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

Cell wall chemistry of carrots (Daucus carota cv Amstrong)during maturation and storage

Ng Annie", Srruth Andrew C. 1, and Waldron Kerth W. 1

Abstract

Carrots are a biannual dicotyledon plant, the edible portionof which is an over-winter storage organ. Maturation andpost-harvest storage involve the modification of cell wallarchitecture which may affect the textural properties. Theaim of this work was to investigate the changes of cell wallchemistry of carrots (Daucus carota cv Amstrong) duringmaturation and storage m relation to textural properties.Alcohol-msoluble residues (AIRs) were prepared and wereextracted sequentially WIth water and CDTA to leave aresidue. These were analyzed for their carbohydratecomposition and the degree of methylestenficanon (DM).Maturation of carrots resulted in a decrease in the relativeproportion of cell wall xylose, and an increase in (highlymethylesterified ) pectic polysacchandes. ThIS wasaccompanied by an increase m estenfied ferulic acid cross-linking which may have significant structural implications.Unlike maturation, there was no significant change in thecarbohydrate composition, the DM of the pectic moieties andthe level of estenfied ferulic acid of AIRs of carrots dunngstorage. Maturation and post-harvest storage resulted m adecrease in the level of water-soluble polymers and anincrease in the level of CDTA-soluble polymers. Storage-induced firmness of carrots at higher temperature enhancedthe tissue firmness during subsequent processing.

Introduction

The state of maturity and the conditions of storage areimportant factors affecting the quality of vegetables whichcan be manipulated in order to meet the continuous marketsupply (Avon, 1979). Most aspects of plant growth anddevelopment involving the modification of cell wall structureand continued maturation during storage may affect texturalproperties (Waldron and Selvendran, 1992). Conventionalstorage of carrots often results in loss of firmness anddeterioration m quality (Burton, 1982).

1 Institute of Food Research, Norwich Research Park, Colney,Norwich NR4 7UA, Umted Kmgdom

Carrots, a biannual dicotyledon plant, are among the mostancient of the vegetable crops grown in Europe, the edibleportion of which is an over-winter storage organ (root).Carrots have been identified as an important source ofdietary fibre (Robertson et al., 1979a; Robertson et al ,1979b). A detailed fractionation of cell walls of carrots hasbeen carried out by Steven and Selvendran (1984) and Ngand Waldron (1997). They have demonstrated that most ofthe extractable pectic polymers may be solubilized byextraction in water and CDTA. Recently, investigationsconcernmg the changes in cell wall chemistry of carrotsduring maturation and storage showed that maturation ofcarrots resulted in an increase m less branched cell wallpectic polysaccharides, and both maturation and storage ofcarrots resulted m a decrease in total pectic polysacchandesof water-soluble polymers and this was accompanied by anincrease in total pectic polysacchandes of CDTA-solublepolymers (Ng et al. 1998a). Hence, the physiologicalchanges during maturation of carrots may play an importantrole m the quality control of stored carrots.There IS growing interest in the demand for ready-made

meals containmg high- fiber processed vegetables.Vegetables often undergo heat treatment dunng foodprocessing. One of the most important changes occurnngduring processmg is heat-induced tissue softening. In orderto reduce an undesirable degree of softness during cooking,carrots can be pre-cooked at moderate temperature for aperiod of time and followed by cooking (Ng and Waldron,1997). ThIS results in greater firmness compared to thosedirectly cooked WIthout precooking. This firrrung effect ISprobably due to an increase m thermal stability of calciumcross-hnking of pectic polysaccharides (Ng et al. , 1998a).Maturation- and storage-related increase in CDTA-solublepolymers of carrots might have significant influence on theirfinal texture and cell wall composition during subsequentprocessing, particularly the precooking-induction offirmness.

Effect of Maturation onCarrot Cell Wall Chemistry

Maturation of carrots resulted in an increase of freshweight, dry weight and total root length (Ng et al. , 1998).This was accompanied by an mcrease in cell-wall pectic

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

polysaccharides, as indicated by the levels of rhamnose,arabinose, galactose and uronic acid, and a concomitantdecrease m other cell-wall polysacchandes particularlyxylose and glucose (Table 1). These changes can beexplained by the lateral growth of the carrot resulting m arelatively lower contribution of thin highly vascularisedirutial root. The ratio of uronic acid (UA): arabinose +galactose (NS) increased during maturation mdicating less-branched pectic polysacchandes (Table 1). This bearssimilarity to the breakdown of the galactan and arabman sidechams in asparagus during maturation (Waldron and

Sel vendran, 1992). The maturation-related increase in thedegree of methylesterification (DM) of the uronidesuggested that cell extension probably involves thedeposition of methylesterified pectic-rich polysaccharides(Ng et al. , 1998a). The maturation-related decrease m thelevel of water-soluble polysaccharides (WSP) and anincrease in the level of CDTA-soluble polysacchandes (CSP)could be due to continued synthesis and insertion of CSP andreduced synthesis and/or turnover of water-solublepolysaccharides (WSP) dunng maturation (FIgure 1; Ng etal. , 1998a).

~ ~ 30"E~a ~-5 -5 25~ g..o~8.a 20~~Co) Co)

8. 8. 15~e:j:E<C~e 10 +---------+---------1----------1

I-wspl~CSP

8 14 18 22

Weeks

Fig. 1. Solubility of pectic polysacchandesl % AIR) dunng maturation of carrots.

Pre-soaking fresh carrot tissues in CDTA for 16 h did notmduce vortex-induced cell separation (Ng and Waldron,1997). This indicated that, apart from calcium-cross-Iinkmgpolysaccharides, other characteristics of cell-wall polymersintegrity and cross-linking, such as SImple phenolics, mayalso have important implication in mechanical properties,involving cell-cell adhesion. Recent studies on the ester-linked phenohc components of carrot cell walls havehighhghted their possible role m influencing the texturalproperties (Waldron et al. , 1997; Parr et al. , 1997). Suchphenolics are probably estenfied to cell wall polysaccharides

(Waldron et al. , 1997). Maturation of carrots resulted in anincrease in the proportion of 8-0-4' and 5-8' (benzofuranformi-dehydrodiferulic acids in relation to total ferulic acids(Ng et al., 1998a). The presence of 8-8' (aryl tetralinformr-dehydrodiferulic acid in carrot cell walls may have aspecial role m cell-cell adhesion (Waldron et al., 1997).Maturation-related increase in cell wall interpolymenc cross-linking, involving simple phenohcs, of asparagus and cauli-flower have been reported (Waldron and Selvendran, 1992;Femenia, Waldron and Selvendran, unpublished).

Table 1. Yield, carbohydrate compoSItion and degree of methylesterification of cell wall matenals of carrot during maturation.

Weeks Carbohydrate (mol %) Total DM Ratio

Rha Fuc Ara Xyl Man Gal Glc UA fig/mg % UA'NS

8 1 1 12 3 2 12 37 32 631 18 1.3

14 1 1 12 3 2 12 37 32 656 30 1.3

18 1 1 11 2 2 12 37 34 647 34 1.5

22 1 1 10 1 2 1033 41 622 40 2.1

The values are the mean of duphcate determinations and the vanation withm duphcates was less than 5% (Sources: Ng et al. )Abbrevia nons.Rha =: rhamnose, Fuc =: fructose, Ara =: arabmose , Xyl =: xylose, Man =: mannose, Gal =: galactose. Glc =: glucose, UA =: uromc acid,and UA NS =: uromc acid neutral sugarsfarabmose f galactose)

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

~ '00' 40"'d vfa]-fi..c:= 30(,) (,)~ (,).c~8. 8. 20~~(,) (,)

