Abstract
Hydrolysis of native starch by a-amylase is a complicated process which is highly affected by compositional and architectural properties of starch granules that affect accessibility of enzymes to the interior of the granule. In view of the paucity of knowledge in this area regarding the control of amylolysis, the following study was undertaken using different potato starches (grown under the same conditions). The focus was to understand how cultivar specific variation regulated hydrolysis with a-amylase. The composition of the starches was determined using classical chemistry techniques while structural aspects were determined by specific physical techniques including differential scanning calorimetry (DSC), particle sizing (Coulter conter), high performance size exclusion chromatography (HPSEC) and nuclear magnetic resonance (NMR). Different amylase enzymes were employed (fungal and pancreatic) and possible inhibition of activity of the enzymes was studied.Chemical composition revealed some differences amongst the potato starches with amylose ranging from 25.23 to 29.05% and phosphorus 52.59 to 66.20mgl00g-l. Despite having a close range of protein content, the surface protein had a great impact on starch properties, especially amylolysis. The HPSEC results showed that amylopectin molecules of potato starches was less branched and consisted of more B1-chains but less A-chains comparing to rice starches. Gélatinisation onset (TO), peak (Tp) and conclusion (Tc) temperatures of the native potato starches ranged from 58.71 to 62.45°C, 62.52 to 66.05°C and 68.67 to 72.27°C respectively while the gélatinisation enthalpies ranged from 15.13 to 18.37 Jg-1. The gélatinisation temperatures of starches increased with amounts of short and intermediate size amylopectin chains. The NMR and XRD data (30.56 % ± 0.22 crystallinity on average) showed little variance amount the samples. Particle sizing results, however, revealed great variance among potato starches granule size (20.57 to 30.92pm in diameter). The granule size and surface protein content were found to be key factors that controls the hydrolysis rate of potato starches. Fungal and pancreatic amylolysis showed similar patterns of hydrolysis although pancreatic a-amylase showed more effective towards hydrolysis of native starches. The protein content of the starches also affected hydrolysis. As a result of starch modification (annealing) all the gélatinisation temperatures were shifted towards higher values except for the gélatinisation temperature range (T0-Tc) which was decreased significantly. Annealed starch were more resistant to enzymatic hydrolysis.
| Date of Award | 2009 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Richard Tester (Supervisor) & Kofi Aidoo (Supervisor) |