INDUSTRIES DEPENDING ON PLANT PRODUCTS 



1681 



Aldehyde is formed in the control specimens as in those containing 

 sulphur dioxide, but in much smaller quantities. 



The sulphur dioxide is recovered at the end of the experiment \\ith 

 but slight loss, 10 nigms. out of 200, and 15 out of 405. This proves that 

 as fast as the sulphur dioxide is set free from the sugar bj- the latter's 

 breaking up, it enters into combination with the aldehyde and is thus re- 

 tained; whereas, if an}- considerable piopcrtion remained free it would 

 be carried off by the carbon dioxide and much would be lost. 



The fermentation of the must is sometimes seen to take place in the 

 presence of free sulphur dioxide, even when a test portion taken from 

 the top of the cask contains a fairly high percentage of sulphurous acid. 

 To explain this, it is suggested that in the lees at the bottom of the cask 

 there is a layer where the free sulphur dioxide combines with the aldehyde 

 produced by the incompleteh' sterilised yeast. Here fermentation begins, 

 at first but slight, and then with greater and greater evolution of carbo- 

 nic acid gas according as the layer in which the sulphur dioxide enters 

 into combination, grows irK extent'. It has been possible to prove this by 

 experiment. 



III. — The influence on fermentation of sulphurous acid 

 WHEN chemically COMBINED. — Changes in the rate of fermentation. — 

 The results of experiments undertaken to elucidate this question are shewn 

 in Table 1\. 



Table IV. — The influence on fermentation of sulphurous acid 

 when chemicallv combined. 



The antiseptic influence of combined sulphurous acid on wine yeast 

 is thus shewn to be perceptible in certain conditions; nevertheless, when 

 the proportion does not exceed 200 mgms. per litre, which is the maximum 

 for musts purified in the ordinary way, the rate of fermentation is but 

 slightly decreased, unless the temperature sinks to below the normal. 



Changes in the selective affinity of yeast. — Normally, yeasts which 

 cause grape sugar to ferment shew a marked preference for glucose. But 



