148 LOUIS PASTEUR 



by microscopic examinations, the contamination of 

 beers by foreign micro-organisms could be largely 

 avoided. In this and in many other ways Pasteur 

 was able to control the process of beer making so 

 as to improve the flavor and keeping qualities of 

 the product, and his methods came to be widely 

 adopted in French breweries. 



Wishing to extend his knowledge of beer making 

 he paid a visit in 1871 to the great breweries of 

 London. In one of the largest of these in which 

 he was being shown about he requested a sample 

 of the porter then being made and examined it with 

 a microscope. Finding in it a considerable amount 

 of foreign ferments which he pointed out to the 

 managers ; he remarked that the product must be 

 considerably inferior to what was desired and 

 would probably bring complaint from some of its 

 purchasers. The managers, surprised at this criti- 

 cism, admitted that they had just decided to re- 

 place their yeast by a new supply. A visit to the 

 same brewery a week later showed that they had 

 installed a microscope and were beginning to test 

 with this instrument the purity of their supplies of 

 yeast. On his return to France he inspected many 

 breweries and initiated their managers into methods 

 by which they might improve their products and 