8.8.10~~o~(J:} ~ 0 +-------+---------11----------1

o

Effect of Storage onCarrot Cell Wall Chemistry

Commercial storage of carrots at Ice Bank resulted m a lossof 15% fresh weight over the 3 months period (Ng et at. ,1998a). However, at high temperature storage, freshweights of carrots were dramatically decreased by up to 80%at 1O'C. This may be due to moisture loss, the rate andextent of which was greater at the lugher temperatures, andresulted m the shrinking of the roots. Storage resulted in alarge increase in firmness and this was accompanied by adarkening of colour.Unlike maturation, there was no significant change m the

carbohydrate composition of cell wall materials and the DMof the uronide of carrots during storage (Ng et al. , 1998a).Storage of carrots resulted in a decrease in WSP (Figure 2).ThIS was accompanied by an increase in esp. However, theoverall composition of cell wall material from carrots did notshow any change during storage, yet there was a decrease in

I~wspl!--CSp 1

WSP and an increase m CSP. Whilst contmual synthesis andturnover can not be ruled out durmg maturation of carrots,the results could also be explamed by the conversion of WSPto esp. Although the mechamsm IS not clear, such amechanism could involve a number of factors, for example,the availability of calcium required for cross-linkingpolysacchandes. Ng et al ( 1998b) have demonstrated inonion that increasing the calcium availability does reduceWSP and increase esp. On the other hand, the conversionof WSP to esp durmg maturation of carrots could also reflectthe iomc movement mto cell walls during storage.Carrots sustam stress durmg post-harvest handling and

commercial storage. Induction of several phenoliccompounds of carrots dunng storage and stress conditionshave also been reported (Coxon et at., 1973; Sarker andPhan, 1979; Lafuente et al , 1989). An mcrease in thelevel of esterified 4-hydroxybenzoic acid in carrot cell wallsdunng storage (Ng et at., 1998a) may be due to thepresence of pathogen-related elicitors (Schnitzler et at. ,1992) .

2 3

Fig. 2. Solubility of pectic polysacchandes (% AIR) dunng storage of carrots.

Months

Effect of Processing

In order to provide a continuous supply of vegetablesthroughout the year, processing, such as cooking orcanning, is often used by the food industry. Long termstorage of many vegetables prior to processing results in anotable decrease in firmness, colour and the development ofoff flavours (Okoh et at., 1988; Woolfe, 1991; Collins andWalter, 1992) which makes it difficult for food processors tomanufacture products of consistent quality throughout theyear. Heating-induced tissue softening of vegetables isaccompanied by a dissolution of pectic polymers through 13-eliminative degradation (Brett and Waldron, 1996). Heatingof carrots involves cell separation (Ng and Waldron, 1997)which is consistent with a heat-related weakening of cell-cell

adhesion (Brett and Waldron, 1996) Ng and Waldron(1997) have investigated the reduction in cookmg-mducedtissue softening of carrots by pre-cookmg at moderatetemperatures for a period of time. Such pre-cooked andcooked carrots exhibit a firmer texture than those directlycooked without precookmg. ThIS firmmg effect IS probablydue to an increase in thermal stability of calcium cross-linking of pectic polysaccharides.Storage of carrots for 3 months at 100e showed an

enhancement of carrot firmness subsequent to cookmg,subsequent to pre-cookmg and also resulted m an mcrease inthe firmness of tissue which was pre-cooked followed bycooking (FIgure 3; Ng et aI. , 1998a). These may suggestthat storage-mduced firmness may modulate the firmnessduring subsequent processmg. This may involve an increasein the thermal stability of the cross-linked pectic

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Proceedings of the 7th Internatumal Workmg Conference on Stored-product Protection - Volume 2

polysaccharides. A reduction of heat-induced solubilization ofwall polymers by increasmg the availability of calcium ionsto cell wall matenals from omons has been reported (Ng etal. • 1998b). Hence. the conversion of WSP to CSP dunng

40

30--00~rI)rI) 20

~~10

0

Fresh Precooked(50 C, 30nun)

storage of carrots probably reduce the propensity for 13-eliminative degradation and depolymerisation of wallpolymers dunng subsequent heating.

o Control• Stored (10 C, 3 months)

Cooked(l00 C,30mm)

Precooked& Cooked

Fig. 3. Effect of storage(lO'C , 3 months) on firmness of carrots dunng subsequent processmg.

