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Changes in Flavor Components in γ-Irradiated FreshGinger (Zingiber officinale) Rhizomes During StoragePrasad S. Variyar a , A. S. Gholap a & Arun Sharma aa Food Technology Division, Bhabha Atomic Research Centre, FIPLY, Trombay, Mumbai, 400085, IndiaPublished online: 22 Sep 2008.

To cite this article: Prasad S. Variyar , A. S. Gholap & Arun Sharma (2006) Changes in Flavor Components in γ-Irradiated FreshGinger (Zingiber officinale) Rhizomes During Storage, Journal of Herbs, Spices & Medicinal Plants, 12:1-2, 25-35

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Changes in Flavor Componentsin �-Irradiated Fresh Ginger

(Zingiber officinale) RhizomesDuring Storage

Prasad S. VariyarA. S. GholapArun Sharma

ABSTRACT. Fresh ginger rhizomes were gamma irradiated at sprout-inhibiting doses of 60 Gy and stored for two months at ambient tempera-tures (28-30�C) in perforated low-density polyethylene bags. Changes involatile aroma constituents and pungent principles (gingerols) weremonitored during the storage period at intervals of one month. No signif-icant qualitative and quantitative differences could be noted in the vola-tile aroma constituents of the control (non-irradiated) and irradiatedsamples any time during storage. The major constituents identified in theoil by GC/MS analysis were zingiberene, �-sesquiphellandrene andar-curcumene. A decrease in gingerol content was observed in the irradi-ated samples on storage. This decrease was approximately 21%, 22%and 10% in irradiated ginger stored at 0, 1 and 2 months, respectively,compared with their corresponding non-irradiated controls. Gamma ir-radiation at a dose of 60 Gy was found to prevent sprouting and extendthe shelf-life of fresh ginger under ambient conditions without affectingits flavoring principles. doi:10.1300/J044v12n01_03 [Article copies availablefor a fee from The Haworth Document Delivery Service: 1-800-HAWORTH.

Prasad S. Variyar, A. S. Gholap, and Arun Sharma are affiliated with Food Technol-ogy Division, Bhabha Atomic Research Centre, FIPLY, Trombay, Mumbai-400 085,India.

Address correspondence to: Prasad S. Variyar (E-mail: prasadpsv@rediffmail.com) at the above address.

The authors would like to thank Mr. V. N. Sawant for the technical assistance ren-dered during the course of this work.

Journal of Herbs, Spices & Medicinal Plants, Vol. 12(1/2) 2006Available online at http://jhsmp.haworthpress.com

© 2006 by The Haworth Press, Inc. All rights reserved.doi:10.1300/J044v12n01_03 25

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E-mail address: <docdelivery@haworthpress.com> Website: <http://www.HaworthPress.com> © 2006 by The Haworth Press, Inc. All rights reserved.]

KEYWORDS. Flavor, gamma irradiation, ginger, pungency, volatilearoma constituents

INTRODUCTION

Ginger is an important spice of commerce, valued for its aroma andpungency. The spice is available in the market both in fresh and dryforms. While dry ginger is primarily meant for the export market, freshginger is mostly consumed locally where it finds extensive use in sev-eral culinary preparations both as a spice and as a delicacy (2). Thearoma of the spice is contributed by its volatile essential oils. These oilsare reported (2) to be composed of a complex mixture of mono- andsesquiterpene hydrocarbons, with minor amounts of oxygenated ter-penoids. The phenolic ketones namely, gingerols, are the major pun-gency principles of fresh ginger while their dehydration products,shogaols, are formed during extended storage or thermal processing (2).Gingerol homologues, 6-, 8- and 10-gingerol, account for about 75, 8and 11%, respectively, of the total pungency fraction of fresh ginger (2).

A major problem associated with storage of fresh ginger is its suscepti-bility to sprouting under ambient conditions, resulting in an inferior-quality product of lower market value. Exposure to ionizing radiationsuch as gamma rays at doses of 40-60 Gy (2,6) has been shown to be ef-fective in inhibiting sprouting and extending the shelf-life of the spice.However, there are very few reports (9) on the effect of such a treatmenton the flavor-contributing constituents of the fresh spice particularly dur-ing storage. Studies on the aroma quality of fresh ginger rhizomes imme-diately after irradiation (60 Gy) treatment have been reported by us earlier(7). Here, we report the changes in flavoring constituents of gamma-irra-diated (60 Gy) fresh ginger during storage under ambient conditions.

