Differential scanning calorimetry of raw and annealed starch isolated from normal and mutant maize genotypes

Abstract

Cereal Chern. 64(3): l8i-190 Differential scanning calorimetry was used to study gelatinization of laboratory-isolated starcb granules from normal and mutant maize varieties. Samples were studied in their native condition and after annealing by heating at 50° C in excess water for 48 hr. Native starches exhibited considerable variation in enthalpy and temperature range ofgelatinization in a pattern consistent with the composition and degree of crystallinity of Differential scanning calorimetry (DSC) was first used for measuring gelatinization of starch by Stevens and Elton (1971). As part of their study. they examined maize starches from normaL waxy, and high-amylose genotypes and found significant variation in temperature range and enthalpy of gelatinization. From those studies they concluded that the role of amylopectin was more important than that of amylose in gelatinization. DSC has since become an important tool for study of starch gelatinization, and numerous studies have been published. The work of Donovan (1979) was especially important for developing a well-founded thermodynamic approach to gelatinization based on DSC measurements. Recently, we reported the use of DSC for studying the effects of annealing on the gelatinization characteristics of corn starch (Krueger et al 1987). Annealing, defined as the heating of starch in excess water at subgelatinization temperatures, was first shown by the birefringence studies of Gough and Pybus (1971) to narrow the temperature range and raise the peak temperature of wheat starch gelatinization: however, quantitative data such as enthalpy measurements could not be obtained by their technique. In our studies, we found that annealing increased the enthalpy of gelatinization. and that annealed (or partially annealed) starches from different maize varieties varied in gelatinization charactcristics. We also demonstrated that commercially prepared starches showed characteristics of annealed starch. indicating that the temperature and moisture conditions employed in the wet milling process cause annealing of the starch. In this study, we are extending our investigation to starches from mutant genotypes in order to determine the effect of annealing on starches of varying composition and degree of crystallinity. Several maize endosperm mutants have their primary effect on the synthesis of starch or of a particular protein. Many mutants in the group, including shrunken (517), shrunken-2 (5172). brittle (hI). brittle-2 (hI2), waxy (H'x). amylose-extender (ae), sugary (su), sugary-2 (su2), and dull (du), cause variation in amylose percentage or the total amount of starch accumulation. In addition to their effects on polysaccharide composition, these mutants alone and in combination have noticeable effects on kernel development or on starch granule development and morphology (Brown et aI1971). The group of mutants including opaque-2 (02), opaque-6 (06). opaque-7 (07). floury (jl), floury-2 ((/2). and floury­ 3 (/73) change endosperm protein production. The prolamin (zein)