THE CREAMING ABILITY OF MARKET MILK 9 
EFFECT OF PARTIALLY COOLING PASTEURIZED MILK IN VAT 
OR TANK 
At many plants using tanks or vats for pasteurization it is cus- 
tomary to cool the milk partially in the tank or vat before it is run 
over the cooler. In this way the capacity of the apparatus is in- 
creased and considerable time is saved. At some plants the milk is 
entirely cooled in the vat. In the laboratory tests reported in Tables 
19 to 21 it is shown that holding or agitating pasteurized milk for 
any length of time at temperatures between 110° and 60° F. had a 
detrimental effect on its creaming ability. 
In Table 6 is shown a summary of the results of partially or wholly 
cooling the pasteurized milk in the tank or vat. The results shown 
in this table corroborate the laboratory results shown in Tables 19 
to 21. The control samples upon which the percentage differences 
in this table are computed were from the pasteurized milk at the end 
of the 30-minute holding period, immediately cooled. The table 
shows the time and the temperature at which the milk was pas- 
teurized and the agitation to which it was subjected during the 
subsequent period prior to cooling over the cooler, and the range of 
temperature at which it was held. At some plants where large tanks 
or vats were used the milk, after the 30-minute holding period, was 
held for over an hour in the tank or vat before it was finally cooled. 
It will be noted that reductions in cream volume where the entire 
cooling was done in the vat were 20 per cent in one instance and 23.9 
per cent in another instance. Of the 27 tests made, 10 showed no 
reduction for milk held at various temperatures before cooling as 
compared with the milk that was immediately cooled. It will be 
noted that in onty 4 instances where there Avas no reduction in 
cream volume was the milk held at a temperature as low as 110° 
F., and in only 2 instances was it below 110° F. At temperatures 
ranging from 145° to 115° the reductions were very small. The 
results indicate that it is not advisable to cool the pasteurized milk 
below 115° to 110° in the vat, and that it should be cooled quickly 
from this temperature to below 50° over a separate cooler. 
There is considerable range in the reduction in the cream volume 
on milk held at various temperatures after pasteurization for various 
lengths of time. A decrease in creaming ability during this stage 
of the processing results in variations in the depth of the cream vol- 
ume in the bottled milk, as the first milk leaving the holding vat will 
show a deeper cream volume than the last milk discharged. The 
maximum reduction obtained would be noticeable to consumers and 
is perhaps one of the causes of complaints received by dealers on 
the lack of uniformity in the cream volume of their product. How- 
ever, the data obtained seemed to indicate that the temperature at 
which the milk is held after pasteurization is much more important 
than the length of time it is held. Milk held for nearly 1 hour after 
the 30-minute holding period at 145° F. in many cases showed a 
better cream layer than milk held after pasteurization for only a few 
minutes at temperatures between 110° and 60° F. 
43946°-25 2 