MATERIALS AND METHODS

Reagents

Chemicals and solvents of analytical reagent grade were obtainedfrom E-Merck India Ltd. (Bombay, India). All solvents were re-dis-tilled. Diethyl ether was made peroxide free before use.

26 JOURNAL OF HERBS, SPICES & MEDICINAL PLANTS

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Materials

Fresh mature ginger rhizomes with well-set skin were procured fromthe local market, washed free of adhering soil and air dried overnight atroom temperature. The rhizomes were packed (200 g/pack) in perfo-rated 150-gauge low-density polyethylene bags (26 × 16 cm2). Half ofthe lot was subjected to gamma irradiation in air at 25°C to a minimumabsorbed dose of 60 Gy in a Co60 Food Package Irradiator (AECL, Ot-tawa, Canada) at a dose rate of 31 Gy min–1. The other lot served asnon-irradiated control samples. All bags were stored under ambientconditions (temp, 30 ± 2�C, relative humidity, 80-85%) and samplesfrom both the lots were analyzed for weight loss, volatile oils andgingerol content at monthly intervals.

Isolation of Volatile Oils

Fifty-gram portions each of the control and irradiated ginger samplewere cut into small slices, crushed using a pestle and mortar and thenseparately subjected to simultaneous distillation-extraction (3) for twohours. Peroxide-free diethyl ether was used as the extracting solvent.Solvent was removed by passing a slow stream of nitrogen to obtain thevolatile oils. Percent oil was calculated on a wet-weight basis.

Gas Liquid Chromatography

Volatile oil obtained from the above samples was analyzed by GLCon a Shimadzu GC-15A gas chromatograph equipped with a flame ion-ization detector, a stainless-steel column packed with 10% carbowax20 M on an 80-100 mesh Gas chrome QAW and a Shimadzu C-R6Achromatopack integrator. Nitrogen was used as a carrier gas (50 ml/min).The temperature was programmed from an initial temperature of 60�Cfor 8 min, followed by an increase of 4�C/min to 200�C and holding for5 min. Gas chromatography-mass spectrometry (GC-MS) was carriedon a Shimadzu GCMS-QP5000 system equipped with a Shimadzu GC-17A gas chromatograph. A capillary DB-1 column was used. Analyticalconditions were essentially the same as above, except for the following:temperature program: 80�C to 220�C; carrier gas: helium; ionization volt-age: 70 eV and electron multiplier voltage: 1500V.

Variyar, Gholap, and Sharma 27

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Isolation of Pungent Principles

Isolation of pungent principles was carried out according to the meth-od previously described (8). In short, freshly cut ginger (10 g) previ-ously soaked in dichloromethane (30 ml, 2 h) was ground in an Omnimixer for 2 min. at a speed of 10,000 r.p.m. The filtrates obtained afterrepeated extraction were pooled and transferred to a separating funnel.The lower dichloromethane layer containing compounds of interest wasremoved and dried with sodium sulfate. The residue after removal ofsolvent was then dissolved in chloroform to obtain a 10 g L–1 solution.This solution containing pungency component was directly analyzed forgingerol content by TLC (thin-layer chromatography) and fluorimetry.

Isolation and Purification of Gingerols

A part of the above extract (130-150 mg solids) was loaded on asilicagel column (25 g, 70 cm × 1 cm i.d.) and then eluted with n-hex-ane, followed by increasing proportions of diethyl ether (8). Fractionsthat eluted with n-hexane: diethyl ether 50:50 and 40:60 were pooledand evaporated completely (100%) to dryness to obtain 40 mg of resi-due that was made to a 1% solution in chloroform. This solution, con-taining more than 700 g L–1 6-gingerol, as determined by HPLC (HighPerformance Liquid Chromatography), was used as standard gingerolin the present study. HPLC analysis of the above purified solution wascarried out on a Pharmacia LKB HPLC system equipped with anRP-C18 analytical column, a VWM 2141 double-wavelength UV de-tector set at 282 nm and a Rheodyne model 7125 injector with a 200 µLsample loop. Peaks were registered on an REC-1 recorder. Sampleswere eluted with 2% acetic acid in water as solvent A and methanol assolvent B using a linear gradient elution from 0 to 100% B in A over aperiod of 30 min at a flow rate of 2.0 ml min –1. Peaks were identified bycomparison of their Rt with that of standard compounds reported in theliterature.

