Factors affecting the inactivation of micro-organisms by intense light pulses

Authors


Frank Devlieghere, Laboratory of Food Microbiology and Food Preservation, Ghent University, Coupure Links, 653, 9000 Ghent, Belgium
(e-mail: frank.devlieghere@ugent.be).

Abstract

Aim:  To determine the influence of several factors on the inactivation of micro-organisms by intense light pulses (ILP).

Methods and Results:  Micro-organisms on agar media were flashed 50 times under different conditions and their inactivation measured. Micro-organisms differed in sensitivity to ILP but no pattern was observed among different groups. Several enumeration methods to quantify the effect of ILP were investigated and showed relevant differences, shading effect and photoreactivation accounted for them, the strike method yielded the most reliable results. Higher decontamination efficiencies were obtained for Petri dishes located close to the strobe and inside the illumination cone. Decontamination efficacy decreased significantly at contamination levels >6·85 log10. After 13 successive treatments, no resistance to ILP could be demonstrated. Media warming up depended on the distance from the strobe and the number of flashes.

Conclusions:  For an industrial implementation: the position and orientation of strobes in an unit will determine the lethality, products should be flashed as soon as possible after contamination occurs, a cooling system should be used for heat-sensitive products and flashed products should be light protected. No resistant flora is expected to develop.

Significance and Impact of the Study:  Conclusions derived from this work will allow a better implementation of this decontamination technique at industrial level.

Introduction

Intense light pulses (ILP) is a novel technique proposed to decontaminate surfaces by killing micro-organisms using short time high frequency pulses of an intense broad spectrum, rich in UV-C light. It has potential application for the treatment of food, packages, medical devices, packaging and processing equipments for the food, medical and pharmaceutical industries, and water and air (Dunn et al. 1997). The following names have been used by different authors to denominate techniques that can be included in the definition of ILP: pulsed UV light (Sharma and Demirci 2003), high-intensity broad spectrum pulsed light (Roberts and Hope 2003), pulsed UV disintegration (Wekhof 2000) (this term does not comprehend all kinds of ILP treatments), pulsed light (Rowan et al. 1999) and pulsed white light (Marquenie et al. 2003). This technology is grouped by food scientists in the frame of the so called nonthermal technologies, which are intented to produce stable and safe food products without those damages provoked by heating.

Different mechanisms have been proposed to explain the lethal effect of ILP, all of them related with the UV part of the spectrum and with its photochemical and/or photothermal effect (Anderson et al. 2000; Wekhof 2000; Wekhof et al. 2001; Takeshita et al. 2003; Wuytack et al. 2003).

The literature on ILP is actively growing but a gap between basic and applied research is notorious in its application to food decontamination. Additionally, results from applied research on the effect of ILP are frequently explained using the current knowledge from studies on the effect of continous UV; however, despite the spectrum of light used in ILP studies it has a very important UV component and both mechanisms of microbial inactivation are not necessarily equivalent. As a simple example, MacGregor et al. (1998) wrapped their plates in aluminium foil after flashing to avoid photoreactivation, although this phenomenon has not been reported for ILP-treated microbes.

In vitro studies by MacGregor et al. (1998), Rowan et al. (1999) and Anderson et al. (2000) showed the capability of ILP to achieve high lethality on bacteria and mould spores, and Roberts and Hope (2003) showed its capacity to inactivate viruses important in therapeutic biological products. Fine and Gervais (2004) have recently used the term ‘mean energy level threshold’ to identify a fluence level beyond which the inactivation is increased dramatically.

However, in vivo studies have been focused on specific applications. Huffman et al. (2000) reported the inactivation of Klebsiella terrigena, the parasite Cryptosporidium parvum and strains of polio and rotavirus in water. Hoornstra et al. (2002) achieved more than 2 log10 reductions in the aerobic count of four from five vegetables after two flashes, while Marquenie et al. (2003) found no effect of ILP on the suppression of Botrytis cinerea rotting of strawberries. Jun et al. (2003) modelled the inactivation of Aspergillus niger spores on maize meal, reporting up to 4 log10 reduction, that the shorter the distance and the higher the time and voltage the higher the lethality, and too high sample temperatures under some experimental conditions. Similar conclusions regarding distance, time and temperature have been reported by Sharma and Demirci (2003) when modelling the inactivation of Escherichia coli O157:H7 on alfalfa seeds, and additionally finding that the thicker the seed layer the lower the inactivation. Fine and Gervais (2004) failed to achieve 1 log10 reduction in the natural flora of wheat flour and black pepper, yet the treatment burned both products.

