180 EXPERIMENT STATION RECORD. 



The 2>olnt at which ni'dk may he cotisldered sour and the rate of i<our- 

 iiKi in the presence or ahsence of jyreservatives. — Stokes (E. S. R., 3, p. 

 l'.'.")) states that milk which has not reached an acidity of 0.8 per cent 

 of lactic acid (33.3°), or near it, will coagulate on boiling. The 

 authors made a series of experiments, the results confirming almost 

 absolutely the figures of Stokes. They found that milk tastes sour, 

 on an average, when it has an acidity of 45°, although the variations 

 are fairly wide. Fresh milk is stated to have an acidity of 20°; when 

 the acidity reaches 33° the milk curdles on boiling. 



"It is quite certain that the 'acidity' of milk is not wholly due to lactic acid; 



indeed, the 'acidity' of fresh milk is due to the mono- and di-basic phosphates, and 



N 

 not to free acid at all. Seeing that 9.7 cc. ^ lactic acid will curdle milk on boiling, 



while it requires a development of about 13° 'acidity,' it is highly probable that 

 another acid very much weaker than lactic is produced, and we venture to think 

 that carbonic acid is responsible for a portion of the acidity of sour milk; we know 

 that carbonic acid is produced, and we have found that when milk is sufficiently 

 sour to develop gas about half the acidity, as indicated by phenolphthalein, is 

 shown to litmus (to which both milk and carbonic acid are approximately neutral). 



"We have based a hypothesis on the facts that different acids do not give the 

 same result, that salts of polybasic acids are present in milk, that both casein and 

 alV)umin have acidic functions, and that the coagulation of milk at temperatures 

 between 17 and .35° does not appreciably vary with the temi^erature; it appears to 

 us that curdling of milk is due to an amount of acid being present to set up an equi- 

 librium between the acids and bases present, such that certain acids, e. g., casein and 

 albumin, are liberated. At a boiling temperature we are inclined to think that the 

 curdling is determined by the coagulation of the albumin, the equilibrium being 

 destroyed by the removal of one acid (albumin) from solution, and fresh amounts of 

 allHimin, and finally perhaps casein, are liberated. 



"When milk tastes sour, it would appear that the equilibrium is such that a sour- 

 tasting free acid exists in solution; while when milk curdles spontaneously the 

 equilibrium is such that the insoluble acid casein is produced." 



Experiments were made to determine the rate of souring of milk 

 with and without the addition of preservatives (boric acid or formal- 

 dehyde). 



"At high temperatures (say 80° hot summer weather) preservatives are compara- 

 tively useless unless added in relatively large quantities; the minimum quantities 

 used by us, and also by Kideal, only increase the life of milk a few hours, and are 

 equivalent only to a lowering of temperature of about 5° F. Unless milk can be 

 made to keep at least 12 hours longer than without preservatives, we do not think 

 much is gained by their use, and to do this in summer we think that the nunimum 

 amounts are 0.09 per cent boric preservative and 0.004 per cent formaldehyde. 



"We would also draw attention to the increased rate of souring as time goes on, 

 when preservatives are added. This indicates a possible danger in using preserva- 

 tives in milk, as it seems far from improbable that succeeding generations of micro- 

 organisms become in the presence of preservatives more active and more virulent, 

 and if the use of preservatives were universal, there is a probability that they would 

 cease to act. The work of Effront on yeast grown in the presence of sodium fluorid 

 shows that this view is not a mere hypothesis." « 



