Softening of Durian Cultivars

J Plant Physiol. Vol.

B JOUR.AL OF B

154. pp. 408-411 (1999)

Plan1 Phys~oloay

I Short Communication I

© 1999 URBAN&: FISCHER

Softening of Durian Cultivars SAICHOL KETsA 1

* and TIRA DAENGKANIT2

1

Department of Horticulture, Faculty of Agriculture, Kasetsart University, Chatuchak, Bangkok 10900, Thailand

2

Surat Thani Rubber Research Center, Ministry of Agriculture and Cooperatives, Amphur Tachana, Surat Thani 84170, Thailand

Received March 30, 1998 · Accepted July 1, 1998

Summary

The activities of the enzymes polygalactosidase, pectinesterase, ~-galactosidase and cellulase were assayed during aril softening of durian (Durio zibethinus Murray) cvs. Chanee and Monthong. The aril of both and durians softened continuously throughout the ripening period and coincided with a marked increase in water-soluble pectin, polygalacturonase and pectinesterase activities, while ~-galactosidase and cellulase activities were relatively high at picking time and slightly increased during ripening. The aril of durian was firmer and contained less water-soluble pectin and pectinesterase activity than that of durian, while their polygalacturonase activities were comparable during ripening.

Key words: Durian fruit, cellulase, [J-galactosidase, pectinesterase, polygalacturonase, ripening, softening.

Introduction

Durian is an economical fruit of the South East Asian region and there are two commercial cultivars grown mostly in Thailand, namely and . is an early cultivar and ripens rapidly whereas is a late cultivar and ripens slowly (Tinggal et al., 1994). Durian fruit is stalked pendulous, round to oblong, commonly 200 mm long by 175 mm thick, but may be up to 400 mm long for some Thai cultivars. The rind (husk) of the fruit is extremely tough and covered with thick sharp pointed coarse spines. The fruit is actually a capsule and dehisces when ripe into 3 to 5 segments of irregular width, each of which contains from 1 to 5 seeds. Each seed is embedded in a rich cream or yellow coloured aril (pulp). The aril is the edible portion and varies in colour, texture and thickness among cultivars (Watson, 1984). Durian is a climacteric fruit (Tongdee et al., 1989). The ripening of durian is indicated by the aril becoming soft, creamy and sweet with the presence of strong adour and flavour typical of the fruit. These changes are the result of some

* Corresponding author. Fax: 66-2-579-1951, 66-2-579-6152; e-mail: [email protected]

ripening processes that take place in the fruit (Ketsa, 1997). Durian aril softens very rapidly during ripening and becomes mushy and unfit for fresh consumption. The firmness of durian aril is in part, as in other fruits, due to the presence of pectic substances (Ketsa and Daengkanit, 1998). The softening is the result of degradative changes in pectic substances due to the activity of cell wall hydrolases (Huber, 1983). However, there is scant information available on the enzymes involved in the aril softening of ripening durian (Siriphanich et al., 1994). The present paper reports on the investigation of changes in polygalacturonase, pectinesterase, ~-galactosi­ dase and cellulase activities of two durian cultivars in relation to softening during ripening.

Materials and Methods

Experimental ftuit Durian (Durio zibethinus Murray) fruits cvs. Chanee and Monthong used in this study were purchased from a commercial orchard in Rayong Province in eastern Thailand. Fruits were harvested at 109 ± 2 and 123 ± 2 days after full bloom for cvs. Chanee and Monthong, respectively, and transported by truck to the laboratory within a day after harvest. Fruits were selected for uniformity of size 0176-1617/99/154/408 $ 12.00/0

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Fig. 1: Changes in aril firmness (A), water-soluble pectin (B) and activities of polygalacturonase (C), pectinesterase (D), ~-galactosidase (E) and cellulase (F) in the aril of durians cvs. Chanee ("') and Monthong (•) during ripening. Data are mean values± S.D.

410

SAICHOL KETsA and TIRA DAENGKANIT

and then dipped in fosetyl aluminium at 4 g/L for 2 min to control Phytophthora palmivora fruit rot. Fruits were held at ambient conditions of 29 ± 2 ·c and 72 ± 4% RH during ripening. Fruits were observed every 2 days for firmness and then aril collected and frozen for enzyme and cell wall assays. Aril from 6 fruits of each treatment was pooled and frozen at -80 ·c until use.

