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Changes in Bacterial Counts and Flavor of Concentrated and Recombined Milks during Storage at Low Temperatures¹

Department of Dairy Husbandry, University of Minnesota, St. Paul

ABSTRACT

From the results of these studies it is evident that bacterial growth took place at an appreciably greater rate in recombined milk than in the concentrated milk from which it was prepared. Bacterial growth was associated with flavor deterioration, as poorer keeping quality was obtained with recombined than with concentrated milks.

In studies which were designed to indicate what might be expected by way of keeping quality of concentrate which had been prepared under ideal sanitary conditions after proper pasteurization, approximately 2, 3, and 5 weeks of satisfactory keeping quality were obtained after storage at 7°, 4°, and 1° C., respectively. Generally, these periods of satisfactory keeping quality were longer than those observed with commercially processed concentrate, although in certain trials, including those reported in Tables 1 and 2, commercial concentrated milk showed excellent keeping quality. Differences which have been observed undoubtedly have been influenced greatly by the amount and type of post-pasteurization contamination which occurred.

Coliform bacteria did not appear to play a role in flavor deterioration of the products used in this study. As a matter of fact, coliforms were found to increase in only one of the trials preliminary to those reported. This was expected, for considerable work, notably that of Dahlberg (2), has shown that coliform bacteria do not grow rapidly in pasteurized whole milk at or below the storage temperatures used in these studies. Similar results have been obtained in our laboratories (4), although occasional samples have indicated the presence of coliforms capable of rapid growth.

Since concentrated milk is used mainly in the form of recombined milk, the results obtained during storage of that product are of some interest. It is evident that recombined milk may remain acceptable in flavor for considerably longer than the usual storage period in the household refrigerator before ultimate consumption. In this connection it should be emphasized that the data reported were from results obtained using water for recombination which was at the temperature used for storage of the respective products. Consumer handling may vary considerably. During summer months it is conceivable that water used for recombination may in some areas reach a temperature of 70° F., or higher. This and subsequent conditions of handling may influence greatly the extent of bacterial growth and keeping quality. Studies are in progress to assess the effect of various factors which might influence the keeping quality of recombined concentrated and dried milks.

¹ Paper No. 2968, scientific Journal Series, Minnesota Agricultural Experiment Station. This work was done under the sponsorship of the Commision on Environmental Hygiene, Armed Forced Epidemiological Board, and was aided in part by the Office of the Surgeon General, Department of the Army.