1920.] 



Pink Discoloration of Cheese. 



521 



tint just under the coat of the cheese. It only penetrated to a 

 depth of about 0.5 mm., so that successive thin layers of tinted 

 portions could be sliced off. leaving a white surface, which in 

 turn became coloured after exposure for a few hours. 



Examination of the Cheese. — Examination made of small 

 portions of the coloured cheese showed that large numbers of 

 yeast cells of various shapes, as well as bacteria, were present. 

 Portions of the cheese, both coloured and uncoloured. were thru 

 subjected to gelatine and agar, and whey agar tests. Tt was 

 found that colonies of the usual cheese bacteria of the lactic- 

 acid group, and others, appeared, as well as colonies of yeasts 

 and torula. The yeast was isolated, and grown in liquids and on 

 solid media, in all of which it grew and produced colours of 

 various shades of rose, pink and salmon, the colour deepening 

 as the culture became older. 



Trials with the yeast were also made on three small cheeses, 

 weighing from 1-2 lb. each. One cheese (No. 1) was made 

 from milk which was inoculated just before renneting with 

 ground -up portions of the original cheese sent from Yorkshire, 

 and the two other cheeses (Nos. 2 and 8) were made from milk 

 which was inoculated with the pure culture of the pink yeast. 



No. 1 cheese was made on the Cheddar system, a lactic acid 

 starter being used, and just before renneting a quantity of the 

 original cheese, grated up and mixed with sterilized whey, was 

 added to the milk. The high scald (102°F.) was used as in 

 Cheddar practice. Cheese No. 2 was made in the same manner, 

 except that, instead of the ground-up cheese, a pure culture of 

 the pink yeast grown on bread was added to the milk beforr 

 renneting. 



The cheese^ all ripened normally, and were examined daily 

 and cut at about 8 weeks old. Tn Xos. 1 and 2 no growth of 

 the yeast or pink coloration was noticed, the cheese being 

 somewhat dry and verv firm in texture. It was concluded that 

 the dryness and texture and the high scald of the curd bad 

 resulted in conditions unsuitable for the growth of the yeast. 



On the coat of No. 3 cheese, which was made with a lower 

 scald, resulting in a softer curd, small spots and patches of 

 pink and violet were observed at a very early stage. These 

 followed the cracks and openings in the coat, and even in 

 M»me places penetrated a short distance into the curd. As the 

 cheese became firmer and dryer, these spots did not spread 

 rapidly, but remained confined to the coat on the clothing of 

 the cheese. 



