation of Escherichia coli ATCC 25922 and Escherichia coli O157:H7 in apple juice and apple cider, using pulsed light treatment. Journal of Food Protection 72, 937-944.
8. Rodov, V, Vinokur, Y, Horev, B (2012). Brief postharvest exposure to pulsed light stimulates coloration and anthocyanin accumulation in fig fruit (Ficus carica L.). Postharvest Biology and Technology 68, 43-46.
9. Beeleman, R, Kalaras, M (2009). Methods and compositions for improving the nutritional content of mushrooms and fungi. Patent number US 2009/269441 A1.
10. Arboleya, JC, Artiguez, ML, Fernández, E, Martínez-de-Marañón, I (2011). Method for improving functional properties by means of pulsed light, samples with improved functional properties and uses thereof. Patent number WO 2011/113968 A1.
11. Fernández, E et al. (2012). Effect of pulsed-light processing on the surface and foaming properties of β-lactoglobulin. Food Hydrocolloids 27, 154-160.
12. FDA (1996). Code of Federal Regulations 21 CFR, part 179.41.
13. claranor.com
14. guildoffinefood.com/ffdonline/news.asp?StoryID=254
15. xenoncorp.com/markets/process-development

About the authors

Dr Eduardo Puértolas is a researcher in the Food Research Division of AZTI-Tecnalia (Spain). He obtained a PhD in Food Science and Technology from the University of Zaragoza, Spain and is co-author of more than 25 peer reviewed papers. He has participated in more than 30 public and private projects relating to novel food technologies. His research interests are focused on the study, validation and industrial implementation of emerging technologies for food processing, such as pulsed light, pulsed electric fields, electromagnetic field assisted freezing and high pressure processing.

Dr Iñigo Martínez de Marañón is R&D Director of the Food Research Division of AZTI-Tecnalia (Spain). He has more than 20 years of experience in R&D project management, participating in more than 100 projects. He obtained his PhD in Food Science and Technology from the University of Bourgogne, France. His research is mainly focused on studying the effects of novel technologies, such as pulsed light or high pressure processing, in microbial inactivation and the physicochemical properties of food. This activity is reflected in more than 40 publications and more than 70 conference contributions.