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doi:10.1016/j.biosystemseng.2006.07.008PH—Postharvest Technology

Biosystems Engineering (2006) 95 (2), 219–225

Effect of Gamma-ray Irradiation on Drying Characteristics of Wheat

Y. Yu; J. Wang

Department of Biosystems Engineering, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR China;e-mail of corresponding author: jwang@zju.edu.cn

(Received 30 August 2005; accepted in revised form 14 July 2006; published online 30 August 2006)

Wheat, pre-treated by gamma-ray irradiation, was air-dried and the influence of irradiation dose, airtemperature, and initial moisture content of wheat on drying rate and surface temperature was investigated.Irradiation dose, air temperature, and initial moisture content of wheat affected drying characteristics. Dryingrates increased with an increase in dose and temperature. Surface temperature of wheat samples increased withincreasing dose at the same drying rate but decreased with increasing dose at the same moisture content. Thesechanges of drying characteristics of irradiated wheat samples were the result of wheat cell structure changescaused by gamma-ray irradiation. It was clear from the microscopic observation that increasing irradiationdose reduced cell-wall thickness or eventually destroyed the cell-wall, causing the cytoplasm to leak from itsown cell.r 2006 IAgrE. All rights reserved

Published by Elsevier Ltd

1. Introduction

Gamma irradiation has long been employed todecontaminate and extend the shelf- life of food. Onthe basis of the extensive scientific evidence reviewed,the Food and Agriculture Organisation (FAO)/Interna-tional Atomic Energy Agency (IAEA)/World HealthOrganization (WHO) Joint Expert Committee on FoodIrradiation has unconditionally declared foods irra-diated up to 10 kGy as wholesome and nutritionallyadequate and safe for human consumption (FAO/IAEA/WHO, 1991a, 1991b, 1992, 1994). A joint FAO/WHO/IAEA group study also concluded that foodtreated with doses greater than 10 kGy could beconsidered safe and nutritionally adequate when pro-duced under established good manufacturing practice(FAO/IAEA/WHO, 1999). Many studies have beenreported on the advantageous effect of irradiation onprocessing characteristics and physicochemical proper-ties of irradiated products. Wang and Chao (2002,2003a) reported that the interior tissue structure ofsliced apple was changed and injured by 60Co gamma-ray irradiation in the dose range of 0–10 kGy, and thesechanges in structure brought about changes in dryingcharacteristics and affect drying rate and dried qualities

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of the dried products. Wang and Chao (2003b), andWang and Du (2005) also showed that the drying rate,content of vitamin C, and rehydration ratio of driedpotato were greatly affected by irradiation dose(0–10 kGy) prior to the drying process. The study onthe effect of gamma irradiation on physicochemicalproperties of three types of dried wheat cultivars showedthat the apparent amylose content was reduced and gelconsistency was improved with increasing dose(0–12 kGy) (Wu et al., 2002). Four major parametersdetermined by a rapid visco analyser (RVA, Model-3D,Newport Scientific Inc., Australia), peak viscosity, hotpasting viscosity, cool pasting viscosity, and setbackviscosity, were considerably decreased with increasingdose (Wu et al., 2002). The effect of gamma irradiationon the viscosity of two dried barley cultivars showedthat the viscosity values were reduced by 25%, 50%,65%, 72% and 74% in the Local Black barley cultivar,while, in the Shoaa cultivar the reductions were 15%,30%, 52%, 69% and 67% at 10, 50, 100, 150 and200 kGy, respectively (Al-Kaisey et al., 2002).

However, there was little study of wheat dryingcharacteristics after irradiation. The objectives of thisresearch were to: (1) study the effect of irradiation,drying temperature, and initial moisture content on

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Y. YU; J. WANG220

wheat drying characteristics; (2) study the effect ofirradiation and initial moisture content on surfacetemperature of drying wheat; and (3) use the electronmicroscope to study the effect of irradiation on cellstructure of wheat.

Fig. 1. Sketch of wheat

2. Materials and methods

2.1. Wheat

Wheat harvested in May, 2004, from the experimentalfarm of Agronomy, Zhejiang University, was used forthis experiment immediately after harvest. The initialmoisture content was determined by drying five replicatesamples of wheat at 105 1C in a constant temperatureoven till the weight of the samples became constant(GB/5497-85, National Standard of China), and was0�25 kg [H2O] kg�1 [dry matter, DM].

