Journal of Food Engineering 90 (2009) 365–371

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Journal of Food Engineering

journal homepage: www.elsevier .com/locate / j foodeng

Improving cranberry shelf-life using high voltage electric field treatment

V. Palanimuthu, P. Rajkumar, V. Orsat, Y. Gariépy, G.S.V. Raghavan *

Department of Bioresource Engineering, Macdonald Campus, McGill University, Canada

a r t i c l e i n f o

Article history:Received 17 January 2007Received in revised form 7 July 2008Accepted 8 July 2008Available online 16 July 2008

Keywords:High voltage electric fieldCranberryShelf-lifeRespiration

0260-8774/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2008.07.005

* Corresponding author.E-mail address: vijaya.raghavan@mcgill.ca (G.S.V.

a b s t r a c t

Cranberries (Vaccinium macrocarpon Aiton) were treated with high voltage electric fields (HVEF) of 2, 5 or8 kV cm�1 in strength for 30, 60 or 120 min in a parallel plate electrode system. The treated berries werestored at ambient conditions (23 �C and 65% RH) for three weeks to study the effect of treatments on theirrespiration rate, physiological loss of mass (PLM), colour, total soluble solids (TSS) and skin puncturestrength. Resulting respiration rates were in the range of 11.69–14.56 mL CO2 kg�1 h�1 after the firstweek of storage, and increased to 13.95 and 21.33 mL CO2 kg�1 h�1 by the end of third week. For bothtwo and three weeks of storage, HVEF-treated cranberries showed significantly lower respiration ratesthan the control. This particular attribute indicates the potential of HVEF for improving shelf-life. ThePLM of HVEF-treated cranberries were in the range of 23.2–30.4% after three weeks of storage. Therewas no significant difference between treated and untreated berries in terms of absolute L*, a* and b* col-our values; however, the colour difference value DE*ab of treated berries was somewhat greater. The TSScontent of various HVEF-treated cranberries was in the range of 7.27–7.69 �B, similar to the TSS contentof untreated berries (7.4 �B) before storage. The skin puncture strength of different HVEF-treated cranber-ries was in the range of 11.7–14.3 N; while the untreated berries (11.2 N) showed lower values prior tostorage.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Cranberry (Vaccinium macrocarpon Aiton) is highly valued for itsnutritional and medicinal properties. It prevents many ailments,which include scurvy (Eck, 1990) and bladder infections in elderlywomen (Avorn et al., 1994). Bringing this high-value crop to mar-ket is plagued by fruit rot, which is caused by a number of fungaland bacterial microorganisms (Oudemans et al., 1998). Storagetemperature, relative humidity, air circulation and gas compositionplay a major role in the quality deterioration of fruits and vegeta-bles during storage (Raghavan and Gariépy, 1985). Respirationkinetics also plays a major role in affecting the shelf-life of a fruitas respiration provides all the necessary energy for biochemicalprocesses. Reducing the respiration rate by the modification ofthe storage environment, i.e., lowering the temperature, changingthe gas composition (like in CA/MA) can significantly increaseshelf-life (Ratti et al., 1996; Raghavan et al., 2003). However, anumber of factors like type of fruit, variety, maturity, storage timeafter harvest, injury, etc., were also reported to affect respirationrates (Fonseca et al., 2002).

Recently, several studies have investigated the use of high volt-age electric field (HVEF) in drying food materials and for shelf-lifeimprovement. Asakawa (1976) first observed enhanced water

ll rights reserved.

Raghavan).

vaporization under the influence of HVEF. A number of studies(Chen and Barthakur, 1994; Bajgai and Hashinaga, 2001a, b; Caoet al., 2004a, b) have reported an enhancement of drying ratesfor food materials (e.g., potato (Solanum tuberosum L.) slabs, radish(Raphanus sativus L.), spinach (Spinacia oleracea L.), rough rice(Oryza sativa L.) and wheat (Triticum �stivum L.)) using single ormultiple corona electrodes HVEF systems. Bajgai et al. (2006b)have presented a concise overview of electro-hydrodynamic drying,covering various aspects of HVEF drying.

