Effect of Pre-heating Treatment on Texture and Pectic Substances of Peeled Japanese Chestnut

Abstract

Peeled chestnuts, which were held for more than 30min in water at 50-70°C and subsequently boiled, attained a harder texture than the untreated control. The cause of the hardening effect was investigated in relation to pectinesterase activity of the raw tissues, the characteristics of pectic substances and the content of divalent cations(Ca and Mg ions)in the tissues with or without the pre-heating treatment. Pectinesterase extracted with sodium acetate solution from the peeled chestnuts had an optimum pH and temperature of 8.O and 50°C, respectively. The activity wa shigher in immature fruit than in mature one. The degree of esterification of polyuronides of the tissues decreased by 4 to 8 percent after the pre-heating treatment, though the increased carboxyl groups of the polyuronides were insufficiently combined with divalent cations. The degree of the divalent cation saturation to the free carboxyl groups was higher in less hard tissues. There was no correlation between hardeness of the boiled chestnuts and the degree of saturation of the polyuronides with divalent cations. It was presumed that the pre-heating treatment caused apparently the decrease of the degree of esterification in the polyuronides with-out rapid combination with divalent cations and the resulted lower esterified polyuronides probably resisted to heat-degradation of glucoside bond to inhibit softening of the tissues.