Before irradiation and drying experiments, wheatsamples were air-dried in ambient conditions to differentinitial moisture contents (0�25, 0�24, 0�22, 0�20, &0�19 kg [H2O] kg�1 [DM]). The initial sample mass foreach drying test was 250 g.

2.2. Experimental procedure

Wheat samples were irradiated by 60Co gamma-ray inthe Institute of Nuclear-agriculture Sciences, ZhejiangUniversity (Hangzhou, Zhejiang). The doses were 0 kGy(non-irradiation), 0�6, 1�5, 2�4, and 3 kGy, with dose rateof 1 kGy h�1.

The irradiated samples were evenly placed in a sifter.Drying experiments were conducted at five air tempera-tures (40, 45, 50, 55, and 60 1C), from five initialmoisture contents. A fixed-bed hot-air dryer (101A,Anling Apparatus Inc., Shanghai, China) was used withan automatic weighing system (70�01 g) with a controlof temperature within 71 1C of a set temperature.During all drying experiments, air velocity was kept at0�5m s�1 (70�1m s�1) and relative humidity was kept at50% (72%). Drying experiments were stopped whensamples reached a final moisture content of0�13570�001 kg [H2O] kg�1 [DM], which represents asafe moisture level for grain storage at ambienttemperature. The mass of each sample was detectedevery 5min. The drying rate after drying for each 5minwas calculated basing on the lost moisture in 5min andits unit is kg [H2O] kg�1 [DM] h�1. The surfacetemperatures of samples were periodically measured byinfrared thermoscope (MiniTemp FS, Raytek, USA)with an accuracy of 71 1C, and the initial water activity(indication of the equilibrium state of water within

material) was measured by a water activity apparatus(HD-A-II, Bibo Electronic Apparatus Inc., Wuxi,Jiangsu, China) with an accuracy of 70�015. TheANOVA analysis was carried out with SAS 6�12software. All drying and water activity experimentswere replicated three times.

After drying, samples were cut open and coated in anIB-5 ion coater (Philips, Amsterdam, The Netherland).The miroscopic observation of sample structure wasconducted at the Institute of Plant Protection Sciences,Zhejiang University with an electronic microscope (XL30-ESEM, Philips, Amsterdam, The Netherland). The wheatgrains used in this experiment were cut along their creaseto the middle of the crease with a knife (Fig. 1), andlaniated (torn in two) along the crease by hand.

3. Results and discussion

3.1. Effect of irradiation dose on cell structure

The effect of irradiation dose on seed coat cellstructure of wheat grain is shown in typical microscopyimages in Fig. 2. In this experiment, each sample hadtwo duplicates, and the same result was obtained fromsamples with the same dose. The cell structure of a non-irradiated wheat grain is intact, and its cell wall is thickand cytoplasm is contained in the cell wall [Fig. 2(a)].The cell structure of 0�6 kGy irradiated wheat grain waschanged, and its cell wall became thin and demonstratedsome breakpoints, but its cytoplasm was still containedin its cell wall [Fig. 2(b)]. When the dose increased to1�5 kGy, some cell walls disappeared and cytoplasm as awhole flowed out of cell wall. Besides, some cavitiesappeared due to the outflow of cytoplasm [Fig. 2(c)].When the dose was increased to 2�4 kGy, most cell wallsdisappeared and the cytoplasm appeared to be de-stroyed [Fig. 2(d)]. When the dose was increased to3 kGy, nearly all cell walls disappeared and thecytoplasm joined with that of other cells [Fig. 2(e)].These changes of cell structure were due to the

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Fig. 2. Effect of irradiation on cell structure of wheat at an initial moisture content of 0�22 kg [H2O] kg�1 [DM]: (a) 0 kGy; (b)0�6 kGy; (c) 1�5 kGy; (d) 2�4 kGy; (e) 3 kGy

0.10

0.08

0.06O]

kg−1

[D

M]

h−1

EFFECT OF GAMMA-RAY IRRADIATION 221

susceptibility of large molecules to damage by gamma-ray irradiation, and the cell wall and cytoplasm aremainly composed of large molecules (Wu et al., 2002).Based on the above observation, the seed coat cell

structure of wheat grain could be destroyed by gamma-ray irradiation, and the destructiveness is increased withthe increase in dose.

