Thermostable and theriviolabile isoforms in commercial orange peel pectinesterase

Thermostable and thermolabile pectinesterase isoforms in commercial Valenci a orange peel pectinesterase (Sigma Chemical Co., Sr. Louis, MO) were separ ated by affinity chromatography on a Heparin-Sepharose CL-6B column. Among the seven putative PE isoforms separated, three (P1, P2 and P3 with 70 kDa, 60 kDa and 27 kDa, respectively) were thermostable and four [P4, P5, P6 (3 5 kDa) and P7 (35 kDa)] were thermolabile, according to the qualitative and quantitative studies of thermal stability. The commercial PE showed 83.4% and 98.3% inactivation after heat treatment at 70C for 5 min and at 90C for 1 min, respectively. P2 was the most heat stable, with 85.5% inactivation at 70C for 5 min and 93.0% inactivation at 90C for 1 min. The thermolabile P6 and P7 could be inactivated at 70C for 5 min or at 90C for 1 min. Thermo stable PE activity was 16.6% of total activity of the commercial PE based o n heat treatment at 70C for 5 min, but only 1.5% based on heating at 90C fo r 1 min. The 90C for 1 min heat treatment might be better than the 70C or 5 min heat treatment to distinguish thermostable PE and thermolabile PE isof orms in Valencia oranges.