Date of Award
Doctor of Philosophy
Food Science and Human Nutrition
This study was conducted with aim to understand how planting dates and drying conditions affected starch properties and dry-grind ethanol production of corn kernels. Three corn varieties with planting dates between 4 April and 11 June during 2007–2009 growing seasons as the treatments were used in this study. The maximum grain yield and kernel starch content were obtained with corn planted between late April and mid–May; both decreased when planting was delayed to late May and June. Later planting dates resulted in larger proportions of short amylopectin branch–chains for one of the studied varieties, less amylose contents of starch, lower starch gelatinization and pasting temperatures, and greater peak viscosity of starch paste for both corn varieties. While these changes may impact the quality and yield of starch, they might not be of sufficient magnitude to impose major problems in processing of products containing starch.
Starches isolated from corn kernels planted on a late date (11 June) were hydrolyzed at comparable rates as those planted on early dates (late April and mid–May) using a raw starch hydrolyzing enzyme. Consequently, kernels planted in late May and in June gave similar ethanol yields (on kernel dry weight basis) as those planted on earlier dates. The results showed that the planting date of corn did not affect the ethanol yield on the basis of kernel weight. The grain yield, however, decreased with late planting dates, and thus, reduced the ethanol yield on the basis of unit planting area (g ethanol/hectare of planted area).
Freshly harvested kernels of three commercial corn hybrids were dried at low (10 °C), ambient (25 °C), intermediate (45 and 65 °C), and high (85 °C) air temperatures to 14% moisture content to assess how kernel drying temperatures impacted functional properties of starch and ethanol yield of the kernels. The air drying temperature at 10 °C increased starch gelatinization temperature and enthalpy change and reduced the swelling power of starch compared with the control (25 °C). This can be attributed to that the drying temperature at 10 °C was close to the optimum temperature of starch crystallization (4 °C) and, thus, enhanced starch crystallinity. The intermediate air drying temperatures (45 °C and 65 °C) increased starch gelatinization temperature and enthalpy change and narrowed the gelatinization temperature range of starch compared with the control. These results suggested that starch molecules annealed during the drying at 45 °C and 65 °C and perfected the crystalline structure of starch. The drying temperature of 85 °C partially gelatinized the starch granules during the drying and reduced their crystallinity. Consequently, starch isolated from kernels dried at 85 °C exhibited higher gelatinization temperature and reduced swelling power of starch compared with the control.
Changes in the starch structure reduced the granule susceptibility to the enzyme hydrolysis and, thus, decreased the ethanol yield of ground kernels dried at the intermediate (45 °C and 65 °C) and high (85 °C) air temperatures. Kernels dried at 85 °C produced the least yield of ethanol, which might be resulted from the most severe reduction in the starch swelling power that inhibited enzyme penetration into the starch granule in addition to the loss of the endogenous enzyme activity of kernels during drying at 85 °C.
Kernels dried at temperatures up to 65 °C air temperatures displayed similar levels of endogenous amylase activity, whereas those dried at ≥85 °C contained partially reduced amylase activity as indicated by the reduced amount of reducing sugars produced in the ground kernel suspensions incubated at 40 °C for 20h. Among the endogenous amylases, β-amylase was most heat-labile and showed reduced activity after the kernel was dried at 45 °C. Pullulanase and isoamylase showed reduced enzyme activity in kernels after the drying at 85 °C. The α-amylase was relatively stable up to 85 °C but significantly lost its activity after drying at 105 and 125 °C air temperatures.
Medic, Jelena, "Starch properties, endogenous amylase activity, and ethanol production of corn kernels with different planting dates and drying conditions" (2011). Graduate Theses and Dissertations. 10106.