The Use of Thiamin Disulfide for the Estimation of Reducing Substances in Processed Milk

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Abstract
Thiamin disulfide was used to determine the amt. of certain reducing substances in fresh skim milk and non-fat dry milk solids. Details for the prepn. of the thiamin disulfide reagent are given. A soln. of the thiamin disulfide is prepd. by dissolving 200 mg. in 75 ml. of soln. containing 10 ml. of [image]/10 HCl. 1 ml. of the disulfide soln. is added to 2 ml. of fresh milk or 200 mg. of dry milk plus 2 ml. of water in a test tube. After mixing and covering with 3 drops of isobutanol, the mixture is allowed to stand 2 hrs. at room temp. 2 ml. of 10% tri-chloracetic acid are added to the mixture and the proteins separated by centrifuging at high speed for 5 min. The liquid is decanted through cotton and 2 ml. of the filtrate diluted to 100 ml. The liberated thiamin is detd. on a 1-to 5-ml. aliquot of the soln. by the thiochrome method. Reducing substances equivalent to approx. 20 mg. of cysteine-HCl per liter are liberated when skim milk is heated to 90[degree]-95[degree] C for 5 min. Unheated milk does not reduce thiamin disulfide. Spray process non-fat milk solids contain substances that reduce thiamin disulfide equivalent to 0.0-11.5 mg. of cysteine-HCl per 100 g. Heating skim milk longer than 10 min. at 80[degree] C or above destroys reducing substances.