High sugar content of extracts interferes with colorimetric determination of amino acids and free proline

Sugars were found to interfere with the determinations of alpha-amino nitrogen and free proline with ninhydrin. Suitable modifications are proposed for the quantitative analysis of these compounds in the presence of large amounts of carbohydrates. In this new method of proline assay, the improvement consists of using a citrate buffer adjusted to pH 4.6. Free proline is assayed by ninhydrin reagent prepared without phosphoric acid. This latter procedure uses less ninhydrin and appears to be simplified in comparison with the commonly used method. Both procedures permit a simple, sensitive, and specific determination of nitrogenous compounds in crude extracts. Elimination of carbohydrate interference makes our procedure applicable to crude extracts from various food products such as fruits and plants that accumulate soluble carbohydrates.