A colorimetric ninhydrin assay, reportedly specific for asparagine, ha
s been published (Sheng et al. (1993). Anal. Biochem., 211, 242-9). Th
e suitability of this assay for determining asparagine in food samples
was tested by using it to measure the asparagine levels in tips of ha
rvested asparagus spears. Compared with our standard chromatographic p
rocedure (x), the colorimetric assay (y) severely overestimated aspara
gine in asparagus (y = 129 . 6 + 1 . 12x, S-y/x = 32 . 2) even though
the results from the two methods were highly correlated (r = 0 . 997,
P = 0 . 003). By testing other amino acids in the assay, we showed tha
t they also react with ninhydrin under the conditions of Sheng et al.
In particular, proline gave a response two-fold greater than asparagin
e and the response with phenylalanine was half that of asparagine. It
is believed that the overestimation of asparagine by the ninhydrin met
hod is due to the additive response of the other amino acids present i
n asparagus. It is concluded that the assay is unsuitable for the rout
ine determination of asparagine in asparagus, or in complex mixtures o
f amino acids in other foods.