308 F'lxiitcet: 7v.il/. Ihi ma r< sij : 



On the otJief liuiid, a cifaiu of k^ss fat cuiitont may, if acid, l)e- 

 apparently rich, and How with difficulty. 



In order to study the effect on viscosity of rise in acidity uiicom- 

 pli(;ated liv other disturbing factors, a (|uantity of cream, oi)tained 

 by sei^aration from fresh milk, was gradually soured artificially, 

 and its conse(}U(Uit increase in viscosity at a constant tcniperature- 

 experimentally determined. 



In the majority of instances, the cream employed was obtained 

 by immediately machine-separating milk fresh from one i^ajticular 

 cow (new milk). Several experiments, liowever, were niade with 

 cream separated from milk as ordinarily supplied to the consumer 

 in the city. These latter experiments are duly noted in the tables 

 as having been made on commercial mill\. The means employed to 

 acidify the cream were as follow : — 



(1) The addition to the sample of cream of minute quantities of 

 pure lactic acid. 



(2) The introduction of a small amount of a pure lactic culture 

 to the cream, which was stdjsoquently maintained for a prolonged 

 period at a temperature of ,'52 deg. C, readings of its viscosity 

 being taken at short intervals. The first method presents consider- 

 able difficulty, as local clotting is apt to occur upon the additicui 

 of the pure acid. Addition of dilute^ acid was attempted, but was 

 abandoned owing to the diluting effect, with the consj(juent 

 hydrolysis of the calcium cascinogenate of the cream. 



Ultimately the following procedure was adopted :— A given 

 (plant ity of cream, of ]ircviously determined fat content, was 

 i-apiiliy stiri'cd with a glass rod just moistened with pure lactic 

 acid. TIk; additicuis thus rlTecti-d were necessarily somrwliat hap- 

 hazard, and iicnci' wvw subscipicnl ly estimated l)y titration of a 

 sample of the acidilied cream with X/I(l alkali, using pheiiolph- 

 tlialein as indicator. 



The viscosity detenninations were madi' with an Ostwald viscosi- 

 inctcr, surroumled l)y a wati'r-jacl\ct kt_'i)i at a temperature of 

 25 deg. ('. throughout the experiment, this temperature being the 

 lowist that coiihl be maintained appro\iniately constant in the 

 lalioiatoiy during the smnmer. 



Preliminary experiments soon showed tiiat. whilst ii]i to a (■ertain 

 point gra(hiai small additi(ms of acid itroduce very slight increases- 

 in tJK' viscosity, Iher.' is an acid ily-vahu', at which the viscosity 

 of the ci"eani. as measured by tiie tinn" iiilerval rc(piired for tlie 

 l)idb of the viscosinieter to enijity, lajiidly lises. This aiidity is 

 approxinudely e(|uivalent to that at wliieh cream may be considered 

 rii>e ft)r chui"ning. 



