Proceedings of the 7 th International Workmg Conference on Stored-product Protectwn - Volume 2

Effect of tissue type, variety and storage on cell wallchemistry of onion (Allium Cepa L . )

Ng Annie", Smith Andrew c.' and Waldron Keith W.l

Abstract

Onion IS one of the major vegetable crops grown m Europeand a large proportion of them is processed. In the EuropeanUnion, 450 000 tons of onion waste are produced annuallymainly from the UK, Holland and Spain. Industnal onionwaste comprises a mixture of damaged whole ornons andouter, dry leaves and fleshy tissues. There IS a concern overthe production of large quantities of industrial oman waste,and Its disposal. The aim of tlus investigation IS tounderstand the cell wall chemistry of oman waste, With theintention of converting the environmentally-unfnendly omanwaste into food ingredients. Alcohol-insoluble residues(AIRs) were prepared as the source of cell wall matenalsand were analyzed for their carbohydrate compositions. On awhole organ baSIS, the cell wall carbohydrate compositionwas similar to each variety studied, and no significantchange resulted from commercial storage, with or WIthoutsprout suppressant. However, there were significantdifferences in the carbohydrate composiuon of cell wall fromdifferent component tissues. Cell walls of inner leaf basescontained galactose-rich pectic polysacchandes. The outerlayer had less pectic galactose, and the outer brown skincontained virtually none. This was accompanied by changesin cell wall polymers and may be related to the dry andprotective nature of the non-lignified outer skin. Storage didnot induce any significant change in the YIeld, totalcarbohydrate and carbohydrate components of AIRs. Thedifferences m cell walls from different tissues provide thebasis for developing processing methods for exploiting omanwaste.

Introduction

Onion (Allium Cepa L. cv Sturon , Durco , Hysam,Grano de Oro and Caribo), a npened bulb, is one of themajor vegetable crops grown m Europe and a largeproportion of them are processed. Because of the growing

1 Institute of Food Research, Norwich Research Park, CoIney,NOrwich NR4 7UA, Uruted Kmgdom

interest m current environmental issues, there IS a need toidentify means for converting the vast quantity of mdustrialoman waste (450 000 tons produced annually mainly fromthe UK, Holland and Spain) into useful products. Industrialonion waste mainly consists of brown dry skin, two outerfleshy leaves, and top and bottom parts of the onion.Omans have been Identified as an Important source of

dietary fibre and pectin (Fenwick, 1985). There have beenseveral studies on the cell-wall polymers of oman bulbtissues (Redgwell and Selvendran, 1986). In addition,investigations concernmg the cell wall chemistry of oniontissues different m cultivar (Allium Cepa L cv Sturon,Durco, Hysam, Grano de Oro and Canbo , 5 commonlyprocessed vaneties in Europe) or of their component tissueshave shown that cell walls of inner leaf bases containedgalactose-rich pectic polysaccharides, and the outer layerhad less pectic galactose, and the outer brown skin containedVIrtually none; however, the cell wall carbohydratecomposition was similar to each vanety studied (Ng et al.1998a). As part of a large study for exploitmg oman wasteinto profitable food-grade products, one of the objectives isto mvestigate the use of processing methods (such aspressure-cooking and extrusion-cooking ) to modify thephysiochemistry of oman cell wall materials. Lecain et al.(1998) have demonstrated that cell walls of outer tissuesfrom Hysam onions were much more strongly degraded thanthose from the Delta variety dunng pressure-cooking.Hence, the differences of cell wall composition betweenvane ties and their component tissues will have sigmficantinfluence on the degree of heat degradation of onion tissues.

Onion Varieties

Since onions are largely cross-fertilized, they show somevariation m the colour and shape of the bulb in differentvarieties. Sturon, Durco, Hysam, Grano de Oro and Caribovarieties studied by Ng et al. (l998a) were the most popularinter-mediate globe shape and the skin colour of these onionvarieties were yellow, except brown for Sturon and reddish-yellow for Grano The carbohydrate compositions of cell-wall materials from whole onion was rich m pecticpolysacchandes, as indicated by the levels of rhamnose,arabinose, galactose and uronic acid: they also containedglucose and relatively small amounts of xylose and mannose

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

(Table 1). The Yieldand total carbo-hydrate of the cell wallmaterials of whole oruons and their component tissues werenot significantly affected by vanety