0.04

0.02

0.14 0.16 0.18 0.20 0.220

Moisture content, kg [H2O] kg−1 [DM]

Dry

ing

rate

, kg

[H2

Fig. 3. Effect of irradiation dose on drying rate at an airtemperature of 50 1C and at an initial moisture content of 0�22 kg[H2O] kg�1 [DM]: m, 0 kGy; X, 0�6 kGy; %, 1�5 kGy; J,

2�4 kGy; ’, 3 kGy

3.2. Effect of dose on dehydration characteristics

The drying rate of wheat samples irradiated withdifferent doses, at a drying temperature of 50 1C and aninitial moisture content of 0�22 kg [H2O] kg�1 [DM], wascalculated and plotted against moisture content asshown in Fig. 3. The drying rate of the irradiatedsample was larger than that of the control (0 kGy), and ahigher drying rate was observed at higher dose at thesame moisture content. As the tissue and structure of thewheat grain were altered by irradiation (Fig. 2), it is

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Table 1

The water activity and average drying rate of wheat at different

doses (air temperature, 50 1C; initial moisture content, 0.22 kg

[H2O] kg�1

[DM])

Dose, kGy Water activity� Average drying rate�� kg[H2O] kg�1 [DM] h�1

0 0�78570�005 0�02170�0010�6 0�80070�005 0�02470�0011�5 0�84070�015 0�03170�0022�4 0�85570�005 0�03670�0013 0�87070�010 0�03970�002

F, ratio between variances; P, probability.�F ¼ 49�219; Po0�0001:��F ¼ 80�045; Po0�0001:

0.10

0.08

0.06

0.04

0.02

0.14 0.16 0.18 0.20 0.22 0

Moisture content, kg [H2O] kg−1 [DM]

Dry

ing

rate

, kg

[H2O

] kg

−1 [

DM

] h−1

Fig. 4. Effect of air temperature on drying rate with anirradiation dose of 1�5 kGy and at an initial moisture contentof 0�22 kg [H2O] kg�1 [DM]: m, 60 1C; X, 55 1C; %, 50 1C;

J, 45 1C; ’, 40 1C

Table 2

The average drying rate of wheat at different air temperature

(dose, 1.5 kGy; initial moisture content, 0.22 kg [H2O] kg�1

[DM])

Air temperature, 1C Average drying rate�, kg[H2O] kg�1 [DM] h�1

40 0�01470�00245 0�02370�001

Y. YU; J. WANG222

likely that the moisture inside the irradiated wheat iseasier (needs less energy) to migrate to the surface. Withincreasing damage caused by the increase in irradiation,the transport rate of moisture also increased (Jain &Pathare, 2004). This has been supported by values ofwater activity (Table 1) that wheat with a higher dose hasa higher transport rate of water. The analysis of variance(ANOVA) showed that the water activity (the ratiobetween variances probability F ¼ 49�219; Po0�0001)and average drying rate (F ¼ 80�045; Po0�0001) weresignificant affected by irradiation dose (Table 1). Thecurves of drying rate versus moisture content at other airtemperature and initial moisture contents had the sametrend as in Fig. 3 but were omitted.

50 0�03170�00255 0�03870.00260 0�04970�003

F, ratio between variances; P, probability.�F ¼ 123�75; Po0�0001:

3.3. Effect of air temperature on dehydration

characteristics

The effect of different drying air temperatures ondrying characteristics of irradiated wheat at an irradia-tion dose 1�5 kGy and initial moisture content 0�22 kg[H2O] kg�1 [DM] is shown in Fig. 4. At the samemoisture content, the drying rate of samples increasedwith the increase in air temperature because the higherair temperatures can improve the transport rate ofmoisture in irradiated wheat (Yang et al., 2003;Erenturk et al., 2004; Igathinathane & Chattopadhyay,2002; Wu et al., 2004). During the later drying stages(when moisture content is smaller), values of drying ratestill have a relatively large difference (compared with theresult of Fig. 3). This implies that air temperature has alarger effect on drying rate of irradiated samples thanirradiation dose. The ANOVA analysis showed that theaverage drying rate (F ¼ 123�75; Po0�0001) weresignificant affected by air temperature (Table 2). Thecurves of drying rate versus moisture content at otherirradiation doses and initial moisture contents had thesame trend as in Fig. 4 but were omitted.

