Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2000
Publication Date: April 1, 2000
Interpretive Summary: Wheat hardness, a physical property that is a manifestation of the biochemical interaction between the proteins of the endosperm and starch, is important in the determination of milling throughput, equipment design, energy requirements, and equally important to the baking and processing industries that rely on the textural properties of hard and soft wheats in forming food products. An extensive study was performed on ten well characterized, internationally available wheat samples that form the basis for the commonly accepted definition of wheat hardness. Individual wheat kernels were machined into precise geometrical cylinders, conditioned to one of five controlled humidities, then loaded in uniaxial compression until failure. Maximum compressive stress, Young's (tangent) modulus, energy to maximum stress, and strain at maximum stress were calculated. Regression analyses were performed on each property to determine the effect of moisture content on the property in each of the ten samples as well as in poolings of samples such as soft vs. hard. Results are presented in an easily interpretable form such as the fractional change in physical strength property when moisture content is changed from a level that is safe for storage (i.e., 12% wet basis) to a dryer (3-12%) or wetter (12-21%) condition. Essentially, negative linear relationships were found for maximum compressive stress vs. moisture content and Young's modulus vs. moisture content, with the first relationship being the best. Hard wheats were more sensitive to moisture change than soft wheats. The results of this study are of benefit to wheat millers and processors, especially those who use commercial wheat hardness instruments.
Technical Abstract: The effect of moisture on the physical strength properties of wheat kernel endosperm on an in situ basis were studied using five soft and five hard wheat samples available from the U.S. National Institute of Standards and Technology's Standard Reference Materials Program. Geometrically precise cylinders of wheat (1 mm diam x 3 mm height) were conditioned to one of five controlled humidities (ca. 0 to 93% rh), then tested for the following compressive strength properties: maximum compressive stress (Smax), tangent modulus of elasticity (E), work to maximum stress [W(Smax)], and strain at maximum stress [e(Smax)]. Large variations in strength properties occurred within the same sample. Mean strength values, based on 8-11 endosperm cylinders per sample x humidity setting, were calculated, then normalized with respect to their mean value at the humidity (53% rh) considered most similar to ambient storage conditions. Normalized strength value vs. moisture content (3-28% db) response curves were linearly fitted and statistically tested to determine the similarity in response among samples. Smax was most linearly related to moisture content (r2 = 0.911-0.994). When samples within a hardness class were pooled, the slopes of the normalized Smax regression equations were -0.036 (r2 = 0.856) and -0.055 (r2 = 0.954) for soft and hard wheats, respectively, and significantly different (P<0.05). A similar pooling procedure for normalized E resulted in a nonsignificant difference between slopes of soft (-0.413, r2 = 0.838) and hard (-0.0448, r2 = 0.885) samples. The response of the two remaining physical properties, W(Smax), and e(Smax), to moisture content was not statistically significant for most samples.