lable at ScienceDirect

Food Control 50 (2015) 297e303

Contents lists avai

Food Control

journal homepage: www.elsevier .com/locate/ foodcont

Ultraviolet-C light inactivation of Penicillium expansum on fruitsurfaces

Roopesh M. Syamaladevi a, Achyut Adhikari b, Shari L. Lupien c, Frank Dugan c,Kanishka Bhunia a, Amit Dhingra d, Shyam S. Sablani a, *

a Biological Systems Engineering Department, Washington State University, P.O Box 646120, Pullman, WA 99164-6120, USAb School of Food Science, Washington State University, P.O Box 6463760, Pullman, WA 99164-6376, USAc USDA-ARS Western Regional Plant Introduction Station, Washington State University, P.O. Box 646402, Pullman, WA 99164-6402, USAd Department of Horticulture, Washington State University, P.O. Box 646414, Pullman, WA 99164-6414, USA

a r t i c l e i n f o

Article history:Received 9 June 2014Received in revised form3 September 2014Accepted 5 September 2014Available online 16 September 2014

Keywords:Blue moldInactivation kineticsHydrophobicityPathogenicitySurface roughnessSurface energy

* Corresponding author. Tel.: þ1 509 335 7745; faxE-mail address: ssablani@wsu.edu (S.S. Sablani).

http://dx.doi.org/10.1016/j.foodcont.2014.09.0060956-7135/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Understanding the influence of fruit surface morphology on ultraviolet-C (UV-C 254 nm) inactivation ofmicroorganisms is required for designing effective treatment systems. In this study, we analyzed UV-Cinactivation of Penicillium expansum that was inoculated onto the surface of organic fruits. Resultsshow that maximum reductions of 1.8 (apple), 2.4 (cherry), 2.6 (strawberry) and 2.8 (raspberry) log CFU/g were observed after 1.2, 2.1, 3.3, and 3.3 kJ/m2 of UV-C doses, respectively. The UV doses required toreduce 2 log CFU/g of P. expansum population on apples, cherries, strawberries, and raspberries estimatedwith the Weibull equation were 1.03, 1.28, 1.39, and 1.61 kJ/m2, respectively. Findings also show that thehydrophobic nature of raspberries, along with high surface roughness, resulted in a lower inactivationrate of P. expansum. This study shows that UV-C effectively reduces P. expansum populations on fresh fruitsurfaces; however, the efficacy of treatment is dependent on fruit surface morphology.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Hot water treatments, biological control, and chemical appli-cations have been shown to reduce postharvest loss of fresh fruitsby spoilage fungi (Eckert, 1990; Nunes, 2012; Schirra, D'Aquino,Cabras, & Angioni, 2011). However, each of these techniques hasdrawbacks that can limit their commercial applicability. Manychemical solutions cannot be used to sanitize organic produce;hence the surface sanitation of fresh organic fruits poses a chal-lenge. Spoilage by fungi amounts to up 5e10% of fresh fruit loss,evenwhen fungicides are used (Cappellini & Ceponis, 1984; Sapers,2001). In organic fruits, postharvest losses are even greater (Poratet al. 2000). This necessitates development of a cost effective andnon-toxic surface disinfection method for organic fruits.

Penicillium expansum Link (Blue mold) is an important spoilagefungus in apple and pear; however, it may grow on many otherfresh fruits and reduce their shelf life (Snowdon, 1990). Along withfruit spoilage, P. expansum poses health risks in humans by pro-ducing mycotoxins (Morales et al., 2008). Neri, Donati, Veronesi,

: þ1 509 335 2722.

Mazzoni, and Mari (2010) reported that P. expansum can producesignificant amounts of patulin, a mutagenic mycotoxin, in hostssuch as apples, peaches, strawberries and kiwi. However, consti-tutive characteristics of the host can influence the rate of conidialgermination and growth of P. expansum. Therefore, identifying thepathogenicity of P. expansum in organic fruits is important to un-derstand their growth and expansion in specific hosts and developtheir growth reduction strategies.

The ultraviolet light is an effective method for decontaminatingfood surfaces (US-FDA, 2011). Effective doses range between 200and 280 nm (UV-C). More specifically, UV-C at 254 nm is a viablenonthermal alternative, approved by the Food and Drug Adminis-tration (US-FDA) for surface decontamination of foods (US-FDA,2011). In our previous study, we observed that interactions be-tween microorganisms and fruit surfaces affect the UV-C inactiva-tion kinetics (Syamaladevi et al. 2013). That study focused oninactivating generic Escherichia coli on two surfaces. However, moreresearch is needed to determine how fruit surface morphologyaffects UV-C inactivation of other microorganisms in order to applyUV-C to effectively sanitize fruits. Therefore, the current studydetermined the UV-C inactivation rate of P. expansum on selectedorganic fruit surfaces of different morphological characteristics.

