142 MILK 



solids and plasma solids need not be estimated unless very accu- 

 rate results are desired. 



Fresh milk contains considerable quantities of gas in solution, 

 and therefore should be permitted to stand for at least ten hours 

 from the time of milking before the specific gravity is taken. Dur- 

 ing this time excess of CO 2 has escaped and the specific gravity is 

 nearly constant. It is higher than immediately after milking and 

 continues to rise for about two days, although slightly. This 

 phenomenon is known as the Recknagel phenomenon. The rise 

 of specific gravity during the first two days may also be due in 

 part to a change in the volume of the proteins. The increase is 

 within the limits of 0.0008 and 0.0015. 



The specific gravity may be determined by three methods, 

 namely: 1, a lactometer; 2, a pycnometer, and 3, by means of the 

 Westphal balance. A lactometer is a hydrometer which has 

 been constructed especially for use in milk work. The tempera- 

 ture of the milk should always be determined with the specific 

 gravity, since increase of temperature reduces the specific gravity. 

 The actual reading is reduced to a standard temperature, usually 

 60 F. or 15.6 C. 



There are three styles of lactometer commonly in use: 1, the 

 Quevenne (Fig. 35); 2, the New York Board of Health (Fig. 36), 

 ' and 3, Shaw and Eckles' lactometer (Fig. 37). 



The Quevenne lactometer is probably used to a greater ex- 

 tent than either of the other two. The scale is divided into 

 25 parts, commencing with 15 and ending with 40. These figures 

 represent the second and third decimals. They aVe sufficiently 

 below and above the normal specific gravity of milk to cover all 

 - cases. A thermometer is combined with the Quevenne lactom- 

 eter, so that specific gravity and temperature can be read at the 

 same time. 



The New York Board of Health lactometer has an arbitrary 

 scale, but since the subdivisions of degrees are -smaller than on 

 the Quevenne lactometer, accurate reading is facilitated. The 

 scale is divided into 120 parts, representing the mark to which 

 the lactometer sinks when immersed in water. The 100 mark 

 is equal to 29 on the Quevenne lactometer, which means a specific 

 gravity of 1.029. The upper limit is 120 or 34.8 on the Quevenne 

 lactometer. Milk with a specific gravity higher than 1.0348, 

 such as skimmed milk may be, cannot be measured with the New 

 York Board of Health lactometer. A separate thermometer may 

 be used in connection with this lactometer, although a New York 

 Board of Health lactometer provided with a thermometer can be 

 purchased. 



A lactometer of somewhat different construction has been 