stages of ripening except day 6. This suggests that cell wall hydrolases other than polygalacturonase may be involved in the difference in the aril softening between and durians during ripening (Ketsa and Daengkanit, 1999). High pectinesterase activity was detected at picking time (day 0) and increased at a fairly constant rate in both durian Firmness determination cultivars (Fig. 1 D). This indicates that pectinesterase activity Aril firmness was determined with an Effegi firmness tester with a was detectable at early maturation, before polygalacturonase spherical diameter plunger of 0.5 em. The plunger was inserted to a activity was detected. This supports the hypothesis that pectidepth of 0.5 em and the force necessary to do this recorded in new- nesterase action is required before polygalacturonase can be tons. At each observation 9-12 fruits from each cultivar were meas- fully effective. Pectinesterase removes the methyl groups of ured. the galacturonic acid polymers and polygalacturonase preferentially degrades deesterified pectic substances, thereby producing water-soluble pectin (Whitaker, 1972). An increase in Assay of water-soluble pectin and cell wall hydrolases pectinesterase activity showed a correlation (r = -0.93 for Water-soluble pectin of the cell wall material was extracted ac- and r = -0.84 for ) with the progressive cording to Robertson (1979) and the galacturonic acid content in change in firmness loss. The correlations support the earlier the fraction was measured by the method of Blumenkrantz and Assuggestion that pectinesterase provides deesterified pectic hoe-Hansen (1973). Extraction and assay of polygalacturonase, pectinesterase, ~-galacruronase and cellulase were as described by Yos- substances for polygalacturonase to depolymerize them more hida et al. (1984), Hagerman and Austin (1986), Ross et al. (1993) effectively, leading to cell separation and aril softening. Pectiand Abeles and Takeda (1990), respectively. Total protein content nesterase activity in the aril of durian was greater was determined using the method described by Bradford (1976). than that of durian at all stages of ripening (Fig. The analysis was repeated 6 times for each cultivar. 1 D). A greater demethylation by pectinesterase in the aril of durian could result in a greater number of carboxyl groups, which might facilitate polygalacturonase activity, thus Results and Discussion increasing more water-soluble pectin and aril softening in durian. A close association between textural changes At picking time (day 0), the aril of durian was and pectinesterase activity during fruit ripening has been resignificantly firmer than that of durian. During ri- ported in banana, tomato and strawberry (Huber, 1983). pening the two durian cultivars had a similar steady decline Regardless of durian cultivars, both ~-galactosidase (Fig. in firmness. However, the aril of durian was fir- 1 E) and cellulase (Fig. 1 F) activities were relatively high at mer than that of durian at all stages of ripining until picking time (day 0) and slightly increased at the end of the day 6 when their firmness was almost the same (Fig. 1 A). study period. Cellulase activities in the aril of and The decline in firmness coincided with an increase in water- durians were not significantly different at all stasoluble pectin. The increase in water-soluble pectin in the aril ges of ripening. Apparently ~-galactosidase activity in the aril of durian was less than that of durian of durian was higher than that of at all stages of ripening (Fig. 1 B). The progressive increase in durian but ~-galactosidase activity of durian water-soluble pectin showed a correlation (r = -0.96 for tended to increase more rapidly than that of durian and r = -0.98 for ) with the decline in (Fig. 1 E). Loss of aril firmness of durian was greater firmness of the aril. Because the water-soluble pectin is iden- than that of durian (Fig. 1A); in addition, ~-ga­ tified with the cell wall pectin, the correlations support the lactosidase activity in the aril of less mature durians was suggestion that aril softening results from the increase in higher than that of more mature durians (Ketsa and Daengchanging water-insoluble pectin to water-soluble pectin kanit, 1999). The results suggest that both ~-galactosidase (Huber, 1983). However, the great differences in firmness and cellulase may not be necessarily required for the aril sofand water-soluble pectin of the aril between and tening of ripening durian (Ketsa and Daengkanit, 1999). durians may reflect their difference in cell wall hydrolase activity (Huber, 1983). Polygalacturonase activity in the aril of both and References was very low at picking time (day O) and increased markedly after day 2 during ripening (Fig. 1 C). The ABELES, F. B. and J. M. TAKEDA: Scientia Hortic. 42, 269-275 (1990). increase in polygalacturonase activity showed a correlation (r = -0.99 for and r = -0.96 for ) with BLUMENKRANTZ, N. and G. AsBOE-HANSEN: Anal. Biochem. 54, 484-489 (1973). loss of firmness. This relation supports the view that aril sofBRADFORD, M. M.: Anal. Biochem. 72, 248-254 (1976). tening results from degradation and depolymerization of cell HAGERMAN, A. E. and P. J. AusTIN: J. Agric. Food Chern. 34, 440wall pectin (Huber, 1983). However, the polygalacturonase 444 (1986). activity in the aril of and durians was HUBER, D. J.: Hort. Rev. 5, 169-219 (1983). comparable during the most notable softening, which occur- KETsA, S.: Durian. In: MITRA, S. (ed.): Postharvest Physiology and red between day 2 and 4, while the aril of durian Storage of Tropical and Subtropical Fruits, pp. 323-334. CAB was significantly firmer than that of durian at all International, Wallingford (1997).

Softening of Durian Cultivars .l
577.

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