Vitamin methods

Abstract

The agreement between the results of chemical titration and biological assay has again been confirmed in tests on many fruits and vegetables including fresh and stored potatoes, fresh and canned sieved black currants, cabbage "cooking water" and dried cabbage. Detailed directions are given for the exacting procedure with emphasis on representative sampling, complete extraction, inactivation of oxidases and prevention of oxidation, and rapidity of titration. The alleged increase of ascorbic acid after cooking, supposedly a release of "bound ascorbic acid," could not be confirmed. Faulty technique is held responsible for such an apparent rise, including failure to protect against the action of oxidases, or incomplete extraction from raw fibrous foods as compared with the cooked products. The methods measure the total antiscorbutic activity; any existing combinations are broken down by the extraction process. Interfering substances were not found in measurable quantities in any of the materials. Large amounts of SO2 used as preservative, or abnormal contamination with tin, were both accurately differentiated from ascorbic acid. The quantity of dehydroascorbic acid normally found was too small to have practical significance; contrary results from a few workers indicate failure to guard against oxidation. With the specified precautions, direct titration of the acid extract against 2.6-dichlorophenolindophenol gives the total antiscorbutic activity of plant materials. Tables show the close correspondence between the chemical and biological values.