Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: 3/20/2006
Publication Date: 3/20/2006
Citation: Hall, M. 2006. Laboratory methods of analysis for feedstuff starch content and availability. Journal of Dairy Science. 89:1867. Interpretive Summary:
Technical Abstract: The relationships that starch has with profitable production or with health disorders in cattle advise closer accounting of its quantity and quality in diets. Native starch is an -(1-4)-linked-glucan with -(1-6) linked branch points found in crystalline granules in plants. In feedstuffs, it can be analyzed by enzymatic hydrolysis, or by polarimetry, though both suffer from interferences. Analysis by enzymatic hydrolysis requires gelatinization, hydrolysis with enzymes specific for starch and detection of glucose. Gelatinization breaks hydrogen bonds and the crystalline structure of starch granules, making starch more available to enzymatic attack. It has been accomplished using water + heating, alkali, and dimethyl sulfoxide among other methods. Enzymatic hydrolysis requires enzymes of adequate specificity to release glucose only from starch, and appropriate conditions to take hydrolysis to completion. Heat-stable, -amylase, an endoamylase, can partially hydrolyze starch before amyloglucosidase, an exoamylase, completes the hydrolysis to glucose. Detection of released glucose is best done by assays specific for glucose. Recoveries of pure starch often range from 90% to 98% on a dry matter basis, and analyses typically have a repeatability of + 2% units. Pure starches from different plant sources may differ in recovery values. Most oft noted errors include use of enzyme preparations that release glucose from non-starch molecules, lack of correction for free glucose, and incomplete hydrolysis. Indices of rate and potential extent of digestibility of starch could be useful to formulate diets that promote both good production and health. Assays using enzymatic glucose release from starch and disappearance of starch during microbial fermentation have both been used to assess these characteristics. Method of sample processing for analysis affects the results. Relative indices of rate or extent of starch digestion could be useful in the field. However, for these assays to be applied to describing quantitative differences among starch sources, they need to be linked to a ration formulation or evaluation system that is calibrated to the values they provide.