Few studies have reported on the effect of HVEF on shelf-life offood materials. Toda (1990) found that the respiration rates of let-tuce (Lactuca sativa L.), spinach and komatsuna (Brassica rapa var.perviridis L.H. Bailey) were reduced by HVEF treatment. Tanaka(1991) postulated that the water activation under corona dischargeduring HVEF treatment was responsible for maintaining foodfreshness. Kharel et al. (1996) treated pear (Pyrus communis L.),plum (Prunus domestica var. domestica L.), banana (Musa sapientumL.), apple (Malus domestica Borkh) and sweet pepper (Capsicumannuum L.) with HVEF and noted a reduction in respiration ratesduring the climacteric period of pear, plum and banana and in postclimacteric respiration in the case of apple. Under the same treat-ment sweet pepper freshness and shelf-life were improved. Zhangand Hashinaga (1997) on treating Satsuma mandarin (Citrus reticu-lata Blanco) fruits with AC HVEF observed that there was a delay inchlorophyll degradation. Li (1998) observed that the enzymaticactivity could be lowered with HVEF treatment. While studying

Nomenclature

a* colour coordinate value of HVEF-treated cranberry afterstorage

a*control colour coordinate value of untreated cranberry before

storageb* colour coordinate value of HVEF-treated cranberry after

storageb*

control colour coordinate value of untreated cranberry beforestorage

CIELAB Commission Internationale d’Eclairage L*a*b* colour sys-tem

d distance between top plate electrode and top surface ofcranberry, cm

E electrical field strength, kV cm�1

HVEF high voltage electric fieldL* lightness value of colour of HVEF-treated cranberry

after storage

L*control lightness value of colour of untreated cranberry before

storageL*, a* and b* colour coordinate values under CIELAB Colour SpaceRRCO2

respiration rate in terms of carbon dioxide evolution,mL kg�1 h�1

RRO2respiration rate in terms of oxygen consumption,mL kg�1 h�1

TSS total soluble solids, �BrixV potential difference between parallel plate electrodes,

kVDE*ab colour difference of cranberries under CIELAB Colour

SpacePLM physiological loss of mass, %Ti treatment labels, i=1, 2, . . . 10

366 V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371

the effect of corona discharge on decay of fruits and vegetables,Hildebrand et al. (2001) observed that the corona discharge maybe an effective tool for reducing germination, decay and spore pro-duction of pathogens of stored produce. However, it was also indi-cated that this treatment may not be effective on all commodities.Atungulu et al. (2004) reported intermittent and continuous directcurrent electric field treatment of apples to have a potential to re-tard CO2 evolution at the climacteric peak. Both a parallel plateelectrode system and a multiple (n = 11) needle type negativecorona discharge electrode system retarded respiration. More re-cently, Bajgai et al. (2006a) studied the effect of both AC and DCHVEF on emblic (Phyllanthus emblica L.) fruits using parallel copperplate electrode system and observed that AC HVEF could be usedfor shelf-life extension. Since many studies have indicated thatthere is a potential to reduce the respiration rate of fruits by HVEFtreatment and possibly enhance their shelf-life, a study was initi-ated to investigate the effect of HVEF treatment of cranberry onits respiration and other physical attributes like PLM, colour, TSSand skin puncture strength that may have a direct bearing on itsshelf-life.

2. Materials and methods

2.1. Experimental sample preparation

Fresh cranberries for the experimental study were obtainedfrom a local super market (Fresh1 Brand, C.H. Robinson Co.,Minnesota, USA). Care was taken to obtain packages having thesame batch number. The commercial packages (LDPE with punchedholes for ventilation) were stored at 2 �C until trials. The packagedberries were taken out from the cold storage a couple of hours be-fore the experiment, allowed to equilibrate with room conditionsand were sorted using plastic screens to remove undersized(<13 mm) and oversized (>17 mm) fruits. Thus relatively uniformsized fruits with a mean diameter of 15.3 ± 0.78 mm received HVEFtreatments.

