Structure and Composition of the Corn Kernel

Corn kernels contain, by weight, approximately 70% starch, 9% protein, 4% fat and oil, and 9% fiber on a dry basis [10]. Most corn grown for ethanol production is #2 yellow dent corn, so named because of the indentation in the top of the dried kernel. Energy is stored in the seed in the form of starch and oil, which are segregated to the endosperm and germ, respectively (Figure 4.1). Different pro­teins are contained in the endosperm, germ, and tip cap. The gluten protein fraction is found in the endosperm, bonded to the starch. The seed contents are protected by a waxy coat and fibrous outer layer (the pericarp). Fiber is also present in the germ and tip cap.

Ethanol yield potential varies among corn hybrids [11, 12], and also depends on agronomic practices and environmental factors. Corn hybrids are being devel­oped and marketed specifically for enhanced ethanol production, and seed corn is labeled for sale with high extractable starch (for wet milling) or high ferment­able starch content (for dry grind ethanol processing). Total starch content and total extractable starch content do not necessarily correlate with ethanol yields obtained in dry grind processing of the whole kernel; instead, high-performing varieties have been identified by empirical testing. Grain from high ethanol-

yielding hybrids reportedly results in ethanol yields up to 4% higher than the yield from mixed commercial grain, representing an additional one to two million dollars to a 40-million-gallon-per-year dry grind ethanol facility [13]. Spectro­scopic methods using near-infrared (NIR) technology have been developed for use at ethanol plants to predict the ethanol yield potential of samples of whole corn kernels [14].