Evaluation of a rapid method for estimating free amino acids in silages

A modified ninhydrin colorimetric assay was evaluated for quantification of free amino acids in silage extracts. The assay gave a similar response with the range of amino acids tested (excluding imino acids) with responses relative to leucine, on a molar basis, varying from 90 to 109%. Amines, short peptides and ammonia also gave a reaction to the ninhydrin reagent. A range of experimental silages were analysed for free amino acid content by both the ninhydrin assay and by ion-exchange chromatography (IEC). The correlation between results obtained with these methods was good (r2=0.95) with slope values close to 1 (i.e. 0.97 and 1.04). Protein degradation during ensilage of birdsfoot trefoil was monitored using the ninhydrin assay. Silages were either untreated, inoculated with Lactobacillus plantarum or inoculated with L. plantarum with added sugar. Untreated silages and silages treated with L. plantarum alone showed a greater release of free amino acids compared with silages treated with L. plantarum and added sugar, and these differences appeared to be related to the rate and extent of decline in silage pH. We propose that this technique has application for routine assessment of the extent of protein degradation in silage with a high degree of accuracy, and has the advantage of using small sample volumes and minimising the quantities of reagent required.