2.2. Experimental HVEF set up

The experimental set up (Fig. 1) for HVEF treatment of cran-berry consisted of a high voltage step up transformer (CanadianGeneral Electric Co., Ltd, Toronto, Canada; Model 15A7) that couldgive a maximum secondary AC voltage of 10 kV and 23 mA cur-rent. The necessary high voltage output for HVEF treatment couldbe obtained by controlling the input voltage to the high voltage

transformer using a Powerstat Autotransformer (Superior ElectricCo., USA; Model 3PN226) and the generated output voltage wascontinuously monitored with an oscilloscope (Agilent, USA; Mod-el 54621A). The rectangular HVEF treatment chamber was madeup of perforated aluminium mesh with a door, inside whichtwo parallel copper plate (disc) electrodes of 120 mm in diameterwere centrally placed one above the other. The grounded bottomelectrode was fixed while the top electrode was movable so thatthe distance between the electrodes could be adjusted. An exten-someter (Digimatic Indicator, Mitutoyo Corp., Japan; Model IDU-1025E) measured the distance between the electrodes accurately.From this the distance between the top electrode and the top sur-face of the fruit could be deduced by subtracting mean fruitdiameter. The temperature and relative humidity inside the treat-ment chamber was continuously recorded using a type-K thermo-couple (Omega Engineering, Inc., USA) and relative humidityprobe (Mamac Systems, USA; Model HU-224-3-MA), respectively.Data acquisition occurred through a Data Logger (Agilent Tech-nologies, Inc., USA; Model 34970A). A fiber optic temperatureprobe (Meoptix Inc., Canada; Model-T1) was also used to recordthe fruit temperature during HVEF treatment. However, surfacetemperature of the fruit was not observed during the treatmentperiod.

2.3. HVEF treatment procedure

In each trial, about 30 g pre-graded cranberries were placed onthe bottom disc electrode in a natural rest position and the topelectrode was adjusted such that the gap between this electrodeand the top surface of the fruit was 1 cm. Therefore, the potentialdifference between the electrodes will be numerically equal tothe electric field strength (E = V/d).

Cranberries were treated at controlled laboratory ambient con-ditions (temperature 23 �C; RH, 65%), under a factorial combina-tion of three electric field strengths (2, 5 or 8 kV cm�1 andthree durations of exposure (30, 60 and 90 min), with three rep-lications. The control treatment was also replicated as othertreatments.

The weight of berries was measured just before and after HVEFtreatment using an electronic platform scale (Denver Instruments,USA; Model-APX 602) and there was no significant weight varia-tion observed. The HVEF-treated berries were then placed insideperforated paper bags and stored under controlled ambient condi-tions (23 �C and 65% RH) for three weeks to study respiration andother physical parameters.

1. Powerstat Autotransformer 8. Extensometer 2. High Voltage Step Up Transformer 9. Temperature Probe 3. Oscilloscope 10. Relative Humidity Probe 4. Earthed Base Copper Plate Electrode 11. Fiber Optic Temperature Probe 5. Cranberries 12. Signal Conditioner 6. Adjustable Top Copper Plate Electrode 13. Data Logger 7. Shielded Treatment Chamber 14. Personal Computer

Fig. 1. Schematic diagram of experimental set up for high voltage electric field treatment of cranberry.