More extensive basic research is needed to understand how ILP affects micro-organisms in order to make the best profit of this decontamination technique. This study therefore (i) includes a critical evaluation of enumeration methods to determine the number of survivors of ILP, verifying the existence of the photoreactivation phenomenon in flashed micro-organisms, (ii) extensively surveys the lethal effect of ILP on a wide variety of micro-organisms, (iii) investigates how the relative position of the sample with respect to the strobe affects the lethality of the process, (iv) the influence of the size of microbial populations on the level of decontamination by ILP, (v) the possible development of resistance to ILP and (vi) the heating of the supporting medium by ILP treatment; emphasizing the implications of the results for the food processing industry.

Materials and methods

ILP equipment

The test assembly consisted of a housing, a stroboscope and a lamp. The housing was a rectangular parallelepiped stainless steel box (length: 60 cm, width: 40 cm, depth: 30 cm). The stroboscope was a 100 W apparatus (ST100-IE, Sysmat Industrie, St. Thibault des Vignes, France), pulse duration of 30 μs and pulse intensity of 7 J. The stroboscope has two modes for controlling flash trigger, a manual mode or an automatic mode, in the first each flash is generated by pressing a switch, in the second successive flashes at a frequency of 15 Hz are triggered after pressing the switch and stopped when the switch is pressed once more. The lamp was a 14 cm cylindrical Xenon flash lamp (Hamamatsu, Shizuoka-ken, Japan), with an emitted spectrum ranging from UV-C to infrared, with 50% of the light in the UV region; Fig. 1 shows the spectral distribution of the light as reported by the fabricant. The lamp was horizontally placed at 1 cm of the axis of a stainless steel half cylinder of 15·5 cm of arc, 14 cm of length and 10 cm of width; and this cylinder was horizontally placed without protruding at the centre of the ceiling oriented along the longitudinal axis. As the lamp was longer than the 8·5 cm diameter plastic Petri dishes used in the tests which were placed directly below the lamp unless otherwise noted, they received homogenously the light. A mechanical device allowed to change the distance between sample and probe by moving the shelf where plates were placed.

Figure 1.

Spectral distribution of the used xenon lamp. Source: Hamamatsu (1999)

Standard treatment

A standard treatment was used in the experiments, unless noted otherwise, using a distance between the strobe and the surface of spread-inoculated Petri dishes of 8·4 cm, and 50 flashes per treatment; every flash was manually started at a rate of one pulse per second. The lethality of the process was evaluated establishing the degree of micro-organism inactivated by subtracting the logarithm of the count after processing from the logarithm of the count before processing. All the experiments were carried out in triplicate using the same culture on the same day for all three trials to avoid sample variability.

Microbial strains and spore preparation

The species of micro-organisms used in these experiments, their origin, culture media and incubation temperature are summarized in Table 1. Bacteria from pure cultures stored on polystyrene pearls at −72°C were transferred to 5 ml of a suitable liquid medium (Table 1) and grown at optimal temperature (Table 1) for 48 h, then transferred to 5 ml of the same medium and grown at optimal temperature for 24 h. They were stored at 7°C on slants of the suitable medium (Table 1) and monthly checked for purity and renewed.

Table 1.  Origin and culture conditions of the used micro-organisms
NameCode*MediaIncubation temperature (°C)
Liquid†Solid‡
  1. *Culture collection codes: ATCC, American Type Culture Collection, Rockville, MD, USA; CBS, Centraal Bureau voor Schimmelcultures, Baarn, the Nederlands; LMG, Laboratorium Microbiologie, Universiteit Gent, Belgium; NRRL, Northern Regional Research Laboratory, USDA, Peoria, IL, USA; WS, Bakteriologishes Institut, Süddeutsche Versuchs- und Forschungsanstalt für Milchwirtschaft, Techische Universität München, Freising-Weihenstephan, Germany.

  2. †Liquid media codes and sources: BHI, brain heart infusion (Oxoid, Hampshire, UK); MRSB, de Man, Rogosa and Sharpe broth (Oxoid); OSB, orange serum broth (Beckton Dickinson, Cockeysville, MD, USA); SAB, sabouraud liquid medium (Oxoid); TGM, thioglycolate medium (BBL, Le Pont De Claix, France).

  3. ‡Solid media codes and sources: MRSA, de Man, Rogosa and Sharpe agar (Oxoid); NA, nutrient agar (Oxoid); OSA, orange serum agar (Beckton Dickinson); PDA, potato dextrose agar (Oxoid); YGC, yeast glucose choramphenicol (Sanofi Diagnostic Pasteur, Marnes-La-Coquette, France).

