MANUFACTURING MILK QUALITY: A RE-EVALUATION

Abstract

The bacteriological quality of manufacturing-grade milk is very similar to that marketed a decade earlier when bulk tanks first came into general use. Milk grading programs usually relied on reduction tests. These tests indicated that most milk supplies were good quality. Based on the Standard Plate Count, data is presented that show approximately one-third of the samples tested, in 1969–70 and in 1957–59, exhibit counts <200,000/ml. Considerable quantities of milk, received at processing plants have plate counts exceeding 1,000,000/ml. Dairy farmers learned they could substitute cooling for cleaning because psychrotrophic bacteria predominated the microflora of most high count bulk milk. These bacteria do not readily reduce resazurin and methylene blue. Psychrotrophs also tend to grow in clumps preventing an accurate evaluation of milk quality using the Direct Microscopic Count (DMC). USDA uses the DMC to test check manufacturing plant's milk supplies. Laboratories are recognizing the value of plating procedures, including the Plate Loop Count, to determine milk quality. Manufacturing-grade milk must be evaluated with a plating procedure before progress can be made in milk quality improvement. One grade of milk is far from being a reality if present levels of manufacturing-grade milk quality are considered.