Milling Yields and Physicochemical Properties of Long-Grain Rice Thickness Fractions

Abstract

Thickness grading of rice ( L.) can divert thin, chalky kernels to a secondary processing stream, thus improving milling yields of the primary stream. However, the quantity and functionality of the secondary stream may impact processing operations. Rough rice of multiple long-grain cultivar lots was either left unfractioned (Unf) or thickness graded into fractions comprising A ( 2.05 mm). Mass distribution of rough rice; milled rice yield (MRY), and head rice yield (HRY); kernel dimensions of brown rice; crude protein content (CP) and chalkiness of brown and head rice; and paste viscosities of head rice flour were determined for each cultivar lot/fraction. For all cultivar lots, MRY, HRY, and kernel dimensions increased with increasing thickness fraction, while chalkiness and CP decreased. Milling yields were the greatest for the C/D/E fractions. Including the C fraction (C/D/E) with the primary stream resulted in an average 17.4 percentage point mass increase compared to D/E alone; this with minimal impacts to milling yields of the primary stream. Weighted-average MRYs and HRYs of the combined C/D/E fractions increased by up to 6.9 and 12.8 percentage points, respectively, compared to Unf rice. The A/B fractions were generally of greater chalkiness and CP; however, paste viscosities remained relatively consistent with those of the C/D/E fractions. Thickness grading could be justified on the merits of improved milling yields, kernel uniformity, and reduced chalkiness of the primary processing stream. A small change in the thickness grading procedure decreased the quantity of the secondary stream; moreover when milled, the secondary stream retained functionality similar to the primary stream.