Abstract
Despite many years of research, the alteration in beer flavour upon storage remains challenging for the brewing sector. One of the most relevant changes during beer storage is the appearance of staling offflavours. Even a slight increase is perceptible, leading consumers to rate stored beers with elevated offflavour levels lower in both taste and drinkability. This poses a major limitation for beer producers, who want to uphold a consistent flavour profile for their products. Beer flavour instability becomes even more relevant in view of globalisation, as conditions during transportation and distribution, e.g. high temperatures and vibrations, adversely affect flavour stability of beer.
This PhD study aims to provide a comprehensive insight into the behaviour of staling compounds during brewing and beer storage. Particular focus was given to staling aldehydes, including volatile forms and non-volatile adducts with cysteine. A state-of-the-art literature research, covering chemical and technological aspects affecting aldehyde dynamics was conducted. Malt has been identified as the primary source of staling precursors for aldehyde formation and several brewing strategies have been proposed to minimise aldehyde formation during wort and beer production. The past research often focused on lipid oxidation aldehydes, while more recent findings point to the importance of other types such as Strecker and Maillard aldehydes. The concept of bound state aldehydes, particularly at the brewhouse level, remains underexplored. Addressing these gaps formed the core focus of this PhD.
From the experimental point of view, the work was divided into three parts. The first illustrates the dynamics of volatile and cysteine-bound aldehydes monitored on pilot-scale brewing trials from malt to final beer, including storage. The results confirmed malt serving as the primary origin of staling aldehydes entering the brewing process. Both volatile and cysteine-bound aldehydes were detected in the wort samples, with the highest concentrations observed during mashing-in. Majority of the aldehydes were reduced during wort boiling (except for furfural) and fermentation. Despite marginal levels found in the fresh beer, volatile aldehydes evidently appeared in aged beers. While some of the aldehyde markers reached a plateau after one month of forced ageing, furfural continued to creep up with a linear trend. Cysteine-bound aldehydes were not found either in the fresh nor stored beers.
In a following research set up, aldehyde dynamics was monitored in parallel with an extensive set of flavour instability parameters and sensory staleness. After 90 days of forced ageing all volatile aldehydes were detected below their corresponding sensory thresholds, yet, sensory staleness doubled, indicating a synergistic effect of carbonyls in staleness perception. Additionally, a PLS model predicting beer staleness was established based on four selected parameters, among which two were aldehydes: furfural and 2-methylpropanal. These two aldehydes were confirmed as relevant markers in relation to beer staleness. The other two parameters were trans/cis iso-α-acids ratio and total reactive antioxidative potential (TRAP).
Finally, staling phenomena were explored during mashing at varying pH conditions. As a result of buffering capacity, wort samples displayed similar biochemical and staling properties, regardless of pH adjustment within the range 5.0 to 6.0. However, mashing with the very low pH of 4.5, resulted in the sweet wort, which stood out as an outlier with few attributable downsides i.a. lower extract, higher dextrin and amino acid content, protein precipitation, higher wort cloudiness, higher solubilisation of transition metal ions and higher reducing potential. Moreover, an intriguing negative correlation was observed between electron spin resonance (ESR) determined rate of radical formation with the content of transition metal ions and the reducing potential of the sweet wort, which was thoroughly discussed.
This PhD study aims to provide a comprehensive insight into the behaviour of staling compounds during brewing and beer storage. Particular focus was given to staling aldehydes, including volatile forms and non-volatile adducts with cysteine. A state-of-the-art literature research, covering chemical and technological aspects affecting aldehyde dynamics was conducted. Malt has been identified as the primary source of staling precursors for aldehyde formation and several brewing strategies have been proposed to minimise aldehyde formation during wort and beer production. The past research often focused on lipid oxidation aldehydes, while more recent findings point to the importance of other types such as Strecker and Maillard aldehydes. The concept of bound state aldehydes, particularly at the brewhouse level, remains underexplored. Addressing these gaps formed the core focus of this PhD.
From the experimental point of view, the work was divided into three parts. The first illustrates the dynamics of volatile and cysteine-bound aldehydes monitored on pilot-scale brewing trials from malt to final beer, including storage. The results confirmed malt serving as the primary origin of staling aldehydes entering the brewing process. Both volatile and cysteine-bound aldehydes were detected in the wort samples, with the highest concentrations observed during mashing-in. Majority of the aldehydes were reduced during wort boiling (except for furfural) and fermentation. Despite marginal levels found in the fresh beer, volatile aldehydes evidently appeared in aged beers. While some of the aldehyde markers reached a plateau after one month of forced ageing, furfural continued to creep up with a linear trend. Cysteine-bound aldehydes were not found either in the fresh nor stored beers.
In a following research set up, aldehyde dynamics was monitored in parallel with an extensive set of flavour instability parameters and sensory staleness. After 90 days of forced ageing all volatile aldehydes were detected below their corresponding sensory thresholds, yet, sensory staleness doubled, indicating a synergistic effect of carbonyls in staleness perception. Additionally, a PLS model predicting beer staleness was established based on four selected parameters, among which two were aldehydes: furfural and 2-methylpropanal. These two aldehydes were confirmed as relevant markers in relation to beer staleness. The other two parameters were trans/cis iso-α-acids ratio and total reactive antioxidative potential (TRAP).
Finally, staling phenomena were explored during mashing at varying pH conditions. As a result of buffering capacity, wort samples displayed similar biochemical and staling properties, regardless of pH adjustment within the range 5.0 to 6.0. However, mashing with the very low pH of 4.5, resulted in the sweet wort, which stood out as an outlier with few attributable downsides i.a. lower extract, higher dextrin and amino acid content, protein precipitation, higher wort cloudiness, higher solubilisation of transition metal ions and higher reducing potential. Moreover, an intriguing negative correlation was observed between electron spin resonance (ESR) determined rate of radical formation with the content of transition metal ions and the reducing potential of the sweet wort, which was thoroughly discussed.
| Originalsprog | Engelsk |
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| Forlag | Department of Food Science, Faculty of Science, University of Copenhagen |
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| Antal sider | 156 |
| Status | Udgivet - 2024 |