Changes in unreacted starch content in corn during storage

• Seasonal variation in unreacted starch content in corn that occur during storage may help explain the variation in ethanol yields in dry grind facilities.
• Unreacted starch content ranged from 2.2 to 14.1% over a 48 wk storage period.
• Unreacted starch decreased at 0.31% per wk during the first 10 wk of storage; afterwards, it increased at 0.16% per wk.

In the dry grind process, corn is ground, mixed with process water and cooked; starch is enzymatically hydrolyzed to sugars, and subsequently fermented to ethanol by yeast. The conversion of starch into ethanol, however, is not complete as distillers dried grains with solubles (DDGS) often contain more than 5% starch. The amount of unreacted starch represents inefficiencies in the process and reducing this amount is important to improving profitability of the ethanol industry. Additionally, dry grind facilities have reported seasonal variation in ethanol yields. In order to minimize variation in ethanol yields and the amount of unreacted starch in DDGS, it is important to understand the effects that storage temperature and time have on the digestibility of the corn starch. While starch quality is largely controlled by genetics and growing conditions, postharvest practices (handling, storage, and processing conditions) also affect starch composition and structure. In this study, changes in unreacted starch content of corn during storage were monitored to provide an explanation for the seasonal variation in ethanol yields observed by dry grind facilities. Yellow dent corn was harvested in 2011 and 2012 and stored indoors, outdoors, and under refrigeration for 5–12 mo. Results with the 2011 harvest corn showed unreacted starch content ranging from 2.17 to 14.1% over a 48 wk period. Unreacted starch was more influenced by storage time, initially decreasing at a rate of 0.31% per wk during the first 10 wk and steadily increased at an average rate of 0.16% per wk for the duration of storage. Results with the 2012 harvest corn, however, showed a higher average unreacted starch content of 13.0% during 20 wk of storage and no appreciable change in unreacted starch content regardless of storage temperature.