ARS | Publication request: Enzymatic method for measuring starch gelatinization in dry products in situ

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 8, 2012
Publication Date: March 8, 2012
Repository URL: http://riley.nal.usda.gov/nal_web/digi/submission.html
Citation: Liu, K., Han, J. 2012. Enzymatic method for measuring starch gelatinization in dry products in situ. Journal of Agricultural and Food Chemistry. 60:4212-4221.

Interpretive Summary: Starch gelatinization is one of the important physical, chemical and biochemical changes during processing starch-containing foods or feeds. The extent of starch gelatinization not only determines the textural and organoleptic properties of final products but also affects human and animal nutrition through changing enzymatic access to glucosidic linkages and consequent digestibility. Many methods have been described to measure starch gelatinization, including polarization microscopy, x-ray diffraction, amylograph, differential scanning colorimetry (DSC), pulsed nuclear magnetic resonance, absorbance of iodine binding, and enzymatic susceptibility. However, most of these techniques, including some popular enzymatic methods, are applicable only to purified starch. For processed products, starch has to be isolated first, making the method laborious and/or prone to errors. Others, such as DSC, require instrument that is not easily accessible. Still others, like dye-binding methods, are less reliable. The enzymatic method proposed in this study has an edge over many previously reported methods in that it can measure starch gelatinization of dried processed food or feed in situ, without need of expensive instrument or a unique device. The present study was able to achieve the new objective by not only optimizing the steps on which previous workers have focused but also adding new steps relating to sample handling before enzymatic hydrolysis of starch to D-glucose. Starch-containing foods and feeds that have been heated and then cooled often contain substantial amounts of retrograded starch. They also contain some native starch (not fully gelatinized). The new steps, which included sample particle size reduction and a unique mechanical solubilization of starch, address the issues of retrograded starch insusceptibility as well as native starch susceptibility to enzyme hydrolysis, and thus effectively eliminate the need for starch isolation or an expensive instrument. The proposed method is simple, accurate, and reliable.

Technical Abstract: An enzymatic method based on hydrolysis of starch by amyloglucosidase and measurement of D-glucose released by glucose oxidase-peroxidase was developed to measure both gelatinized starch and hydrolyzable starch in situ of dried starchy products. Efforts focused on the development of sample handling steps (particle size reduction of dry samples followed by a unique mechanical resolubilization step) prior to the enzymatic hydrolysis using native and fully gelatinized flours of corn and rice. The new steps, when optimized, were able to maximize resolubilization of gelatinized/retrograded starch while minimizing solubilization of native starch in dried samples, thus effectively addressing issues of insusceptibility of retrograded starch and susceptibility of native starch to enzymatic attacks and eliminating the need to isolate starch from dry samples before using an enzymatic method. Various factors affecting these and other steps were also investigated, with the objectives to simplify the procedures and reduce errors. Results are expressed as the percentage of the total starch content. The proposed method, verified by measuring mixed samples of native and fully gelatinized flours of five grain species (corn, rice, barley, oat, and wheat) at different ratios, is simple, accurate, and reliable, with a relative standard deviation of less than 5%.