Wheat endosperm compressive strength properties as affected by moisture.
Tempering, the addition of water to wheat before milling, is routinely performed to toughen the pericarp of the 
kernel and thus improve the efficiency of flour extraction. In an effort to better understand how tempering affects the 
kernel’s internal structure, the effect of moisture on the physical strength properties of wheat kernel endosperm on an in 
situ basis was studied. Five soft and five hard wheat samples, available from the U.S. National Institute of Standards and 
Technology’s Standard Reference Materials Program for the purpose of standardizing wheat hardness instruments, were 
examined. Geometrically precise cylinders of wheat endosperm (1 mm diameter ×3 mm height) were conditioned under 
one of five relative humidities (~0 to 93% RH, producing moisture contents of 3 to 28%, dry basis), then tested for the 
following compressive strength properties: maximum compressive stress (S max ), modulus of elasticity (E), work to 
maximum stress [W(S max )], and strain at maximum stress [e(S max )]. Large variations in strength properties occurred 
within 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 a 
reference moisture content of 13% d.b. Normalized values of the first three properties were linearly fitted to moisture 
content and statistically tested to determine the similarity in response among samples. S max was most linearly related to 
moisture content. When samples within a hardness class were pooled, the slopes of the normalized S max regression 
equations were significantly different (P < 0.01) for hard and soft wheats. A similar pooling procedure for normalized E 
revealed a moderately significant (P < 0.05) difference between slopes of soft and hard classes. For W(S max ), most 
samples did not exhibit a sensitivity to moisture; however, when pooled into soft and hard classes, the difference between 
these classes was highly significant (P < 0.01). The response of e(S max ) to moisture was constant except at the highest 
moisture content (24 to 28% d.b.), where the values were approximately twice as large.