Crit. Rev. Food Sci. Nutr. – “Postprint” version, i.e. accepted manuscript, for release in HAL
determination of amino acids released by hydrolysis can be performed using different
methods. The reference method is ion-exchange liquid chromatography with ninhydrin post-
column derivatization because it assays all amino acids in the same run and has been
proved to be repeatable. The somewhat low sensitivity of this method is not a practical
problem since the sample size is rarely limiting. But because this method is time-consuming,
other techniques have been applied which are based on amino acid separation by gas
chromatography, RP-HPLC, ULPC and capillary electrophoresis with pre- or post-column
derivatization (Peace and Gilani, 2005). Recently, liquid chromatography coupled to tandem
mass spectrometry (ESI-MS/MS) has demonstrated higher amino acid resolution and
detection than classical methods (Piraud et al., 2005 ; Qu et al., 2002).
Real specific conversion factors have been overlooked and Jones’ factors remain
The conversion factors computed by this method using data from the literature vary
products because of differences in their amino acid composition and because non alpha-
amino nitrogen levels
are higher in plant pr
oduc
ts than in animal products. It is noteworthy
that differences within animal and plant products are more marked than the average
difference between animal and plant products. For a given foodstuff, the conversion factor
may differ according to species and nitrogen content (Huet et al., 1988, Mossé, 1990) and
depends on the purification rate of protein extracts (Mossé, 1990) (Figure 1). However, in a
given specie, reported values are very close. Furthermore, the K' conversion factor for milk is
quite similar (when computed from an amino acid sequence which excludes prosthetic
groups but includes non-protein nitrogen (5.80, Table 1)) to the factor k calculated from
amino acid and nitrogen compositions (5.85, Table 2). When the most recent values are
considered, the factors range from 5.85 to 6.15 for milk products, from 5.38 to 5.74
(averaging 5.6) for meat, fish and eggs, and an average of 5.5 for soybean. For other
legumes (pea, lupine, dry beans), estimates range from 5.24 to 5.64 (average: 5.4). For most
cereal products, the range is between 5.3 and 5.8 (average: 5.4), despite quite considerable
discrepancies in the literature. For most fruits and vegetables, the factor varies as a function
of the product and the authors. It is low (4.3) and could even be considered lower if k
P
is
used, which may be more relevant than k
A
for these low protein sources rich in non-protein
nitrogen (Fujihara et al., 2001, Jones, 1941, Mossé, 1990, Sosulski and Imafidon, 1990).
In practice, these data are rarely used even though they are more relevant than those
applied at present. Indeed, the conversion factors most frequently employed are those of
Jones (Table 3), which were first published in a USDA report in 1931 (slightly revised in
1941) and consist in a compilation of published and unpublished results.