3.4. Effect of initial moisture content on dehydration

characteristics

The drying characteristics for different initial moist-ure contents, at a drying temperature of 50 1C and doseof 1�5 kGy, are shown in Fig. 5. At the beginning ofdrying, the drying rate of the irradiation sample isgreater at a higher initial moisture content. This isbecause the sample with a higher initial moisture contenthad a higher water activity (Table 2), and the transportrate of water inside the sample is also higher. At thesame moisture content, however, the drying rates ofsamples with higher initial moisture contents are smallerthan those of samples with lower initial moisturecontents (Fig. 5). It is possible that, at the same sampleaverage moisture content, the sample with a higherinitial moisture content takes more time to reach to thismoisture content, and its internal diffusion and dryingrate are more reduced. The ANOVA analysis showed

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0.08

0.06

0.04

0.02

0.15 0.18 0.21 0.240

Moisture content, kg [H2O] kg−1 [DM]

Dry

ing

rate

, kg

[H2O

] kg

−1 [

DM

] h−1

Fig. 5. Effect of initial moisture content on drying rate with anirradiation dose of 1�5 kGy and at an air temperature of 50 1C m,0�25 kg [H2O] kg�1 [DM]; X, 0�24 kg [H2O] kg�1 [DM];%, 0�22 kg [H2O] kg�1 [DM]; J, 0�20 kg [H2O] kg�1

[DM]; ’, 0�19 kg [H2O] kg�1 [DM]

Table 3

The water activity and average drying rate of wheat at different

initial moisture contents (Dose, 1.5 kGy; Air temperature, 50 1C)

Initial moisturecontent, kg [H2O]kg�1 [DM]

Water activity� Average dryingrate��, kg [H2O]kg�1 [DM] h�1

0�19 0�75070�005 0�02770�0010�20 0�81070�010 0�02870�0010�22 0�84070�015 0�03170�0020�24 0�87070�010 0�03270�0010�25 0�89570�010 0�03270�002

F, ratio between variances; P, probability.�F ¼ 86�455; Po0�0001:��F ¼ 7�5; Po0�005:

45

40

35

30

250 30 60 90 120

Drying time, min

Surf

ace

tem

pera

ture

, °C

Fig. 6. Effect of irradiation dose on surface temperature at anair temperature of 50 1C and at an initial moisture content of0�22 kg [H2O] kg�1 [DM]: m, 3 kGy; X, 2�4 kGy; %,

1�5 kGy; J, 1�6 kGy; ’, 0 kGy

EFFECT OF GAMMA-RAY IRRADIATION 223

that the water activity (F ¼ 86�455; Po0�0001) andaverage drying rate (F ¼ 7�5; P ¼ 0�005) were significantaffected by initial moisture content (Table 3). The curvesof drying rate versus moisture content at other airtemperatures and irradiation doses had the same trendas in Fig. 5 but were omitted.

3.5. Changes of surface temperature

The effect of dose on surface temperature at a dryingtemperature of 50 1C and an initial moisture content of0�22 kg [H2O] kg�1 [DM] is shown in Fig. 6. The surfacetemperatures of wheat samples tend to increase withincreasing dose. Based on many current studies (Sree-

narayanan & Chattopadhyay, 1986; Shyamal et al.,1994; Subramanian & Viswanathan, 2003), the specificheat of grain increases with the increasing moisturecontent. During the same drying time, the moisturecontent of wheat sample irradiated with a higher dosewas lower than that of wheat sample irradiated with alower dose because of the different drying rate. That is tosay, the specific heat of the wheat sample irradiated witha higher dose was lower than that of the wheat sampleirradiated with a lower dose after the same drying time.It is well known that lower specific heat express samplesneed lower energy to increase the internal temperature.As all the samples were drying at the same temperature,the wheat sample with a lower specific heat could reach ahigher temperature than samples with a higher specificheat after the same drying time.

