During the past decade numerous experiments have been conducted with a view to determining the relative nutritive values of a large number of individual proteins. The proteins studied were in the greater number of instances those which are the easiest to prepare in quantity in a state of purity from substances of animal or vegetable origin. The most prominent investigators who have pursued this line of inquiry are Osborne of the Connecticut Agricultural Station and Mendel of Yale University.

It has been pointed out in Chapter II (A Biological Method For The Analysis Of A Foodstuff. 17. Mccollum'S Experiments Not Verified By Osborne And Mendel) that these investigators first sought to make comparative studies of isolated proteins by adding them to a mixture of purified carbohydrates, fats, mineral salts and water, and invariably met with failure, for in every instance the animals suffered steady decline. After their failure with the diet composed entirely of purified food substances, every component of which was known, they began a series of studies with what they believed to be essentially the equivalent of a purified food mixture. This involved the inclusion in the diet of about 28 per cent of a product which they called "protein-free milk." On several occasions it was pointed out by McCollum that the properties of this product were such as to seriously impair their results (27). Osborne and Mendel apparently thought that this criticism was not well founded, for they continued to employ "protein-free milk" in almost all of their work during a period of seven years, and without successfully justifying its use. They finally abandoned the use of the product in 1917, at which time they adopted essentially the method of study which had been described by McCollum and Davis in 1915 (28).