Strategies to Reduce the Iron Intake During the Brewing Process with respect to Flavour Stability

Flavour stability represents a key aspect of beer quality, but remains a major challenge for brewing science and the brewing industry. The gradual deterioration of the organoleptic qualities of beer during ageing significantly diminishes its value in several respects, including market appeal, palatability and overall drinkability. While beer is inherently unstable due to its complex composition, strategic measures can be implemented to enhance the initial freshness of the product and slow the rate of flavour deterioration. The main objective of this thesis was to address this issue by specifically targeting the detrimental effects of transition metals (iron, copper and manganese), which act as catalysts for the generation of damaging radicals through Fenton and Haber-Weiss reactions. By sequestering and subsequently removing these metal ions using chelating agents, their deleterious effects can be avoided from the mashing stage onwards. A total of nineteen chelating agents were investigated for their ability to form filterable complexes with transition metals in brewing relevant setups. These included EDTA, citric acid, tartaric acid, quercetin, chlorogenic acid, ferulic acid, gallic acid, phytic acid, tannic acid, as well as extracts derived from green tea, pomegranate, grapeseed, reishi, cinnamon, curcuma, milk thistle, ginkgo, grapefruit seed and raspberry.

In the initial model study, it was revealed that a typical wort pH of 5.60 provided a more favourable environment for the removal of transition metals through complex formation than a beer pH of 4.30. Of the first nine chelators listed, tannic acid (a high molecular weight polyphenol) showed the greatest efficacy. At wort pH, it significantly removed iron and copper from solution after 0.2 µm microfiltration (-94.0 % Feᴵᴵ, -96.8 % Feᴵᴵᴵ, -98.3 % Cuᴵᴵ) compared to chelator-free controls. Other chelators also exhibited noticeable effects at pH 5.60, albeit to a lesser extent, namely quercetin (-34.6 % Feᴵᴵ, -96.2 % Feᴵᴵᴵ), gallic acid (-72.6 % Feᴵᴵᴵ, -39.9 % Cuᴵᴵ), chlorogenic acid (-90.2 % Feᴵᴵᴵ) and ferulic acid (-9.4 % Feᴵᴵ, -11.6 % Feᴵᴵᴵ). Phytic acid demonstrated an undesirable property in chelating zinc (- 70.3 % Znᴵᴵ). Despite forming complexes, EDTA, citric acid and tartaric acid did not lead to reductions in metal concentrations, as the chelates were able to pass through the microfilter.

A subsequent study, evaluating the performance of all nineteen chelators in wort during laboratory-scale mashing, reaffirmed the significant impact of pH on chelator efficacy and wort metal load. Acidified mashing led to worts with higher levels of iron, manganese and zinc; with increases of 230 %, 320 % and 150 %, respectively, when the mash pH was lowered from 6.0 to 5.0. The effectiveness of tannic acid, as observed in the wort buffer solution, was confirmed in wort for iron, but not for copper. While none of the other chelators from the initial study retained their ability to substantially reduce transition metal levels in lautered wort, two novel chelating agents, green tea and pomegranate extract, were discovered among the ten alternative extracts tested in this study. In particular, pomegranate extract (containing 90 % ellagic acid) exhibited remarkable capability in decreasing iron content and suppressing radical formation in the wort. With a mash addition of 60 mg/L, it achieved approximately 80 % reductions in both variables compared to chelator-free controls.

The final study investigated the effects of incorporating tannic acid, green tea and pomegranate extract into pilot scale mashes and found that all three additives were effective in reducing iron concentrations during brewing. Among the twelve trials conducted, additions of pomegranate extract again demonstrated the highest efficacy, resulting in a reduction of almost 90 % in iron level and 80 % in radical concentration in the final wort. On average, the inclusion of these chelators led to 40-60 % reduction in total post-boil aldehydes. Overall, the findings suggest that natural chelators have the potential to improve beer quality and flavour stability by reducing radical formation during brewing and lowering the concentration of transition metals and aldehydes in the final product.