Tissues Types

Inner tissues contnbuted the major average weight of onions(Ng et al., 1998a). The cell-wall materials of outer skintissues gave a very high Yield (on a dry weight basis) thantheir component tissues. The carbohydrate compositions ofcell-wall matenals from brown dry skin, outer two fleshyleaves, inner tissue, or top and bottom parts of oman werealso nch in pectic polysaccharides (Table 1). Interestingly,

there were sigmficant differences m the carbohydratecomposition of cell wall from different tissues (Ng et al. ,1998a). Cell walls of inner leaf bases contained galactose-nch pectic polysacchandes. The outer layer had less pecticgalactose, and the outer brown skin contained virtuallynone. The relative Sizes of the arabinose and/or galactoseSide-chamsmay relate to the degree of solubihty of pecticpolymers, reflectmg the maturity of onion tissues. Inaddition, Durco, Hysam and Caribo varieties of onion skinhad significantly lower carbohydrate yields compared toSturon and Grano varieties. This probably reflects the post-harvest handling, which may have differentially removedouter brown layers.

Table 1. Carbohydrate composition of CAIRsof fresh and stored ornons.

Carbohydrate (mol %) Total RatioRha Fuc Ara Xyl Man Gla Glc UA [.Lg/mg UA NS

FreshWhole 2 1 3 4 3 10 30 47 807 4Skin 2 1 1 4 2 1 32 547 828 30Cuter 2 1 3 4 3 12 28 48 793 3Inner 2 1 4 4 3 17 29 39 816 2Top and Bottom 2 1 3 6 2 4 28 53 791 8StoredSkin 1 1 1 4 2 1 37 52 762 26Cuter 1 1 2 4 2 13 33 45 739 3

Values were represented as the mean of the 5 omon vaneties, value m parentheses are expressed as the standard deviation of omon vaneties(Soyrce: Ng et al 1998)Abbreviations

Rha - rhamnose, Fuc - fructose, Ara - arabmose, Xyl - xylose, Man - mannose, Gal- galactose, Glc - glucose, VA - uromc acid, and VA NS- uromc acid neutral sugars (arabmose + galactose).

Storage and Sprout Suppressant

On a whole organ basis, the weight of SiXmonths storedonions (0"( ) was Similar to the fresh onions indicating thatlittle or no water loss by evaporation occurred duringcommercial storage, Withor without sprout suppressant (Nget al., 1998a). Storage did not mduce any Significantchange m the Yield, total carbohydrate and carbohydratecomponents of cell wall matenals.

Extractability ofCell-Wall Components

A detailed fractionation of cell walls of whole onions andtheir component tissues has already been earned out by Nget al. (1998a). Most of the extractable pectic polymers ofonion tissues were solubilized by the CDTA-1 and Na2C03-1extraction, whilst relatively httle was released by thewater, imidazole, CDTA-2, Na2C03-2and KOH extractionIn addition, skin of onions solubihzed relatively less uromcacid of water-soluble polymers compared to the outer and

mner tissues (Ng et al., 1998a). This difference msolubility may reflect the maturity of oman tissues.However, skin of onions contained relatively higher levels ofextractable uronic acid of CDTA-solublepolymers comparedto the outer and inner tissues. Sturon, Duree, Hysam,Grano de Oro and Caribo varieties showed a similar trend(Ng et al., 1998a). Furthermore, there were no majoreffects of sprout suppressant or storage.

Effects of Heating

Industrial processing often involves heat treatment. Heatingof onions not only induced tissue softenmg but also had aconsiderable effect on the extractability of the cell wallpolymers (Lecain et al. ,1998) Heat-induced modificationof the plant cell walls probably involves dissolution of pecticpolymers through i3-ehmmatIve degradation (Sajjaanantakulet al. , 1993; Greve et al. , 1994a and b; Ng and Waldron,1997a and b; Ng et al., 1998b) and also results in cellseparation (Van-Buren, 1979; Jarvis and Duncan, 1992;Parker and Waldron, 1995; Brett and Waldron, 1996).Recently, Lecam et al. (1998) have demonstrated that the

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cell walls of outer tissues from Hysam onions were muchmore strongly degraded than those from the Delta varietyduring pressure-cooking. This may be due to differences inthe degree of methylestenfication of the wall uronideHowever, there is httle information on the effects of heattreatment on the chemistry of cell-wall polysacchandes ofonion waste, particularly the brown dry skm and the twofleshy outer tissues.In order to investigate the effect of heat treatment on the

solubility of cell-wall polysaccharides of skm and outertissues of onions, alcohol-insoluble residues of skin and outertissues were prepared, as the source of cell-wall matenals,and were extracted with hot water (lOO·C, 30 min; methodand materials see Ng et al. , 1998b). The cell wall matenalsfrom skin reported here contamed much lower levels of hot-

8

,.........-0"2 6- ()() t':lt':l ()() -- ~§ § 4

~~t':l-0< 2E-'$.

