Physicochemical properties of waxy and normal corn starches treated in different anhydrous alcohols with hydrochloric acid
Introduction
Acid hydrolysis of starch in alcohol has high recovery of starch and uses less amount of acid (Lin, Lee, & Chang, 2003). The average degree of polymerization of treated starch was affected by the botanical source and concentration of starch, type and concentration of alcohol, acid concentration and treatment temperature (Ma and Robyt, 1987, Fox and Robyt, 1992, Robyt et al., 1996, Chang et al., 2004). The change on number-average degree of polymerization (DPn) of starch after acid-alcohol treated has been deeply concerned by Robyt and his co-workers (Ma and Robyt, 1987, Robyt et al., 1996) using reducing value method. The molecular weight distribution of acid-alcohol treated starch determined by high-performance size-exclusion chromatography (HPSEC) illustrated that the amylopectin fraction content of starch obviously decreased and shifted toward amylose fraction after treated (Lin et al., 2003, Chang et al., 2004). The chain length distribution of acid-alcohol treated starch also indicated that amylose and long chain of amylopectin degraded after treated (Lin et al., 2003). Therefore, the examination of molecular weight and chain length distribution is helpful for the alteration of molecular structure of starch after acid-alcohol treatment.
Ma and Robyt (1987) indicated that potato and waxy maize starches treated by acid-alcohol with different anhydrous alcohols had distinct DPn, and the molecular size of starch after treated progressively decreased in the order of methanol>ethanol>2-propanol>1-butanol. Chun, Lim, Takeda, and Shoki (1997) showed that the degree of hydrolysis and average DP (degree of polymerization) of modified rice starch decreased as the ethanol concentration increasing from 70 to 99% with the same acid concentration. The result indicated that the molecular size of starch after acid treated in different types or concentrations of alcohol was distinct. However, the molecular structure (molecular weight and chain length distributions), granular structure and physicochemical properties of starch after acid treated in different alcohols were still absent.
In this study, waxy and normal corn starch were treated in anhydrous methanol, ethanol, 2-propanol or 1-butanol containing 0.36% HCl at 45 °C for 1 h. The molecular weight and chain length distributions of starch was then examined by HPSEC, and the physicochemical properties including the particle size, granular structure, λmax, blue value, solubility and gelatinization thermal properties of starch were examined. The differences on the alternation of molecular structure among the starches after treated in different anhydrous alcohols with hydrochloric acid were elaborated. The changes on granular structure and physicochemical properties among the starches after treated in different anhydrous alcohols were also compared.
Section snippets
Materials
Waxy corn starch was a product of National Starch and Chemical Company (Bridgewater, MA) and normal corn starch was purchased from African Products Ltd. Co. (Germiston, South Africa). The moisture content of waxy maize starch was 7.8% (w/w) and normal corn starch was 8.3%. All reagents used were of analytical grade.
Preparation of treated starch
Acid-alcohol treatment on starch was performed according to the procedure described previously (Lin et al., 2003). Starch (25 g, dry basis) was suspended in anhydrous methanol,
Recovery yield, average granule size and granular structure
The recovery yields of waxy and normal corn starches after treated were higher than 96% (Table 1). The average granule sizes of treated starches were slightly lower than their counterpart native starch, and no obvious differences were found among the granule sizes of starches after treated with different alcohols. The 3D images and cross-sections of waxy corn starch granules observed by confocal scanning laser microscopy were shown in Fig. 1 Native waxy corn starch showed polygonal, irregular
Conclusion
Waxy and normal corn starches showed high recovery yield (>96%) after treated in different alcohols with 0.36% HCl at 45 °C for 1 h. After acid-alcohol treated, the DPw, λmax, blue value and gelatinization onset temperature of starch obviously decreased with the increasing carbon number of alcohol, while the solubility of starch profoundly increased. Amylose or long chain of amylopectin of starch degraded after acid-alcohol treated. The internal fissures and cavities were found in the starch
Acknowledgements
We thank the National Science Council, Taiwan, for financial support (NSC 92-2303-B-126-010).
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