Effects of Phosphates Added to Concentrated Milks before Sterilization at Ultra-High Temperatures

Abstract

Summary The microscopically visible particles in sterile concentrated milks were studied in relation to the addition of sodium phosphate salts and changes leading to gelation. Polyphosphates added to concentrated milks before sterilization at 303±2F delayed gelation, whereas orthophosphates speeded gelation. Concentrated milks containing 0.05% added polyphosphate stored at 100F browned, and developed undesirable flavors before gelation. Viscosities of concentrated milks increased with increased additions of polyphosphates and the size of microscopically visible particles became more uniform in the size range 0.7 to 1.4 µ diam, compared with 0.1 to 5 µ diam in the controls. These aggregates of protein and fat, regardless of phosphates added, dissociated rapidly in 6 M urea at pH 8, very slowly in 6 M urea at pH 6.7, rapidly in 0.1 N potassium oxalate, sodium oxalate, and ammonium oxalate, but only slightly when diluted one part of concentrated milk to 50 parts of dilute acetic acid at pH 3.88. Gel structure microscopically was similar in the gelled phosphate-containing samples and the gelled controls. Both noncasein and nonprotein nitrogen increased during storage at 70 and 100F. Titratable sulfhydryl groups decreased during storage. Bond-breaking reagents indicated that disulfide bonds were involved in the gel structure.