66 MILK PRODUCTS 
adulteration may be detected by the change 
in the refractive power of the serum as described 
on page 42. H. C. Lythgoe, who has investi- 
gated this question, finds that samples adulter- 
ated with butter will give a refraction below 36.0. 
Results may be confirmed by taking the ash 
of the sour serum. A large amount of the 
sample is taken (as the yield of serum is small), 
soured with a pure culture of lactic acid bacillus, 
or with a little sour milk, shaken in a bottle 
until the fat and curd have separated, the serum 
drawn off and the ash of 25 cc. taken. It 
should not be below 0.73%. The homogizing 
of cream without the addition of fat can be 
detected by microscopic examination. 
Formic Acid—Revis and Bolton state that 
glucose containing this may be found in cream 
and give the following method for its detection. 
roo grams are diluted with an equal weight of 
water, 20 c.c. of a 20% solution of phosphoric 
acid added, and 100 c.c. distilled, the end of the 
condenser dipping below the surface of milk of lime 
containing at least 1 gram of calcium hydroxid 
and 2 c.c. of 3% acetic acid, free from formic. 
The distillate is evaporated to dryness, sealed 
in a small tube of hard glass, drawn out at one 
end that dips into a small U-tube containing 
2 c.c. of water, arranged so that none of the water 
can be drawn into the tube, and heated until 