<;»

0

Sturon(WIthoutsprout

suppressant)

Sturon (WIthsprout

suppressant)

water-soluble uromc acid than from outer tissues (Fig. 1 andFig. 2). However, Grano skin or outer tissues were muchless degraded than those from Sturon and Hysam vaneties.In addition, there were no major effects of sproutsuppressant or storage. Hence, the substantial variationwhich existed in cell wall composrtion among oman tissues asreported previously by Ng et al. (1998a) Will havesigmficant mfluence on the degree of solubihsation of omantissues dunng subsequent heating. Clearly, the effect ofthermal degradation on pectic polymers of different varietiesand their component tissues requires further work,particularly If the exploitation of cell-wall polymers from amixture of omon vane ties and their component tissues intofood-grade products are to be optimised.

DWSP• HWSP

Hysam Grano

Fig. 1. Effect of heatmg on the solubility of cell wall matenals of oruon skm.

12

.-0 ~0 90 t';lt';l o() -- ~~ ~ 6-~~< 3E-'$.

'--/

0

Sturon(WIthoutsprout

suppressant)

Sturon (WIthsprout

suppressant)

Hysam Grano

Fig. 2. Effect of heatmg on the solubihty of cell wall matenals of onion outer tissues.

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

Conclusion

The cell wall carbohydrate composition was similar amongvaneties, and not affected by sprout suppressant or storage.The cell-wall pectic polysaccharides of oruons consisted of arange of structurally related polymers which differed Widelyin their ease of extraction from the cell-wall complex, inparticular most of the pectic polymers were extracted byCDTA-l and Na2C0:3-1. A substantial vanation exists in cellwall composition between oruon tissues. The differences incell wall polymers from different tissues provides the basisfor developing processing methods (such as pressure-cookingand extrusion-cooking) for exploitmg onion waste.Apart from understanding the cell wall chemistry aiming

at improving the value of the onion waste, the initialobjective IS to produce a flavour-free fibre, usmg acombination of technologies, from the outer-brown skin andouter fleshy leaves. Tills multi-disciplinary researchprogramme brings together scientific expertise of thedifferent participants, mcludmg Agro-technological ResearchInstitute (The Netherlands), Univeristy Autonoma deMadrid (Spain), Bntish Onion Producers Association (UK),Herbstreith and Fox (Germany) and Triumphus OrnonProducts (The Netherlands). This project involves theextraction of oil, fructo-oligosaccharides and sugars, and thesubsequent modification of the remaining residues, by usingphysical, chemical and biochemical techniques to improvethe solubility of fibre components. The modified fibre willeventually be added to texturally-sensitive food such as dairyproducts, drinks, sauces and desserts.

Acknowledgements

This work was funded by the UK BIOtechnology andBiological Science Research Council and the European Union(FAIR-CT96-1184 )

References

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273-377.Greve, L. C., Shackel, K A. , Ahmadi, H. , McArdle, R.N., Gohlke, I.R. and Labavitch, J.M., 1994a. Impactofheating on carrot firmness: Contribution of cellular turgor.Journal of Agricultural Food Chemistry, 42, 2886-2889.Greve, L. C., McArdle, R. N., Gohlke, J. R. andLabavitch, J. M., 1994b. Impact of heating on carrotfirmness. Changes in cell wall components. Journal ofAgricultural Food Chemistry, 42, 2900 - 2906.Jarvis, M. C. and Duncan, H. J., 1992. The texturalanalysis of cooked potato. 1. Physical pnnciples of theseparate measurement of softness and dryness. PotatoResearch, 35, 83 - 91.Lecain, S , Ng, A., Parker, M. L., Snuth, A. C. andWaldron, K. W. In press Modification of cell-wallpolymers of onion waste. Part I effect of pressure-cookingCarbohydrate Polymers.Ng, A. and Waldron, K. W., 1997a. Effect of steammg oncell wall chemistry of potatoes (Solanum tuberosum cvBintje ) in relation to firmness. Journal of AgnculturalFood Chemistry, 45, 341- 3418.Ng, A. and Waldron, K. W., 1997b. Effect of cooking andpre-cooking on cell-wall cherrustry in relation to firmnessof carrot tissues. Journal of Science of Food andAgriculture, 73, 503 - 512.Ng, A. , Smith, A. C. and Waldron, K. W. In press Effectof tissue type and vanety on cell wall chemistry of onion(Allium Cepa L. ). Food Chemistry.

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