Journal of Food Engineering 119 (2013) 254–259

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Journal of Food Engineering

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Decontamination of powdery and granular foods using Continuous WaveUV radiation in a dynamic process

0260-8774/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jfoodeng.2013.05.021

⇑ Corresponding author. Tel.: +32 14 56 23 10; fax: +32 14 58 48 59.E-mail address: jesse.stoops@khk.be (J. Stoops).

Jesse Stoops a,⇑, Matias Jansen b, Johan Claes a, Leen Van Campenhout a

a Lab4Food, Faculty of Engineering Technology, Department of Microbial and Molecular Systems, and Leuven Food Science and Nutrition Research Centre (LFoRCe),University of Leuven, Kleinhoefstraat 4, 2440 Geel, Belgiumb Kemin Europa NV, Toekomstlaan 42, 2200 Herentals, Belgium

a r t i c l e i n f o

Article history:Received 23 November 2012Received in revised form 24 April 2013Accepted 16 May 2013Available online 30 May 2013

Keywords:UV radiationDecontaminationPowdery foodGranular food

a b s t r a c t

The purpose of this study was to investigate the potential of Continuous Wave UV radiation, applied in adynamic process, to reduce Enterobacteriaceae counts in powdery and granular foods. Several food matri-ces were inoculated with a test strain (Escherichia coli LMG 8063) and radiated for 1 h with UV (254 nm,2.88 mW/cm2). The particles were constantly homogenized under the UV source. Depending on the foodmatrix, reductions of 0.7–4.8log cycles were obtained. For meringue chunks, a storage period of 20 h afterinoculation prior to UV treatment caused a smaller reduction of the test strain (2.7log cycles) than whenradiation was applied immediately after inoculation (4.8log cycles). Different initial contamination levelswere tested. Higher inoculation levels tended to yield lower reductions. The results demonstrate thatContinuous Wave UV radiation can be applied for microbial decontamination of specific powdery andgranular food products under continuous homogenization.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Ultraviolet (UV) light is an electromagnetic radiation in a wavelength range from 100 to 400 nm. This spectrum is subdivided intofour regions: UV-A (315–400 nm), UV-B (280–315 nm), UV-C(200–280 nm), and vacuum UV (100–200 nm) (Koutchma et al.,2009; Orlowska et al., 2012). The most important type of UV radi-ation for the inactivation of micro-organisms is UV-C (Gómez-López et al., 2007). UV beams with wavelengths of 200–300 nmpenetrate cell membranes and damage DNA, thus preventing rep-lication and leading to cell death (Rahman, 2007; Unluturk et al.,2008). For decontamination purposes, there are two modes ofapplication of UV radiation, i.e. Continuous Wave UV (CW UV)and Intense Light Pulses (ILP). In CW UV treatment, light of mainly254 nm is applied in a continuous way (Gómez-López et al., 2007;Oms-Oliu et al., 2010; Takeshita et al., 2003). Inactivation with ILPis achieved by using intense (e.g. 35 mW, Oms-Oliu et al., 2010),short duration pulses (1 ls to 0.1 s, Koutchma et al., 2009; Kowal-ski, 2009) of a broad spectrum white light rich in UV (200–1100 nm, Cheigh et al., 2012; Elmnasser et al., 2007). ILP is morerapid and effective for microbial inactivation compared to CWUV, because of its higher intensity (Cheigh et al., 2012; Oms-Oliuet al., 2010). The penetration depths of both CW UV and ILP arelimited, that of ILP being slightly higher than that of CW UV(Kowalski, 2009). As a consequence, the use of UV light is only

established so far for sterilization of water and some other liquids,for air disinfection, and for surface decontamination (Falgueraet al., 2011; Koutchma et al., 2009). In the food industry, applica-tions of UV-C radiation include air disinfection in meat or vegeta-ble processing, disinfection of process water, and reduction ofmicro-organisms in liquids (milk, fruit juice, cider) and on the sur-face of solid foods (meat, poultry, fish, fresh products) (Falgueraet al., 2011; Unluturk et al., 2008). In this study, CW UV was pre-ferred above ILP for legislative reasons. As stated by Gómez-Lópezet al. (2007), the European Union does not approve ILP in general,but it approves (after notification) specific foods and food ingredi-ents that are treated with ILP. Due to the time-consuming registra-tion process, in which physical and chemical equivalence to thenon-treated product must be proven, the food industry is by andlarge not inclined to implement ILP.