V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371 367

2.4. Measurement of fruit respiration

Fruit respiration rate was monitored at weekly intervals by astatic method. The berries were allowed to respire for about 6 h in-side air-tight glass bottles having a silicone septum on the lid (Songet al., 1992; Ratti et al., 1996). The head space gas composition in-side the bottles was analyzed using a Gas chromatograph (SRIInstruments Inc., California, USA; Model-8610A) equipped with athermal conductivity detector. The respiration of cranberries interms of CO2 evolution (RRCO2), O2 consumption (RRO2) and respira-tory quotient (RQ) were calculated as follows

Respiration rateðRRCO2 Þ;ml=kg-h ¼ Change inCO2 concentration in heaFruit mass ðkgÞ � Dura

Respiration rateðRRO2 Þ;ml=kg�h ¼ Change inO2 concentration in headFruit mass ðkgÞ � Durat

Respiratory Quotient ¼ Respiration rate ðRRCO2 ÞRespiration rateðRRO2 Þ

Free volume;m3 ¼ Container volume;m3 � Fruit mass; kg

Fruit density;kg=m3

!" #

2.5. Measurement of physiological loss of mass (PLM)

Physiological loss of mass by HVEF-treated cranberries wasdetermined at weekly intervals and expressed as a percentagereduction of mass of berries from the mass before storage usingan electronic platform scale.

2.6. Measurement of colour

The colour of treated cranberries was measured at weekly inter-vals in terms of L*, a* and b* values under CIELAB Colour System

d space ð%v=vÞ � Free volume ðmlÞtion of respiration ðhÞ ð1Þ

space ð%v=vÞ � Free volume ðmlÞion of respiration ðhÞ ð2Þ

ð3Þ

ð4Þ

368 V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371

using a Spectra QC Spectrophotometer (Minolta Corp., USA; Model-CM 3500d) with D65 illumination and a 10� standard observer. Thecolour difference DE*ab of the HVEF-treated cranberries against theinitial colour of untreated cranberries (control) before storage wasdetermined automatically by the instrument as

DE�ab ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiL� � L�control

� �2 þ a� � a�control

� �2 þ b� � b�control

� �2q

ð5Þ

For each replication (of a treatment), colour measurement wasmade on 3 berries at 3 different locations and the mean value of9 readings was noted.

2.7. Measurement of skin puncture strength

The effect of HVEF treatments on the skin puncture strength ofcranberries was assessed at weekly intervals using Universal Test-ing Machine (Instron Corp., USA; Model – 4500). The berries wereplaced on a flat platform in natural rest position and the skin waspunctured with a cylindrical Teflon probe of 5 mm diameter at acrosshead speed of 10 mm/min. The puncture strength in N wascalculated as a mean of 10 readings (Hietaranta and Linna, 1999;Marquenie et al., 2003).

2.8. Measurement of total soluble solids (TSS)

The total soluble solids of HVEF-treated cranberries were deter-mined, at weekly intervals, using a Hand-held Refractometer(Atago Co., Ltd, Japan; Model – Master-T). The berry was fingerpressed to extract a couple of drops of juice that were placed onthe refractometer prism. The TSS (�Brix) of the juice was measureddirectly from the instrument. The mean of three replicates is re-ported here.

2.9. Statistical analysis

The data pertaining to respiration, physiological loss of mass,colour, skin puncture strength and TSS were statisticallyanalyzed using the AGRES software package of Indian Agricul-tural Statistics Research Institute, New Delhi, using a 5% signifi-cance level. An ANOVA under a Completely Randomized Designand mean separation by LSD method was carried out for allparameters.

0

5

10

15

20

25

30

35

T1 T2 T3 T4 T5TREA

PHYS

IOLO

GIC

AL

LOSS

OF

MA

SS, %

Fig. 2. Cumulative physiological loss of mass of HVEF-tre

3. Results and discussion

3.1. Effect of HVEF treatment on PLM of cranberry

The cumulative physiological loss of mass of different HVEF-treated cranberries was 10.2–12.9% after the first week of storageat ambient conditions and increased to 23.2–30.4% at the end of3 weeks of storage (Fig. 2). No significant difference in PLM be-tween HVEF-treated and untreated control samples were observedat any given time of storage.

