Effects of Hardness and Drying Air Temperature on Breakage Susceptibility and Dry-Milling Characteristics of Yellow Dent Corn' A. W. KIRLEIS and R. L. STROSHINE 2 ABSTRACT Cereal Chem. 67(6):523-528 Kernel density, test weight, Stein and Wisconsin breakage suscepti- bility was primarily influenced by hardness, whereas Wisconsin breakage bilities, stress cracking, and Stenvert hardness were determined for three susceptibility was correlated with stress cracking. Milling quality, as mea- corn hybrids that were classified as soft, hard, and of intermediate hard- sured by a milling evaluation factor (MEF), decreased linearly with in- ness. A stress crack index was developed for quantifying degree of stress creasing drying temperature. For all drying temperatures, the hard hybrid cracking. Severity of stress cracking was directly related to hardness with had the highest MEF and the soft hybrid had the lowest MEF. Kernel the hardest hybrid showing the most severe cracking. Surprisingly, stress density was the best single predictor of MEF (R 2 = 0.773). A two-variable cracking was most severe at the intermediate (600C) drying temperature. model that combined test weight and kernel density improved the predic- Stein breakage was greatest for the soft hybrid and least for the hard tion of MEF (R 2 = 0.907). Incorporation of additional variables into hybrid, whereas Wisconsin breakage was greatest for the hard hybrid the model did not significantly improve the two-variable MEF prediction and least for the hybrid of intermediate hardness. Stein breakage suscepti- model. In the past decade, corn dry millers and food corn processors have become increasingly aware of the variations in quality among shipments of shelled corn arriving at their facilities. Corn dry millers seek a maximum amount of endosperm recovered as large grits. Recent studies by Paulsen and Hill (1985) and Litchfield and Shove (1989) demonstrated that selection of corn lots on the basis of kernel density, breakage susceptibility, or test weight can significantly improve flaking grit yields. Similarly, varietal differences in dry-milling quality among commercial hybrids have been demonstrated, with "hard" hybrids exhibiting more desirable milling characteristics (Stroshine et al 1986, Peplinski et al 1989). Food corn processors also prefer "hard" hybrids (Ellis et al 1984, Pflugfelder et al 1988), which are low in breakage susceptibility (Jackson et al 1988). Drying temperature has substantial effects on breakage sus- ceptibility, stress cracking, and dry-milling quality. Increasing dry- ing temperature increases breakage susceptibility and stress crack- ing (Peplinski et al 1975, Gunasekaran and Paulsen 1985, Guna- sekaran et al 1985). Brekke et al (1973) found that as drying air temperature increased from 15 to 1430 C, yield of first-break grits decreased from 42 to 12%, respectively. Similarly, percentage of kernels with stress cracks increased from 7 to 84%. Examination of milled products revealed that corn dried at 1430C had more germ attached to the large flaking grits than that dried at 150C. Peplinski et al (1989) dried six hybrids at two temperatures (25 and 600C). Corn dried at 600C had the highest breakage sus- ceptibility and stress cracking and gave the lowest grit yields. They observed genotypic differences in these factors at both drying temperatures. Genotypic differences in breakage susceptibility have been demonstrated (Paulsen et al 1983, Stroshine et al 1986). Pomeranz et al (1985) demonstrated that the Stenvert mill could be used for determining the hardness of corn dried at ambient temperature. In a later study, Martin et al (1987) tested combine- harvested corn dried at ambient temperature for relationships between breakage susceptibility and corn hardness. They reported that Stein breakage susceptibility (percent fines) was highly cor- related to Stenvert grinding time. However, they found no corre- lations between corn hardness and Wisconsin breakage sus- ceptibility. Relationships among several of the quality parameters and physical properties have been studied and related to dry-milling quality. However, the interactions among drying temperature, 'Journal Paper 12421 of the Purdue Agricultural Experiment Station. 2 Professor, Department of Food Science, and associate professor, Department of Agricultural Engineering, Purdue University, West Lafayette, IN 47907. © 1990 American Association of Cereal Chemists, Inc. dry-milling quality, and the more important factors have not been determined. Although hardness has been defined for other crops such as wheat, the distinction between kernel hardness and break- age susceptibility of corn has not been clearly drawn. Kernel den- sity, test weight, and breakage susceptibility are useful screen- ing methods for evaluating dry-milling quality. However, it has not been determined which combination of these tests is most useful for predicting dry-milling quality of corn. This study was undertaken to investigate the relationships among corn hardness, test weight, kernel density, breakage sus- ceptibility, stress cracking, and dry-milling quality and to identify those tests that could best be used to screen corn lots for dry- milling quality. Evaluations were performed on three corn hybrids identified as hard, soft, and intermediate types. Grain samples were dried at ambient temperature in still air or artificially dried at temperatures between 28 and 930 C. MATERIALS AND METHODS Materials Three dent corn hybrids were grown on 0.2-ha field plots at Lafayette, IN, during the 1985 crop year. These hybrids were selected on the basis of kernel vitreousness and dry-milling per- formance in previous crop years to represent hard, intermediate, and soft endosperm types (FR23 X FR140, MBS73 X MBS847, FRB73 X MO17, respectively). Corn (approximately 20 kg of grain) was hand harvested at 23.6-26.4% moisture and stored at 2°C for a maximum of one week prior to hand shelling and drying. Control grain samples from each hybrid were placed on a laboratory bench (still air 270C) and allowed to dry on the cob to about 15% moisture content (mc) before hand shelling. Samples for artificial drying were stored at 2 0 C for one week or less prior to hand shelling and drying. The 1985 stress cracking and breakage patterns were unexpected and some of the tests were repeated on corn grown in 1986 to confirm our results. FRB73 X M017 and FR23 X FRl40 were again grown in 0.2-ha plots, harvested by hand, and hand-shelled. However, control samples were shelled prior to drying, placed in wire trays, and held at room temperatures until they reached 15% moisture. Thin-Layer Drying Grain from each hybrid was divided into four sublots of 1 kg each and dried in a laboratory thin-layer dryer (Stroshine and Martins 1986). Each sublot of the corn hybrids was artificially dried at one of the following temperatures in order of increasing temperature: 37.7, 60, 82.2, and 93.31C. Final moisture contents were about 15% (range 14.6 to 15.6%). At each drying temperature the hybrids were dried in a random order. Approximate drying Vol. 67, No. 6,1990 523