Beer flavor stability is the result of the whole brewing process. One of the main initiators of flavor deterioration is the transition metal-catalyzed formation of reactive oxygen species.
Therefore, the levels of iron (Fe), copper (Cu), and manganese (Mn) should be kept as low as possible in wort and beer. Malt is the main source of metal ions, making mashing the critical step for metal uptake. At the same time, only limited amounts of the total metal content in malt ends up in the sweet wort after lautering, indicating that there are specific mechanisms determining the metal levels. The main purpose of this thesis was to evaluate the impact of specialty malts on the metal uptake during mashing and lautering and to use the inherent
differences in wort composition to evaluate said mechanisms.
During this project, experimental malts were produced from Pilsner malt (roasted malts), raw barley (caramel malts) and tritordeum, a hybrid of durum wheat and barley. Increasing temperatures during kilning and roasting increased sweet wort levels of Fe, Mn, and Zn, while decreasing Cu. This can partially be attributed to the drop in pH as well as the change in wort composition. Both spent grains and wort have a strong binding affinity for metal ions, with sweet wort levels being the result of an equilibrium formed between the two systems during mashing and lautering. Tritordeum malts show promising suitability as an alternative to barley
malt, yielding a similar final alcohol content. However, they gave higher Fe levels in worts, retaining it throughout the brewing process.
Cu(II) binding affinity of wort, investigated using ion-selective electrodes, decreased with increasing temperature treatment, and was significantly higher for caramel malts than for roasted malts. Electron paramagnetic resonance spectra indicated dipeptides as the main chelators of Cu(II) in wort. A positive correlation of total Cu and free amino nitrogen was demonstrated.
Cu has predominantly antioxidative effects in dark worts with inherent high rates of radical formation, while exhibiting limited pro-oxidative effects in pale worts with low oxidation rates. High levels of Fe(II) during mashing did not affect the wort oxidative stability if the concentrations after lautering remained the same.
In complementary research, it was demonstrated that monitoring the amount of active amylases during mashing can be used to optimize the time-temperature program, leading to significant reductions in mashing times. The applied novel method may allow brewers to better control the process when complex malt compositions including traditionally enzyme-deficient specialty malts are used.