ESTIMATION OF CARBOHYDRATES 



233 



FIG. 45. 



C. ESTIMATION BY FERMENTATION. 



Sugar is sometimes estimated by fermentation with yeast in an Einhorn 

 fermentation tube. This is a U-shaped tube, one limb of which 

 is closed and the other expanded into a bulb (Fig. 45). 



The closed limb of these tubes is filled with the sugar solu- 

 tion to which some yeast has been added. Mercury is placed 

 at the bend. The carbon dioxide evolved collects in the 

 closed limb and drives down the solution into the other and 

 wider limb. The narrow limb is graduated in percentages of 

 glucose, so that the amount of sugar can be directly read off. 



This fermentation method is not very accurate and is con- 

 sequently not often used for estimating sugar. 



Another method of estimation is by taking the specific gravity of the 

 solution before and after fermentation. 



The most accurate results by fermentation are obtained with Lohnstein's 

 apparatus (Fig. 46). 



A U-shaped tube is employed. The straight limb is left 

 open, but the bulb is closed, after filling, with a stopper. The 

 straight limb is narrower at the base than at the upper end, 

 and upon the end rests a wooden scale graduated in percent- 

 ages of glucose ; the graduations on one side are for working 

 at room temperature, on the other side at 37. 



A definite weight of mercury is placed in the bend ; it 

 almost fills the bulb and reaches up the narrow part of the 

 limb to about the zero mark on the scale. Upon the surface 

 of the mercury in the bulb is placed 0*5 c.c. of glucose solution 

 and 1-2 drops of a suspension of yeast in water. The stopper 

 (carefully greased) is inserted. It is perforated with a small 

 hole which coincides with a similar hole at the neck of the 

 bulb. With the two openings together so that air can enter or 

 be displaced the apparatus is tilted so as to put the mercury 

 in the limb at the zero point of the scale. The stopper is 

 then turned to close the apparatus and it is set aside for the 

 fermentation to proceed for 12-24 hours at room temperature 

 or for 3-4 hours at 37. To prevent the stopper being forced 

 out by the pressure of the gas it is covered with a weight. 



The percentage of glucose is read off on the scale when 

 the fermentation has ended and the apparatus has returned 

 to room temperature, if the fermentation took place at 37. 

 Lohnstein shows that a more accurate result is obtained if the 

 readings are taken at 37 and 20 and if the percentage is calculated from an 

 equation. 



The apparatus should be cleansed immediately after use ; the opening on 

 the stopper is slowly made to coincide with the opening of the bulb. The 

 mercury level takes the original position. The liquid is removed with a small 

 pad of cotton wool, and the surface of the mercury washed with a little 

 water, which is drained away by fresh cotton wool. 



FIG. 46. 



