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Abstract:

Beer is a complex carbonated beverage that is in essence made from water, malt, hops and yeast. In the beer-making process, malting, milling, mashing, filtration and boiling can be viewed as a series of consecutive batch processes that are aimed at barley carbohydrate conversions to produce a balanced mixture of fermentable sugars and soluble polymers prior to fermentation of wort into beer. It is mostly the action of barley and malt enzymes that affects carbohydrate solubilisation and hydrolysis. While fermentable sugars are related to brewing efficiency and yield, the soluble polymers are more important in the context of beer quality, including mouthfeel. Although the beer-making industry can rely on a profound tradition of research, a holistic view on the fate of cereal carbohydrates throughout malting and mashing is missing. Such knowledge is, however, necessary if we want to mobilise these carbohydrates to the advantage of process and product improvement. Therefore, this dissertation aimed to study the conversions of starch, arabinoxylan and β-glucan in the process from barley to beer, and use the obtained knowledge to identify opportunities and investigate methods to improve the resource efficiency of the brewing process and the mouthfeel of beer. In the first part of this dissertation, the focus was on the conversions of barley carbohydrates in the process from barley kernel to beer. As a base-measurement at the start of this section, an industrial-scale barley malting process (150 metric tons) and a pilot-scale brewing process (500L) were analysed with a focus on carbohydrate content and structure in the various process streams. Both processes were executed as simply as possible, using only the main ingredients. All streams were analysed and a mass balance was established. The efficiency of the mashing process was expressed as the conversion of malt to fermentable sugars, which was 41.0%. Results showed that degrading all β-glucan present in beer to fermentable sugars by the addition of exogenous enzymes would increase the efficiency only to 41.1%. Some small starch granules were found to withstand mashing and were retained in the spent grain fraction. The full conversion of these granules would lead to an increase of the yield to 42.0%. This might be established by decreasing the particle size of the grist or the addition of cell wall degrading enzymes. Conversion of all dextrin in the wort to fermentable sugars would improve efficiency to 50.3%. However, it is commonly accepted that dextrin plays an important role in beer mouthfeel. It was hypothesised that arabinoxylan, due to its viscosifying properties, might replace dextrin, if dextrin were to be converted to fermentable sugars. This strategy was further elaborated in the second part of this dissertation. The architecture of the barley endosperm cell wall and its degradation as a result of malting were investigated next. Cell walls of barley and malt from the industrial-scale malting process were visualised using cryo-SEM and 3D-multiphoton-CLSM and analysed for their composition. For the first time, pectin was visualised in barley endosperm cell walls as a minor constituent using antibody staining. Its presence was backed by chemical analysis. Barley and malt endosperm contained 0.24 and 0.32% of uronic acids, respectively, which are the main building blocks of pectin. Results allowed to improve the existing model of the matrix polysaccharide structure in barley endosperm cell walls. To increase our knowledge of the dynamics of starch hydrolysis during mashing, the impact of starch granule size on fermentable sugar and dextrin production during lab-scale mashing was evaluated. It was shown microscopically that mainly small granules gelatinised at temperatures exceeding 62 °C. Although it is known that small and large barley starch granules gelatinise at different temperatures, the difference that was observed was larger than expected. External factors during mashing like limited water availability and the possibility of annealing at sub-gelatinisation temperatures can be hypothesised to contribute to this phenomenon. As a result, small granules contribute less to fermentable sugar content in wort compared to large granules and are mainly converted to dextrin. In the second part of this dissertation, the impact of carbohydrates on the quality of beer and more specifically, the aspect of mouthfeel was studied. The observation that the conversion of dextrin to fermentable sugars has the potential to improve the resource efficiency of the beer brewing process profoundly was the starting point of this research. Despite the contradicting information in literature, dextrin is believed to play a major role in beer mouthfeel. The conversion of dextrin in the production of light beers is well-established and believed to be one of the major causes of lower mouthfeel appreciation. Besides light beers, also the non-alcoholic and low-alcohol beers (NABLAB's) suffer from a perceived watery mouthfeel. Therefore the potential of arabinoxylan and β-glucan as a mouthfeel contributor was assessed. A lab-scale brewing experiment was set up that made use of non-malted adjuncts (barley, rye and oats) in order to increase the amount of high molecular weight arabinoxylan or β-glucan. The beers in which 20% of the malt was substituted by rye, showed a high kinematic viscosity of 1.85 mm/s² compared to 1.48 mm/s² for the control beer with 100% malt. This was attributed to the elevated content of high molecular weight arabinoxylan. Neither extensive milling nor the addition of xylanases improved the solubilisation of arabinoxylan to such an extent that the viscosity was increased even more. As a proof of concept, two light and low alcohol beers were brewed in a food-grade set-up. The experimental beer consisted of 70% malt and 30% rye and was compared to a 100% malt beer by a sensory panel. The experimental beer showed a significantly improved fullness compared to the control. As sensory tests are expensive and time-consuming, new methodologies are developed to evaluate mouthfeel of food systems instrumentally rather than using a test panel. Exploratory tests were conducted using ball-on-three-plates tribology to predict the lubrication behaviour of beer. Together with the tribological data, kinematic viscosity and density were measured for a data set of six commercial beers and were linked to their chemical composition and sensory scores. Due to the complex background of the commercial beers, the formerly established relationship between arabinoxylan content and beer viscosity was not confirmed. The strong intercorrelations between the original extract, alcohol, glycerol, protein and total polyphenol content of the selected beers impeded us to draw more accurate conclusions regarding their impact on mouthfeel. To gain more insight in the complex phenomenon of beer mouthfeel, the data set could be expanded with more beers or methodologies that might predict mouthfeel. However, model systems with additions of pure constituents should also be investigated. In order to improve the resource efficiency of a modern-day brewery, three strategies were outlined. The architecture of the barley endosperm cell walls was investigated, which revealed that an arabinoxylan scaffold persisted malting, which might entrap starch granules during mashing. Secondly, the small barley starch granules tend to gelatinise at higher temperatures, which results in a relatively reduced fermentable sugar production compared to large starch granules. The most impactful strategy would be the degradation of dextrin to fermentable sugars. While it is still debated whether dextrin plays an important role in the mouthfeel of beer, this PhD-study shows that a possible lack of fullness of the beer can be compensated by increasing the amount of high molecular weight arabinoxylan, for example by the addition of non-malted rye as an adjunct. Future research could focus on the exact root of the difference in gelatinisation characteristics between small and large barley starch granules, as well as on the factors that determine the relative ratio of both fractions in a barley kernel. Secondly, although being present in very low amounts in barley, pectin might play an important role in the malting or brewing process. Finally, the complex phenomenon of mouthfeel has to be elucidated in order to support the development of innovative beer styles like NABLAB's or low-calorie beers. Instrumental techniques that can predict mouthfeel will be very valuable in this context.