The influence of initial moisture content on surfacetemperature, at a drying temperature of 50 1C and adose of 1�5 kGy, is shown in Fig. 7. The surfacetemperatures of samples decreased with increasinginitial moisture contents. This effect of initial moisturecontent may be because that, during the same dryingtime, samples with a higher initial moisture content havemore moisture loss, and the quantity of heat lost withmoisture evaporation is increased.

3.6. Relationship between surface temperature and drying

rate/moisture content

The relationship between surface temperature anddrying rate/moisture content of irradiated wheat, with

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45

40

35

30

250 30 60 90 120

Drying time, min

Surf

ace

tem

pera

ture

, °C

Fig. 7. Effect of initial moisture content on surface temperaturewith and irradiation dose of 1�5 kGy and at an air temperature of50 1C m, 0�25 kg [H2O] kg�1 [DM]; X, 0�24 kg [H2O] kg�1

[DM]; %, 0�22 kg [H2O] kg�1 [DM]; J, 0�20 kg [H2O]kg�1 [DM]; ’, 0�19 kg [H2O] kg�1 [DM]

45

40

35

30

250 0.03 0.06 0.09

Surf

ace

tem

pera

ture

, °C

Drying rate, kg [H2O] kg−1 [DM] h−1

Fig. 8. Surface temperature as a function of drying rate fordifferent doses at an air temperature of 50 1C and at an initialmoisture content of 0�22 kg [H2O] kg�1 [DM]: m, 3 kGy; X,

2�4 kGy; %, 1�5 kGy; J, 1�6 kGy; ’, 0 kGy

45

50

40

35

30

25

Surf

ace

tem

pera

ture

, °C

0.14 0.16 0.18 0.20 0.22

Moisture content, kg [H2O] kg−1 [DM]

Fig. 9. Surface temperature as a function of moisture contentfor different doses at an air temperature of 50 1C and at an initialmoisture content of 0�22 kg [H2O] kg�1 [DM]: m, 3 kGy; X,

2�4 kGy; %, 1�5 kGy; J, 1�6 kGy; ’, 0 kGy

Y. YU; J. WANG224

an initial moisture content of 0�22 kg [H2O] kg�1 [DM]and an air temperature of 50 1C, at different doses areshown in Figs 8 and 9. It is evident that the specific heatof agricultural product will decrease with the decreasingmoisture content (Gooding et al., 2003; Erenturk et al.,2004; Xu et al., 2004; Zhang et al., 2002, 2005). Duringdrying, when all wheat samples reached a same drying

rate (Fig. 8), the sample irradiated with higher dose hada lower moisture content (Fig. 3), and its surfacetemperature was easier to increase with a lower specificheat. However, when all wheat samples reached a samemoisture content (Fig. 9), the sample irradiated with ahigher dose had a higher drying rate (Fig. 3), and itssurface temperature was harder to increase, with morethermal energy being removed from the surface atthe higher drying rate. Besides, in Figs 8 and 9,surface temperature increased with the decreasingdrying rate and moisture content for all samples dueto less thermal energy being removed from the surface ofthe samples.

4. Conclusions

(1)

Irradiation dose, air temperature, and initial moist-ure content affect drying characteristics of irradiatedwheat, and drying rate increased as the values ofthese factors increased.

(2)

Surface temperature increased with increasing irra-diation dose at the same drying rate, but decreasedat the same moisture content.

(3)

The changes in drying characteristics and surfacetemperature of irradiated wheat are due to theincreasing destruction of cell structure by increasingirradiation. With the increasing irradiation dose, cellwalls become thin or disintegrate, and cytoplasmflowing from a cell is destroyed or mixes withcytoplasm from an other cell.

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EFFECT OF GAMMA-RAY IRRADIATION 225

Acknowledgements

The authors acknowledge the financial support ofChinese National Foundation of Nature and Sciencethrough project 3047000 and the Research Fund for theDoctoral Program of Higher Education through Project20020335052 and the financial support of Program forNew Century Excellent Talents in Chinese University(NCET-04-0544).

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