196 



milk as delivered to the London trade is always acid, the acidity 

 being equal on an average to 0.2 per cent of lactic acid. 



Such an amount of acidity usually develops in the milk within 8 hours of its com- 

 ing from the cow. In England, according to the temperature, in from 30 to 40 

 hours after it has reached an aciility of 0.2 per cent its acidity rises to 0.35 or 0.4 

 per cent, at which point it has acquired an acid taste and is said to be sour. Usually 

 at an acidity of 0.6 to 0.7 i)er cent it separates or coagulates. If kept for a long 

 period milk rarely develops a greater quantity of lactic acid than 2 per cent. In 

 some milk, the acidity of which I determined, the highest acidity found was 2.34 

 per cent after 117 days of keeping. The reason of this is that when this amount of 

 acidity is reached the acid formed is destructive to the fungus forming it. Milk that 

 has not yet developed an acidity of 0.3 per cent but is near it, will coagulate on 

 boiling; it is therefore customary in the trade to test the freshness of a milk, if it ia 

 suspected to be stale, by boiling it. 



To study the eftect of different preservatives on milk not sealed up, 

 a large quantity of the same milk was divided into 11 parts. One part 

 was kept in its original state, another part was boiled, and to the other 

 parts carbonate of soda, potash, salicylic acid, borax, boracic acid, 

 and a mixture of equal parts of borax and boracic acid were added. 

 The acidity was determined in all the samples at intervals of about 4 

 hours, the sam])le being tlnnunglily mixe<l each time the acidity was 

 determined. In these expernnents boracic acid proved the best preserv- 

 ative. When this was used at the rate of two parts per thousand the 

 milk ke))t sweet lor 4L' hours longer than milk without any preservative, 

 lioiling milk [iroved as eUieaeious as any preservative nse<l at the rate 

 of one part per thousand. 



His method of detecting borax in milk is given as follows: 



On a porcelain slab jdace one ilrop of the milk with two drops of strong HC'I and 

 two drops of a saturated turmeric tincture. Dry tliis on the water bath, take it off 

 directly it is dry, cool, and add a drop of ammonia by means of a glass rod. A 

 slaty blue color changing to green is prodnce«l [if borax is present]. I found that 

 a drop of milk containing the thousandth of a grain of borax would give the reac- 

 tion; even less than this will give the green tint, but not the blue. The turmeric 

 tincture must bo fresh, otherwise it is l>est to use th<' powdered tnrmeric. 



For rapidly determining the acidity of milk in the trade he recom- 

 mends the use of compressed pellets containing weighed amounts of car- 

 bonate of soda and jihenol i>hthalein. 



In the discussion which followed Dr. P. \'ieth said he was quite sure 

 that if the experiments were repeated during the summer and autumn 

 the results would be very different from those obtained by the atithor 

 from January to ^lay. There was n(» doubt milk soiu'ed miu-h more 

 ([iiickly in summer and autumn, October and November being the most 

 troublesome months. Farmers attributed thisto thedecayingleaves, and 

 this was perhai)s not far from the truth. His own experience was that 

 as soon as an acidity of from 1 to 1.5 per cent was reached fermentation 

 practically ceased, or at any rate procefded with extreme slowness. 

 With reference to the retarding action of boracic acid on lactic fernieu- 

 tatiou, other exi)crimentcrs had not got such good results as ^Ir. Stokes. 



