Modified Reduction Test to Predict the Shelf Life of Cottage Cheese

Abstract

Summary One hundred and one commercial samples of Cottage cheese were subjected to a modified reduction test designed to measure the correlation between shelf life and the reduction time required for resazurin, methylene blue, and 2,3,5-triphenyl tetrazolium chloride (TTC). Prior to testing, the samples were neutralized to a pH of 6.5±0.5, bile salts No. 3 (Difco) were added to partially inhibit the Gram-positive lactic organisms, and trypticase soy broth was added to stimulate the growth of the psychrophiles. The reduction times varied from 1.3 to 40 hr., depending on the quality of the cheese and the type of dye used. The reduction times in hours at 24° C, plotted against keeping quality in days, gave correlation coefficients as follows: resazurin pink, 0.80; resazurin white, 0.77; methylene blue, 0.75; and TTC 0.74. Reduction to resazurin pink gave the most rapid results as well as the highest correlation. The correlations between shelf life and initial psychrophile and total count were -0.67 and -0.78, respectively. The test is useful for identifying Cottage cheese with a short shelf life. Nineteen of 20 samples which developed an organoleptically detectable defect within two days when stored at 10° C., reduced resazurin to pink in 7.75 hr. or less, and all samples requiring more than 23 hr. to reduce resazurin retained their original organoleptic quality for six or more days.