Freie und triglycerid-gebundene Hydroxyfettsäuren in Gerste und Malz und ihre Bedeutung für die Geschmacksstabilität des Bieres

Abstract

The sum of different fractions of free and triglyceride-bonded hydroxy fatty acids in barley, green malt and finished malt was characterized using a GC/MS analysis. The fraction of monohydroxy octadecadienoic acids (HOD) could be found in the highest concentration in barley and malt followed by dihydroxy octadecenoic acids (DHOE) and trihydroxy octadecenoic acids (THOE). The amount of free hydroxy fatty acids in the raw materials was always low. Different barley varieties and types from different proveniences gave varying rates of hydroxy fatty acids. Amongst others the formation of such acids is linked to the activity of lipoxygenases that have specific plant physiological functions. An activation of the enzymatic lipid peroxidation is particularly described by biological processes like defence of pathogens, ageing (senescence), and germination. It could be demonstrated that all these processes really have an obvious influence on the hydroxy fatty acids in barley and malt, whereas germination and the following kilning procedure had the most significant effect. Beers which were brewed using stored raw materials gave no poorer flavour qualities and flavour stability than those produced by fresh barley respectively malt. The direct wounding of plants is reported to lead to an activation of the LOX pathway. This could neither be confirmed for already cropped barley kernels nor for the later brewing malt that is milled in the brewhouse. Miscellaneous experiments concerning the chemical stability of purified fractions of hydroxy fatty acids indicated that the HOD in comparison to DHOE and THOE are comparatively pH-stable but more temperature sensitive. Especially the pH and temperature ranges that are of importance in the brewery influence the amount of hydroxy fatty acids significantly. This was additionally confirmed by experiments regarding different mashing-in temperatures in the brewhouse. An improvement of beer flavour stability by the usage of so called low-LOX-barleys could not be proven and should be evaluated critically due to the physiological functions of lipoxygenases (e. g. defence against pathogens). The nonenal-potential which is reported to be directly linked to lipid peroxidation had a strong dependence on pH level and sample preparation. A correlation of the nonenal potential and other simple analytical methods (electron spin resonance spectroscopy, chemiluminescence, absorption integral) to beer flavour stability could not be found. Furthermore it did not suceed to predict beer flavour stability based on the measurement of the sum of separate fractions of free and/or triglyceride-bonded hydroxy fatty acids.