Kinetics of hydrolysis and changes in amylose content during preparation of microcrystalline starch from high-amylose maize starches

Two commercial sources of high amylose (∼65% amylose) maize starch, Hi-maize and Hylon VII, were acid-hydrolyzed to produce microcrystalline starch and hydrolyzed starch sugar solution. The yield of microcrystalline starch was lower than 50% of the original starch weight when hydrolysis was carried for up to 8 days. The kinetics of hydrolysis was divided into three stages. The slope of the linear graph ranged from 11.11 to 11.36 mg/(ml days), 4.24 to 4.55 mg/(ml days), and 2.15 to 3.96 mg/(ml days) in the first, second and third stage and corresponded to a rapid, slow and very slow hydrolysis rates, respectively. HPAEC-ED analyses of the hydrolyzed starch solutions revealed 14 and four major sugar and oligosaccharide components when the hydrolysis was carried for 1 and 8 days, respectively. There was a good linear relationship between glucose content and hydrolysis time (R > 0.992). Oligosaccharide components (dp ⩾ 2) attained highest levels and then decreased with further increase in hydrolysis time. For hydrolyzed Hi-maize starch solution, the highest levels were 35.9, 12.4, 8.3, 7.6, 2.8, 2.3, and 2.0 mg/ml for glucose (dp1), maltose (dp2), maltotriose (dp3), maltotetraose (dp4), maltopentaose (dp5), maltohexaose (dp6), and maltoheptaose (dp7), respectively. Similarly, hydrolyzed Hylon VII starch solution contained the highest levels at 31.8, 12.7, 7.2, 5.6, 4.3, 2.7, and 2.2 mg/ml for glucose, maltose, maltotriose, maltotetraose, maltopentaose, maltohexaose, and maltoheptaose, respectively. The critical times (rapid to slow transition) for hydrolysis of Hylon VII and Hi-maize starches should be between 5 and 6 days under the present conditions for preparation of microcrystalline starch. The hydrolysis process also significantly increased amylose content of microcrystalline starch.