3 



as determined by measurement of the gas evolved on fermenting 

 the flour with yeast. 



In testing flour in the laboratory one takes twenty grammes of 

 flour and mixes with twenty cubic centimetres of water in which is 

 suspended half a gramme of fresh yeast. These are thoroughly 

 mixed together in a small conical flask or wide-mouthed bottle until 

 a uniform dough is produced. The flask is furnished with a well- 

 fitting cork and delivery tube. By means of a water-bath the flask 

 is maintained at a temperature of 35 C.; this is the optimum 

 temperature for the fermentation by yeast. The gas evolved as 

 fermentation proceeds is collected over water in a graduated cylinder. 

 If readings of the volume of gas are now taken at regular intervals, 

 we get the data necessary to plot such a curve as shown in Figure 1. 

 To get over the difficulty introduced by variations in the fermenta- 

 tive power of different specimens of yeast, it is desirable to make a 

 number of tests at the same time with different flours. The best 

 plan is then to weigh out some yeast and mix it with sufficient 

 water to give a mixture containing half a gramme of yeast in 

 twenty cubic centimetres; the necessary quantity of this mixture 

 can then be added to each weighed portion of flour by means of a 

 pipette. Figure 1 shows the results obtained with three varieties of 

 imported flour. It is interesting to note how exactly the curves 

 correspond to the merchant's description of the samples. The Rou- 

 manian wheat was reported as being a very strong wheat. Of the 

 two English wheats one was stated to have moderate strength while 

 the other was considered as weak. It will be seen that the quantity 

 of gas evolved is in accordance with the merchant's description. 



In examining a number of Egyptian wheats it was soon found 

 that owing to the variation in the quality of the yeast and the diffi- 

 culty of obtaining yeast in a perfectly fresh state, the time taken for 

 the fermentation to come to an end was sometimes unduly prolonged, 

 and indeed occasionally fermentation did not commence until several 

 hours after the mixing of the dough. Under these circumstances 

 it seemed desirable to set up some automatic device which would 

 secure a time record of the evolution of the gas. In the apparatus 

 employed the gas, as evolved from the dough, was caused to displace 

 an equal volume of water from a second flask. This water, as it fell 

 from the delivery tube of the flask, was caught in a small tipping spoon. 