3.2. Effect of HVEF treatment on respiration rate and respiratoryquotient

Respiration rate of HVEF-treated cranberries expressed in termsof CO2 production (RRCO2) during the three week storage period un-der ambient conditions is presented in Fig. 3. For the various treat-ments respiration rates ranged between 11.7 and 14.6 ml/kg�h

after the first week, and increased to 13.9 and 21.3 ml/kg�h bythe end of third week of storage. After the first week, there wasno significant difference in respiration rates between HVEF-treatedand the untreated control samples. However, after two and threeweeks of storage, the respiration rates of HVEF-treated cranberries(T1 to T9) were significantly (P 6 0.05) lower when compared tountreated berries (T10). It was also interesting to note that all theHVEF treatments were found to be statistically on par as far asCO2 evolution was concerned. That is, within the experimentalrange, the variation in severity of treatments with different electricfield strengths (2–8 kV cm�1) and time (30–90 min) combinationsdid not show any significant difference as far as cranberry respira-tion rate was concerned. The reduction in respiration rate (RRCO2)of HVEF-treated cranberries during storage when compared to un-treated sample could be attributed to some physiological changesthat might have occurred in the berries. It may also be due to somepartial if not complete retardation of microbial activity of both fun-gi and bacteria owing to the HVEF treatment, that otherwise wouldhave continued unabated, reducing the shelf-life of berries. Thereduction in respiration rate indicates that there is a potential forextending shelf-life of cranberry with HVEF treatment. However,the mechanism involved in controlling the respiration by HVEF isnot clear. Further study regarding this issue can give us a betterunderstanding

T6 T7 T8 T9 T10TMENTS

WEEK-1WEEK-2WEEK-3

ated cranberries at ambient storage (23 �C; 65% RH).

0

5

10

15

20

25

30

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10TREATMENTS

RES

PIR

ATI

ON

RA

TE (C

O2)

, ml/k

g-h WEEK-1

WEEK-2WEEK-3INITIAL

Fig. 3. Respiration rate (CO2) of different HVEF-treated cranberries during storage at ambient conditions (23 �C; 65% RH).

0

1

2

3

4

5

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10TREATMENTS

RES

PIR

ATO

RY

QU

OTI

ENT

WEEK-1WEEK-2WEEK-3INITIAL

Fig. 4. Respiratory quotient of different HVEF-treated cranberries during storage at ambient conditions (23 �C; 65% RH).

V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371 369

The respiratory quotient (Fig. 4) was in the range of 1.69–2.21for various HVEF-treated cranberries after one week of treatmentand significantly increased to 1.88 and 3.48 range by the end ofthird week. The higher respiratory quotient for the cranberry mightbe due to its greater organic acid content as reported for blueber-ries (Beaudry et al., 1992). Gunes et al. (2002) also reported higherrates of CO2 production (almost double) than O2 consumption incranberry during CA storage. However, statistically there was nosignificant difference between treated and untreated berries withrespect to respiratory quotient at any given time of storage.

Cranberry is a non-climacteric fruit (Abdallah and Palta, 1989)and its respiration rate is generally expected to decline (for maturedfruit) with storage time until the onset of physiological breakdownand decay by microorganisms. But the increase in respiration rate ofCO2 and respiratory quotient with storage duration in all the treat-ments in the present study indicated that a few berries in each lotmight be the site of anaerobic respiration due to internal fruit dam-age, physiological breakdown or fungal decay. Since cranberries aregenerally harvested by raking of the fruits in flooded bogs with awater reel, it is quite possible for some berries to have internal inju-ries that would hasten the onset of physiological and fungal break-

down (Stretch and Ceponis, 1986). The visual observation of berriesin each replication also supported the argument that some berriesin the same lot tend to become softer than the others, indicatingthe onset of physiological breakdown. Patterson et al. (1967) re-ported that the activity of polygalacturonase, a cell wall degradingenzyme, is triggered by bruising, resulting in extensive softeningand tissue breakdown in cranberries. Oudemans et al. (1998)blamed more than 10–15 species of fungi and bacteria, many ofthem originating in the field itself, for cranberry fruit rots. ThoughHVEF treatment might have retarded the growth of above microbesinitially, it did not completely arrest their growth, particularly thatof spores. The increased microbial respiration with time might alsohave contributed to the increase in respiration rate of cranberries.However, further investigation may be necessary with fresh hand-picked cranberries in order to understand the mechanismsinvolved.

