Changes in Cell Wall Polysaccharides of Kiwifruit and the Visco-elastic Properties Detected by a Laser Doppler Method.

Abstract

Visco-elastic properties of kiwifruit were measured by a laser Doppler method over an eight day ripening period. After ethylene treatment, fruit flesh elasticity decreased exponentially. However, viscosity of the flesh increased from day 0 to 2 and then decreased. Fruit tissue was boiled in 80% ethanol to extract soluble sugars (EtOH fraction) ; the EtOH-insoluble residue was treated with α-amylase to solubilize starch. The amount of sucrose, glucose, fructose and inositol in the EtOH fraction on unit FW basis increased from day 0 to 3, while starch decreased during the ripening period. Cell wall material was fractionated into the hot water-soluble (WS), hot EDTA-soluble (pectin), alkaline soluble (hemicellulose) and residual (cellulose) fractions. The quantity of hydrolyzed sugars in the WS fraction decreased exponentially during ripening, whereas that in the pectin and hemicellulose fractions increased slightly on day 1 and then decreased rapidly. The amount of cellulose remained nearly constant during ripening. The iodine method revealed appreciable amounts of xyloglucan in the hemicellulose fraction. Gel permeation chromatography of polymers in the hemicellulose fraction revealed that xyloglucan had degraded rapidly by day 2. The molecular weights of both total sugar and uronic acid components of the pectin fraction increased from day 0 to 2, and then decreased. The amount of total cell wall polysaccharides and starch decreased by 80% during ripening, which indicates that a decrease in the molecular weight of xyloglucan correlates with a loss in fruit tissue elasticity, while the changes in the molecular weight of pectin corresponds with changes in viscosity. A substantial portion of the cell wall polysaccharides was hydrolyzed to soluble sugars in the early phase of ripening.