Diaz Jerome V, Anthon Gordon E, Barrett Diane M
Department of Food Science and Technology, University of California, Davis, California 95616, USA.
J Agric Food Chem. 2007 Jun 27;55(13):5131-6. doi: 10.1021/jf0701483. Epub 2007 Jun 6.
The underlying mechanisms governing nonenzymatic pectin and pectate degradation during thermal treatment have not yet been fully elucidated. This study determined the extent of nonenzymatic degradation due to beta-elimination, acid hydrolysis, and demethylation during prolonged heating of citrus pectins and its influence on physicochemical properties. Solutions of citrus pectins, buffered from pH 4.0 to 8.5, were heated at 75, 85, 95, and 110 degrees C for 0-300 min. Evolution of methanol and formation of reducing groups and unsaturated uronides were monitored during heating. Molecular weight and viscosity changes were determined through size exclusion chromatography and capillary viscometry, respectively. Results showed that at pH 4.5, the activation energies of acid hydrolysis, beta-elimination, and demethylation are 95, 136, and 98 kJ/mol, respectively. This means that at this pH, acid hydrolysis occurs more rapidly than beta-elimination. Furthermore, the rate of acid hydrolysis is diminished by higher levels of methyl esterification. Also, citrus pectin (93% esterified) degrades primarily via beta-elimination even under acidic conditions. Acid hydrolysis and beta-elimination caused significant reduction in relative viscosity and molecular weight.
热处理过程中果胶和果胶酸盐非酶降解的潜在机制尚未完全阐明。本研究确定了柑橘果胶长时间加热过程中因β-消除、酸水解和去甲基化导致的非酶降解程度及其对理化性质的影响。将pH值缓冲在4.0至8.5之间的柑橘果胶溶液在75、85、95和110摄氏度下加热0至300分钟。加热过程中监测甲醇的释放、还原基团和不饱和糖醛酸的形成。分别通过尺寸排阻色谱法和毛细管粘度测定法测定分子量和粘度变化。结果表明，在pH 4.5时，酸水解、β-消除和去甲基化的活化能分别为95、136和98 kJ/mol。这意味着在该pH值下，酸水解比β-消除发生得更快。此外，较高水平的甲酯化会降低酸水解速率。而且，即使在酸性条件下，柑橘果胶（93%酯化）主要通过β-消除降解。酸水解和β-消除导致相对粘度和分子量显著降低。