The difficulty to decontaminate powdery and granular foods isgenerally recognized (Fine and Gervais, 2004, 2005). In this study,the possibilities and limitations of CW UV radiation to decontami-nate powdery and granular foods were investigated. More specifi-cally, powdery and granular foods were studied that are used in anice cream production plant to coat individual ice cream portionsbefore they are packaged. In the production process, coating isaccomplished by dropping individual ice portions in a rotating cir-cular trough which is filled with the coating material (ground nuts,cacao powder, etc.). The coating material is constantly beinghomogenized by several baffles mounted in the trough. As thetrough is open, the coating material can be contaminated withmicro-organisms and in this way also the finished product. The

J. Stoops et al. / Journal of Food Engineering 119 (2013) 254–259 255

micro-organisms (mainly Enterobacteriaceae in ice cream factories)can be airborne and contaminate the coating material directly. Inaddition, as was observed in the production facility, micro-organ-isms can also contaminate the material via ice or water dropletsfalling from equipment situated above the trough. It was ques-tioned in this study whether the use of CW UV lamps above thetrough leads to a sufficient reduction of the microbial numbers ofthe coating material. This was investigated by constructing a labo-ratory-scale model of the rotating trough, mounting lamps abovethe trough and conducting inoculation experiments with coatingmaterials using target micro-organisms.

The specific case considered in this study leads to the formula-tion of a more general hypothesis. As indicated before, UV radia-tion has only a limited penetration depth. For powdery andgranular foods, however, continuous movement and homogeniza-tion increases the product surface exposed to the radiation andmay thus improve the efficiency of the application. For some liquidfoods UV radiation has been implemented as a dynamic process toenhance the microbiocidal effect (Chen and Han, 2009; Franz et al.,2009). This study investigates the potential of a similar mode ofapplication for some particulate dry foods.

2. Materials and methods

2.1. Experimental design

A first series of experiments was conducted with an Escherichiacoli strain obtained from a culture collection. The reduction inmicrobial numbers by dynamic UV treatment was investigatedwhen the test strain was inoculated onto a range of food matrices.In the first place, a high inoculation level of the strain was com-bined with a UV treatment immediately after inoculation. Sec-ondly, a high inoculation level was combined with a postponedUV treatment allowing the inoculum to adapt to the matrix. Inthe third place, experiments were performed with a low inocula-tion level of the target organism.

A second series of experiments was carried out using a mixtureof Enterobacteriaceae strains which were isolated from the produc-tion environment of an ice cream company.

2.2. Food matrices

Several food matrices, used to coat ice cream portions, were in-cluded in the experiments: Brésilienne nuts (0.0–0.9 mm), merin-gue chunks (0.0–0.5 mm), cacao powder (<75 lm), small groundhazelnuts (0.0–2.2 mm), large ground hazelnuts (0.0–4.0 mm)and a hazelnut-with-waffle-mixture (2.2–5.0 mm). Brésiliennenuts included a mixture of sweeted particles of peanut and hazel-nut. They were purchased from a local retail supermarket. Theother coating materials were obtained from an ice cream company.Initial levels of the background microflora (aerobic total viablecount and Enterobacteriaceae) of the coating materials were deter-mined as described in Section 2.7.

2.3. Microbial strains and inoculum preparation

E. coli LMG 8063, obtained from the Belgian Co-ordinated Col-lections of Micro-organisms (BCCM) was used as E. coli test strain.It was stored in glycerol (50% w/w) at �80 �C.

To prepare an E. coli inoculum, 17 ml Nutrient Broth (NB, BiokarDiagnostics, France) was inoculated with a stock culture of E. coliLMG 8063 and incubated at 28 �C for 16 h in a shaking water bath(GFL mbH 1083, Belgium, 190 rpm) to obtain a cell density of about109 cfu/ml.

