Freezing Point Determination of Milk
(Freezing point determination should be made on fresh milk having the titrable acidity less than 0.18 %. Rinse the tube with about 25 ml of milk sample which was cooled to 10 0 C lower, measure into tube and insert the tube into the apparatus.(sample calculations) Laboratory thermometer No. 2.
Water 7 gm sucrose to 100 ml 10 gm sucrose to 100 ml
Av. + 0.0550 – 4.25 0 C – 0.621 0 C
Interval = 0.196
0.196 equivalent = 0.199
Corrective factor = 0.055
Laboratory thermometer No. 24
Freezing point depression sample with = 0.584 0 C
(0.584 – 0.420) 0.791 = 0.124
True freezing point = 0.422 + 0.124 = 0.546.
Maintain below 0 0 C the temperature if cooling bath at 2.5 0 C below the probable freezing point of the sample. Make the determination of the milk adopting the same procedure employed in determining freezing point of water.
If necessary to start the freezing action in the milk by inserting freezing starter at the time when mercury column has reached 1.0 to 1.2 0 C below the probable freezing point.nRemove the starter and manipulate the stirrer slowly and carefully two or three times while the mercury approaches its highest point. Complete the adjustment of the mercury column in the same manner as in earlier determination then avoiding parallax observe the exact reading on the thermometer scale and estimate to 0.0001 0 C.
The algebraic difference between the average of reading obtained for the water and sample of milk represents the freezing point depression. The freezing point depression of 7 percent sucrose solution as determined by the correction factor for the thermometer. Add to the product 0.422 (freezing point of 7 percent sucrose solution by Bureau of standard thermometer) see example.
Ascertain the percentage of added water (w) corresponding to the determined freezing point according to ISI specifications by mean of the following formula.
The percentage added water in the sample of milk is given below.
W = —————————– x (100 – Ts)
W = Percentage of added water by weight.
= The freezing point depressions of the guanine milk (0.530 0 C).
= Observed freezing point depression after adulteration of milk.
Ts. = Percentage of total solids by weight.