Studies on the carbohydrates of jackfruit (artocarpus heterophyllus L.) and their role on its ripening

Abstract

The main purpose of this work is 10study the curbuhydrdlcs of jackrruil pcrianths in details and to a certain extent lhe physico_bio-chemicul changes the fruit undergo<:s due tu enzymic actiolls during its maturation and ripening. In the initial stage uflhc study, pcrianlhs from hard and soft variety haIr ripe and immature jacHruits were extracted with water, This aqueous treatment splitted the perianth materials into fhe major fract!uns--- A, II, c, D and E. Of the five fractions A and II are the low mol.wl. free sugars including allied substances. The C fractions are the water soluble llondiulysable carhohydrate malerials. The D fractions arc high mol.wt. waler insoluhle carbohydrates while E fractions are the water insoluble residual materials. H.p.l.e and g.l.e analysis of the low mol-wI. A and .Ii fractions show lhal sucrose and fructose form the major sugars followed by traces of glucose in lhe ripe perianths of both hard and soft fruits. No appreciable amount of any of the frcc sugars was present in the immature perianths. From the ripe soft perianlhs - 7 % V_mannitol was quantified by h. p.l.c while - 3 % was isolated (confirmed by "C-n.m.f spectrum). The finding is significant since Vmannitol is found only ill traces in plants and fruits. Again it is striking \>nnugh that unlike in other fruits, _ 1 % riiJitol has bccn quanlilalCl1 by g.l.c in ripe SOI'1pcrianths. Myo-inosilo1 ( _ 0.1 %) is present in lhe immalure penanlhs bul nol in the ripe perianlhs. In addition, Succinic acid ( _ 0.2 %, confirmed by llC_n.rn.r) was isolated rrom the ripe soft perianths. The nondialysablc C fraclions were separated into their neutral and acidic components by lOn-exchange chromatography. Chemical struclural analysis followed by IH-n .m. r, lJC-n. m.rand IR spectral studies establish the pre<;ence of a 1,6-a-V-glllcan ( dextran) in the neulral ( void volome ) fraclions from both immature and ripe hard perianths as well as from one of the sofl ripe perianths. The nnding is rcmarlmhle as 1,6-1:>'- V_gluclln hllS not !Jcen prniously "CIIOJ1cd from plant kingdom. vm Preliminary analysis of the large number of acidic ( pectic) fractions obtained from ionexchange chromatography of the G fractions show that basically many of them share common sll'uc!urai featuTes as they arc lhe degraded soluble pectic materials of the parent giant pectic polymer. Hence out of these large number of fractions,only two suitable fractions were subjected to further analysis. Glycosyl composition and linkage analyses followed by "C-n.m.T studies identified onc of these two fractions as a type II arabinogalactan and the other as a rhanmogalac1l1l"onan I ( RG I ). Methylation analysis followed by enzymic digestion established that the major proportion of the high mo1.wl. waler insolllhle D fractions aTCstuITh. The minor parts remaining after termamyl digestion are found to be highly rich in glucose residues and hence might form an 1,4- linked glucan as no evidence of 1,6-1inked glucose residues was found from the methylation of one of the parent D fractions. But further studies on lhe fractions could not be pursued for want of adequate amount of materials. As determined by enzymic method, the water insoluble residual n fraclions of illlmatul'e perianths contain _ 30-33 % starch while the ripe perianths contain only - 2-11 %.. So, it is clear that there is a massh'e synthesis ( anabolism) of starch in growing jackfnlit while there is a drll~1ic h)'drolysis ( catabolism) of starch in ripe jackfruils. This suggests that hydrol)'si~ of starch, is perhaps a prerequisite for the ripening associated changes of jaekfruil, the changes being aho observed during ripening of other major starch bearing fruits like banana and hreadfruit. Many fnJits grow l"llllnd the world hut a very few of them contain starch and that tun in a very negligihle amount. l'rom that point of view, the finding is highly promising. Breadfruit, a native to Polynesia is the only [ruitin thewholcofthe fruit kingdom which for its high starch content is regarded a~ the staple food in Polynesia. Thus the presence of high pcrcenlage or starch in jackfn,it perianths cannot be a gross ove,.