STRUCTURAL CHARACTERIZATION OF AN APPLE JUICE ARABINOGALACTAN-PROTEINWHICH AGGREGATES FOLLOWING ENZYMATIC DEARABINOSYLATION

An arabinogalactan-protein (AGP) was isolated from a cider apple juice and contained arabinose and galactose as the main sugars (Ara/Gal mol ratio 0.67), with some uronic acids (1.4%) and protein (1.7%). Its we ight-average molecular weight (M(W)) was 80 kDa. Enzymatic dearabinosy lation of AGP with a purified alpha-L-arabinofuranosidase released 95% of arabinose and left a galactan-protein GP(1). Enzymatic degalactosy lation of GPI(1) with a beta-D-galactosidase released 42% of galactose giving GP(2). Methylation analysis of AGP, GP(1), and GP(2) revealed that AGP is built of inner chains of (1-->3)-linked galactosyl residue s 6-substituted by (1-->6)-linked galactan outer chains 3-substituted by terminal arabinofuranosyl units. After dearabinosylation, beta-D-ga lactosidase hydrolysed specifically 76% of (1-->6)-linked galactan out er chains. Removal of arabinofuranosyl substituents led to partial agg regation of GP(1) upon freeze-drying, and further elimination of (1--> 6)-linked galactan outer chains by beta-D-galactosidase did not alter the aggregation phenomenon. The formation of aggregates reveals the po ssibility that enzymatic degradation of AGPs can lead to haze formatio n in fruit juices. Copyright (C) 1996 Elsevier Science Limited.