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The Reducing Capacity of Milk as Measured by an Iodimetric Titration¹

Division of Agricultural Biochemistry, University of Minnesota, St. Paul 1

ABSTRACT

The reducing systems of milk have been studied by an iodimetric titration employing o-iodosobenzoate. The fat phase, the serum proteins and the dialyzable portion all exhibit reducing capacity in this method. Since milk fat emulsified in gelatin has no reducing capacity, the materials constituting the natural "fat globule membrane" must be responsible for reduction by the fat phase. Titrations of purified crystalline ß-actoglobulin indicate that it probably is the principal reducing constituent of the serum proteins. Undoubtedly, ascorbic acid is the chief dialyzable reductant.

Considerable variability was found in the reducing capacity of the serum proteins in various samples of milk; commercial raw milks tended to give lower values than fresh milks.

Sulfosalicylic acid as used by Gould does not precipitate quantitatively the serum proteins from raw milk, but the efficiency of precipitation is greater in heated milk. Thus, the decrease produced by heat treatment of milk in the iodimetric titration values of sulfosalicylic acid filtrates is due to both decreased reactivity of protein sulfhydryl groups and increased precipitability of the serum proteins. The similarity in decreases of the reducing capacity for skim-milk, purified serum proteins and crystalline ß-lactoglobulin again suggests that ß-laetoglobulin is the principal reducing component of the milk proteins. The decrease in reducing titer upon heat treatment probably is due to oxidation by molecular oxygen, since heat treatment of deaerated samples in the presence of nitrogen produces little or no decrease.

¹ Paper no. 2520, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul 1.

² The data in this paper are to be included in a thesis to be submitted by Bruce L. Larson to the University of Minnesota in partial fulfillment of the requirements for the Ph.D. degree. This investigation was supported in part by a research grant from the American Dry Milk Institute, Inc.