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303298

2. Materials and methods

2.1. Pathogenicity testing of Penicillium expansum associated withUV-C treatment of fruit surfaces

Apples (Fuji Organic Ranier 94131), sweet cherries (TangaroVentures Ltd. Eastern British Columbia), strawberries and rasp-berries (USDA organic Driscoll's Associates, Inc. Watsonville, CA),were purchased from local retail outlets the day before thepathogenicity trials. Inoculum was freshly prepared from single-spore fungal cultures grown on ½ V8 agar for approximately7e14 days at room temperature under a 12 h diurnal cycle ofcombined cool white and near-UV light. A single strain ofP. expansum (CLX1499) was tested on apples, cherries, straw-berries, and raspberries to confirm pathogenicity of P. expansumon these fruits. Inoculum stocks were generated by gentlywashing conidia from an actively sporulating culture on an agarplate with 1e2 ml of sterile water. Conidial suspensions wereserially diluted and quantified using a hemocytometer, and thentransferred to a vial of sterile semi-liquid water agar (0.2% Bacto™agar in diH2O). The uniform conidial suspension had a finalconcentration of 5.0e5.5 � 107 cells per ml in 0.2% liquid agar.Prepared inoculum was stored at 5 �C for 12e24 h during thepathogenicity tests.

The surfaces of whole fruits were disinfected with a 0.5% hy-pochlorite solution, rinsed with sterile deionized water andallowed to dry in the culture hood, from several hours to overnight.Prepared fruits were wounded on a single site, midway on theoutside of the fruit, using a sterile probe of 4.5 mm long and 1.4 mmwide in such a way that the fruit would not roll in the incubationchamber (keeping the wound in an upright position) in order tomaintain the inoculum on top of the wound site. Then, the path-ogenicity experiments were repeated.

Surface disinfected and wounded fruits were inoculated with35 ml of either sterile 0.2% semi-liquid water agar, pipetted ontothe wound site for controls and 35 ml of conidial suspension of thedesignated fungal strainwas prepared in 0.2% of semi-liquid wateragar for inoculated treatments (Chen & Zhu, 2011). Inoculated andcontrol fruits were incubated (one fruit per container) in indi-vidual, 4 oz. polypropylene specimen containers (VWR®, Radnor,PA) with a single 4 mm hole drilled through the center of thespecimen container cap or similarly in commercial 4 � 4 freezerboxes for larger fruit. Containers with a single fruit type (treat-ments inoculated with a single designated fungal isolate and theassociated controls) were randomized within one incubation tray(Nalgene product #6900-0020, 21¾ � 16¾ � 5¼00) using auto-claved deionized water to maintain, a humidity near 95% RH.Apples were tested at room temperature (23 �C), while straw-berries, raspberries and cherries were tested at 15 �C to avoid non-specific rot and spoilage. During pathogenicity tests, the fruitswere monitored for the presence of rot and allowed to incubate for7e14 days. At that time, the maximum lesion diameter wasmeasured and recorded for all fruit types inoculated on a giventrial date.

Lesion diameters at the wound site or simply the wound siteitself in the absence of rot were measured and recorded usingMax-Cal® digital calipers. Results were analyzed as the calcu-lated rate of lesion development using Systat 10.2 (SYSTATSoftware Inc. Richmond, CA). Analysis was conducted with theGLM estimate model (including the constant), with GLMpair-wise comparisons grouping by treatment using a post hoctest of Fisher's LSD and Error Term ¼ Model MSE. Probabilityvalues corresponding to differences between control and inoc-ulated treatments for each fruit type on a given date wererecorded.

2.2. UV-C inactivation of P. expansum

Spore suspensions of P. expansum (average of 2 � 107 CFU/ml)were then prepared, serially diluted and quantified as describedabove. Before UV-C inactivation experiments, the fruits were sur-face disinfected by using 0.5% hypochlorite solution and air-driedinside a hood for 1 h at ~23 �C. Small fruits (cherries, straw-berries and raspberries) were bisected longitudinally. For apples,discs of 5.7 cm diameter, ~1 cm thick (~30 g, with ~25 cm2 intactepidermis) were prepared along the longitudinal axis. Fruit pieceswere placed on sterile Petri dishes with the cut surface on thebottom and the epidermal surface on top. The epidermal surfaces ofthe fruits were spot-inoculated with 0.5 ml of P. expansum conidialsuspension and kept for minutes inside a hood for equilibration(Chen& Zhu, 2011). UV-C treatments of the organic fruit discs wereconducted with a UV-C Emitter™ table-top System (Reyco Systems,Meridian ID) comprised of two Steril-Aire™ 16SE food-grade,shatter-resistant, sleeved UVC Emitters™ at a wavelength of254 nm at 23 �C. UV-C emitters were fixed 10 cm above the inoc-ulated fruit discs.

