Wheat bran is a major source of fiber as an ingredient in cereal-based products. However current utilization of bran represents only a small portion of total bran production as most is actually sold for animal feed at a reduced price. During wheat milling, bran is separated from the endosperm, though a clean separation is not possible and there is always some starch adherent to the bran. This bran starch still remains one of the main components of the bran fraction. Understanding the physicochemical properties of bran starch and its relationship with bran tissue is required to maximize bran utilization. Bran starch was hypothesized to have different properties than endosperm starch, and it was hypothesized that bran chemical composition and bran thickness are each related to bran particle size and bran starch quantity. The hypotheses were tested by following aims: (1) to develop methods of isolating bran starch and endosperm starch from wheat bran and endosperm flour, respectively, and identify a common method to isolate both types of starches to ensure comparability of results of further investigation, (2) to characterize bran starch properties compared with those of endosperm starch from the same wheat sample, (3) to determine bran particle thickness and biochemical composition of bran tissue and their relationships with bran particle size and bran starch quantity, and (4) to investigate the ultrastructure of milled bran particles and the starch morphology of isolated bran starch and endosperm starch. Significantly different physicochemical properties of bran starch compared with endosperm starch were found. Bran starch was found to have higher percent B-type small granules, higher amylose content, higher crystallinity, broader gelatinization temperature range, higher enthalpy of gelatinization, lower retrogradation degree, and lower pasting peak and setback viscosities than the counterpart endosperm starch. A-type X-ray diffraction patterns were found for both bran starch and endosperm starch. Bran starch of variety Aubrey had highest crystallinity and gelatinization temperature, while bran starch of variety D8006 had highest percent B-type granules and lowest retrogradation degree. SEM images revealed that small sized starch granules were not lost during isolation steps. For both bran and endosperm starches, the A-type starch granules displayed a disk shape with diameters of 10-30 fÝm, and the B-type granules displayed a spherical shape with diameters of about 2 fÝm. A greater proportion of small granules was observed in bran starch than in endosperm starch. All granules seen in SEM images had smooth surfaces and intact structure, which may indicate that the isolation procedure did not damage the morphology of the starch granules. Bran starch content was found to be negatively correlated with percent large bran particles. The neutral saccharide profile of the wheat bran was dominated by arabinose, xylose, and glucose, whereas mannose and galactose were present in small amounts. Bran thickness, bound ferulic acid (BFA) content and BFA to xylose ratio showed significant correlations with percent large bran particles and bran starch content. SEM images revealed that the outer layers of wheat bran were deformed after milling and the aleurone layer was no longer visible. Milled bran tissue was about twice as thick as intact outer layers of the wheat kernel. Observed relationship between bran characteristics and bran starch content explained why there was correlation between percent large bran particles and bran starch quantity.
Read
