l6o BACTERIOLOGICAL METHODS 



cidity or, to state it more accurately, the quality of the product 

 dependent upon age is in direct proportion to the quantity of crys- 

 talline clusters present. It would appear that the quantity of 

 crystals present is not proportional to the amount of bacterial 

 contamination and decomposition. The indications are that it is 

 possible to determine the source of the fat from the color, size and 

 arrangement of the fat crystal aggregates. For example, the 

 crystal clusters of lard are smaller than those of the fat of the do- 

 mestic fowl. The fat crystal aggregates of the hen are compara- 

 tively large and the individual crystals are long and slender. The 

 inexperienced analyst is apt to mistake the crystal clusters for 

 mold colonies (Leptothrix) . This mistake can very readily be 

 avoided by applying heat which causes the prompt melting of the 

 fat crystals whereas the mold hyphae are not greatly disturbed or 

 changed. The differential characteristics which would be con- 

 cerned in the microscopical examination of fat crystals may be 

 given as follows: 



1. Differences in the size of the aggregates. 



2. Differences in the length of the individual crystals. 



3. Differences in the diameter of the individual crystals. 



4. Differences in the form of the ends of the individual crystal. Ends may be 

 rounded or pointed. 



5. Differences in color. These will in all probability pertain to different races or 

 families of the animal kingdom. For example, lard crystals are colorless whereas 

 those of the domestic fowl are yellowish. 



The use of certain chemicals will aid in the microscopical find- 

 ings. For example, sulphuric acid produces characteristic color 

 reactions with certain fats. If two drops of concentrated sul- 

 phuric acid are added to twenty drops of goose fat, a greenish -yel- 

 low color is produced which changes to reddish brown on stirring. 

 Under the same conditions cod-liver oil turns a violet color whereas 

 turtle oil turns brown. Castor oil turns yellowish to yellowish 

 brown and finally wine red with a very distinct zone. A similar 

 reaction is observed with neats foot oil. Raw linseed oil turns a 

 deep reddish brown to very dark brown. Lard oil shows a distinct 