{'~timatinn since jackiruit, Artocarpus helroph)'Ifus L. and breadfrllit, Arlocarpus altilis L. belong to the same family Morocca.Now this high percentage of starch in jacHruit perianths has raised the question r IX of its probable utilization as a food stuff like breadfruit starch in Polynesia and its surrounding islands. But this will require further Shldies on jackfruit from various angles but with particular reference to the cWllomic viability. Sequential extraction of the starch free cell wall materials (CWMs ) with CDT A and dilute alkali separated them into 3 fractions--- (a) CDTA soluble pectins, (b) alkali soluble hemicelluloses and (c) alkali insoluble cellulosic mat erials. Determination of sugar composition and Uronic acid contenl of the CDTA solubilized pectins show that they are mainly homogalacluronan (HG) (reported by Sen-Gupta & Rao,1963 and Sen-Gupta & Das, 1964) and pectic galact:lIIs. But their presence in relatively higher percentages in the immature perianths as wcll as the higher uronie acid cOlltcnls of the immature perianlhs in comparison to the ripc perianlhs, elcar! y indicatcs that the dissolut ion of these wall cements followed by destmction of the texture of the perianths, might pla~' the vital role in the ripening of jackfmit, the phcnomenon as<;ociated with the ripening of all other fruits. Preliminary studies on alkali solubilized hemicclluloses establish that they contain a glucan different from the dextran isulated from the C fractions. These studies also give some evidences in favour of the presence ofax~'loglucan in ripe jackfruit perianlhs. The absence of any 1,6-linked glucose residues in lhe methylation products of the alkali solubilized materials further confirms that the dextran isolated from the nondialysable water soluble C fractions is not a wall polymer (structural polymer) of tile jacfruit penanlhs. So, the dextran might be a storage polysaccharide of the jackfruit periantils like starch. As expected, when subjectcd to analysis, the dilute alkali free CWMs are found to bc mainly cellulosic in nature. These alkali insoluble cellulosic CWMs along with the alkali solubilized hemicelluloses were also found to be hadly contllminated with left o\'er pectic substances from starch free CDTA insoluble CWMs. This finding supports that the pcctic substances of jackfruit pcriantbs arc knut1~d to thc ",all matrix hulll by covalent assuciation and aggregation with ea+l ion "egg bux" model of Rees and his colleagues. • II Microscopic studies (both LM find SEM) supports that the pcrianths of mature fruits arc - made of thin cell walls. The cells are found to be packed with starch granules, some occurring in well del'ined clusters. The micrographs are also found to contain some crystals of unknown composilion. Extensive cell separation and dissolution of starch granules can be seell in the micrograph of the ripe soft perianth. Thus the study is in full conformity with the chemical analysis thai drastic hydrolysis of starch and solubilization of waU cements OCCIII"during l'ipclling of juckfrllit, The finding is also in agreement with the findings from other ripening fruits. High PG, Be lind PE activities can he followed in the CHAPS extracts ofbolh hard and soft ripe perialllhs ( more in soft perianths) . Thus it appears that these three major enzymes in conjunction with many other enzymes, either activated or synthesized de-llovo at the on~ct of ripening play the k('y role in undoing the wall knots as well as in chopping the slrings (polymer chains) randomly wilh concomitant generation of numerous monomeric, oligomeric and low mol. wt. polymeric soloble carbohydrates. So, the ripe perianths are found to contain more high soluble carhohydrates and dry matter than the immature perianths. Thus the generation of the soluble dry matter and carhohydrales ( dne to enzymic action or for any other reason It may be ) has a tremendous bearing upon the ripening of jackfruit.