70
TEMPERATURE CORRECTIONS FOR THE LACTOMETER
[Vol. XIX, Part I,
This paper, therefore, deals with the results of investigations carried out to
achieve the above object, as also with a study of the effect of fat content and varia-
tion in temperature on the specific gravity of milk. It is a well known fact that
a rise in temperature decreases the specific gravity of a milk sample and a fall in
temperature increases its specific gravity. Secondly, solids not fat increase the
specific gravity, whereas the fat decreases it. Vieth and others have studied the
variation of specific gravity mainly with reference to temperature only. Perhaps
they have assumed either that fat and solids not fat in milk are almost the same
universally or that they vary only within a very narrow range as in the Western coun-
tries. In India, however, cow milk shows a wide variation in fat percentage (usually
from 3 to 6 per cent), and in buffalo milk a fat percentage as high as 8 per cent is not
very uncommon. This makes the study of the problem all the more complicated.
EXPERIMENTAL
Effect of fat and temperature on the correction of lactometer reading
Samples of milk were taken from the dairy herd at the Institute, at 4-30 A.M.
and kept under refrigeration at 40ºF. till 8-30 A.M. to avoid Recknagel phenomenon.
Fat percentage was determined by the Gerber method. The milk samples were
then toned to various lower fat percentages with separated milk which usually
contained less than 0.1 per cent of fat. Separated cow milk was used for toning
cow milk and buffalo milk for buffalo milk.
Generally 200 ml. of the toned milk were taken in a 250 ml. conical flask. After
cooling or warming it to the desired temperature it was mixed thoroughly by slow
rotation, taking care that no air bubbles were incorporated due to violent shaking.
The milk was then poured into a 250 ml. measuring cylinder, the lactometer dipped
upto its entire scale length and then left free. When the lactometer came to rest
the reading was taken at the top of the meniscus, an addition of 0.2 being made
to the reading to make up for the error due to meniscus. Simultaneously with the
noting of the lactometer reading the temperature also was noted with a Fahrenheit
thermometer.
Of the lactometers available in the laboratory, Gerber's thermolacto-densimeter,
graduated from 20 to 40 with a sensitivity of 0.5 and having an attached thermo-
meter ranging from 0º to 40ºC., was the only one found to record the same specific
gravity as determined by a specific gravity bottle or pyknometer. Another advant-
age of this lactometer was that its scale divisions were well apart. As, however,
the attached thermometer was graduated in degrees C. and as it took considerable
time to record the temperature when it was rising or falling, a separate Fahrenheit
thermometer was used for noting the temperature of the milk.
Having taken the lactometer reading at 50°F. the temperature of the milk
sample was gradually raised to 60ºF. by pouring the milk back into the conical
flask and heating it on a water bath. Again the lactometer reading was taken as
before. These readings were repeated at intervals of 10ºF., from 50ºF. to 110°F.
Whenever the temperature had to be brought down it was done by cooling the
sample in ice. After finishing one reading the measuring cylinder and the lactometer
were washed and drained completely before another reading was made.