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Enzyme digestibility and acid/shear stability of heat–moisture treated canna starch

Abstract
The stability of heat–moisture treated (HMT) canna starches (starches with moisture contents of 15, 18, 20, 22, and 25%, treated at 100°C for 16 h) against acid, high shearing forces, and enzyme digestion was investigated. Micrographs of starch gels taken from a Rapid Visco Analyzer showed that granules of untreated native starch subjected to pH values of 7.2 and 4.6 were highly swelled, whereas granules exposed to higher acidic pH (3.0) values fragmented into small pieces. Yet, less swelling was found for the HTN15%, HMT18%, and HMT20% samples. Even at pH 3.0, HMT22% and HMT25% granules were still in an intact granular form. Similar findings were observed when native and HMT canna starches were agitated at various rates (160, 240, 320, and 480 rpm). Starch samples treated under higher moisture levels exhibited a higher tolerance to shearing forces acting on them. Comparative investigation of HMT and chemically crosslinked starches showed that pasting properties of HMT22% and HMT25% were equivalent to those of canna starches crosslinked by sodium trimetaphosphate at 0.2 and 0.5% (dwb), respectively. In regards to the enzyme digestibility of treated canna starches, there was no recognizable trend. The extent of starch hydrolysis (at 24 h) as a function of moisture level during HMT conformed to the following order: HMT25% > HTN15% > HMT18% ≈ native > HMT20% > HMT22%.

Keywords
Acid/shear stability; Canna starch; Crosslink; Enzyme digestibility; Heat–moisture treatment

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