Microbial strains were isolated from the production environ-ment of an ice cream company using contact plates (APTACA155, Italy) and the swab method (APTACA 2160/SG, Italy). Sampleswere collected at the following sites: the rotating circular trough,the engine block of the conveyor belt, the conveyor belt as such,and the cables above the conveyor belt. In an ice cream productionplant, Enterobacteriaceae are important indicators for process hy-giene. The objective was to isolate and study Enterobacteriaceaefrom the production environment. Therefore, contact plates werefilled with Violet Red Bile Glucose agar (VRBG; Biokar Diagnostics)and pressed for 10 s to the surface to be examined. The plates wereincubated at 37 �C for 24 h. Swabs were moistened with 10 mlpeptone physiological salt solution (0.85% (w/v) NaCl, 0.1% (w/v)peptone, Biokar Diagnostics) and rubbed over a surface of approx-imately 10 cm2. After sampling, swabs were transferred into thepepton physiological salt solution, the solution was homogenizedand 0.1 ml was inoculated on VRBG. The plates were incubatedat 37 �C for 24 h. Three presumptive Enterobacteriaceae colonieswere isolated from the engine block of the conveyor and subjectedto confirmation tests. After streak plating on Nutrient Agar (NA;Biokar Diagnostics) and incubation at 37 �C for 24 h, oxidase testswere performed using Oxidase Identification Sticks (Oxoid, Bel-gium) according to the manufacturers’ prescription. Negative colo-nies were investigated for glucose fermentation by picking themfrom the NA plates and stabbing into Glucose Agar Tubes (BiokarDiagnostics, at 37 �C for 24 h). A yellow colour throughout thewhole tube indicated a positive reaction. All three colonies wereconfirmed to be Enterobacteriaceae.

To prepare an inoculum of the Enterobacteriaceae strains, eachisolate was inoculated individually in 17 ml NB (Biokar Diagnos-tics) and incubated at 28 �C for 16 h in a shaking water bath(190 rpm). After incubation, equal aliquots of each individual iso-late were combined to produce a mixed culture.

2.4. UV Sensitivity of the target organisms

Prior to the dynamic radiation experiments, it was confirmedthat the test strains were sensitive to UV radiation when growingon the flat surface of a solid medium in a Petri dish. Two UV lamps(VL-206 G, Vilber Lourmat GmbH, Germany) emitting light ofmainly 254 nm were mounted in a wooden box (20.0 cm �25.0 cm � 19.5 cm). The box was covered with aluminum foil onthe inside. A Petri dish could be placed at the bottom of the boxand the distance of the lamps to the medium surface was14.5 cm. The UV-C light intensity was measured using a radiome-ter (UV–VIS Radiometer RM-21, Dr. Grobel UV-Elektronik GmbH,Germany) with UV-C sensor 811010 (Dr. Grobel UV-ElektronikGmbH) and was 3.36 mW/cm2. The radiometer was placed at asimilar distance from the UV lamps as the medium surface. Inoculawere prepared from the target organisms as described before (withthe three Enterobacteriaceae isolates being tested individually aspure cultures). The inocula were diluted with peptone physiologi-cal salt solution to achieve a cell density of approximately 103 to104 cfu/ml. Petri dishes containing 20 ml NA (Biokar Diagnostics)were inoculated on the surface with 0.1 ml of the diluted cell sus-pension. Prior to radiation, the UV-lamps were preheated for10 min. Then the plates were placed one by one under the UVlamps and radiated for 0, 2, 4, 6, 8, 12 or 15 s. Each exposure timewas replicated five times. After radiation, plates were incubated at37 �C for 24 h. Results are expressed as log cfu/cm2.

2.5. Equipment for the dynamic radiation experiments

For the dynamic radiation experiments, a laboratory-scale mod-el of the rotating trough used for the industrial coating process wasconstructed. Figs. 1 and 2 show the apparatus for the dynamic UV

Coating material

8.0 cm

35°

Rotating circular trough

16.5 cm

UV lampUV lamp

Baffle

4.0 cm

36.0 cm

8.0 cm

Fig. 1. Schematic drawing of apparatus for dynamic UV treatment.

Fig. 2. Apparatus for dynamic UV treatment.

256 J. Stoops et al. / Journal of Food Engineering 119 (2013) 254–259

treatment. The apparatus consisted of a rotating circular trough,baffles and UV lamps. The rotational speed of the stainless steeltrough in the downscale model was 5 rpm. In this way, the rota-tional speed of the food particles in the laboratory installationwas the same as in the industrial process. Homogenization of theproduct was performed in the same way as in the industrial pro-cess by two baffles (PVC in the downscale model). Two UV lamps(the same as described above) were mounted above the trough.The lamps and the treatment area were enclosed in a woodenbox which was covered on the inside with aluminum foil (Fig. 2).The UV light intensity in the dynamic experiment was measuredwith the radiometer described above and was 2.88 mW/cm2.