Aeromonas hydrophilaLMG 3771BHINA30
Alicyclobacillus acidoterrestrisATCC 49025OSBOSA44
Aspergillus flavusCBS 131·61 10/8/9 PDA30
Bacillus cereusLMG 6924BHINA30
Bacillus circulansIsolated from potato puree of a refrigerated mealBHINA30
Botrytis cinerea  PDA30
Brochotrix thermosphacta 1Isolated from cut degreased hamBHINA30
Candida lambicaIsolated from mixed lettuce stored at 7°CSABYGC30
Clostridium pasteurianumLMG 3285TGMNA37
Clostridium perfringensLMG 11264TGMNA37
Clostridium sporogenesLMG 8421TGMNA37
Enterobacter aerogenesLMG 2094BHINA37
Escherichia coliLMG 8223BHINA37
Klebsiella oxytocaLMG 3055BHINA37
Lactobacillus sake subsp. carnosisIsolated from cooked hamMRSBMRSA30
Leuconostoc mesenteroïdesNRRL 1335MRSBMRSA30
Listeria monocytogenesLMG 13305BHINA30
Photobacterium phosphoreumLMG 4233BHINA30
Pseudomonas fluorescensLMG 1794BHINA30
Rhodotorula mucilaginosaIsolated from mixed lettuce stored at 7°CSABYGC30
Salmonella typhimuriumLMG 10396BHINA37
Shewanella putrefaciensLMG 2250BHINA37
Shigella flexniiLMG 10472BHINA37
Staphylococcus aureusLMG 8224BHINA37
Yersinia enterocolitica O:9WS 24/92BHINA37

Conidia suspensions were obtained by growing moulds on potato dextrose agar at 30°C. Aspergillus flavus produced sufficient spores after 6 days, B. cinerea after 3 weeks. The slants were afterwards washed with 5 ml of physiological saline solution (PSS) [1 g peptone (Oxoid) and 8·5 g NaCl (Vel, Leuven, Belgium) per litre of water] with 0·01% Triton X-100 (Sigma, USA). This suspension was filtered to remove mycelium fragments and then centrifuged at 3·700 g. Finally, the detergent was replaced by a 10 mmol l−1 phosphate buffer pH 7·2 and the spores were used immediately.

Bacteria endospore suspensions were obtained plating a 24-h culture and incubating for 4 days to get sporulation. The plates were washed with 10 ml of PSS and this suspension was heated at 80°C for 10 min to kill vegetative cells. Alicyclobacillus acidoterrestris was incubated for 5 days, the culture was washed with 10 ml of sterile apple juice and this suspension was heated for 20 min at 70°C. The heat-shocked endospore suspensions were stored at 0–2°C and checked by microscopy before starting experiments to assure that they had not germinated. Serial dilutions were made by transferring 1 ml of culture to 9 ml of PSS.

Enumeration methods

Three enumeration methods to determine the number of survivors were tested in this study.

Incubation method.  A quantity of 15 g ± 0·1 g of the appropriate agar for each micro-organism (Table 1) contained in Petri dishes was rubbed with 0·1 ml of the different dilutions of a 24-h culture. After drying for 1 h on the laboratory table to avoid light attenuation due to PSS, the plates were flashed, then immediately wrapped in aluminium foil (unless otherwise noted) to prevent photoreactivation and incubated without further treatment for 48 h. Then the separate colonies were counted. The limit of detection was one colony.

Strike method.  The strike method is similar to the incubation method but after flashing, 0·1 ml of PSS with 0·5% Tween-20 (Merck, Hochenbrunn, Germany) was brought on the agar and this fluid was spread thoroughly. Then the plates were wrapped in aluminium foil, incubated for 48 h and then the separate colonies were counted. The limit of detection was one colony.

Plating method.  In the plating method, the agar (15·0 g ± 0·1) was immediately removed after flashing aseptically from the dishes, and weighted and mixed with PSS in a Stomacher bag making a 10-fold dilution. This suspension was homogenized thoroughly by means of a Colworth Stomacher 400 (Steward Laboratory, London, UK) and 10-fold dilutions were prepared, plated, incubated for 48 h, and the individual colonies were afterwards counted. The limit of detection was 10 colonies.

Sensitivity of micro-organisms to ILP

For this assay, the micro-organisms showed in Table 1 were flashed using the standard treatment and survivors were enumerated according to the plating method on the appropriate agar. The lower the degree of inactivation the more resistant was the micro-organism. From these results, one bacteria was chosen as indicator micro-organism.

Evaluation of enumeration method

Enumeration method reliability.  To compare the reliability of the plating method and the incubation method to enumerate survivors of ILP, plates containing the appropriate agar were inoculated with Enterobacter aerogenes, Lactobacillus sake or Listeria monocytogenes, flashed according to the standard treatment and survivors were enumerated by the incubation method or the plating method. Shading effect and photoreactivation experiments were designed in order to explain the observed differences.