The UV-C dose corresponding to each treatment time wasdetermined with a UV radiometer (EIT UVICURE PLUS II, EIT, Inc.,Sterling, VA, USA). Preliminary experiments were conducted tolearn about the effectiveness of UV-C so that suitable UV-C doses(kJ/m2)/treatment times for the desired level of P. expansum inac-tivation could be identified. Preliminary UV-C inactivation experi-ments were conducted in all the selected fruits as the surfacecharacteristics of the fruits vary and which can influence the UV-Cinactivation efficacy. UV-C doses ranged from 0.13 to 3.3 kJ/m2,corresponding to treatment times of 10e360 s for fruits. Specificamounts of UV-C intensity and treatment times used for eachorganic fruit are shown in Table 1. Fruit pieces not treated with UV-C light were considered as controls. At least three UV-C treatmentswere conducted, corresponding to each treatment time, in a Class IIlaminar hood.

The fruit surfaces were rubbed with 100 ml of sterile waterusing Stomacher bags (Stomacher® 400 CIRCULATOR, SewardLaboratory Systems Inc. Port Saint Lucie, FL, USA) for 3 min tofacilitate detachment and quantify the P. expansum conidia. A 100 mlportion of the supernatant was collected and mixed with 900 ml ofsterile water and serially diluted. Later, triplicate samples of 100 mldiluted sample was spread on potato dextrose agar (TSA, HardyDiagnostics, Santa Maria, CA). After UV-C experiments, the agarplates were incubated for 72e96 h at 23 �C, and colony formingunits (CFU) were counted.

2.3. Kinetics of UV-C inactivation of P. expansum

The Weibull equation (Peleg & Cole, 1998) was used to describethe UV-C inactivation kinetics of P. expansum. The equation is.

N ¼ No exp

"��ta

�b#

(1)

Where N is the number of surviving conidia after UV-C treatmenttime t, No is the initial number of conidia, a is the scale factor and b

is the shape parameter determining the shape of the curve. Thevalues of a and gwere determined by non-linear optimizationwiththe Statistica® version 5 computer program. The time required for a90% reduction in the number of the target microorganism or thereliable life time (tR) was determined using Weibull parameters(van Boekel, 2002). The value of tR can be determined as:

tR ¼ að2:303Þ1b (2)

Table 1Average logarithmic reduction levels of P. expansum on selected organic fruit surfaces (N ¼ 3).

Time (s) Apple Cherry Strawberry Raspberry

UV-Cdose (kJ/m2)

Log reduction UV-Cdose (kJ/m2)

Log reduction UV-Cdose (kJ/m2)

Log reduction UV-Cdose (kJ/m2)

Log reduction

0 0 0 0 0 0 0 0 010 0.13 0.24 ± 0.15a 0.14 0.20 ± 0.05a 0.14 0.79 ± 0.23bcd e e

20 0.24 0.44 ± 0.26ab 0.30 0.58 ± 0.17abc 0.30 0.99 ± 0.30cde e e

30 e e 0.46 0.98 ± 0.08cde 0.46 1.3 ± 0.2ef 0.28 0.54 ± 0.28abc

40 0.47 0.81 ± 0.26bcd e e e e e e

60 0.67 1.3 ± 0.3def 0.86 1.8 ± 0.11gh 0.86 2.0 ± 0.3ghi 0.59 1.3 ± 0.11def

90 0.95 1.6 ± 0.2fg 1.4 2.1 ± 0.10hij e e e e

120 1.2 1.8 ± 0.3gh 2.1 2.4 ± 0.15ijk 2.1 2.5 ± 0.4jk 1.2 1.5 ± 0.6fg

180 e e e e 3.3 2.6 ± 0.3jk 1.7 1.8 ± 0.3gh

240 e e e e e e 2.2 1.7 ± 0.5fgh

360 e e e e e e 3.3 2.8 ± 0.1k

Different superscripts represent statistical significant differences between log reduction values in number of P. expansum population obtained at selected UV doses and UVtreatment times (P < 0.05). For the same treatment time, UV-C dose was varied by changing the number of UV lamps).

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303 299

2.4. Surface roughness of fruits

A section of apple and cherries (~10 mm by 10 mm) wasremoved and the flesh was cut back to ~2e3 mm from the skin andmounted on a microscope slide with wax of low melting temper-ature. The sample was placed on the Tencor P15 stylus profilometerstage with the stylus centered on the sample. A scan of1mm� 1 mmwasmade using 12 parallel scans on a 92 mm spacingat a rate of 200 mm/s. For raspberries, a similar size piece wasdissected and the flesh was cut back to a similar dimension. Scansweremade over the peak of the drupelet using a 500 mmby 500 mmscan area, with 12 scans at a 46 um spacing at a rate of 100 mm/s. Forstrawberries, a triangular piece of ~6 mm � 10 mm was removed,with the flesh cut back to 2e3 mm thick. A scanwas made betweentwo achenes using a 500 mm by 500 mm scan area and the sameparameters as for the raspberries.

