Het begrijpen en verbeteren van de bubbel- en schuimeigenschappen van alcoholvrij bier

Global demand for non-alcoholic beers is rising strongly, while the consumption of alcoholic beers is declining. Many breweries have responded to this by launching non-alcoholic variants of their popular beers. Although various innovations have already significantly improved the taste of non-alcoholic beers, creating non-alcoholic beers that match the quality of their alcoholic variants remains a challenge. Removing ethanol affects various beer parameters, including taste, mouthfeel (body and warming effect), and bubble and foam properties (due to changes in surface tension and viscosity). Despite consumers frequently indicating that the bubble and foam properties of non-alcoholic beers are not equivalent to those of alcoholic beers, fundamental scientific research into these differences and the causes behind them is lacking. Therefore, the aim of this doctoral research was to gain a mechanistic understanding of the bubble and foam characteristics of alcoholic and non-alcoholic beers and to develop strategies to improve these in non-alcoholic beers. Screening of commercial alcoholic and non-alcoholic beers using a self-developed bubble measurement setup and sensory evaluation showed that non-alcoholic beers produce fewer bubbles, resulting in a lower perceived bubble intensity. Although the bubbles in non-alcoholic beers were larger, the size of the nucleation sites seems to be a more dominant factor for bubble size than the beer composition itself. These differences in bubble size were also not sensorially perceptible. Despite the differences in bubble formation, both beer types had comparable CO2 levels, resulting in a similar tingling sensation (sting) in the mouth. Furthermore, the foam properties of commercial beers were analysed using four distinct foaming methods, including a novel technique featuring foam replenishment (the Ultrasonic Beer Foamer). These foaming tests indicated that the reduced bubble formation in non-alcoholic beers also led to less foam formation and a drier foam. Additionally, this foam was less stable due to increased coalescence, disproportionation and drainage. Notably, within this study, the commercial non-alcoholic beers produced with alternative yeast strains had better foam stability than non-alcoholic beers produced by removing alcohol. The four foaming methods generated foams with significantly different (initial) foam volumes, liquid contents, and lifespans. Consequently, these tests target different foam destabilisation mechanisms, leading to limited correlation between the foam properties measured by the different methods. Validation of these techniques against the foam properties determined after manual pouring by 106 consumers revealed that the self-developed Ultrasonic Beer Foamer produces a foam that accurately reflects real-world consumption conditions, unlike the three other foaming methods, including the industry-standard (NIBEM method). The findings suggest that less vigorous foaming methods, which yield a drier foam structure, better matches consumer poured foam. Given the high multicollinearity between the concentrations of beer components, an unhopped, alcohol-free model beer was used to study the influence of ethanol and iso-α-acids on bubble and foam properties. This revealed that the lack of ethanol is the main reason for the lower bubble formation in alcohol-free beers. Ethanol lowers the surface tension at a bubble formation rate relevant to beer (200 ms). Without ethanol, more energy is therefore required to form a bubble, leading to fewer bubbles and, consequently, less foam formation. However, the lack of ethanol did not appear to be the cause of the lower foam stability. On the contrary, in the model beers, ethanol decreased stability. This effect was caused by reduced adsorption of surface-active components at the interface (lowering viscoelasticity), increased coalescence and disproportionation, and, remarkably, higher drainage despite the viscosity-increasing effect of ethanol. Furthermore, ethanol diminished the effect of iso-α-acids, which improve foam stability by lowering surface tension, increasing viscoelasticity, and reducing drainage, coalescence and disproportionation. Ethanol causes iso-α-acids to concentrate less in the foam, presumably by reducing interactions between iso-α-acids and beer proteins in the bulk phase. Based on a Response Surface Model developed using commercial beers, strategies were developed to optimise bubble formation. A first strategy is to increase the CO2 content. The model predicts that an increase up to 7 g/L is necessary to match the bubble formation of a pilsner beer in an alcohol-free beer. However, this will increase the sting and sourness as well, while alcohol-free beers are already perceived as more sour. A second strategy focuses on lowering the surface tension at 200 ms. The addition of 0.9 g/L protein hydrolysates from wheat gluten, pea protein or brewers' spent grain to the alcohol-free model beer decreased the surface tension. Although this led to more bubble formation with some hydrolysates, the effect was smaller than adding 5 %v/v ethanol. A higher concentration of protein hydrolysates could likely further enhance bubble formation. Remarkably, this strategy worked counterproductively in an alcoholic model beer, actually causing bubble formation to decrease. The addition of certain protein hydrolysates also proved effective in increasing interfacial viscoelasticity and foam stability of non-alcoholic beers. This research shows that the lack of ethanol is not responsible for the lower foam stability of alcohol-free beers. However, the concentration of iso-α-acids in alcohol-free beer is more critical for foam stability than in alcoholic beer. Adding a low concentration of iso-α-acids (10 mg/L) to the unhopped alcohol-free model beer decreased the interfacial viscoelasticity and foam stability, an effect that did not occur in the alcoholic model. In contrast, higher concentrations of iso-α-acids (starting from 20 mg/L) resulted in an increase in interfacial viscoelasticity and foam stability, which was even more pronounced than in the alcoholic model beer. Overall, this research provides viable strategies for brewers to improve the bubble and foam characteristics of non-alcoholic beers.