Shading effect.  Agar medium plates inoculated with L. monocytogenes (106 CFU cm−2) were flashed according to the standard method and the survivors were enumerated with the incubation method or the strike method. Three tests under identical conditions were run.

Photoreactivation.  Foods and food samples are exposed to light during processing and microbiological analyses respectively. This experiment was designed to evaluate the effect of sunlight and artificial light on the reactivation of damaged cells under conditions that simulate what could occur during real-life conditions. To test the possibility that photoreactivation occurs, agar medium plates inoculated with L. monocytogenes were flashed according to the standard method and subjected to two tests. In the first, three dishes were wrapped with aluminium foil immediately after flashing, and three others were placed on the laboratory table and let them get illuminated by sunlight for 4 h. The same scheme was used in the second test, but with fluorescent light coming from eight TL-lamps sited 2·5 m above the dishes under normal conditions occurring in a microbiological analysis laboratory. The survivors were enumerated by the incubation method.

Influence of the relative position between sample and lamp on the decontamination efficacy

Vertical distance. Listeria monocytogenes was flashed at vertical distances between Petri dishes and strobe ranging from 3 to 27 cm, according to the strike method. The inactivation of micro-organisms by ILP is based on two mechanisms: the photochemical inactivation by the UV portion of the light of the lamp and the photothermal inactivation by the total spectrum of this light. When inactivation is only due to UV light, the relationship between surviving micro-organisms and the distance from the lamp can be described by the equation:

image

which is a simplification of an equation of Hiramoto (1984).

The photothermal inactivation is dependent on the received light intensity. When it is assumed that the lamp is a point source, the received light intensity is inversely dependent of the distance. The number of survivors is therefore inversely related with the distance. For the photochemical effect, data were therefore fitted to the equation:

image

where No and N are the number of micro-organisms respectively before and after UV treatment; x is the distance from the lamp to the agar surface (cm); A is a dimensionless constant; B is a constant specific to each micro-organism (cm−1); C is a constant specific to each micro-organism (cm2).

Position with respect to the lamp.  Twelve agar media in plates were inoculated with L. monocytogenes and placed on a 43 × 32 cm flat shelf in a 3 × 4 arrangement and flashed. The experiment was carried out at 6 and 13 cm vertical distances between shelf and lamp and the inactivation at different positions on the shelf (see Fig. 3) was measured for each distance by the strike method. Data were collected in triplicate.

Figure 3.

Effect of the relative position of inoculated Petri dishes on a shelf, on the inactivation of Listeria monocytogenes after 50 flashes at 6 cm (a) and 13 cm (b). Numbers inside the Petri dishes are log10 CFU cm−2 reductions attained in each dish. The rectangle at the centre of each panel represents the lamp

Influence of the population size on decontamination efficacy

To determine the influence of the bacteria population size on the decontamination efficacy of ILP, several agar media in Petri dishes were inoculated with 0·1 ml of the 10-fold dilution of a 24-h culture of L. monocytogenes and stored at 7°C, temperature in which this bacterium can grow. Plates were taken out at determined times, flashed and survivors were enumerated by means of the plating method. As an agar medium, nutrient agar 5·12% of NaCl (aw = 0·970) was used to ensure the presence of a significant lag phase in the growth curve.

Development of resistance

To evaluate the possibility of building resistance to ILP, three subcultures from a L. monocytogenes culture were taken and three dilutions prepared out of each one, these subcultures were flashed and enumerated by the strike method. After an incubation time of 48 h and survivors enumeration, one colony from each subculture and dilution was picked up, cultured for 24 h in Brain Heart Infusion broth, plated and flashed again; the procedure was performed 13 times. The significance of the linear regression model (inactivation = D × repetition + E) was evaluated by an F-test.

Media heating

To examine the heating of the surface and the distribution of heat throughout the agar, three optical fibres (Reflex, Quattro model, Aims Optronics, Kraainem, Belgium) linked to a computer with the appropriate software (Noemi Assistant 1·5, Aims Optronics) were inserted through the bottom of Petri dishes completely filled with fresh nutrient agar (agar layer 12 mm high) whose colour is pale yellow and placed at a certain depth in the agar (1, 5 and 10 mm from the surface, measured by a caliper). The plates were treated with 50 flashes generated manually, or 195, 495 and 990 flashes generated automatically; and the computer registered the temperature increase during treatment. Each data point represents the mean of three measurements.

Statistical analyses

Data were analysed by using spss 11·0 (SPSS Inc., Chicago, IL, USA), with P ≤ 0·05. A t-test was used to compare pairs of means; when multiple mean comparison was made, one-way anova and the Duncan test were used.