2.6. Dynamic radiation experiments

At the start of each radiation experiment, the treatment areawas cleaned and disinfected. The dynamic experiment was per-formed by transferring 100 g (meringue chunks and cacao powder)or 200 g (small ground hazelnuts, large ground hazelnuts, hazel-nut-with-waffle mixture and Brésilienne nuts) of coating materialin the rotating circular trough. The difference in weight for the foodmatrices was necessary to obtain the same height of the food layerin the trough for all experiments. Prior to inoculation, the food ma-trix was homogenized in the trough by rotating the trough for 36 s

(3 rotations). The coating material was inoculated with eitherE. coli LMG 8063 or the mixture of the three isolates, as describedbelow. Then the product, which was continuously blended in thedish, was radiated for 1 h. The reduction was expressed as log(N/N0), where N is the microbial count after the treatment (cfu/g)and N0 the initial microbial number (cfu/g). Each experiment wasreplicated five times.

For E. coli LMG 8063, the effect was investigated for all coatingmaterials with a high inoculation level and immediate radiation.The products were inoculated with 2 ml from the stationary phaseculture having a cell density of approximately 108 to 109 cfu/ml. Toachieve a homogenous inoculation, the coating material was inoc-ulated with 20 aliquots of 0.1 ml of the inoculum. In addition, formeringue chunks a high inoculation level was also combined witha postponed UV treatment allowing the inoculum to adapt to thematrix. The inoculated meringue was stored at 25 �C for 20 h be-fore UV treatment. Furthermore, the importance of the contamina-tion level to the microbiocidal effect of the treatment was studiedwith cacao powder as a matrix. In order to obtain a lower inocula-tion level, the stationary phase culture was diluted to a cell densityof approximately 106 cfu/ml prior to inoculation. Only 1 ml (in 10aliquots of 0.1 ml) of the inoculum was applied on the matrix.

For the Enterobacteriaceae isolates, meringue chunks were inoc-ulated with 2 ml of the mixture of three isolates, by adding 20times 0.1 ml of the mixed culture.

2.7. Microbial analysis of the food matrix

Microbial counts were carried out according to the ISO Stan-dards for microbial analysis of food as compiled by Dijk et al.(2007). For cacao powder and meringue, a sample of 10 g wastransferred aseptically to a sterile stomacher bag and 90 ml of pep-tone physiological salt solution was added. The mixture washomogenized for 2 min at high speed using a stomacher (Stom-acher 400, LED Techno, Belgium). For small and large ground hazel-nuts, hazelnut-with-waffle-mixture and Brésilienne nuts, a sampleof 20 g was transferred aseptically to a sterile stomacher bag and180 ml of peptone physiological salt solution was added. The mix-ture was homogenized for 15 seconds using a pulsifier (Pulsifier,LED Techno). Decimal dilutions were prepared using the same dil-uent and plated in duplicate on appropriate growth media. Countsof Enterobacteriaceae were determined on VRBG using the pour

Table 1Counts of E. coli LMG 8063 on different powdery or granular food matricesimmediately after inoculation (logN0) and reduction of the counts (log(N/N0)) after1 h UV radiation.

Food matrix Log N0 (log cfu/g) Log (N/N0) (log)

Meringue chunks 6.1 ± 0.6a �4.8 ± 1.0a

Cacao powder 6.5 ± 0.4a �0.8 ± 0.5b

Small ground hazelnuts 6.4 ± 0.3a �0.8 ± 0.5b

Large ground hazelnuts 6.2 ± 0.6a �0.7 ± 0.3b

Hazelnut-with-waffle mixture 6.6 ± 0.5a �0.9 ± 1.3b

Brésilienne nuts 6.1 ± 0.3a �1.7 ± 0.3b

Data are the means ± standard deviations of five replicates.a,b Means with the same superscript within the same column do not differ signif-icantly (p > 0.05).

J. Stoops et al. / Journal of Food Engineering 119 (2013) 254–259 257

plate technique. Plates were incubated at 37 �C for 24 h. Aerobicmicro-organisms were enumerated on pour plates of Plate CountAgar (PCA; Biokar Diagnostics) and incubated at 30 �C for 72 h. Re-sults are expressed as log (N/N0).