2.5. Environmental scanning electron microscopy

An environmental scanning electron microscope (ESEM)(Quanta 200 ESEM, FEI Co. [Field Emission Instrument], Hillsboro,OR) with magnifications from 100 to 1000 mmwas used to analyzethe surface morphology of selected fruits. Small fruit surface sec-tions (2e3 mm) were used for ESEM analysis.

2.6. Contact angle and surface energy

A sessile drop method was used to determine the contact angleof fruit surfaces, which relates surface hydrophobicity. A face con-tact angle set-up equipped with a camera (VCA Optima, ASTProducts Inc., MA, USA) was used (Bernard, Balla, Davies, Bose, &Bandyopadhyay, 2011). Small sections of selected fruits approxi-mately 2 � 2 cm2 and 1 mm thick were cut with a sharp knife. Asmall amount (0.5e1.0 ml) of a polar liquid (double-distilled water)or a non-polar liquid (diiodomethane 99% purity; SigmaeAldrich)were dropped onto the fruit surfaces using a microliter syringe anda 0.5-mm diameter needle at 23 �C. At least twenty data pointswere used for each fruit sample (N¼ 20). The surface energy valuesof the selected fruit surfaces were calculated from contact anglemeasurements using Fowkes' equation (Bernard et al., 2011;Ribeiro, Vicente, Teixeira, & Miranda, 2007).

2.7. Quality changes in fruits after UV-C treatment

The fruit surfaces were treated with UV-C light at specific UV-Cintensities (specific treatment times) to achieve two log reductions

in P. expansum. Selected physicochemical changes, such as colorand soluble solid content in the organic fruits before and after UV-Ctreatments, were analyzed to investigate the effects of UV-Ctreatment on quality. Untreated fruits were used as controls. AMinolta Chroma CR-200 color meter (Konica Minolta SensingAmericas, Inc. Ramsey, NJ) was used to determine color character-istics as changes in L value (lightness of color from zero (black) to100 (white); a value (degree of redness (0e60) or greenness (0to �60); and b values (yellowness (0e60) or blueness (0 to �60)(N ¼ 10) (Chen, Zhu, Zhang, Niu, & Du, 2010). A pocket PAL-1refractometer (Atago USA. Inc. Bellevue, WA) was used to deter-mine the total soluble solid (TSS) content (g/100 mL) of UV-treatedand untreated fruits in duplicate measurements.

2.8. Statistical analysis

The data for inactivation of E. coli by UV-C were analyzed forstatistical significance using SAS 9.1 (SAS Institute, Inc., Cary, NC). Avalue of P < 0.05 was selected as statistically significant using procGLM by Least Square Difference (LSD). The experiment involved acompletely randomized design with two-way ANOVA by Fisher'sLeast Square Difference (LSD)method where a value of P < 0.05 wasselected as statistically significant. Two-way ANOVA analysis wasconducted to find out significant two-way interaction between UV-C dose and type of surface on logarithmic reduction in P. expansumpopulation.

3. Results and discussion

3.1. UV-C inactivation kinetics

P. expansum strain CLX1499 was found to be pathogenic to allfruit types tested in repeated pathogenicity trials (P ¼ 0.000). Theaverage P. expansum conidia counts on apple, cherry, strawberryand raspberry surfaces before UV-C treatments were 3.0 � 105,1.0 � 105, 3.5 � 105, and 6.0 � 105 CFU/g, respectively. Significantreductions (P < 0.05) were observed in P. expansum populationwith1.8 (apple), 2.4 (cherry), 2.6 (strawberry), and 2.8(raspberry) log CFU/g after 1.2 kJ/m2, 2.1 kJ/m2, 3.3 kJ/m2, and 3.3 kJ/m2 of UV-C treatment, respectively (Table 1). The population ofP. expansumwas reduced significantly with 1.6 and 2.1 log CFU/g onthe surfaces of apples and cherries during the initial 90 s treatment(0.94 kJ/m2 UV-C dose for apples and 1.4 kJ/m2 dose for cherries).On the surfaces of strawberries and raspberries, the P. expansumconidia were reduced by 0.8 and 1.2 log CFU/g during the first 10 s(1.1 kJ/m2 dose) and 60 (0.59 kJ/m2 dose), respectively (Table 1). A

Table 2Weibull model parameters for P. expansum inactivation on selected fruit surfaces by UV-C.

