Starch is the most important poly-glucide and the main source of digestible car-bohydrate in the human diet. It is formed from α-D-gluco-pyranose with a high polymerization degree of thousands order. The importance of starch for the food industry makes it necessary to develop easy, fast, economic and accurate methodologies for its quantification. In some cases, determination of starch content is not made directly. It is calculated as the difference between the dry matter and all the other components (e.g. proteins, lipids, sugars, fibers and minerals).
The starch content in plants varies from 14% to 75 % depending on the plant species. Several chemical and physical methods have been developed to estimate starch contents, such as headspace gas chromatographic method, differential thermal analysis, and determination of the solution poalrimetric deviation. However, the most efficient and easiest method is the enzymatic methods. This method is based on a complete enzymatic conversion of the starch into glucose, followed by an enzymatic determination of the glucose. Gelatinized starch is partly hydrolyzed with thermostable α-amylase. A complete degradation of starch oligomers into glucose is performed using amyloglucosidase. The released amount of glucose is determined quantitatively by an enzymatic method using glucose oxidase. The enzymatic hydrolysis and glucose colorimetric determination allows the quantification of starch content.
Figure 1. Schematic showing a reaction scheme of alpha-amylase and amyloglucosidase on starch [2]
Starch is formed by α-glucose binding chains and is comprised of two glucose polymers, amylopectin, a large branched molecule comprising chains of α-(1→4) linked anhydroglucose resides linked by α-(1→6) branch points, and amylose, an essentially linearpolymer made of α-D-glucose units, bonded to each other through α-(1→4) glycosidic bonds. Amylose chain can form helical coils which traps iodine and turn blue.
Starch from different sources contains different amount of amylose and amylopectin, which generally affects physicochemical properties of starch such as paste viscosity, gelatinization, and water absorption. For those reasons, the amylose content must be measured accurately.
Starch-iodine colorimetry is a simple and rapid method for the measurement of amylose content. Starch-iodine binding leads to a blue solution that mainly depends on the amylose/amylopectin ratio in starch. Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. This amylose-iodide complex has a distinct blue-black coloration, as opposed to the orange-red color of iodine. This method is rapid, simple, accurate and does not require the use of multi-component analysis of spectra. It has been widely used to determine amylose content.
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References
1. DEMIATE, Ivo Motim, KONKEL, Francisco Enéias, & PEDROSO, Ricardo Alexandre. (2001). ENZYMATIC DETERMINATION OF STARCH IN DOCE DE LEITE USING DIALYSIS. Food Science and Technology, Vol21(3), 339-342.
2. Frederick J. Warren, Bin Zhang, Gina Waltzer, Michael J. Gidley, Sushil Dhital (2015) The interplay of α-amylase and amyloglucosidase activities on the digestion of starch in in vitro enzymic systems. Carbohydrate Polymers, Vol117(6), 192-200.
3. McGrance, S.J., Cornell, H.J. and Rix, C.J. (1998), A Simple and Rapid Colorimetric Method for the Determination of Amylose in Starch Products. Starch/Stärke, 50: 158-163.