2.8. Statistical analysis

SPSS (IBM� SPSS Statistics ver. 19, New York) was used for sta-tistical analyses. One-way ANOVA was performed to investigatethe effect of UV radiation on E. coli LMG 8063 inoculated on differ-ent coating materials. Multiple Comparison was performed byDuncan’s post hoc test. The effects of the postponed radiation, theinoculation level and the target organism(s) were determined byperforming the Independent Samples T-Test. For all statistical anal-yses, a significance level of 0.05 was used.

3. Results and discussion

3.1. Background microflora of the food matrices

Coating materials obtained from the ice cream company (mer-ingue chunks, cacao powder, small ground hazelnuts, large groundhazelnuts, hazelnut-with-waffle mixture) showed initial levels oftotal aerobes as well as Enterobacteriaceae below the detectionthreshold (1.0 log cfu/g, calculated according to Dijk et al., 2007).The results were in conformity with the product specifications ofthe supplier.

Enterobacteriaceae counts of Bréselienne nuts obtained from thesupermarket were also below the detection limit. The initial countof aerobes was 2.9 log cfu/g.

3.2. UV Sensitivity of the target organisms

Prior to dynamic radiation experiments on complex food sur-faces, the UV sensitivity of the target organisms on the flat surfaceof a well-defined growth medium in a Petri dish was investigated.When no radiation was applied (treatment time of 0 s), the micro-bial load on the medium surface was about 300 cfu/plate or 4.7 cfu/cm2 for all four strains. After an exposure time of only 2 s, UV radi-ation prevented growth of the test strains almost completely, sincenumbers below the detection limit (0.02 cfu/cm2) were obtained.Obviously, no colonies of the target organisms were detected whenlonger exposure times were applied (4, 6, 8, 12 and 15 s). This indi-cated that E. coli LMG 8063 and the three Enterobacteriaceae iso-lates were very sensitive to UV radiation and therefore they weresuitable target organisms.

3.3. Radiation experiments with E. coli LMG 8063

The effect of UV radiation on the reduction of E. coli LMG 8063immediately after inoculation in different coating materials is gi-ven in Table 1. All coating materials contained initial counts ofEnterobacteriaceae between 6.1 and 6.6 log cfu/g immediately afterinoculation. These numbers can (almost) completely be attributedto the presence of the inoculated E. coli LMG 8063, as the Entero-bacteriaceae count of the uninoculated food matrices was below1.0 log cfu/g. There was no statistical difference between the initialcounts of the matrices (p = 0.55). The effect of the one hour radia-tion treatment was different for the food matrices (p = 0.00). A 1logreduction was observed for all products except for meringuechunks (4.8 log reduction) and Brésilienne nuts (1.7log reduction).For an inactivation technology to be commercially viable, reduc-tions of at least a few log cycles are aimed at. The reduction ob-tained for meringue was statistically higher than the reductionobtained for the other materials. Several reasons can be postulated

to explain the fact that the food matrix affects disinfection perfor-mance, the most important being (1) the topography of the foodsurface and (2) the tendency of the materials toward lump forma-tion. Previous research has addressed the role of the physical loca-tion of the micro-organisms and the (micro)structure of the foodsurface in the efficiency of UV radiation, both for CW UV and ILP(Allende et al., 2006; Chun et al., 2009; Woodling and Moraru,2005; Yaun et al., 2004). Surface roughness and crevices of dimen-sions comparable to the size of micro-organisms can shield micro-organisms from UV radiation and enable them to survive (Koutch-ma et al., 2009). In addition, food particles can shadow each otherwhen treated together (Gómez-López et al., 2007). The latter effectwas minimized in this study by agitation of the particles by therotation of the trough and the inclusion of baffles. Another impor-tant factor determining the UV radiation efficiency was thetendency of the products toward lumping. In the process at indus-trial scale, lumping was observed in several types of coating mate-rials. This phenomenon was also perceived in the laboratoryexperiment. Excellent results were obtained for meringue chunks.This can be explained by the fact that there was almost no lumpformation of the meringue chunks compared to the other materials(Fig. 3). Meringue showed only lump formation at the start of thedynamic experiment immediately after inoculation. The lumpsconsisted of large particles that were held together with inoculum(Fig. 3A). During the UV treatment, the lumps rapidly collapsedinto the original particles. Therefore, the test organism was thor-oughly exposed to UV light. Brésilienne nuts showed little or nolump formation, resulting in a moderate reduction of about 2logcycles. In contrast, for cacao powder the inoculation droplets weresurrounded by a layer of powder particles (Fig. 3B), forming lumpsthat remained throughout the whole radiation period. Micro-organisms in the centre of the lumps could not be reached by UVlight, which resulted in a smaller reduction. For small and largeground hazelnuts and for the hazelnut-with-waffle mixture, theinoculum droplets caused the food particle to form agglomerates(Fig. 3C). These were also persistent throughout the whole experi-ment, thus protecting microbial cells from UV rays.