Fruit surface a, kJ/m2 (sec) b tR, kJ/m2 (sec)

Apple 0.522 ± 0.165 (48.5 ± 13.1)a 0.80 ± 0.12a 1.41 ± 0.23 (137 ± 19)b

Cherry 0.469 ± 0.035 (28.2 ± 0.7)ab 0.68 ± 0.04a 1.73 ± 0.10 (97 ± 8)b

Strawberry 0.247 ± 0.064 (16.1 ± 6.1)b 0.44 ± 0.05b 1.94 ± 0.59 (108 ± 35)b

Raspberry 0.497 ± 0.260 (46.0 ± 22.5)a 0.51 ± 0.13b 2.35 ± 0.31 (230 ± 27)a

Different superscripts represent statistical significant differences between Weibull parameter values column-wise (P < 0.05).

Table 3Root mean square surface roughness (Rq) and average surface roughness (Ra) valuesof selected fruits.

Fruit Rq (mm) Ra (mm)

Apple 30.3 25.4Cherry 10.7 8.79Strawberry 296 287Raspberry 78.6 62.4

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303300

decrease in the rate of log reduction of the P. expansum populationwas observed after this initial sharp decline. No statistically sig-nificant interaction effect (P � 0.05) between the fruit surface andUV-C dose on P. expansum population was observed. Similar ob-servations on the effectiveness of UV-C in reducing the microbialpopulations on food surfaces have been reported recently. UV-Cwith 2.2 and 4.4 kJ/m2 intensity showed effectiveness in reducingthe Botrytis cinerea population on bell peppers (Mercier, Baka,Reddy, Corcuff, & Arul, 2001). Syamaladevi et al. (2014) reported2.8 log reduction in P. expansum population on the pear surfaceafter a UV-C dose of 1.7 kJ/m2.

The UV-C inactivation kinetics of P. expansum was fitted with anonlinear Weibull equation. The scale factor (a) values for the UV-Cinactivation kinetics of P. expansum on apple, cherry, strawberryand raspberry surfaces were 48.5, 28.2, 16 and 46 s, respectively(Table 2). The shape factor (b) values for apple, cherry, strawberryand raspberry surfaces were 0.80, 0.68, 0.44, and 0.51, respectively(Table 2). The reliable life time (tR) values for apple, cherry,

Fig. 1. Surface profile of (A) apple (B, C) raspberry (D, E) strawberry (F)

strawberry, and raspberry surfaces were 1.41, 1.73, 1.94 and 2.35 kJ/m2, respectively (Table 2). The greater values of a and tR for rasp-berry surfaces indicate the lower rate of inactivation of P. expansumduring UV-C treatment.

In our previous study (Syamaladevi et al., 2013), we foundsignificant differences in the a and tR of UV-C inactivation ofgeneric E. coli on peach surfaces compared to those on intact pearsurfaces. This is attributed to the shielding of microorganismsagainst UV radiation by trichomes on peach surfaces (Syamaladeviet al., 2013). For all the selected surfaces, the b < 1 showing tailing(upward concavity) of the UV-C inactivation curves (Chen & Zhu,2011). The weaker P. expansum cells were inactivated faster thanthe higher UV-C resistant P. expansum cells, which gained theability to adapt to UV-C radiation, resulting in the tailing effect onUV-C inactivation curves (Chen & Zhu, 2011). A higher value of bfor apple surfaces (0.80) indicates that the UV-C inactivation ofP. expansum on apple surfaces parallels first-order kinetics, wherethe b value is 1. The UV doses required to achieve a 2 log CFU/greduction in P. expansum population on apple, cherry, strawberry,and raspberry estimated by the Weibull equation were 1.03, 1.28,1.39, and 1.61 kJ/m2, respectively. This indicates that greater UV-Cdoses are required to inactivate P. expansum on raspberry andstrawberry surfaces.

3.2. Surface roughness and morphology

The root mean square (Rq) and average surface roughness (Ra) ofstrawberries (296 and 287 mm) and raspberries (79 and 62 mm)

cherry observed with environmental scanning electron microscopy.

Table 5Color change in untreated and UV-C treated organic fruits during storage (N ¼ 10).

Sample L a b

Apple Untreated 53.2 ± 5.6h 20.5 ± 9.2i 25.2 ± 4.5j

UV-C treated 50.9 ± 6.9h 22.0 ± 8.8i 24.4 ± 4.9j

Cherry Untreated 31.7 ± 2.2k 21.5 ± 6.1l 6.9 ± 2.5n

UV-C treated 33.9 ± 2.8k 27.3 ± 5.5m 10.0 ± 3.3�

Strawberry Untreated 35.1 ± 2.7d 34.2 ± 4.8f 17.1 ± 4.3g

UV-C treated 38.0 ± 3.1e 35.9 ± 2.8f 20.7 ± 4.8g

Raspberry Untreated 26.9 ± 1.9a 19.2 ± 2.9b 4.7 ± 1.1c

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303 301

were greater than those of cherries (11 and 9) and apples (30 and25 mm), as determined with a stylus profilometer (Table 3). Thesurface roughness values are dependent on the size and nature ofscanning area using the profilometer. The surfaces of apple andcherries exhibited smooth morphology in comparison to rasp-berries and strawberries, as observed through environmentalscanning electron microscopy (Fig. 1). We observed the presence oftrichomes (100e1000 mm) and irregular valleys on raspberry andstrawberry surfaces.