Conclusion

Maturation of carrots involves the modification of thechemical composition of the cell wall and extracted cell wallpolymers. This. in addition to an increase m esterifiedferulic acid cross-Iinkmg of cell wall matenals, may havesignificant structural Implications. Furthermore. water lossduring post-harvest storage of carrots at highertemperatures greatly affect their textural quality ThISmodulates the firmness during subsequent processing.

Acknowledgement

This work was funded by the UK Biotechnology andBiological Science Research Council and the EuropeanCommunities (AIR Project NI CT92-0278) for their financialsupport.

References

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processed products. In Fifty Tears of CooperativeSweetpotato Research, A. Jones, J. C. Boukamp (Ed),Southern Cooperative Senes Bulletin 309, Louisiana StateUniversity Baton Rouge. LA. p. 71 - 87.Coxon. D. T .• Curtis. R F. , Price, K. R. and Levett. G.•1973. Abnormal metabohtes produced by Daucus carotaroots stored under conditions of stress. Phytochenustry,12. 1881-1885.Lafuente. M. T.. Cantwell. M.. Yang. S. F. andRubatzky, V .• 1989. Isocoumann content of carrots asinfluenced by ethylene conentration storage tempeatureand stress conditions. Acta Horticulture 258. 523 - 534.Ng. A. and Waldron. K. W • 1997. Effect of cooking andpre-cooking on cell-wall chemistry in relation to firmnessof carrot tissues. Journal of SCIence of Food andAgriculture. 73. 503 - 512.Ng, A.• Parr. A. J .• Ingham. L. M .• Rigby. N. M. andWaldron. K. W.• In press. Cell wall chemistry of carrots(Daucus carota cv Amstrong) during maturation andstorage. Journal of Agricultural and Food Chemistry.Ng. A., Lecam, S.. Parker, M. L.. Smith, A. C. andWaldron. K. W.. In press. Modification of cell-wallpolymers of onion waste. Part II. effect of divalentcations Carbohydrate Polymers.Okoh, E. C.• Nmorka, O. G. and Unaegbu, M. E.. 1988.Techmcal note: Blanching and storage of some Nigerianvegetables. International Journal of Food Science andTechnology, 23. 639 - 641.Parr. A.J .• Ng, A. and Waldron, K W. , 1997. The ester-lmked phenolic components of carrot cell walls. Journal ofAgncultural Food Chemistry. 45. 2468 - 2471.

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Robertson, J. A , Brydon, W. G., Tadesse, K, Wenham,P , Walls, A. and Eastwood, M. A. , 1979a. The effect ofraw carrot on serum lipids and colon function. AmencanJournal of Clinical Nutrition, 32, 1889-1892.Robertson, J. A. , Eastwood, M. A. and Yeoman, M. M. ,1979b. An investigation into dietary fibre content of namedvarieties of carrot at different developmental ages. Journalof Science of Food and Agriculture, 30, 388 - 394.Sarker, S K. and Phan, C T. , 1979. Naturally- occurringand ethylene- mduced phenohc compounds m the carrotroot. Journal of Food Protection, 42, 526 - 534.Sharma, S. c.. Wolfe, R. R. and Wang, S. S., 1975.Kmetic analysis of post-harvest texture changes inasparagus. Journal of Food Science, 40, 1147 -1150.Snuth, J. L and Stanley, D. W., 1987. Nonenzymatichgmfication of asparagus. Journal of Texture Studies, 18,

339- 358Stevens, B. J. H. and Selvendran, R. R. , 1984. Structuralfeatures of cell-wall polysaccharides of the carrot (Daucuscarota). Carbohydrate Research. , 128, 321 - 327.Waldron, K. W. and Selvendran, R. R., 1992. Cell wallchanges in immature Asparagus stem tissue after excision.Phyto-chemistry, 31, 1931 -1940.Waldron, K. W , Smith, A. C., Parr, A. J. , Ng, A. andParker, M. 1., 1997. New approaches to understandingand controlhng cell separation in relation to fruit andvegetable texture. Trends in Food SCIence andTechnology, 8, 213 - 221.Woolfe, J. A., 1991. Sweet potato: An Untapped FoodReserve p. 251, 317 - 320. Cambridge University Press:Cambndge, U K.

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