THE EFFECTS OF RADIOFREQUENCY TREATMENT ON MICROORGANISMS INVOLVED IN

FOOD SPOILAGE

Georgiana-Aurora ŞTEFÃNOIU, Elisabeta Elena TÃNASE, Amalia Carmen MITELUŢ, Mona Elena POPA

University of Agronomic Sciences and Veterinary Medicine of Bucharest, Faculty of Biotechnologies, 59 Mărăşti Blvd, District 1,

011464, Bucharest, Romania, Phone: +4021.318.25.64, Fax: + 4021.318.25.67

INACTIVATION MECHANISM OF RFThe mechanism of radio frequency heating relates to the fact that the molecules within a product placed in an RF environment re-orient

themselves continuously in response to the applied field (dipole heating). This response initiates volumetric heating within the entire product

due to frictional interaction between the molecules. Radio frequency heating is accomplished through a combination of dipole heating and

electric resistance heating resulting from the movement of dissolved ions present in the food. Although identical to the microwave in terms of

its heating characteristics, radio frequency has the added advantage of uniform heating in homogeneous foods, and most important of all, high

penetration depth that could be used to pasteurize or sterilize liquid products.

Microbial inactivation is based on electroporation of cell membranes, causing reversible or irreversible pore formation depending on the

electric field intensity. When a cell is exposed to an external electric field, charge is accumulated on the cell membrane resulting in an

artificial increase of the transmembrane potential (TMP). If such TMP increase is large enough, and sustained for long enough, cell membrane

permeability to ions and macromolecules will increase very significantly.

Escherichia coli

Saccharomyces cerevisiae

Listeria monocytogenes

Salmonella typhimurium

Bacillus subtilis

Aspergillus parasiticus Penicillium citrinum Erwinia carotovora

APPLICATIONS OF RADIOFREQUENCY TREATMENT IN FOOD INDUSTRY

Microbial and pest reduction by dielectric heating has been studied in many experiments, including meat

and meat products; poultry; eggs and egg products; fish and shellfish; fruit and vegetable products such as

canned fruit, fruit juice, and jam; soy milk; sugar beet molasses; pea protein concentrates; ready-cooked

meals; milk and its products; puddings; cereals; breads; cakes; pasta; starch; and spices (Mitelut and

colab., 2011; Orsat and Raghavan, 2014).

Exposure of liposomes to frequencies of 27 and 100 MHz resulted in increased lysis of vesicles (Trujillo

and Geveke, 2014).

Radio frequency dielectric heating is now widely used in industrial applications such as drying textile

products (spools, rovings, and skeins), final drying of paper, final dehydration of biscuits at outlets of

baking ovens, and melting honey.

Bottled juices including peach, quince and orange moving through an RF applicator offered better

bacteriological and organoleptic qualities than juices treated by conventional thermal processing methods

(Wang et al., 2003) .

RF heating for 50 s resulted in 2.80 to 4.29 log CFU/g reductions of S. typhimurium and E. coli O157:H7

in black peppers and RF heating of red peppers for 40 s reduced pathogens by 3.38 log CFU/g to more

than 5 log CFU/g (below the detection limit) without affecting the color quality change (Kim et al., 2011).

Tofu was produced experimentally using RF-FH processed soybean milk and conventionally heated

soybean milk. Comparative studies revealed that the tofu made by RF-FH processing had higher gel

strength than the tofu made by conventional heating (Uemura et al., 2010) .

RF heating can be applied to control pathogens in peanut butter products without affecting quality (Guo et

al., 2006) .

The non-thermal process of radio frequency electric fields (RFEF) has been shown to inactivate bacteria in

apple juice at moderately low temperatures, but has yet to be extended to inactivate bacteria in orange

juice. No loss in ascorbic acid or enzymatic browning was observed due to RFEF processing (Geveke et

al., 2007).

Heating bread to 58°C or higher resulted in 4-log reduction of P. citrinum spores isolated from moldy

bread. The storage life at room temperature (23°C) was extended by 28 ± 2 days for the treated white

bread (Liu et al,. 2010) .

Radio-frequency heating, coupled with appropriate packaging, can improve the storability of repacked

hams by reducing the bacterial load, reducing moisture loss and maintaining an overall greater product

sensory quality and acceptance (Orsat et al., 2004).

RF treatment was also investigated in naturally infected fruit where the Monilinia spp. development was

completely inhibited in both ‘Summer Rich’ and ‘Placido’ peaches. No brown rot control was observed in

nectarine fruit artificially inoculated or with natural inoculum (Casals et al., 2010).

CONCLUSIONS The reality today is that these novel processing technologies are being tested for use in the food industry to improve the foods we eat, as they are capable of inactivating microorganisms, changing

cell permeability, promoting chemical reactions, and even inactivating enzymes.

In fact, RF heating has been successfully applied in the food industry for drying, baking and thawing, but controversial data are present on the effect of RF on biological systems

ACKNOWLEDGEMENTS This paper was published under the frame of Partnerships in priority areas Programme, PCCA Contract no. 164 / 2014, RAFSIG.

SELECTIVE REFERENCES

Geveke D. J., Kozempela M., Scullena O. J., Brunkhorst C., (2002). Radio frequency energy effects on microorganisms in foods. Innovative Food Science and Emerging Technologies, 3 (2), 133–138.

Marra F., Zhang L., Lyng J.G., 2008. Radio frequency treatment of foods: Review of recent advances. Journal of Food Engineering 91, 497–508.

Mitelut A., Popa M., Geicu M., Niculita P., Vatuiu D., Vatuiu I., Gilea B., Balint R., Cramariuc R., (2011). Ohmic treatment for microbial inhibition in meat and meat products. Romanian Biotechnological Letters, 16 (1), 149-152.

Orsat V., Raghavan G.S.V., (2014). Radio-Frequency Processing. Emerging Technologies for Food Processing (Second Edition), 385–398.

INTRODUCTION Preventing food spoilage is a challenge for the food industry, especially when applying mild preservation methods and when they are trying limit the use of preservatives. Therefore, it is essential

to explore the boundaries of preservation by better understanding the causative microbes, their phenotypic behaviour and their genetic makeup. Food decomposition is a metabolic process which

may cause some undesirable organoleptic changes in food, even though decomposed food is not a cause of food borne illness, the changes in its appearance, taste or consistency are matters that

affect the retail value of the food. Various microorganisms are responsible for the decomposition of food products which may induce food poisoning as well. Research in novel heating of foods, for

applications such as cooking, pasteurization/ sterilisation, defrosting, thawing and drying, often focuses on areas such as the assessment of processing time, the evaluation of heating uniformity, the

appraisal of the impact on quality attributes of the final product as well as the prediction of the energy efficiency of these heating processes. Radio-frequency (RF) heating offers a solution to this

issue, since it uses electromagnetic energy of a longer wavelength than microwaves (MWs), which is of greater industrial interest. RF heating targets the product, not the air surrounding it. In fact,

because the interior of the product gets hot faster than the surface, RF treatment tends to drive the moisture from inside outward, equalizing moisture throughout the product and avoiding

overheating and over-dehydration of the surface of the product. Many applications using RF heating as supplemental heat have been developed successfully in the food-drying industry for pasta,

crackers, and snacks.

Fig. 2. A schematic

sketch of the 2-kW,

27.12 MHz radio

frequency unit

(Awuah, 2005)

Fig. 1 Dipole heating (http://www.stalam.it/)

RADIOFREQUENCY

TREATMENT