UV-C treated 26.6 ± 1.7a 21.8 ± 2.8b 5.4 ± 1.4c

Different superscripts represent statistical significant differences between eachcolor parameter value of untreated and UV-C treated individual fruit surfaces only(P < 0.05).

3.3. Contact angle and surface energy

The contact angle for water and diiodomethane on raspberrysurfaces was greater than those on the other selected surfaces(Table 4). For all the selected fruits, the water contact angles weregreater than 65�, indicating their hydrophobic nature (Vogler,1998). The raspberry surfaces were more hydrophobic than theother fruits (Table 4). The water contact angles of strawberry(76.3 ± 9.2), cherry (84.1 ± 5.4) and apple (81.8 ± 12.5) surfaceswere less than 90�. This indicates higher surface spreading of theliquid compared to raspberry surfaces. However, on raspberrysurfaces, the water drop forms a bead, as its contact angle(100 ± 10.0) was greater than 90� (Woodling & Moraru, 2005).

All selected fruit surfaces were low-energy surfaces, with sur-face energy values of the solid (gs) at less than 100 mN/m (Table 4).The gs of was smaller than the other selected fruits (Table 4). Asimilar trend of smaller gs for more hydrophobic peach surfacescompared to pear surfaces was observed by Syamaladevi et al.(2013). Strawberry surfaces exhibited greater spreading coeffi-cient/wettability adhesion (Ws ¼ �55.6 ± 11.4 mN/m andWa ¼ 90.2 ± 11.7 mN/m) than other selected fruits.

3.4. Influence of surface properties on UV-C inactivation kinetics ofP. expansum

On the strawberry and raspberry surfaces, greater UV-C doseswere required to achieve similar log reductions in P. expansumpopulation compared to apple and cherry surfaces, as predicted bytheWeibull equation. The greater survival of P. expansum conidia onraspberry and strawberry may be attributed to their higher,rougher surfaces, which shield conidia from direct exposure of UV-C light. Even though strawberries have a higher Rq than raspberries,the greater number of trichomes and irregular valleys contributesto a lower reduction of P. expansum population on raspberries(Figs. 1 and 2). Syamaladevi et al. (2013) reported that the bettershielding of conidia fromUV-C radiation due to trichomes on peachsurfaces reduces UV-C inactivation of E. coli compared to smootherpear surfaces.

Furthermore, other surface morphological parameters such assurface hydrophobicity may influence the inactivation kinetics ofP. expansum on these fruits. During inoculation of P. expansum, thesurface distribution of conidia could be broader on hydrophilicsurfaces compared to hydrophobic surfaces (Choi, Park, Ahn, Lee, &

Table 4Average and standard deviation values of surface energy parameters of selected fruits (N

Fruit surface Contact angle (q) g

Water Diiodomethane

Apple 81.8 ± 12.5bc 42.5 ± 8.4 4Cherry 84.1 ± 5.4b 65.2 ± 6.9 3Strawberry 76.3 ± 9.2c 35.6 ± 11.0 4Raspberry 91.0 ± 10.0a 77.3 ± 8.0 2

where gS ¼ Surface energy of the solid (mN/m), Wa ¼ Reversible work of adhesion (mN

Lee, 2002; Syamaladevi et al. 2013). This leads to greater exposureto UV-C light, resulting in higher inactivation rates. The greater UV-C inactivation rate and smaller reliable life time (tR) of P. expansumon strawberry may be related to the higher spreading coefficientand hydrophilic nature of strawberries compared to other selectedfruit surfaces (Table 4). The lower P. expansum inactivation rate onraspberry is partly attributed to their greater hydrophobic nature,resulting in smaller surface spreading and greater aggregation incomparison to the other fruits (Syamaladevi et al. 2013).

