Model studies on the precursors and formation of the metallic smelting (E,Z)-2,6-nonadienol during the manufacture and storage of buttermilk

Application of Aroma Extract Dilution Analysis to the volatiles generated from the degradation of 9-hydroperoxy-(E,Z,Z)-10,12,15-octadecatrienoic acid under aqueous acidic conditions at room temperature, and in the presence of butter starter cultures, showed that the metallic-smelling (E,Z)-2,6-nonadienol (NDOH) was the most odour-active compound. Quantitative studies on its generation during storage of sweet-cream buttermilk which had been spiked with several potential precursors, suggested the following reaction pathway for the formation of NDOH during the manufacture of buttermilk: during fermentation of the sweet-cream, the α-linolenic acid present in the neutral triacylglycerides is peroxidized by lipoxygenases of the starter cultures into glyceryl 9-hydroperoxy-(E,Z,Z)-10,12,15-octadecatrienoate. During separation of the butterfat, the polar peroxide is transferred into the buttermilk and is subsequently cleaved into (E,Z)-2,6-nonadienal under the acidic conditions in the buttermilk matrix. Reductases of the starter cultures, which are still active during storage, finally produce (E,Z)-2,6-nonadienol and the attendant metallic off-flavour.