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Cereal Chem 37:436 - 455.  |  VIEW ARTICLE

The Relation of Particle Size to Certain Flour Characteristics.

B. Sullivan, W. E. Engebretson, and M. L. Anderson. Copyright 1960 by the American Association of Cereal Chemists, Inc. 

A hard-wheat, 90% patent flour was air-classified in six particle size ranges and the separated fractions analyzed for particle size distribution, ash, protein, maltose value, gassing power, and viscosity as measured by the amylograph. The analysis of different samples of the same flour did not vary as greatly as does the analysis of the different size particles, themselves, that comprise the given sample of flour. Both ash and protein content showed marked fluctuations with particle size ranges. At a very low micron size, the ash content was, roughly, double that of the original flour. Ash decreased rapidly to about 15 microns where it was well below the original ash content. From 15 to 30 microns, the ash content increased, probably because the ratio of small endosperm cells and peripheral cells to more-or-less free starch granules increases. From 30 to 70 microns ash content again decreased, followed by an increase above 70 microns because there is a greater number of covered cells and aggregates in this size range. Protein content followed the same general pattern, showing a wide range from 5.5% in the region around 15 to 30 microns (where free starch granules occurred in larger amounts) to well over 25.0% in the region under 5 microns. The exact size at which these fluctuations in analysis occur and their extent depend on the wheat, the grade of flour milled from it, and the milling process used. Maltose value and gassing power decreased with increasing particle size to approximately 70 microns where there was a definite increase. Specific surface, which can be calculated from the particle size distribution curve, correlates very well with maltose value and gassing power for most flours---in fact, better than starch damage as it is presently measured. Starch damage is dependent on the type of grinding and does not necessarily parallel fineness of grind. Maltose values showed a log linear relationship with specific surface for a flour ground to varying degrees of fineness. The ratio of maltose value to gassing power increases as the particle size decreases. Viscosity as measured by the amylograph showed an increase from the very small micron range to a peak around 20 to 30 microns, where there was the greatest concentration of free starch and the lowest protein content. There was then a drop in the curve, followed by an increase in viscosity in the coarsest size range. Further work on the structure of the endosperm in relation to milling practices should lead to a better basis for assembling flour streams and to more realistic flour specifications.

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