3.3. Effect of HVEF treatment on colour

The colour of different HVEF-treated cranberries measured interms of L*, a* and b* values under CIELAB colour system after

Table 1CIELAB colour values (L*, a*, b*) and colour difference (DE*ab) of HVEF-treatedcranberries after a week of storage at ambient conditions (23 �C; 65% RH)

Treatment L* a* b* DE*ab

T1 26.57 13.70 3.85 7.84T2 26.14 13.26 3.60 4.64T3 27.26 17.54 5.35 1.96T4 25.88 12.53 3.22 5.50T5 27.81 18.74 5.81 4.65T6 26.75 16.43 5.03 4.76T7 26.71 16.13 4.66 2.46T8 26.75 16.44 4.73 2.68T9 26.52 15.66 4.54 2.28T10 26.59 15.87 4.61 2.06F-test(@ a = 0.05) NS NS NS *CD @ 5% – – – 3.26CV% 3.86 24.62 33.97 49.32SEd 0.84 3.14 1.26 1.56

NS, not significant.*, significant.

0

2

4

6

8

10

T1 T2TREA

TOTA

L SO

LUB

LE S

OLI

DS,

°B

T5T4T3

Fig. 5. Total soluble solids content of HVEF-treated cranberries aft

0

5

10

15

20

25

TREA

SKIN

PU

NC

TUR

E ST

REN

GTH

, N

T1 T2 T5T4T3

Fig. 6. Skin puncture strength of HVEF-treated cranberries after

370 V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371

one week of storage is given in Table 1. There was no significantdifference between treated and untreated berries in terms ofabsolute colour values L*, a* and b*. However, the colour differ-ence value, DE*ab of various samples calculated against the origi-nal colour of cranberry before any HVEF treatment, showed asignificant difference. The untreated berries showed relativelyless colour change than the HVEF-treated ones, though the effectof higher field strength or treatment time was not discernible inthis study.

3.4. Effect of HVEF treatment on total soluble solids content

The total soluble solids content of various HVEF-treated cran-berries varied between 7.27 �B and 7.69 �B (Fig. 5) one week aftertreatment and was similar to that of untreated berries (7.4 �B)before storage. Statistically, there was no significant differencebetween the treatments as far as TSS was concerned.

TMENTT10T9T8T7T6

er one week of storage at ambient conditions (23 �C; 65% RH).

TMENTT10T9T8T7T6

one week of storage at ambient conditions (23 �C; 65% RH).

V. Palanimuthu et al. / Journal of Food Engineering 90 (2009) 365–371 371

3.5. Effect of HVEF treatment on skin puncture strength

The break load at skin puncture for different HVEF-treated cran-berries after the first week of treatment varied from 11.7 to 14.3 N(Fig. 6) which was slightly higher than the puncture strength of11.2 N recorded for untreated berries prior to storage. The reasonmight be attributable to the ageing effect. However, with respectto skin puncture strength, there was no significant difference be-tween the treatments suggesting that the HVEF treatments withinthe experimental range did not affect the berry skin mechanicalproperties.

4. Conclusions

In this study, the potential of reducing the respiration rate ofcranberry with HVEF treatment was clearly demonstrated. How-ever the mechanism through which the respiration is controlledby HVEF is not clear and further study is necessary to describethe mechanism. Since this method of treatment does not consumeany significant amount of energy, it can be exploited to enhancethe shelf-life of cranberry without affecting fruit quality. In thepresent study, the selected combinations of electric field strengthand treatment time of cranberry did not show significant differ-ences amongst themselves.

Acknowledgements

The authors acknowledge the financial support given byCanadian International Development Agency for Tier-I Project:Consolidation of Food Security in South India, under which thisresearch study was carried out at McGill University, Canada.

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