Proceedings of the 7th luternational Working Conference on Stored-product Protection - Yolume 2

Microwave treatment of rape to ensure seed quality duringstorage

E. Pawelzik", I. Irfanl and W. Luecke'

Abstract

The aim of our research was to investigate the mfluence ofmicrowave-pre-treatment of rape seed on the storability ofthe seeds and the oil quality. For tlus purpose rape seedsamples with different water content of 6.9 and 9.5 % wereheated in a laboratory microwave applicator, until thetemperature level of 80, 90 and 100"( respectively werereached. These differently treated seeds were stored for sixmonths at 30"( and at relative humidity rangmg between 50and 70%. Microwave-pre-treatment reduced water contentand metabolic activity of the seeds, while oil yield increasedby pressing. Before storage, OIls from microwave treatedseeds had lower acid values and slightly higher peroxidevalues than those of the controls. After six months thetreated seeds had higher 011 content and higher 011 yieldcompared to untreated seeds. In general, quality criteria ofoils from treated seeds did not considerably changed. Thepresent results show an improvement of some processingcharacters of rape seeds and higher quality during storageeven under unfavourable environmental conditions aftermicrowave treatment.

Introduction

Rape seeds are an important source of edible OIls. A highcontent of unsaturated fatty acids (mainly oleic, linoleic andlinolenic acids) and active lipid splitting enzymes can lead toundesirable changes m quality durmg the storage of rapeseeds. Deterioration of rape seeds durmg storage prior topressing for oil has been a problem for the rape seedprocessing industry.Several drying methods are practised to ensure the

storabihty of cereals and 011 seeds. Over the past thirtyyears, the use of microwave energy has become popular as afood processmg technique as well as technique for

1 Institute of Agricultural Chenustry, Dept Of Plant ProductsQuality, Georg-August-Uruversity, Von-Siebold-Str 6, 0-37075Goettmgen, Germany

2 Institute of Agricultural Engmeenng , Georg-August-Umversity,Gutengerg-Str 33, 0-37075 Goettmgen, Germany

inactivating enzymes in stored plant products to preventtheir detenoration (Tao et al., 1993, Conkerton et al.,1991, List et al. , 1990) Maheshwary et al. (1981) studiedthe effect of microwave application on microstructure ofdehulled rape seeds and they assumed it facilitates oilextraction in the seeds.The objectives of our study were to investigate the

Influence of microwave-pretreatment of rape seeds on thestorabihty of the seeds, the oil quality and yield.

Materials and Methods

German double zero rape seeds of Brassica napus, L. wereadjusted to moisture contents of 9, 5 and 6, 9 % byequilibrating at room temperature for 48 hr. 670g sampleswere exposed in a rotary teflon dish to 2450 MHz microwaveIrradiation in a laboratory microwave applicator until thedesired temperature of 80, 90 or 100°C was reached. A partof these differently treated seeds were immediately pressed(type of press machine: Komet CA 59G) , and the otherswere stored for SIX months at 30°C and 50 - 70% relativehumidity before pressmg. All treatments were replicatedthree times. Water content, 011 yield, 011 content andparameters of oil quality were analysed (Irian et al.,1997a ). Chlorophyll content was determined withspectrophotometer at A 668 - 671nm (1. Irian, unpublisheddata) .

Results and Discussion

Water content, oil content and oil yield

Water is a polar molecule, which plays a very importantrole in the heating process of rape seeds with microwaves.On the other hand, rape seeds with high water content areeasy spoiled during storage. Through microwavetreatments, water content in rape seeds was reduced. Watercontent decreased with increasing temperatures m themicrowave oven. Despite the same storage condition,microwave treated seeds still had less water contentcompared to untreated ones, even after 6-month storage(Fig. 1). ThIS was probably due to a microstructural andbiochemical changes in the seeds, which were caused bymicrowave treatments.These structural changes in seeds are also one of the

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Proceedings of the 7 th International Working Conference on Stored-product Proteetian - Volurne 2

reasons, why the oil yields of microwave treated seeds atpressing were higher than that of untreated seeds (Ebrahim,1998, Maheshwari, 1981). Even after 6 months storage,the oil yields from microwave treated seeds were highercompared to untreated seeds (Fig. 2). This indicates thatsuch micro-structural changes are irreversible.These structural changes in seeds are also one of the

10

reasons, why the oil yields of microwave treated seeds atpressing were higher than that of untreated seeds (Ebrahim,1998, Maheshwari, 1981). Even after 6 months storage,the oil yields from microwave treated seeds were highercompared to untreated seeds (Fig. 2). This indicates thatsuch micro-structural changes are irreversible.

