- The malt alkaloids N-methyltyramine, N,N-dimethyltyramine
(hordenine), N-methyl-3-aminomethylindole and N,N-dimethyl-3-aminomethylindole
(gramine) are known precursors for N-nitrosamines.
Analytical procedures were developed and applied to extract and
quantify these alkaloids from malt, green malt, malt roots, and
raw barley. A ground malt sample under nitrogen atmosphere was
extracted with 5 percent concentrated ammonium hydroxide in methanol
at room temperature for 24 hours. After purification, the
extracted alkaloids were quantitated by reverse phase high performance
liquid chromatography using both phosphate buffer and
ion-pairing solvent systems. The recoveries obtained for the alkaloids
listed were, respectively, 77.4, 92.0, 74.0 and 80.5 percent
with detection limit of 0.2, 0.2, 0.1 and 0.1 μg/gm. Conformation
of identity of each alkaloid was accomplished by TLC,
UV, and MS.
Twenty-three samples of kilned malts, five samples of green
malts, ten samples of kilned malt roots, five samples of green malt roots, and ten raw barley samples, from a total of 11 varieties
of Hordeum vulgare were analyzed for the alkaloids of interest.
N-Methyltyramine was found in every sample from all
sources with a mean and range, respectively, of 26.8 (15.7-48.5),
21.4 (11.6-37.5), 1959.8 (1358.4-2798.4), 1532.5 (962.3-2091.1)
and 4.7 μg/gm (0.4-17.9 μg/gm) on a dry weight basis. The means
and ranges for hordenine were, respectively, 28.1 (8.9-42.4),
21.2 (9.6-35.5), 4066.4 (2741.2-5180.1), 3363.3 (1956.2-4744.3)
and 0.7 μg/gm (0.5-1.0 μg/gm). Gramine was found in only two
barley malt varieties examined where kilned and green malts contained,
respectively, mean values and ranges of 7.4 (4.9-10.0)
and 6.0 μg/gm (4.7-7.3 μg/gm). N-Methyl-3-aminomethylindole was
not detected in any of the samples examined.
The results of this investigation indicated that the variety
of barley affected alkaloid production during malting. The results
also showed that the kilning step of the malting process increased
the total amount of each alkaloid produced.
Finally, room temperature nitrosation of five varieties of
kilned and green malts, in which the contents of hordenine and
gramine were known, was carried out with an excess of nitrite in
acetic acid at pH 3.2 for 18 hours. The amounts of N-nitrosodimethylamine
(NDMA) obtained from this nitrosation were far greater
than the expected yields for NDMA from the hordenine and gramine
present in the malts. The results strongly suggest that the alkaloids hordenine and gramine were not the only precursors for NDMA and that there must be additional precursors; dimethylamine was
suggested as a possibility.