We also evaluated the influence of spreading coefficients (Ws)and Rq on UV-C inactivation and reliable life time (tR) of P. expansumon fruit surfaces (Fig. 2). For apples and cherries, the lower Rq maybe attributed to the lesser tR of P. expansum (Fig. 2). Further, highWsof apple surface may be related to the lowest tR value ofP. expansum. The lowest Ws and high Rq and the presence of largenumber of trichomes, as observed through ESEM, may havecontributed to the highest tR of P. expansum on raspberries. Eventhough strawberry had the highest Rq, the largest Ws, indicatingbetter spreading of P. expansum, may have contributed to the lowertR of P. expansum (Fig. 2). The Pearson correlation coefficients (r)were determined to observe the correlation between UV inactiva-tion kinetics and surface properties. The r values for surfaceroughness, spreading coefficient and contact angle with tR ofP. expansumwere 0.3,�0.6 (negatively correlated) and 0.5, showingbetter correlation of spreading coefficient and hydrophobiciy withUV-C inactivation kinetics compared to surface roughness.

3.5. Physicochemical quality changes in organic fruits after UV-Ctreatment

Color parameters (L, a, and b values) did not differ significantlybefore and after UV-C treatment on the fruits to achieve a 2 log CFU/g reduction in P. expansum population, except for the increased L-value in strawberries (Table 5). No significant color change wasobserved immediately after UV-C treatment, although browningwas reported during two-month storage of pears after UV-Ctreatment (Syamaladevi et al., 2014). UV-C treatment did notsignificantly alter the soluble solid contents of the fruits, except foran increase from 8.1 to 9.5 g/100 mL in strawberries (Table 6). The

¼ 20).

s � 103 (mN/m) Wa � 103 (mN/m) Ws � 103 (mN/m)

2.9 ± 6.4 84.6 ± 15.4 �61.2 ± 15.11.3 ± 3.3 80.4 ± 6.8 �65.5 ± 6.76.4 ± 6.1 90.2 ± 11.7 �55.6 ± 11.44.4 ± 5.9 71.7 ± 12.7 �74.2 ± 12.3

/m), Ws ¼ Spreading coefficient (mN/m).

Fig. 2. Influence of root mean square surface roughness (Rq) and spreading coefficient (Ws) on reliable life time (tR).

Table 6Soluble solid content of untreated and UV-C treated organic fruits (N ¼ 3).

Sample Soluble solids content (g/100 mL)

Apple Untreated 14.1 ± 0.0d

UV-C treated 14.2 ± 0.2d

Cherry Untreated 18.7 ± 0.2e

UV-C treated 17.9 ± 0.9e

Strawberry Untreated 8.1 ± 0.1b

UV-C treated 9.5 ± 0.3c

Raspberry Untreated 10.1 ± 0.2a

UV-C treated 10.3 ± 0.1a

Different superscripts represent statistical significant differences between eachsoluble solid content parameter values of untreated and UV-C treated individualfruit surfaces only (P < 0.05).

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303302

soluble solid content of pear fruits did not change over twomonths'storage after UV-C treatment (Syamaladevi et al., 2014).

4. Conclusions

UV-C significantly reduced the P. expansum population on alltested surfaces, with greater reductions on apples and cherries. Thehydrophobic nature and the high surface roughness of raspberriessurface were responsible for the lower UV-C inactivation of theP. expansum population. Results of this study show that thespreading coefficient and hydrophobicity correlated better withUV-C inactivation kinetics of P. expansum on fruit surfaces thansurface roughness. UV-C treatment did not change the color andsoluble solid contents of fruits. Therefore, this study demonstratedthat treatment with UV-C light offers a promising alternative toinactivate P. expansum on fruits; however the level of reduction isdependent on fruit surface properties.

Acknowledgments

This research was partially funded with a Biological and OrganicAgriculture (BIOAg) program grant from the Center for Sustaining

Agriculture and Natural Resources at Washington State University.The authors wish to thank Chang-Lin Xiao, USDA-ARS for providingthe isolate of Penicillium expansum CLX1499 and Dr. Michael KhbeisandMr. Paul Schilling, Microfabrication Facility (MFF), University ofWashington for conducting surface roughness measurements offruits. Technical help from Dr. Mahmoudreza Ovissipour, Dr. StanDittrick, Mr. Hongchao Zhang, Dr. Valerie Lynch-Holm, Franceschi,Microscopy and Imaging Center, Mr. Frank Younce is greatlyacknowledged. We thank Ms. Poonam Bajaj for her technicalassistance and comments on the manuscript.

References

Bernard, S. A., Balla, V. K., Davies, N. M., Bose, S., & Bandyopadhyay, A. (2011). Bonecell-materials interactions and Ni ion release of anodized equiatomic NiTi alloy.Acta Biomaterialia, 7(4), 1902e1912.

Cappellini, R. A., & Ceponis, M. J. (1984). Postharvest losses in fresh fruits andvegetables. In Postharvest pathology of fruits and vegetables: Postharvest losses inperishable crops (pp. 24e30). Berkeley: University of California.