....~ 8-.-c:~ 6c:ou 4...J!l~ 2

oSOW9.5 SOW6.9

* Control was without microwave treatments

S6W9.5 S6 W6.9

.80·C

11J1190OC11J11100'C

OControl*

Fig. 1. Influence of microwaves and storage time on water content changes in rape seeds.

40

....

.c:i 30'ti~e..'C 20-ell>=0 10

oSOW9.5 SO'W6.9

Fig. 2 Influence of microwavesand storage time on oil yield.

Microwaves did not affect the oil content of seeds andresulted in more stability during storage in comparison withuntreated seeds. This was another reason for higher oil yieldfrom treated seeds after 6 months storage, as presented inFig. 2.Oil yield from all control sample and treated stored seeds

was higher than that of un-stored ones. This fact was relatedto a better condition during the pressing process, due tolower (optimal) water content of the seeds after storage(see Fig. 1).

oControl.80'C

1IIIl90'C

1IIIl100'C

S6W9.5 S6W6.9

Oil quality

It was noticed tbat microwave treatments had someeffects on tbe quality parameters of rape oils. In general,oils from microwave treated seeds had lower acid value, thesame thiobarbituric acid value (TEA), as well as higherperoxide value and chlorophyll content. These were alsonoted for oils from the stored seeds. The effect ofmicrowave's temperature treatment was more dominant thanthe effect of water content in rape seeds, before treatmentwith microwaves. Fig.3 presents the effect of microwave's

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Proceedings of the 7 th lntematumol Working Conference on Storedrprodv£:t Protection - Vol:ume 2

temperature treatment on acid value, peroxide value andchlorophyll content.The lower acid values of microwave treated seeds were

assumed to be a result of a successful inactivation of lipidsplitting enzymes in rape seeds (Irfan et al. . 1997b).

Enzyme lipase or methyllumeelliferyl-palmitic-acid-ester-hydrolase (MUPase) is responsible for the splitting of esterbonds in lipids, and this leads to a higher content in freefatty acids (Fretzdorff, 1994, Carlson et a1. , 1981).

3.5

3

2.5

2

1.5

1

0.5

oControl 80 90

Temperature of microwaves [ 'C ]

100

DAcid Value [mg/g] I1lIIPeroxide Value [meq/kg) II Chlorophyl [AU)

Fig. 3. Influenceof microwave's temperature treatment on rape seed quality.

In comparison with other parameters of rape oil quality,the effect of microwave on peroxide value was morefluctuated. By one of our previous research with othersamples of rape, the peroxide values of oils from microwavetreated seeds were considerably lower than those ofuntreated seeds, while in another experiment no effect wasdetected (Pawelzik et a1. , 1998). It is highly probable thatchanges in peroxide value depend on the genotype and thequality of rape seeds used.Through microwave treatment, as well as the oil,

chlorophyll was more easily extracted from rape seed.Thus, the colour of oils from microwave treated rape seedswas more green than that of untreated ones, and was similarto the colour of olive oil (Table 1). Though oils from storedseeds had slightly higher chlorophyll content, it did notmean that storage caused increasing of chlorophyll content inoil. As mentioned in this case, the higher chlorophyllcontent related more to the higher oil amount extracted fromthe seed (yield), indicating that production of cool pressedoil did not considerably reduce the oil quality.Other quality characteristics of rape oils also changed

during seed storage (Fig.4). After 6 months seed storage:acid value and TBA value increased, while peroxide value ofall oils decreased. Oils from microwave treated seeds hadlower acid value, similar TBA value and higher peroxide

value than those from untreated seeds. All of the oil qualityparameters were still acceptable (Platek et aI., 1995,Pokorny et al. , 1985).

Table 1. Influence of microwaves and storage time onchlorophyll content in rape oil compared to oliveoil.

Storage (Months) Chlorophyll Content [AU' ]

Control 80'C 90'C 100'C

0.12 0.43 0.53 0.58

0.14 0.46 0.57 0.61

0.46

o

6

Olive oil

• AbsorbencyUnit

Conclusion

There were some effects of microwave treatments andstorage on the water content, oil yield and quality of rapeseeds. The effect of temperature of microwaves treatmentwas more dominant than the effect of water content of rape

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Proceedings of the 7 th International Working Conference on Stored-product Protection. - Volume 2

seeds before treatment with microwaves.Through microwave treatment, oil yield was increased.