Estimation of Pungent Principles

Estimation of pungent principles was carried out in a similar manneras reported earlier (8).

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TLC

Aliquots of the purified gingerol isolate (20-100 µg) obtained abovewere spotted on ammonium sulfate-impregnated silica gel G TLCplates (250 µm) and then developed using hexane: diethyl ether (1:1.5v/v) as the developing solvent system. The spots were visualized byheating the plate at 180�C for 10 min (8). The area of individual spotswas then measured on a Shimadzu dual-wavelength TLC scanner CS910using a 500 nm filter. The sample and reference wavelengths were setat 510 and 460 nm, respectively. A graph of concentration (µg) versusspot area (sq. cm) was then drawn to obtain a standard curve (correla-tion factor, 0.9475). Crude ginger extracts (100 mg) were subjected toTLC exactly in the same manner as for the standard. The concentrationof the spots at Rf values of 0.35 and 0.29 corresponding to 8 � 10 gin-gerol and 6-gingerol, respectively, was then measured from the stan-dard curve. Gingerol content was expressed as the total concentration ofthese compounds in mg per kg of fresh ginger. The TLC method wasfound to be linear in the concentration range of 20-60 µg (8).

Fluorimetry

The fluorescence of standard gingerol as well as the samples wasmeasured as reported earlier (8) at an excitation wavelength of 435 nmand an emission wavelength of 482 nm against chloroform as a solventblank. A graph of concentration (mg/ml) vs. fluorescence intensity wasdrawn to obtain a standard curve (correlation factor, 0.997). The con-centration of gingerol in the individual sample was measured from thestandard curve and expressed as mg per kg of ginger on a fresh-weightbasis. The method was linear in the concentration range of 1-3 mg/ml.

Data Analysis

Statistical analysis was performed using a paired t-test and an ANOVA(Microcal origin 4.1software), and results were expressed as significantor not significant at 1 and 5% levels. In case of volatile oils, the data aremeans of two independent determinations, each analyzed in duplicate.Data on gingerol content are presented as means of three independentdeterminations, each carried out in duplicate.

Variyar, Gholap, and Sharma 29

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RESULTS AND DISCUSSION

All samples stored under ambient conditions showed a decrease infresh weight (P < 0.001), with the irradiated one showing a consistentlyhigher weight loss (P = 0.01) compared with the non-irradiated control atall the time periods analyzed. A loss in weight of about 34% and 38% wasobserved in the control and irradiated samples, respectively, after the sec-ond month of storage compared with the zero-day samples. The controlsamples sprouted within one week of storage. The length of sprouts con-tinued to increase on storage up to two months. Beyond two months, thecontrol samples started to rot and became commercially unacceptable.However, there was hardly any visible sprouting in the irradiated sampleseven on storage beyond two months. In fact, the one-month stored sam-ples were found to be comparable in quality to the zero-day samples.Beyond two months, the irradiated samples showed moldiness and shriv-eling and hence the study was not continued beyond this period.

Loss in weight of fresh ginger rhizomes presently observed is similarto that reported by Yusof (10), wherein a 50% decrease in fresh weightwas observed on storage up to two months. A higher weight loss in irradi-ated samples was also reported in this study. The loss in weight was sug-gested to be due to respiration and/or transpiration losses on storage,which, in irradiated samples, was found to be much higher. The averageshelf-life of fresh ginger was found to be less than two months when keptunder ambient conditions (25-28�C, 76-96% relative humidity).

Volatile Essential Oils

Percent yields of essential oil obtained from control and irradiatedsamples are presented in Table 1. The yield of oil obtained from irradiatedsamples was consistently higher at all the time points analyzed. As notedin the table, a slight increase in the yield of oil was obtained in rhizomesstored up to one month. The oil yield, however, decreased significantly inthe two-month stored samples irrespective of the treatment (10,1).

