Effect of different tempering methods on sorghum milling
Abstract
Current sorghum milling processes result in high milling losses and inconsistent flour quality. Appropriate tempering methods could improve the quality and quantity of flour extracted from sorghum. In this study, the effects of cold water, hot water, and steam tempering on sorghum kernel physical and mechanical properties were studied. Single kernel characteristics (SKCS), abrasive hardness, structural changes, and texture of kernels were evaluated. SKCS hardness indices decreased from 75.41 to a range of 53.50–64.20 after tempering. In terms of reducing the hardness value, hot water tempering was more efficient than the cold water tempering process. The abrasive hardness index, which represents the pericarp properties, did not show any correlation with moisture content. Abrasive hardness of steam tempered kernels increased from 14.61 to 22.11 when the time of treatment was extended. Smaller deformations and larger rupture forces were found for cold water tempered kernels, indicating the brittleness of cold tempered sorghum kernels. The energy required to rupture sorghum kernels increased (63.41–70.04 N.mm) when the duration of steam tempering was extended. Untreated sorghum kernels were associated with the lowest energy (35.41 N.mm). Pericarp thickness decreased from 57.98 mm (untreated) and 58.49 mm (cold tempered) to 24.49–37.60 mm after hot water and steam tempering. The changes in pericarp structure and endosperm texture induced by steam tempering influence the kernel mechanical properties. The effects of cold water, hot water, and steam tempering on the roller milling performance and flour quality of sorghum were also evaluated. Steamed sorghum had high bran yield with a larger proportion of bran particles after the milling process that might be due to the gradual scrapping of endosperm from the bran. Bran yield increased with increasing steam duration (from 1 to 2 min) but decreased at 2.5 min steam treatment. Flour yield was not significantly different among the three treatments. Crude fiber content of the 1-min steam tempered sorghum was 1.28 ± 0.09%, which was significantly higher than that of flour obtained from cold water tempered sorghum (0.87 ± 0.19%). The damaged starch content was the highest in the cold water treated (5.96 ± 0.24%) sorghum and steam tempering resulted in a damaged starch content of 3.63–4.18%. The circularity equivalent diameters of sorghum flour from different treatments mainly below 50 μm. Steam treatments for 1 and 2 min resulted in flour with more convex and circular particles compared to other treatments, indicating a better separation of starch granules from endosperm cells. Viscosity profiles of sorghum flour did not show significant differences among tempering treatments. Steam tempering of sorghum kernels at high temperature and pressure led to a better separation of bran and endosperm, without negatively impacting flour quality.
Degree
M.S.A.B.E.
Advisors
Ambrose, Purdue University.
Subject Area
Food Science
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