The digestive action of the stomach results in the formation of propeptones and peptones from the albuminates. The best test for the presence of propeptone is the addition of an equal part of a saturated solution of sodium chloride to a small quantity of the filtrate. Propeptone then, if present, is precipitated, and the solution becomes the more turbid the greater the quantity of propeptone. In case no precipitate is formed, add a drop or two of acetic acid, then the solution will turn turbid in case propeptone is present. If heated the solution clears up again, and when allowed to cool the propeptone precipitates anew, and the solution again turns turbid.

6. Peptone

A few cubic centimetres of the filtrate (best after having precipitated the propeptone and filtered) are made strongly alkaline by the addition of some sodium-hydrate solution and a few drops of a weak (one-percent) sulphate-of-copper solution added. The presence of peptone gives rise to a purplish or violet-red color (biuret reaction).

7. Pepsin

A thin disc (1 cm. in diameter and about 1 mm. thick) of the white of a hard-boiled egg is put into a test tube containing 5 c.c. of the filtrate and kept at blood temperature. If hydrochloric acid is not present in the filtrate, it is necessary to add two drops of the dilute muriatic acid. The presence of pepsin effects a disintegration or a disappearance of the egg disc in two to six hours.

8. Rennet Ferment

Take about 5 c.c. of milk in a test tube and add three to four drops of the filtrate. Mix thoroughly and keep the tube in a glass of warm water. In about ten to fifteen minutes the milk becomes curdled. In case coagulation does not occur in an hour or two, then no rennet ferment is present, although rennet zymogen may exist. To test for the latter, it is necessary to add to the same specimen of milk a few drops of a one-per-cent chloride-of-calcium solution, and again allow it to stand a few minutes. If the milk remains uncurdled even then, there was no rennet zymogen present, otherwise the coagulation would have taken place.