WHEAT ENDOSPERM COMPRESSIVE STRENGTH PROPERTIES AS AFFECTED BY MOISTURE

Abstract

Tempering, the addition of water to wheat before milling, is routinely performed to toughen the pericarp of thekernel and thus improve the efficiency of flour extraction. In an effort to better understand how tempering affects thekernels internal structure, the effect of moisture on the physical strength properties of wheat kernel endosperm on an insitu basis was studied. Five soft and five hard wheat samples, available from the U.S. National Institute of Standards andTechnologys Standard Reference Materials Program for the purpose of standardizing wheat hardness instruments, wereexamined. Geometrically precise cylinders of wheat endosperm (1 mm diameter 3 mm height) were conditioned underone of five relative humidities (~0 to 93% RH, producing moisture contents of 3 to 28%, dry basis), then tested for thefollowing compressive strength properties: maximum compressive stress (S max ), modulus of elasticity (E), work tomaximum stress [W(S max )], and strain at maximum stress [e(S max )]. Large variations in strength properties occurredwithin the same sample and humidity, as seen by coefficients of variation reaching as high as 56%. Mean values of S max ,E, W(S max ), and e(S max ), based on 8 to 13 endosperm cylinders per sample humidity setting, were normalized to areference moisture content of 13% d.b. Normalized values of the first three properties were linearly fitted to moisturecontent and statistically tested to determine the similarity in response among samples. S max was most linearly related tomoisture content. When samples within a hardness class were pooled, the slopes of the normalized S max regressionequations were significantly different (P < 0.01) for hard and soft wheats. A similar pooling procedure for normalized Erevealed a moderately significant (P < 0.05) difference between slopes of soft and hard classes. For W(S max ), mostsamples did not exhibit a sensitivity to moisture; however, when pooled into soft and hard classes, the difference betweenthese classes was highly significant (P < 0.01). The response of e(S max ) to moisture was constant except at the highestmoisture content (24 to 28% d.b.), where the values were approximately twice as large.