Treated seeds could be longer stored, due to their low watercontent and low enzymatic activity. After six months, the

treated seeds had higher oil content and oil yield compared tountreated seeds. In general, quality criteria of oils fromtreated seeds did not considerably change.

Before Storage4.---------------------,

Storage: 6 month

2

1

oAV PV TV

oControl.80"(;.90'C.100"(;

AV PV TV

AV: Acid Value [mglg], PV: PeroxideValue (meqlkg], TV: lBA Value (AU/g)/10]

Flg. 4. Influence of storage on quality characteristicsof oil.

References

1. Carlson, K. D., Schneider, W. J., Chang, S. P. andPrincen, 1. H., 1981. Vernonia galamensis seed oil: Anew source for epxy coatings. In: Pryde, E. H.,Princen, 1. H. abd Mukherjee, K. D., ed., Newsources of fats and oils, AOCSmonograph No.9, AOCS,Champaign, IL, 297 - 318.

2. Concerton, E. J., Schneider, G. R., Bland, J. M. ,Marshall, n., H. F. and Goynes, n.. W. R., 1991.Microwave heating to prevent deterioration of cottonseedduring storage. JAOCS. 68, 834 - 838.

3. Ebrahim-Nesbat, F., Oberndorfer, C., Pawelzik, E.,and Luecke. W., 1998. Microstrucktural changes ofrapeseed by different methods of thermal pretreatment.In: Deutsche Gesellschaft fuer Fettwissesnchaft (DGF)Congress, Fats & Oils in Science, Technology andNutrition - The Challenge for the Next Century.Magdeburg, Germany, 13 -15 September 1998.

4. Fretzdcrff, B.• 1994. Vergleich zweier Methoden zurBestimmung lipolytischer Enzymaktivitatcn im Brotget-reide. Z. Getreide Mehl und Brot 48. 2, 34 - 38.

5. Irfan, I., Pawelzik, E. and Luecke, W., 1997a.Einfluss einer Mikrowellenbehandlung auf ausgewaehlteEnzyme in Rapssaaaten und die Qualitaet des

gewonnenen Rapsoeles. In: Deutsche Gesellschaft fuerQualitaetsforschung, ed. , Umwelt, Anbau undVerarbeitung-Einfluss auf die Qualitaet , Proceedings No.32, Waedenswil, Swiss, 151-154.

6. Irian, I., Pawelzik, E., and Luecke, W., 1997b.Einfluss thermischer Behandlung von Raps mitMikrowellen auf die Qualitaetseigenschaften des Oels.In: VDI-MEDKolloquium Agrartechnik, ed. , Qualitaetvon Agrarprodukten, Potsdam-Bomim, Germany, 25-26 August 1997 (in press).

7. List, G. R., Mounts, T. 1., Lanser, A. C. andHolloway, R. K., 1990. Effect of moisture, microwaveheating, and live steam treatment on phospholipase Dactivity in soybeans and soy flakes, JAOCS67, 11, 867-871.

8. Maheshwari, P. N. , Stanley, D. W. , Van de Voort, F.R. , and Gray. J. I. 1981. Effect of microwave treatmenton the microstructure of dehulled rapeseed, CerealChemistry, 58, 5, 381 - 384.

9. Pawelzik, E., Irfan, I. and Luecke, W. 1998.Mikrowellenbehandlung von Raps-Einfluss aufVerarbeitungseigenschaften und Qualitaet des Rapsoeles.In: VDLUFA Congress, Einfluss von Erzeugung undVerarbeitung auf die Qualitaet landwirtschaftlicherProdukte, Giessen, Germany, 14 -18 September 1998.

10. Pokorny, ]., Valentova, H. and Davidek., 1985.

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Proceedmqs of the 7 th International Worktng Conference on Stored-product Protecuon - Volume 2

Modified determination of 2-thlObarbitunc acid value infats and oils. Die Nahrung 29, 1, 31 - 38

11. Platek, T., Katzer, A. and Jerzewska, 1995Influence of double zero rapeseed oil m refining processon Its oxidative stability. In: Proceeding of the Ninth

International Rapeseed Congress, Volume 4 of 4,Cambndge, UK, 4 -7 Juli 1995, 1375 -1377

12 Tao, J., Rao, R. and LIUzza, J , 1993. MIcrowaveheating for nee bran stabilization, Journal of MIcrowavePower and Electromagnetic Energy 28, 3, 156- 164.

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