XXI] MEAT AND MEAT PRODUCTS 277 



iodid and 100 of water. In the presence of a considerable amount of glycogen the 

 latter produces a dark brown color, which is destroyed by heating and reappears 

 on cooling. When starch is present, it may be precipitated by treating the water 

 extract with 2 volumes of glacial acetic acid, filtering and applying the test for 

 glycogen to the filtrate. 



Trowbridge and Francis Method". — Tentative. 



20 PREPARATIO>f OF SOLUTION. 



Weigh out by difference about 25 grams of the finely ground and thoroughly 

 mixed sample. Place in a 400 cc. beaker and mix with 50 cc. of potassium hydroxid 

 solution (1| to 1), free from carbonate. Cover the beaker with a watch glass and 

 digest on the water bath for 2 hours, with occasional stirring. At the end of the 2 

 hours, dilute to approximately 200 cc. with cold water. 



Add to the solution an equal volume of 95% alcohol by volume, cover with a 

 watch glass and set aside for 10-12 hours. Decant the supernatant liquid through a 

 folded 18.5 cm. filter, allowing the glycogen to remain in the beaker and wash by 

 decantation with 66% alcohol (2 volumes of 95% alcohol to 1 of water) until the 

 glycogen is white, or nearly so. Usually about 4 washings are required. Transfer 

 the washed precipitate from the beaker to the filter and wash 2-3 times with 66% 

 alcohol. The solution filters slowly and the funnel should be covered with a watch 

 glass to prevent excessive evaporation. The albuminous substance present retards 

 the filtration if permitted to dry on the paper. If the washing by decantation is not 

 made nearly complete, it will be difficult to obtain the glycogen free from the coloring 

 matter. 



After the washing is completed, close the bottom of the funnel by a piece of rub- 

 ber tubing and a pinch-cock. Fill the funnel with warm water, cover with the watch 

 glass and let stand 2-3 hours, or overnight. Open the pinch-cock and allow all 

 of the solution to pass through the filter into a beaker. Close the funnel with the 

 pinch-cock and fill with warm water as before. Allow this water to remain in the 

 funnel for an hour and then filter as before. At first the glycogen solution appears 

 quite turbid. This washing with warm water should be continued until the filtrate 

 becomes perfectly clear. To the solution of glycogen in water, add double its 

 volume of 95% alcohol by volume and let stand overnight to complete the repre- 

 cipitation of the glycogen. Filter and wash as before with 66% alcohol. 



21 DETERMINATION. 



If desired, the last filtration may be made through a tared Gooch crucible and the 

 weight of glycogen determined after drying to constant weight. This gives results 

 that are approximately correct. More satisfactory results are obtained by hydrolyz- 

 ing the glycogen with dilute hydrochloric acid and determining the resultant dex- 

 trose. Dissolve the glycogen on the filter in warm water as directed above, collect- 

 ing the filtrate and washings in a 300 cc. graduated flask and keeping the volume 

 within 225 cc. Add 12.5 cc. of hydrochloric acid (sp. gr. 1.19) to the combined fil- 

 trate and washings, mix and place in a boiling water bath for 3 hours. Cool, neu- 

 tralize with sodium hydroxid solution, cool again, make up to volume with water 

 and determine dextrose in an aliquot of the solution as directed under VIII, 54, 

 determining the reduced copper as directed under VIII, 29. Multiply the cor- 

 responding weight of dextro'se by 0.9 to obtain its equivalent of glycogen and correct 

 this result for dilution to obtain the per cent of glycogen in the sample. 



