Rapid Screening to Identify Unusual Thermal Starch Traits from Bulked Corn Kernels

Cereal Chem. 81(4):527–532 Differential scanning calorimetry (DSC) is used routinely to screen for starch thermal properties. In early generations of line development, the established analysis separately evaluates starch extracted from five, single corn kernels. A thermal property trait carried by a recessive gene would appear 25% of the time; thus, if five separate kernels were evaluated, the likelihood of detecting an unusual thermal trait is high. The objective of the current work was to expedite selection by examining five kernels at a time, instead of one, hypothesizing that we would be able to detect different thermal properties in this blend. Corn lines, all from the same genetic background (ExSeed68 or Oh43), with known thermal functions (amylose-extender, dull, sugary-1, sugary-2, and waxy) were blended with normal starch (control) in ratios of 0:5, 1:4, 2:3, 3:2, 4:1, and 5:0, and analyzed with DSC. The values for each ratio within a mutant type were unique (α < 0.01) for most DSC measurements, especially for gelatinization onset temperature, change in enthalpy of gelatinization, and range of gelatinization. These results support the five-kernel method for rapidly screening large amounts of corn germplasm to identify kernels with unusual starch traits. Differential scanning calorimetry (DSC) was first utilized by Stevens and Elton (1971) to study starch gelatinization. DSC is an excellent method for researching starch gelatinization because it allows the use of a wide range of starch-to-water ratios, is not limited to temperatures <100°C, and estimates transition enthalpies (Biliaderis et al 1980). Also, DSC requires only a small amount of sample, is easy to operate, and is relatively rapid compared with other methods (Sanders et al 1990; Campbell et al 1995). These factors make it conducive for breeding programs in which large numbers of corn genotypes are screened for desirable starch properties such as low gelatinization onset temperature (ToG) or low change in enthalpy of gelatinization (∆HG). Variations in DSC measurements have been demonstrated for a variety of maize mutants including amylose-extender (ae), dull (du), sugary-1 (su1), sugary-2 (su2), and waxy (wx) (Inouchi et al 1984; Brockett et al 1988; Ninomya et al 1989; Sanders et al 1990; Inouchi et al 1991; Wang et al 1992; Campbell et al 1995; Perera et al 2001; Tziotis 2001). These particular mutants cause changes from normal corn starch in amylose percentage and phytoglycogen accumulation (Shannon and Garwood 1984). For example, the ae mutation results in starch with 50–70% apparent amylose content (Ikawa et al 1981; Yeh et al 1981; Shannon and Garwood 1984), which may dilute the crystalline regions, thus causing a loss of cooperative melting (Wang et al 1992). The ae mutation also was reported to increase the chain length of amylopectin (Ikawa et al 1981), which would then require a higher temperature to gelatinize (Wang et al 1992). Therefore, the ae starch typically has a broad gelatinization peak that is not complete until up to 120°C, and a high ∆HG (Brockett et al 1988; Inouchi et al 1991). The du and su1 genotypes also are reported to increase apparent amylose percentage (Ikawa et al 1981; Yeh et al 1981) but do not have broad gelatinization peaks typical of ae starch (Inouchi et al 1984; Brockett et al 1988; Wang et al 1992). They both, however, typically possess a lower ∆HG value and a ToG a few degrees below that of normal starch, which may be a result of slightly lower and less perfect crystallinity in the starch (Inouchi et al 1984). Starch from the su2 genotype also has a higher apparent amylose content than normal starch but gelatinizes at a much lower temperature and ∆HG, which may be a result of the very low percentage of crystallinity and higher amount of short branch-chains of amylopectin in su2 starches than in normal starches (Inouchi et al 1984; Perera et al 2001). The wx genotype causes an elimination of amylose content, unlike the other mutants presented here (Inouchi et al 1984). This mutant results in starch with ≈100% amylopectin, the crystalline component of starch, which requires more energy to gelatinize (Inouchi et al 1984). Recently, there has been interest in developing corn starches that naturally possess properties similar to those of chemically modified corn starches. The Germplasm Enhancement of Maize (GEM) project has developed and identified exotic by adapted lines that are partially from germplasm foreign to corn races grown in the United States and which may be useful for agronomic, nutritional, and industrial reasons (Pollak 2002). Our laboratory routinely screens corn sources for starch traits that may be useful to the starch industry such as low ToG or low percentage of retrogradation (%R), as well as other criteria (Seetharaman et al 2001). As described earlier, DSC is one of the most rapid methods available for such screening. Earlier methods, however, typically involved extracting starch from single corn kernels such as utilized by Ji et al (2003), for a total of up to 10 kernels from one source. Obanni and BeMiller (1995) described a technique of screening corn through ghost structures, which are the remnants of starch granules after autoclaving small amounts of starch. The greatest value for starch, however, is in the thermal properties. It would be advantageous to expedite the procedure by bulk-extracting starch from a pool of kernels instead of only one kernel, while still being able to recognize the presence of starch with different properties through DSC analysis. Obanni and BeMiller (1997) studied properties of blends of different types of starches such as normal corn, waxy corn, amylose-extender corn, potato, and wheat. They reported that the DSC output did not resemble either of the components in the mixture. However, Liu and Lelievre (1992) reported that the DSC endotherms for blends of wheat and rice starch were the sum of the outputs for each of the components when the starch concentration was <30%. Preliminary data in our laboratory indicated similar results when two starch types were blended together. For example, we used a mixture of normal and su2 starch in different ratios in the DSC pan, which resulted in independent peaks on the DSC, both similar to their respective starch type. The objectives of this study were to investigate the use of DSC as a screening method for detecting unique thermal properties in a blend of two starch types, and to determine whether the starches 1 Graduate student and professor, respectively, 2312 Food Sciences Building, Department of Food Science and Human Nutrition and Center for Crops Utilization Research, Iowa State University, Ames, IA 50011. 2 Currently, Food technologist, Tate and Lyle North America, Decatur, IL 62525. 3 Biologist, USDA-ARS, Plant Introduction Research Unit, Department of Agronomy, Iowa State University, Ames, IA 50011. 4 Corresponding author. Phone: 515-294-9688. Fax: 515-294-8181. E-mail: