332 PRACTICAL PHYSIOLOGY 



has been precipitated. The precipitate is filtered off through an ash- 

 free filter paper, and thoroughly washed with distilled water. The filter 

 paper with the precipitate is removed to a Kjeldahl's combustion flask, 

 and the nitrogen estimated as described on p. 279. The result multiplied 

 by 6-37 gives the total amount of protein contained in the sample of 

 milk. 



A simpler and yet fairly accurate method of estimating protein is by 

 titr'ation after the addition of formaldehyde. To 10 c.c. of milk add 



N 

 ^Q NaOH till neutral to phenolphthalein ; 2 c.c. of neutral 40 per 



N 

 cent, formalin are added and the mixture titrated with y^ NaOH 



to the same neutral tint as before. The number of c.c. of tenth 

 normal soda used, multiplied by 0-17, gives a close approximation to the 

 amount of total proteins. If greater accuracy is required it is advisable 

 to use tenth normal strontium hydroxide instead of sodium. 



(3) The Percentage of Fat. The following method (Adam's) will be 

 found very simple and sufficiently accurate for most purposes : 



Measure 5 c.c. milk and drop it on to a strip of Adam's fat-free porous 

 paper j 1 allow this to dry in the air bath at 60 C., then roll it up and 

 place it in the extractor of Soxhlet's apparatus (see p. 299). The weight 

 of the distilling flask is ascertained before beginning the extraction, and 

 then again after the extraction has been allowed to proceed for about 

 one hour and the ether has been distilled off ; the increase of weight 

 gives the amount of fat in 5 c.c. of milk. Sufficient ether should be 

 used to fill the Soxhlet one and a half times, and it should be made to 

 siphon over at least twelve times. 



A more rapid and yet good method of estimating fat in milk is a 

 slight modification of the Werner-Schmidt method. Place 10 c.c. of 

 well-mixed milk in a Stoke's tube, which is graduated to 50 c.c. Add 

 10 c.c. strong hydrochloric acid (to liberate all fat), close the tube 

 with a cork and heat it in a boiling waterbath for about ten minutes, 

 shaking from time to time. The colour of the mixture should be a 

 dark chocolate brown. Cool now under the tap and when quite cold 

 fill up to the 50 c.c. mark with alcohol-free ether (washing ether with 

 water frees it from alcohol). Insert cork and shake mixture vigorously 

 for one minute. When the ether has again risen to the surface a small 

 layer of undissolved matter will be seen between the ethereal and the 

 acid fractions. Read off the volume of the ether from the middle of 

 this layer to its upper surface. Take of the ethereal solution two 10 c.c. 

 samples and place in weighed porcelain dishes. Evaporate to dryness, 

 then heat to constant weight in a hot- water oven. Percentage of fat is 

 calculated from the mean of the two weighings. Weight of fat multiplied 

 by volume of ether solution and divided by 10 gives fat content in 

 amount of milk originally used. 



(4) The Percentage of Sugar. Folin's method of estimating sugar 

 has been found to give good results (see p. 275). 



In determining the amount of lactose in milk it is convenient to dilute 

 with water as follows : Cow's milk 1 : 4, human milk 1 : 5 or, if only a 

 small amount is available, 1:6. 5 c.c. of Folin's reagent = 40-4 mg. 

 lactose. 



(5) The Percentage of Ash. A weighed quantity of milk is evapor- 



1 The paper can be obtained from any of the dealers. 