Results

Sensitivity of micro-organisms to ILP

Table 2 summarizes the results obtained in the sensitivity assay, inactivation levels ranged from 1·2 to >5·9 log10 CFU cm−2. No clear pattern could be observed regarding the sensitivity of the different groups of micro-organisms and very high inactivation levels make impossible statistical comparisons to differentiate sensitivities. Listeria monocytogenes was chosen as an indicator organism for the rest of the experiments because this micro-organism was among the most resistant to ILP and forms an important problem in the food industry. Regarding the results presented for conidia inactivation, it is possible that during the 1 h drying step of the cultures before flashing, some conidia had germinated. As vegetative cells are generally more sensitive to inactivation treatments than spores, part of the observed reduction in the population of A. flavus and B. cinerea could actually be accounted by the killing of vegetative cells coming from germinated spores; therefore the corresponding results have to be evaluated with precaution.

Table 2.  Initial counts and inactivation (log10 CFU cm−2) after 50 flashes at 8·4 cm for different groups of micro-organisms
Micro-organismInitial countInactivation ± SD
  1. *When the inactivation was greater than the detection limit, variance could not be calculated.

Gram-negative spoilers
 Photobacterium phosphoreum4·8>4·4*
 Pseudomonas fluorescens5·64·2 ± 1·0
 Shewanella putretaciens5·13·9 ± 0·8
Gram-positive spoilers
 Alicyclobacillus acidoterrestris5·7>5·2*
 Bacillus circulans4·5>4·1*
 Brochotrix thermosphacta3·73·1 ± 0·3
 Lactobacillus sake5·02·5 ± 0·2
 Leuconostoc mesenteroides5·04·0 ± 0·8
Enterobacteriaceae
 Enterobacter aerogenes5·42·4 ± 0·5
 Klebsiella oxytoca5·14·2 ± 0·6
Gram-negative pathogens
 Aeromonas hydrophila5·52·3 ± 0·3
 Escherichia coli5·34·7 ± 1·3
 Salmonella typhimurium5·43·2 ± 0·7
 Shigella flexnii5·13·8 ± 0·9
 Yersinia enterocolitica4·83·9 ± 0·5
Gram-positive pathogens
 Bacillus cereus3·4>3·0*
 Clostridium perfringens3·3>2·9*
 Listeria monocytogenes5·02·8 ± 0·4
 Staphylococcus aureus5·5>5·1*
Yeasts
 Candida lambica3·42·8 ± 0·4
 Rhodotorula mucilaginosa3·2>2·8*
Conidia
 Aspergillus flavus5·22·2 ± 0·1
 Botrytis cinerea4·11·2 ± 0·1
Bacterial spores
 Alicyclobacillus acidoterrestris3·32·5 ± 0·4
 Bacillus circulans5·73·7 ± 0·3
 Bacillus cereus6·3>5·9*

Evaluation of enumeration method performances

Enumeration method reliability.  The enumeration method affected the quantification of the level of inactivation because of ILP significantly (P ≤ 0·05). The inactivation measured for E. aerogenes by means of the incubation method was 4·9 ± 0·4 log10 CFU cm−2 while by the plating method was 3·6 ± 0·6 log10 CFU cm−2; the results for Lact. sake were respectively 3·5 ± 0·2 and 2·7 ± 0·5, and for L. monocytogenes 3·6 ± 0·4 and 2·4 ± 0·3. These findings demonstrate that the degrees of decontamination measured by the incubation method are on average 1·2 log10 CFU cm−2 higher than those measured by the plating method. To test the hypothesis that the difference can be attributed to the shading effect and/or photoreactivation, two new experiments were performed with L. monocytogenes.

Shading effect.  When the shading effect was tested, the incubation method yielded in a first test an inactivation level of 4·3 ± 0·2 log10 CFU cm−2 and the strike method 3·4 ± 0·6 log10 CFU cm−2, in the second test the results were respectively 4·7 ± 0·2 and 3·8 ± 0·7; and in the third one 4·5 ± 0·3 and 3·4 ± 0·4, all the experiments yielded statistically significant differences, which show that the inactivation was on average 1 log10 CFU cm−2 higher when measured by the incubation method than by the strike method.

Photoreactivation.  To test the possibility that photoreactivation occurs, ILP-treated plates were exposed for 4 h to sunlight or fluorescent light before wrapping in aluminium foil for further incubation. Nonexposed plates were incubated as control. The inactivation measured in the test using sunlight was 4·3 ± 0·3 log10 CFU cm−2 in immediately wrapped Petri dishes and 4·0 ± 0·2 log10 CFU cm−2 in those wrapped after 4 h of illumination. When fluorescent lamps were used, the respective results were 4·7 ± 0·2 and 4·4 ± 0·2 log10 CFU cm−2. Only in the experiment with lamps the differences were statistically significant (P ≤ 0·05) and the inactivation was 0·3 log10 CFU cm−2 higher in the plates immediately wrapped after flashing.