Wu and Yang (9) reported appreciable losses in aroma compounds offresh ginger during storage (25�C) up to 3 months. On the other hand,Paull, Chen and Goo (5) failed to detect any such changes on storage(22�C) even up to 32 weeks. In the present study, the higher oil yield ob-tained from one-month stored samples could be the consequence of aloss in fresh weight. Beyond this period, considerable loss in aromaconstituents leads to a lower oil yield, overriding the effects of in-creased weight loss. An increase in viscosity due to polymerization of

30 JOURNAL OF HERBS, SPICES & MEDICINAL PLANTS

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essential oil constituents and a consequent reduction in yield of oil havealso been reported from stored ginger sample (2). A consistently higheryield of oil obtained from gamma-irradiated ginger as compared withcontrol could be attributed to the higher weight loss observed in irradi-ated samples on storage.

The composition of the major compounds identified in the oil isshown in Table 2. Zingiberene was found to be the major compound,accounting for 30% of the oil; ar-curcumene and �-sesquiphellandrenewere the other significant aroma compounds present. The compositionof the oil is more or less in agreement with the values reported in the lit-erature. No qualitative differences were noted in the essential oil con-stituents obtained from control and irradiated samples during the courseof storage up to two months.

A quantitative decrease in the major aroma significant compoundscould, however, be noted on storage. Geranial and neral are known tocontribute to the citrusy odor of fresh ginger, and their loss during stor-age results in lowering of the lemony character and hence freshness ofthe sample. In the present study, a gradual decrease in the content of theabove compounds with storage is clearly noticed (Table 2). �-phella-ndrene, linalool and -terpeniol, although relatively odorous, are notcharacteristic or unique to ginger and their variation does not affect theoverall ginger flavor. �-sesquiphellandrene and ar-curcumene, on theother hand, are flavor-significant compounds of ginger. An increase incurcumene and a decrease in zingiberene � zingiberol and �-sesqui-phellandrene � �-bisabolene could be noted in Table 2. Zingibereneand �-sesquiphellandrene are known to undergo gradual transformationto curcumene on storage. These compounds also tend to polymerize, re-sulting in a reduction in their content on storage. The above factors

Variyar, Gholap, and Sharma 31

TABLE 1. Effect of gamma irradiation on the volatile oil yield of fresh ginger rhi-zomes during storage.

Period (Months) Percent yield of essential oil (fresh-weight basis)

Control Irradiated

0 0.14 � 0.007* 0.17 � 0.004*

1 0.17 � 0.002** 0.19 � 0.003**

2 0.12 � 0.001* 0.14 � 0.005*

�SE (N 3), three independent determinations.* Significant at 5% level (between columns).** Significant at 1% and 5% levels (between columns).

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could account for the observed decrease in the content of these com-pounds on storage for up to two months.

In a study on the composition of essential oil of gamma-irradiated (60Gy) ginger rhizomes, Wu and Yang (9) reported a significant reductionin major aroma compounds such as ar-curcumene after 3 months ofstorage at ambient temperature (25�C). Such variations were, however,not observed by them in the volatile oils obtained immediately or onemonth after irradiation. The changes observed were suggested (9) to bedue to radiation-induced instability of some of the volatile compoundsduring storage. In the present study, no significant quantitative variationin the majority of the essential oil constituents was noted in gamma-irra-diated fresh ginger compared with the corresponding non-irradiatedsamples. Thus, radiation treatment does not bring about notable changesin volatile aroma constituents of fresh ginger stored up to a period oftwo months.

Pungent Principles

Data on changes in gingerol content of control and gamma irradiatedginger samples during storage under ambient conditions are presentedin Table 3. As can be seen in the table, the gingerol content of both thecontrol and irradiated ginger increased during storage, attaining a nearlytwofold increase in both samples after two months. Paull et al. (5) haveearlier reported a fivefold increase in the 6-gingerol content of the spicestored at 22�C for six months. The increase presently observed could bedue to a decrease in fresh weight as a result of transpiration losses dur-

Variyar, Gholap, and Sharma 33

TABLE 3. Changes in gingerol content of control and gamma-irradiated freshginger rhizomes during storage as estimated by fluorescence (F) and TLC-densitometric (T) methods.