In the case of the ice cream production plant, when the coatingmaterial is contaminated with micro-organisms from the environ-ment, some time may pass before radiation can be applied. In thisperiod, microbial cells may behave on the substrate in differentways. They may die, grow or adapt and change in sensitivity. Thiswas investigated for meringue chunks (since good reductions wereobserved for this matrix in previous experiments) by applying theUV treatment 20 h after inoculation with E. coli LMG 8063 (Table 2).Immediately after inoculation, the microbial number was6.0 ± 0.5 log cfu/g, which is very similar to the count obtained inthe experiment discussed above. After a storage period of 20 h,the number was decreased with 1 log cycle. This indicates thatE. coli could survive in the meringue chunks, but was not able togrow in the product. This is likely due to the small moisture

A CB

Fig. 3. Lump formation for (A) meringue chunks, (B) cacao powder, and (C) large ground hazelnuts, small ground hazelnuts and hazelnut-with-waffle mixture. Greyrepresents the inoculum and black represents the coating material.

Table 2Counts of E. coli LMG 8063 on meringue chunks immediately after inoculation, after20 h of storage and after 1 h UV radiation following storage (microbial counts), andreduction of the counts (log(N/N0)) after storage and radiation.

Time Microbial counts (log cfu/g) Log (N/N0) (log)

Immediately after inoculation 6.0 ± 0.5 /After storage 4.9 ± 0.7 �1.0 ± 0.9a

After radiation 2.2 ± 1.2 �2.7 ± 1.3b

Data are the means ± standard deviations of five replicates.a Calculated using the count observed immediately after inoculation as N0.b Calculated using the count observed after storage as N0.

Table 3Counts of E. coli LMG 8063 and Enterobacteriaceae strains isolated in an ice creamproduction facility immediately after inoculation (logN0) and reduction of the counts(log(N/N0)) after 1 h UV radiation.

Test organism Inoculationlevel

Foodmatrix

Log N0

(log cfu/g)

Log (N/N0)(log)

E. coli LMG 8063 Low Cacaopowder

3.1 ± 0.7a �1.4 ± 0.7a

E. coli LMG 8063 High Cacaopowder

6.5 ± 0.4b �0.8 ± 0.5a

IsolatedEnterobacteriaceaestrains

High Meringuechunks

7.7 ± 0.6A �4.3 ± 0.8A

E. coli LMG 8063 High Meringuechunks

6.1 ± 0.6B �4.8 ± 1.0A

Data are the means ± standard deviations of five replicates.a,b,A,B Means with the same superscript within the same column (lower case andupper case letters represent different experiments) do not differ significantly(p > 0.05).

258 J. Stoops et al. / Journal of Food Engineering 119 (2013) 254–259

content and the high sugar content of meringue (max. 3% and96.8% respectively, according to the suppliers’ specifications). Thereduction of E. coli in meringue chunks by immediate and post-poned UV radiation was 4.8 and 2.7 log cfu/g, respectively. A stor-age period of 20 h prior to UV treatment caused a significantlysmaller reduction (p = 0.02). A possible explanation is that onlythe strongest cells survived the storage period, and that these cellswere also less sensitive to UV light.