Influence of the relative position between sample and lamp on the decontamination efficacy

Vertical distance. Figure 2 shows the relationship between the inactivation and the vertical distance from lamp to sample. Distance clearly influenced negatively the inactivation efficacy of ILP; the longer the distance the lower was the inactivation. When decontamination is assumed to occur only due to the photothermal effect there is an exponential relationship between ln(N/No) and the distance. However, the coefficient of determination between the experimental values and the plot was only 0·69. When the photochemical effect is assumed as only inactivation factor and the inactivation is correlated with the quadrate of the distance, a coefficient of determination equal to 0·88 was obtained. When both inactivation factors were assumed as complementary, then a coefficient of determination equal to 0·96 was obtained, and the equation is as follows:

image
Figure 2.

Effect of the distance between the strobe and the surface of spread-inoculated Petri dishes on the inactivation of Listeria monocytogenes after 50 flashes. Bars indicate ±SD. N and No: number of micro-organisms respectively before and after flashing. Solid line is the trend line

The photochemical inactivation is set out in the term with the exponential form, the photothermal inactivation is retrieved in the term with the quadratic form. The fitting parameters and their 95% confidence interval are: A = −11·34 ± 1·64, B = 0·005 ± 0·021 cm−1 and C = 97·15 ±63·69 cm2.

Position with respect to the lamp. Figure 3 illustrates the effect of the position of Petri dishes on a shelf on the inactivation of L. monocytogenes at two vertical distances from the lamp. The results can be divided in two groups: (i) those from the two central Petri dishes, and (ii) those from plates placed at the periphery. At 6 cm, the inactivation (log10 reductions) for the first group was very high (>5) and for the second was low (0·0–1·0). At 13 cm, the inactivation for the first group was between 3·2 and 4·0 and for the second between 0·5 and 2. At both distances the inactivation is higher directly below the lamp than at the borders of the shelf. When comparing both distances, the inactivation directly below the lamp decreases with the distance, but increases at the borders of the shelf. An idea of the shape of the light can be deducted from overlapping the two panels presented in Fig. 3.

Influence of the population size on decontamination efficacy

Bacteria can possibly react differently to ILP depending on the growth stage and population size. Figure 4 shows the changes in the level of inactivation as a function of the number of micro-organisms present on the surface of an agar medium as a result of surfacial growth. According to results of the multiple mean comparison test, the inactivation was greater during the lag and early stationary phases. After 138 h the counts had increased to approx. 6·85 log10 CFU cm−2 and the inactivation decreased significantly. When after 289 h the count reached approx. 7·5 log10 CFU cm−2, the inactivation kept constant at approx. 1·2 log10 CFU cm−2.

Figure 4.

Mixed effect of the state of growth and population size on the inactivation of Listeria monocytogenes after 50 flashes at 8·4 cm. Initial counts (bsl00001), log10 reductions (○). Bars indicate ±SD

Development of resistance

In Fig. 5, an example of the inactivation of cells of one strain of L. monocytogenes after successive flashing with the corresponding regression line is given. The multiple correlation coefficient examined by an F-test was equal to 0 for the three strains. This means that there was no correlation between the dependent variable, the inactivation, and the independent variable, the number of times that the flashing procedure was applied. This was the case for all tested dilutions.

Figure 5.

Effect of repetitive flashing (50 flashes at 8·4 cm) on the inactivation of one strain of Listeria monocytogenes. Equal symbols represent that the cell treated in a specific repetition was isolated from a colony from the previous repetition represented by the same symbol. The trend line was obtained from all the data showed

Media heating

Due to the presence of visible and infrared light together with UV light in the spectrum emitted by the lamp, the surface of the product can warm up during flashing. The results presented in Fig. 6 show clearly that both, the number of flashes and the location of the sensor on a particular depth in the agar influence the temperature increase; the higher the number of flashes and the lower the depth, the higher the measured temperature increase. The increase of the temperature at 10 mm depth was low in comparison with that at 1 mm. At a depth of 10 mm the temperature difference after 990 flashes was 0·8°C, whereas at a depth of a 1 mm this difference amounted 12·2°C. The link between the temperature increase and the depth in the agar is exponential and depends on the number of flashes. Table 3 shows the corresponding equations.

Figure 6.

