Determination of critical factors in malt production related to the flavour stability of final beer

Filipowska, Weronika (2022) Determination of critical factors in malt production related to the flavour stability of final beer. PhD thesis, University of Nottingham.

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Flavour instability can be regarded as one of the most important quality problems faced by the brewing industry, as beer flavour starts to deteriorate almost instantly upon packaging. The loss of pleasant flavour attributes and appearance of off-flavours will impact palatability and, therefore, reduce drinkability of beer, which in turn may result in economic losses for the brewer. Efforts to prolong beer freshness may only be fully effective when the complex chemistry behind the phenomena of beer ageing is much better understood. Within this context, one of the current areas of active research is an investigation on the gradual appearance of sensory perceivable off-flavours during beer transport and storage, which, chemically, coincides with an increase in levels of a multitude of unwanted compounds. Among them particular volatiles, known as ‘beer staling aldehydes’. This specific group of aldehydes is characterised by highly flavour-active compounds, perceived at low concentrations, such as Strecker degradation aldehydes (derived from amino acids), furfural (derived from pentose), and hexanal, as well as trans-2-nonenal (both derived from oxidation of unsaturated fatty acids). Two major mechanisms have been ascribed to the development of staling aldehydes during beer ageing in a package, namely de novo formation of aldehydes from a precursor compound (e.g. an amino acid) and release of aldehydes from a pre-formed bound-state adduct (e.g. cysteinylated aldehydes).

It is well-established that the quality of brewing raw materials (in particular malted barley, water, hops, and yeast), as well as the brewing process, affect final beer quality and also beer flavour deterioration. Among raw materials, malt, being the major source of extract, appears to play a pivotal role in beer flavour instability as it has been reported that particular malt quality parameters (such as Kolbach Index, free amino nitrogen content, levels of Strecker aldehydes) are connected to beer staling. Previously, this relationship has also been referred to as the ‘beer staling potential of malt’. Interestingly, the same volatile compounds that are directly involved in beer staling, i.e. the above mentioned staling aldehydes, also develop during production of malt. However, unambiguous cause-effect relationships between particular malt constituents and particular compounds in beer (responsible for in-package ageing) have not been proven yet, demonstrating the huge complexity of beer flavour instability and, consequently, the requirement for more research regarding malt and the malting process in relation to this phenomenon. Since during malting both free and bound-state aldehydes already develop, the central objective of this technology-driven PhD is to monitor the malting process in relation to this development and, in particular, to pinpoint critical moments during malting regarding generation of these unwanted compounds. In this regard, in-depth chemical-analytical evaluation of an industrial-scale malting process and industrial-scale pale lager malts in relation to the formation of (bound-state) aldehydes was performed. Next, targeted micro-malting aiming at malt of reduced beer staling potential and high brewing quality was assessed based on experimental design and numerical modelling.

Aiming at a successful assessment of the malting process, the conditions of sample preparation for GC-MS determination of staling aldehydes were optimised for various types of malting samples (i.e. green malt, partially kilned malt, finished pale lager malt). Next, the resulting protocol for determination of volatile, free aldehydes was applied together with an already available method for quantification of non-volatile cysteinylated aldehydes. By applying this approach, we were able to achieve a truly integrated view on both volatile aldehydes and their non-volatile counterparts for malt samples of various origins throughout this whole doctoral study. Moreover, results obtained on (bound-state) aldehydes were assessed in relation to standard quality parameters of various pale lager malts in search for correlations among all of these variables and to improve our understanding of potential relationships that may be relevant to beer flavour instability.

Results obtained on industrial-scale samples demonstrated that the content of (cysteinylated) aldehydes in finished malt is clearly higher compared to its starting material, barley. During germination, strong increases in levels of the fatty acid oxidation aldehydes, i.e. hexanal and trans-2-nonenal, were found. Levels in Strecker aldehydes and furfural were, however, hardly affected at this stage. Next, when approaching the stage of drying at elevated temperature (in particular when arriving at a critical moisture content of approx. 6% - 9%), levels of most (cysteinylated) aldehydes showed a first dramatic increase, except for hexanal. Furthermore, levels of all (cysteinylated) aldehydes continued to increase rapidly during kilning-off, except for hexanal. Clearly, throughout this PhD, it was found that hexanal behaves differently compared to all other aldehydes, including the other fatty acid oxidation marker aldehyde, trans-2-nonenal. Furthermore, a clear effect of process-associated physicochemical gradients (caused by pneumatic processing in relatively thick grain beds) on aldehyde formation in malting was demonstrated for the first time. Except for hexanal, the highest levels of free and bound-state aldehydes were found in samples derived from the bottom layer of the grain bed, which is most exposed to heat load during kilning. Accordingly, samples taken at the same time from the upper layer (exposed to significantly less heat load) showed the lowest levels of aldehydes.

Next to analysis of the industrial-scale malting process as a function of duration (i.e. stages of malting) and position of the grain in the bed (i.e. bottom, middle, top layer), emphasis was put on several, potentially critical malting variables. These results showed that both the degree of grain modification and kilning-off temperature impact – either indirectly (degree of modification) or directly (kilning-off temperature) – the generation of cysteinylated) aldehydes during malting. Consequently, the degree of steeping and kilning-off temperature were selected as key variables to conduct additional, targeted micro-malting experiments aimed to investigate the potential feasibility of producing malt of reduced beer staling potential, combined with satisfying brewing quality. Modelling of individual responses, representing the measured aldehydes and malt quality parameters, as a function of the key variables, demonstrated that the steeping degree especially impacts levels of hexanal, whereas kilning-off temperature mainly affects levels of Strecker aldehydes, furfural, and trans-2-nonenal. Finally, numerical modelling of the selected malting variables (steeping degree, kilning-off temperature), suggested the feasibility of producing pale lager malt of superior overall quality on condition that adequate, mutual adjustment of both grain modification and kilning-off temperature is implemented.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: De Cooma, Luc
Powell, Chris
Cook, David
De Rouck, Gert
Aerts, Guido
Keywords: Malt production, Beer, Flavour
Subjects: T Technology > TP Chemical technology > TP 368 Food processing and manufacture
Faculties/Schools: UK Campuses > Faculty of Science > School of Biosciences
Item ID: 67406
Depositing User: Filipowska, Weronika
Date Deposited: 31 Jul 2022 04:40
Last Modified: 31 Jul 2022 04:40

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