March, 1922. 



SCIENTIFIC AGRICULTURE 



215 



work are to be interpreted in a suggestive 

 sense. The difficulties under which our 

 researches are being conducted will be ap- 

 preciated. While engaged in field work 

 over protracted periods of time, it is not 

 an easy matter to insure that the temper- 

 atures of the incubators shall be exactly 

 uniform from day to day. Even so, dur- 

 ing the whole of our recorded work, we 

 find that the widest limits of variation 

 with respect to the 'blood heat' incubator 

 are between 35.5 deg. C. and 41 deg. C. 

 Taking into consideration all the essential 

 qualifications, it can be seen from our 

 data, that the 'period of usability' of milk 

 — and consequently the rate of spoilage of 

 milk — is a variable entity. The 'period 

 of usability' of milk has come to an end 

 before the reaction of clotting is con- 

 summated. But the phenomenon of 

 Clotting is one which can be readily 

 and easily observed ; and the ratio 

 of 'time occupied in clotting' to 'actual 

 period of usability' can be established when 

 researches have been pursued for a suffi- 

 ciently long time under both standard and 

 va:ying conditions. Milk which clots in 

 48 hours at a temperature of 98 deg. F. 

 has a much longer 'period of usability' 

 than milk which clots in 9 hours at the 

 same temperature. For the present, there- 

 fore, it is to be desired that such proce- 

 dures in methods of production and man- 

 agement shall be instituted, as will insure 

 at least that all milks shall attain to our 

 tentatively recorded 'maximum period of 

 usability. ' 



The Fermentations of Milk. 



Responsible for the changes which take 

 place in milk leading to spoilage oi^'limita- 

 tion of period of usability are many types 

 and varieties of bac*^eria. The most com- 

 mon are organisms of the Streptococcus 

 lacticus (*) group (Kruse) (6) and the 

 ovgani.sms of the colon-aerogenes group 

 (Escherich. (16) (17) (18). That initial 

 contamination, age of milk, and temper- 

 ature, at which the milk may be kept are 



* The almost invaluable functions per- 

 formed by the .strains of this type in the 

 manufacturing and ripening of cheese are 

 not in the slightest sense called in ques- 

 tion. 



conditions which affect the balani-e of flora 

 is a dictum well supported 1)3' the liter- 

 ature. A temperature approximating to 

 3714 deg. C. encourages the prolification 

 of the bacteria of the colon-aerogenes group, 

 while a temperature of 21 deg. C. more 

 leadily provides for the balance of the 

 flora being in favour of the Streptococcus 

 lacticus types. It is to be expected, there- 

 fore, that the prevailing temperature at 

 which milk is held subsequent to produc- 

 tion, will, by reason of the above, exercise 

 a decided influence as to the type of 'clot' 

 formed on incubation. At the same time 

 it must not be overlooked that the associa- 

 tive action of bacteria as pointed out by 

 Marshall (13) is a factor which requires 

 to be con.sidered in any discussioil on tlhe 

 oiganisms responsible for the 'spoilintr of 

 milk.' 



The Dissolving Fermentation. 



In the course of our work up to the pre- 

 sent, we have not attempted to determine 

 the types of organisms and relative fre- 

 quency of the same in the milks after the 

 clots have formed; but the prevalence of 

 the type of fermentation which we have 

 designated 'dissolving' challenged investig- 

 ation. As changes in the milks submitted 

 to the 'clotting' tests proceed, the partial 

 or complete disappearance of the clot is 

 evidenced in a high percentage of the 

 samples under investigation. Usually 'dis- 

 solving' takes place concurrently with, or 

 sub.sequent to coag'ulation ; but in some 

 cases the milks dissolve without previous 

 clotting. That the activities of bacteria 

 may result in the decomposition and dis- 

 solving of the casein of milk has been long 

 a recognized prenomenon. Many referen- 

 ces to the liquefaction of the clot in milk 

 are to be found in the literature, and the 

 reaction is used as one of the bases for the 

 identification and classification of bac- 

 terial tj'pes. Dyar, (19) Conn, Esten & 

 Stocking, (6) Rogers & Davis, (20) Loh- 

 nis, (21) Heineman, (22) Winslow, Roth- 

 berg & Parsons, (23) Degraaf & Schaap, 

 (24) Barthel & Sandberg, (25) Rogers, 

 (26) and others have recorded their re- 

 searches into this aspect of the fermenta- 

 tions of milk. 



The frequency Avitli which the 'dissolv- 

 ing' of the clot takes place, and the fact 

 that the reaction is common to the milks 



