Effect of dry hopping on the oxidative stability of beer.

Electron spin resonance (ESR) spectroscopy was used todeterminethe effect of dry hopping on the oxidative stability and antioxidative potential of beer.Commercial beerwasdry-hopped at 5 °C and 20 °Cwith six hop varieties (Polish and American). The rate of radical formation and lag time were found to depend on the variety of hop used. An increase in the lag time and a decrease in the rate of radical formation occurred when dry-hopping was performed at 20 °C for all hop varieties (at 5 °C in some varieties). The lag time had a strong correlation with the TPC (total polyphenols content) in beer. The rate of radical formation was correlated with the iron content of the beer. A decrease in iron concentration was observed after dry-hopping at 20 °C. Overall, the evaluation of free radical formation using ESR is useful for predicting oxidative changes in beer during storage.

Determination of optimal sample preparation for aldehyde extraction from pale malts and their quantification via headspace solid-phase microextraction followed by gas chromatography and mass spectrometry.

Aldehydes originating from malt play an important role in beer flavour deterioration. In order to better understand the influence of malting process on beer staling, it is necessary to acquire a reliable analytical methodology for determination of beer staling aldehydes in malt. Therefore, the aim of this study was to evaluate extraction parameters, which allow quantification of beer staling aldehydes present in pale malts. The method was validated with respect to linearity (R > 0.9988), limit of detection (0.28 - 0.99 µg/L), limit of quantification (0.92 - 3.31 µg/L), accuracy (± 5%), repeatability (1.3 - 5.3%) and intermediate precision (>20%). The following parameters of sample preparation were evaluated: sample amount, extraction time and temperature, ultrasonication time and oxygen level. Consequently, the best extraction conditions were successfully applied on pale malts. After extraction, the samples were analysed by headspace solid-phase microextraction (HS-SPME) with on fibre carbonyl derivatisation followed by gas chromatography and mass spectrometry (GC-MS). In addition, the salting-out effect during HS-SPME was studied. The method application allowed to identify significant differences (p ≤ 0.05) in the levels of aldehydes among various industrial scale, pale malts. The optimised method could give the information on the aldehyde content introduced into the brewing process and its potential contribution to the overall beer quality.