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Hydroxy fatty acids as indicators for ageing and the influence of oxygen in the brewhouse on the flavour stability of beer
Wackerbauer, K., Meyna, S. und Marre, S.

The concentration of oxygenated fatty acids, especially of trihydroxy fatty acids, increase noticeable during the storage of the brewing raw materials barley and malt. Measurement of these lipid oxidation products could therefore contribute to the detection of barley and malt freshness and could additionally be a helpful tool for an assessment of storage conditions. By different pilot plant brews the influence of oxygen in the brewhouse on lipid oxidation and flavour stability was measured: A noticeable connection to brewhouse conditions was found, but in some cases this effect was overlaid by yeast and fermentation influences. Although there are a lot of publications about beer flavour stability with special regard to lipid oxidation respectively degradation, the real background of beer staling is not revealed yet. That is why terms like "never ending nonenal story" are common and it is also possible to introduce a new corresponding expression called "hydroxy fatty acid story". The occurrence of lipid oxidation by enzymes and radical reactions in barley and malt and their possible relevance for beer flavour is known since the 1960s (1). Special products of such lipid oxidative reactions are hydroxy fatty acids which are detectable by GC/MS-done for the first time by Eglinton and Hunneman (2). It were Drost et al. who began the "hydroxy fatty acid story" by firstly assuming a connection between Burger's, Glenister's and Becker's term "cardboard flavour" and such acids at an EBC Congress 32 years ago (3, 4). Since that time a lot of papers about these oxygenated linoleic acids in all states of beer production were published, but in all cases the research was focussed on free hydroxy fatty acids. Recently Tressl et al. and further on our institute presented another group of such possible stale flavour precursor-the triglyceride-bonded hydroxy fatty acids in barley and malt-and confirmed the results of Holtman et al. who found that lipoxygenase can oxidise esterified storage lipids in germinating barley (5-8). The identification of such triglyceride-bonded oxygenated linoleic acids started a new chapter in the "hydroxy fatty acid story". However, newest research results indicate that furthermore there may be a third group of hydroxy fatty acids in barley and malt and probably - because of their high polarity-in finished beer, too: phospholipid-bonded ones (5). Maybe this group of possible stale flavour precursor opens a new door in flavour stability research on the base of lipid oxidation. The paper consists of two main parts: The first section gives information about the purposes the measurement of hydroxy fatty acids can be used for in general, the second one deals with the role of oxygen in the brewhouse with special regard to lipid oxidation and beer flavour stability. ...

Descriptors: Lipidoxidation, Hydroxyfettsuren, Geschmacksstabilitt

Monatsschrift fr Brauwissenschaft 56, Nr. 9/10, S. 174-178, 2003