Storage period(Months)

Gingerol content (mg/kg)

Control Irradiated

F T F T

0 3.25 � 0.05 3.31 � 0.06 2.54 � 0.13 2.62 � 0.011 3.65 � 0.04 3.99 � 0.07 2.59 � 0.02 3.09 � 0.162 5.9 � 0.06 6.01 � 0.16 5.31 � 0.08 5.39 � 0.08

� SE (N 6).Means of control and irradiated samples as estimated by the two methods are significant at 1% and 5%levels.F Fluorescence method, T TLC-densitometric method.

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ing storage. Further, irradiated ginger contained a significantly lowergingerol content compared with the control at all time points analyzedin spite of a higher weight loss observed in irradiated samples on storage(Table 3). Onyenekwe (4) has also reported such changes in gingerolcontent in irradiated dry ginger (ground as well as whole) stored for 9months at ambient temperature. The content of 6-gingerol in groundginger decreased by 65.6, 67.4 and 70.4% at the end of the storage pe-riod in the 0, 5 and 10 Kgy-treated samples, respectively. The corre-sponding values for whole ginger samples were 37.8, 40.0 and 44.3%.In the present study, this decrease was approximately 21%, 22% and10% in irradiated ginger stored at 0, 1 and 2 months, respectively, com-pared with their corresponding non-irradiated controls. Conversion ofgingerols to zingerone and their dehydrated product shogaols on storagehas been known in the literature. However, in the present study the con-tents of these transformed products were more or less constant, indicat-ing that such conversions do not occur during radiation processing ofthe spice. The decrease in pungency factor in the irradiated spice couldthus be due to the degradation of gingerols when the fresh spice was ex-posed to a radiation dose of 60 Gy. Wu and Yang (9) have reported thatthe sensory properties of gamma-irradiated fresh ginger with respect toaroma, taste, pungency and color were unaffected when stored up to aperiod of 5 months at ambient temperature. Hence, gamma irradiation ata dose of 60 Gy could prevent sprouting and extend the shelf-life offresh ginger up to two months under ambient conditions without affect-ing their flavor-contributing components.

REFERENCES

1. Abd Shukor, A.R., I.A. Aziz, and A.O. Shokri. 1986. Physicochemical changesof fresh ginger rhizomes as influenced by storage temperature and duration. MARDIResearch Bulletin 14(3):243-248.

2. Govindarajan, V.S. 1982. Ginger–Chemistry, technology and quality evalua-tion: Part 1 and 2. CRC. Crit Reviews in Food Science and Nutrition 17:1-96 and189-258.

3. Nickerson, G.B. and S.T. Likens. 1966. Gas chromatographic evidence of theoccurrence of hop oil components in beer. Journal of Chromatography 21:1-5.

4. Onyenekwe, P.C. 2000. Assessment of oleoresin and gingerol in gamma irradi-ated ginger rhizome. Nahrung 44(2):130-132.

5. Paull, R.E., N.J. Chen, and T.T.C. Goo. 1988. Compositional changes in gingerrhizomes during storage. Journal of American Society for Horticulture Science 113(4):584-588.

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6. Sirikilvadhana, S. and C. Prompukesera. 1979. Effect of gamma radiation andtemperature on ginger (Zingiber officinale) sprout and weight. Food 11(1):55-69.

7. Variyar, P.S., A.S. Gholap, and P. Thomas. 1997. Effect of �-irradiation on thevolatile constituents of fresh ginger (Zingiber officinale) rhizome. Food Research In-ternational 30(1):41-43.

8. Variyar, P.S., A.S. Gholap, and P. Thomas. 2000. Estimation of pungency infresh gingers: A new fluorimetric assay. Journal of Food Composition and Analysis13:219-225.

9. Wu, J.J. and Y.S. Yang. 1994. Effect of �-irradiation on the volatile compoundsof ginger rhizome (Zingiber officinale Roscoe) sprout and weight. Journal of Agricul-tural and Food Chem. 42(11):2574-2577.

10. Yousf, N. 1990. Sprout inhibition by gamma irradiation in fresh ginger (Zingiberofficinale Roscoe). Journal of Food Processing and Preservation 14:113-122.

doi:10.1300/J044v12n01_03

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