Determination of starch by iodine colorimetry

Authors

• K. A. Vainio Department of Animal Husbandry, University of Helsinki

DOI: https://doi.org/10.23986/afsci.71700

Abstract

In the iodine colorimetric method of Paloheimo gently dextrinized solutions are prepared of pure starch and of the analysis sample. One of the optical cells (A) of the comparator is provided with a solution made of pure starch and the other (B) with the solution to be analysed. Both solutions have the same iodine concentration. The solution in B must have a intensive colour than that in A. Solution B is then diluted with an iodinewater solution of the same iodine concentration as in the solutions A and B. When these solutions have attained the same colour it is concluded that also the starch concentration is the same and the starch content of the sample can be calculated. The results obtained by this method are compared with those obtained with the amyloglucosidase method of Salo. Table 1 shows that the two methods give very similar results. Different circumstances which might possibly interfere with the colorimetric starch determinations are studied. It was observed that attention must be paid to the intensity of boiling when the 0.05-N H2SO4 dextrinizing solutions are boiled. If the intensity is very different in the comparison solution and the solution to be analysed, considerable errors may occur. If the sample contains added chalk the neutralizing power of the sample should be determined beforehand and the normality of the solution adjusted to 0.05. If the sample contains acid it should be extracted beforehand with 80-% ethanol. —Cellulose and sugars have no influence on the results, nor have plant proteins or proteins of milk. However, if greater amounts of protein were added, a casein preparation intended for laboratory animals showed an obvious disturbing effect, as did gelatin and meat protein. – Faeces did not appear to have an interfering influence in colorimetric starch determination. The iodine colorimetric sensitiveness of starch solutions was also studied. It appeared that 0.18 mg of dextrinized potato starch already deepened the colour of 100 ml dilute iodine solution in room temperature. For wheat starch the corresponding minimum concentration was 0.27 mg/100 ml. In 3° the concentration limit was even lower, 0.05—0.09 mg/100 ml. In all the above mentioned studies the author has used as comparator essential parts of a Pulfrich photometer. A proper comparator (Fig. 1) can also be made by any skilled optician.