Abstract

Transacylation reaction of citrus pectin catalyzed by pectin methyl esterase (PME) from Aspergillus niger observed by laser particle size analyzer was investigated. PME was purified by a highly methoxylation of cross-linked alcohol-insoluble solids (HM-CL-AIS) column chromatography and a subsequent Sephadex G-75 column chromatography from commercial pectic enzyme (CPE) of A. niger. The increases ofparticle size (transacylation reaction) in PME-treated pectin solution are pectin content and PME activity dependent and expressed a maximum at 0.3% pectin and 0.5 U/ml PME, respectively. PME activity (de-esterification reaction) was highly stable below 50°C but lost completely when above 70°C, while the particle size of PME-treated pectin solution decrease at temperature above 40°C. PME activity was optimum at pH 5.0, while the increase of particle size in PME-treated pectin solution reaches a maximum at pH 3.5. The different characteristics between PME from A. niger and the published plant PME reveal that there are at least two kinds of PME-catalyzed transacylation reaction, one for plant PME and the other for microbial PME.

Key words: Transacylation, pectin methyl esterase, Aspergillus niger.