Effect of the number of flashes and the depth of the agar on the temperature increment of agars flashed at 8·4 cm from the strobe. Bars indicate ±SD. bsl00001 50 flashes, bsl00023 195 flashes, bsl00004 495 flashes, bsl00000 990 flashes

Table 3.  Equations and determination coefficients for the relationship between the temperature increment (y, °C) and the depth of the agar (x, mm) for different number of flashes, at 8·4 cm of distance
No. of flashesEquationR2
50y = 0·80x−0·660·99
195y = 3·60x−0·820·97
495y = 8·32x−1·080·99
990y = 13·42x−1·110·96

Discussion

From the results it is clear that high decontamination effects of ILP were observed on inoculated agar media. No clear line could be drawn between the sensitivity of the different investigated groups of micro-organisms. Rowan et al. (1999) reported that Gram-positive bacteria are more resistant than the Gram-negatives although the pattern they presented is not very sharp; the same trend was reported by Anderson et al. (2000) but comparing only the resistance to ILP of three bacteria.

Once the indicator micro-organism (L. monocytogenes) was selected, a protocol had to be stipulated to measure the degree of inactivation by ILP. The only method that appears in the literature to test ILP lethality on agar surfaces is the incubation method (Dunn et al. 1997; MacGregor et al. 1998; Rowan et al. 1999; Anderson et al. 2000). However, when the degree of contamination of a food needs to be measured, the plating method is applied. Our results show that the enumeration method affects the results of the determination of the lethality caused by ILP, i.e. a higher inactivation was measured by the incubation method when compared with the results obtained by the plating method, therefore a critical assessment of the incubation method was designed in terms of possible shading effect and/or photoreactivation. All the enumeration methods included an 1-h drying step between inoculation and flashing to let agars absorb PSS, which could have interfered with the light transmission towards cells; although it is possible that during that time some cells had undergone division, they should likely have been in stationary phase.

The average size of a bacterium is 1 μm × 3 μm assuming that it has a rectangle form. This means that a monolayer of bacteria will have 3 × 107 bacteria cm−2. To test the shading effect, the Petri dishes were inoculated with 106 CFU cm−2. Although its spreading was carried out carefully, it is practically impossible to avoid some overlapping. This implies that during flashing, those bacteria located on top will be easily reached by the light and are consequently inactivated, whilst those that are covered will have a greater chance to survive the treatment due to the screening of the light by the bacteria located on top, although at high fluence rates the light should be able to penetrate deeper. As in the incubation method the survivors are not further treated after flashing, if two or more surviving micro-organisms are situated very close to each other on the surface of the agar they will develop as a single colony, which will lead to an overestimation of the lethality of the process. However, when micro-organisms are once more spread after flashing, as it is carried out by the strike method, they become separated from each other (although the efficiency of this separation is not necessarily 100%), and develop separately, consequently more survivor colonies are counted and the higher inactivation measured by the incubation method in comparison with the strike method can be attributed to the shading effect. For publications on ILP, it is therefore important to realize that the incubation method does not reflect the reality on food products and that it is possible that levels of inactivation from in vitro tests reported in the literature have been overestimated due to the limitations of the incubation method.

The photochemical effects of UV light on some living beings, including the micro-organisms, can be reversed by illumination with longer wavelengths, especially visible light, a repairing mechanism called photoreactivation (Cleaver 2003). As consequence, when a sample which was treated with UV radiation is exposed to common light (sunlight or light bulbs), the survivor counts will be higher than those from samples stored at darkness immediately after flashing. As the lethality of ILP is caused at least partly by the photochemical effect of the UV light, it can be deducted that photoreactivation can occur after flashing, which was observed in our experiments with ILP. To the date, this effect has not been described for ILP, in spite of that some authors (MacGregor et al. 1998; Rowan et al. 1999; Anderson et al. 2000) have taken the precaution of wrapping their Petri dishes in aluminium foil to avoid it.

It is realistic to claim that photoreactivation must have been present during the implementation of the plating method, as this was not performed in darkness, while in the strike method the plates suffered light exposition only the time necessary to pour the solution of Tween-20 and spread it. Moreover, the plating method is time consuming and difficult to perform in complete absence of light, as the agar has to be separated from the Petri dish, weighted, diluted with PSS, homogenized by the Stomacher, a sample has to be taken out, serial dilutions prepared, plated, agar has to be pour and let solidify. Hence, both the incubation method and the plating method have shown to have important disadvantages when applied for the enumeration of light-treated samples. With the first one the lethality is overestimated with about 1 log10, whilst with the second one it is impossible to eliminate photoreactivation, resulting in a part of the damaged cells that are able to repair themselves (0·3 log10). The strike method do not have these disadvantages, although it can suffer from an incomplete separation of all cells. Therefore it is recommended as the most reliable enumeration method for studies on ILP.