Chen, Z., & Zhu, C. (2011). Modelling inactivation by aqueous chlorine dioxide ofDothiorella gregaria Sacc. and Fusarium tricinctum (Corda) Sacc. spores inoc-ulated on fresh chestnut kernel. Letters in Applied Microbiology, 52, 676e684.

Chen, Z., Zhu, C., Zhang, Y., Niu, D., & Du, J. (2010). Effects of aqueous chlorine di-oxide treatment on enzymatic browning and shelf-life of fresh-cut asparaguslettuce (Lactuca sativa L.). Postharvest Biology and Technology, 58, 232e238.

Choi, W. Y., Park, H. J., Ahn, D. J., Lee, J., & Lee, C. Y. (2002). Wettability of chitosancoating solution on 'Fuji' apple skin. Journal of Food Science, 67(7), 2668e2672.

Eckert, J. W. (1990). Recent development in the chemical control of postharvestdiseases. Acta Horticulturae, 269, 477e494.

Mercier, J., Baka, M., Reddy, B., Corcuff, R., & Arul, J. (2001). Shortwave ultravioletirradiation for control of decay caused by Botrytis cinerea in bell pepper:induced resistance and germicidal effects. Journal of the American Society forHorticultural Science, 126, 128e133.

Morales, H., Marin, S., Obea, L., Patino, B., Domenech, M., Ramos, A. J., et al. (2008).Ecophysiological characterization of Penicillium expansum population in Lleida(Spain). International Journal of Food Microbiology, 122, 243e252.

Neri, F., Donati, I., Veronesi, F., Mazzoni, D., & Mari, M. (2010). Evaluation of Peni-cillium expansum isolates for aggressiveness, growth and patulin accumulationin usual and less common fruit hosts. International Journal of Food Microbiology,143(3), 109e117. http://dx.doi.org/10.1016/j.ijfoodmicro.2010.08.002.

Nunes, C. A. (2012). Biological control of postharvest diseases of fruit. EuropeanJournal of Plant Pathology, 133(1), 181e196. http://dx.doi.org/10.1007/s10658-011-9919-7.

R.M. Syamaladevi et al. / Food Control 50 (2015) 297e303 303

Peleg, M., & Cole, M. B. (1998). Reinterpretation of microbial inactivation curves.Critical Reviews in Food Science and Nutrition, 38, 353e380.

Porat, R., Daus, A., Weiss, B., Cohen, L., Fallik, E., & Droby, S. (2000). Reduction ofpostharvest decay in organic citrus fruit by a short hot water brushing treat-ment. Postharvest Biology and Technology, 18, 151e157.

Ribeiro, C., Vicente, A. A., Teixeira, J. A., & Miranda, C. (2007). Optimization andedible coating composition to retard strawberry fruit senescence. PostharvestBiology and Technology, 44, 63e70.

Sapers, G. M. (2001). Efficacy of washing and sanitizing methods for disinfection offresh fruit and vegetable products. Food Technology and Biotechnology, 39(4),305e311.

Schirra, M., D'Aquino, S., Cabras, P., & Angioni, A. (2011). Control of postharvestdiseases of fruit by heat and fungicides: efficacy, residue levels, and residuepersistence. A Review. Journal of Agricultural and Food Chemistry, 59(16),8531e8542. http://dx.doi.org/10.1021/jf201899t.

Snowdon, A. L. (1990). A colour atlas of post-harvest diseases and disorders of fruitsand vegetables. In General introduction and fruits (Vol. 1, pp. 178e179). London:Wolfe Scientific Ltd, 223; 250.

Syamaladevi, R. M., Lupien, S. L., Bhunia, K., Sablani, S. S., Dugan, F., Rasco, B., et al.(2014). UV-C light inactivation kinetics of Penicillium expansum on pear sur-faces: influence on physicochemical and sensory quality during storage. Post-harvest Biology and Technology, 87, 27e32.

Syamaladevi, R. M., Lu, X., Sablani, S. S., Insan, S. K., Adhikari, A., Killinger, K., et al.(2013). Inactivation of Escherichia coli population on fruit surfaces usingultraviolet-C Light: influence of fruit surface characteristics. Food and BioprocessTechnology, 6, 2959e2973.

US-FDA (United States Food and Drug Administration). (2011). Ultraviolet radiationfor the processing and treatment of food. Code of Federal Regulations, 179.

van Boekel, M. (2002). On the use of the Weibull model to describe thermal inac-tivation of microbial vegetative cells. International Journal of Food Microbiology,74, 139e159.

Vogler, E. A. (1998). Structure and reactivity of water at biomaterial surfaces. Ad-vances in Colloid and Interface Science, 74, 69e117.

Woodling, S. E., & Moraru, C. I. (2005). Influence of surface topography o theeffectiveness of pulsed light treatment for the inactivation of Listeria innocua onstainless-steel surfaces. Journal of Food Science, 70(7), M345eMM351.