In the experiments described above, high inoculation levelswere used to enable monitoring the degree of reduction. However,based on internal microbiological analyses in the ice creamcompany, it was evident that these levels were higher than thecontamination prevailing in situ in the production facility. Hence,the question was raised whether, or to what extent, the contami-nation level would affect the degree of reduction achieved by thedynamic UV process. This was tested for cacao powder, becauselow reductions were found at the high inoculation level. When asmaller inoculum was applied to the cacao powder (Table 3), theinitial microbial number was 3.4 log cfu/g lower than for the high

inoculation level, which was a statistically significant difference(p = 0.00). The results in Table 3 show a numerically lower micro-bial reduction at a higher contamination level (p = 0.20). Neverthe-less, it may be conceivable that in case of large microbialpopulations, relatively less cells may be exposed to the UV raysthan in small populations. Where the food matrix may generateshadow zones and shield cells, cells can also protect each otherfor UV rays. Lower reductions at high contamination levels havebeen reported previously (Gómez-López et al., 2005; McDonaldet al., 2000). In addition, Levy et al. (2011) observed that lowerreductions were obtained with pulsed light when polystyrene sur-faces were contaminated at a higher level. This was true when spotinoculation was performed in a comparable way to this study. Thisstudy reveals that homogenization of the product can minimizeshadow effects from the food matrix, but not from microbial cells.Hence, industry must take into account the same rule-of-thumb forUV radiation as for other decontamination processes and aim atlow initial counts by using ingredients with microbial loads aslow as possible and by ensuring good production hygiene (Brennanand Grandison, 2012).

Furthermore, it was found in this study that the reduction to beachieved by the dynamic UV process is dependent on the food ma-trix. Therefore, it is not certain that the results with respect to theimportance of the initial microbial load obtained for cacao powdercan be extrapolated to other food matrices.

3.4. Radiation experiments with Enterobacteriaceae isolates

In the case considered in this work, CW UV was investigated asa possible tool to eliminate contamination of ice cream coatingmaterials originating from the production environment. It wasnot known to what extent cultures obtained from the productionsite could be eliminated. Micro-organisms originating from theso-called ‘‘house flora’’ of a food company may be more adaptedto stress conditions. They originate from the ice mix with a highsugar content or from (dry) surfaces in the production site. Theyare adapted to stress conditions such as a low water activity andthus may be more difficult to eliminate. In this study, Enterobacte-riaceae strains collected from the production site were subjected toradiation experiments using meringue as a substrate (Table 3). Aslightly lower reduction was found for the production strains(4.3log cycles) compared to the E. coli strain used previously(4.8log cycles). The difference was not statistically significant(p = 0.39). Hence, the production strains can be inactivated in asimilar way as the test strain. This supports scale-up to commercialtreatment levels, but the impact of CW UV on the sensory, chemi-cal and physical quality of the coating materials also needs to beestablished prior to implementation. Koutchma et al. (2009) men-tion changes in texture and colour, damage to vitamins and pro-teins, degradation of antioxidants, oxidation of lipids, andformation of off-flavors and aromas as possible undesirable effects.Colour changes were not detected in our study (based on visual

J. Stoops et al. / Journal of Food Engineering 119 (2013) 254–259 259

inspection) but Fine and Gervais (2004) observed that ILP resultedin an undesirable colour modification of black pepper and wheatflour, even before microbial inactivation was completed. This wasattributed to overheating combined with oxidation. Regardingthe effect of UV on chemical quality and safety of foods, to date lit-tle or no (quantitative) data can be found in literature. However,commercial application of UV, either as CW or ILP, requires moni-toring of the generation and eventually quantification of differentreactive chemical species, such as ozone, NOx and radicals.

4. Conclusion

The aim of this study was to find out whether CW UV applied ina dynamic system leads to a sufficient reduction of microbial num-bers in specific powdery and granular foods. This was true for mer-ingue chunks but the reduction obtained for the other matricesinvestigated was not satisfactory to consider implementation inthe production facility. Moreover, this work revealed several fac-tors influencing the treatment efficiency, being the nature of thefood matrix (in particular the tendency toward lump formation),the history of the contamination (recent or old) and the contami-nation level. In particular, results obtained with CW UV for onefood matrix cannot easily be extrapolated to another matrix.Whereas CW UV radiation can be implemented without legalrestrictions, which is a merit for innovations in the food industry,the aspects mentioned above must be taken into account duringthe design of a radiation process.

Acknowledgements

Glacio N.V. (Beerse, Belgium) is acknowledged for providing theice cream coating materials used in the radiation experiments. Thiswork was part of the Feasibility Study IWT-100513 ‘‘Control of postcontamination during production of ice cream desserts using UVradiation’’, performed on request of Glacio N.V., and sponsoredby the government agency for Innovation by Science andTechnology.

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