Our results show and quantify the existence of photoreactivation after ILP treatment, which has two practical consequences. From the experimental point of view, plates that are not kept in darkness after plating will lead to an underestimation of the killing effect or a high variability as noted in the pioneer work by Kelner (1949); from an industrial point of view, it will be important to keep treated products in darkness at least the first hours after treatment or take in to consideration the photoreactivation when estimating the shelf-life. In what extend photoreactivation might have influenced the results reported in the literature on in vivo experiments needs to be evaluated.

When the effect of the vertical distance from agar to strobe was tested, the results showed what was expected, i.e. the longer the distance between sample and lamp, the lower the lethality of the process. Fitting the results to equations also showed that both the photochemical and the photothermal effects are involved in the inactivation by ILP, the equation combining them explained 96% of the sample variation. Sharma and Demirci (2003) and Jun et al. (2003) also reported quadratic equations to model the effect of distance from agar to strobe on the level of inactivation.

If a number of food pieces are treated and the lamp is placed very close, then those pieces located near to the lamp will be efficiently decontaminated, but the others will undergo almost no decontamination. If we can put the group of samples farther from the lamp, the decontamination will be less intense in those samples located closer to it but the rest of the samples will be also decontaminated. This effect has important practical consequences from an industrial point of view, the position and orientation of strobes in an industrial decontamination unit will determine in a great extend the intensity of the light reaching the product, and as such the decontamination efficacy.

When foods get contaminated by bacteria, the cell population is initially low and become higher during time. In this work, the experiment on the effect of the population size on the inactivation efficiency of ILP was performed to simulate what occurs in real food systems, where the effect of population size and state of growth cannot be separated. In this experiment, the inactivation efficiency was strongly decreased when high numbers of L. monocytogenes were reached on the surface of agar medium. The drop in inactivation efficiency from a specific density of micro-organisms (6·85 log10 CFU cm−2) can be explained by the shading effect. During growth, bacteria are situated on top of each other in several layers forming a colony. Bacteria located on the upper layers receive the light directly and are easily inactivated. However, bacteria on the bottom layers are protected by those on the upper layers which screen the incident light and as a result, the survivor numbers are higher. When bacteria arrived to the stationary phase only the bacteria on the upper layer are inactivated and a large and constant part of the contaminating population will survive the treatment, resulting in a constant put poor inactivation. These results show that when ILP is implemented in the industry one must take into account the contamination degree of products. Heavily contaminated products can be less efficiently decontaminated due to the shading effect. The highest efficiency of the ILP will be reached if the food products are flashed as soon as possible after the processing steps where contamination can occur. In the case of fluid products, the same shading effect can be expected at high cell populations as was noted by Gashemi et al. (2003), a thorough mixing should alleviate this effect.

To implement a decontamination technology in the food industry it is important that micro-organisms do not develop resistance towards the decontamination method, as in this way a resistant house flora can develop hampering long-term efficiency of the technique. The degree of inactivation of L. monocytogenes achieved by flashing was independent of the successive flashing, culturing, plating and flashing again, i.e. in our experiments no resistance to ILP was demonstrated. Until now, no other reports on the development of resistance to ILP are available.

When the temperature of the agar during flashing was recorded, a significant warming was measured, especially near the surface of the agar medium and at a high number of flashes. In the literature, Anderson et al. (2000) investigated the temperature increase during flashing and they reported a negligible rise in temperature after it, although the measuring method was not described, while Krishnamurthy et al. (2004) reported about 20°C increment after flashing phosphate buffer and Baird–Parker agar during a 20-s treatment time. It would have been interesting is to measure the temperature increase in the surface of the agar, where micro-organisms actually form colonies, but it was impossible to place the sensor at a depth less than 1 mm without breaking the agar. To estimate the temperature increase in the surface of the agar, the equations given in Table 3 can be used. For example, at 0·001 mm from the agar surface, 50 flashes increase the temperature by 76°C, which will account for the photothermal effect modelled in this article. Nutrient agar is a pale yellow agar medium when solid, according to the spectrum of the light emitted by the lamp (Fig. 1) no special effect of the colour on the energy absorption by the agar could be expected. Despite differences can be expected between the temperature increase in agar and specific food surfaces, the warming up of samples must be taken into consideration for industrial applications. ILP has especially a potential minimal preservation technique, influencing as less as possible the organoleptical and nutritional quality of the treated product. For heat-sensitive products, such as minimally processed fruits and vegetables, products will have to be cooled during treatment, not only to abate the warming up due to light absorption by the product, but also to dissipate the heat generated by the lamp itself.

Acknowledgements

The authors thank the Consejo de Desarrollo Científico y Humanístico of Universidad Central de Venezuela for the PhD scholarship of V. Gómez.

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