Beer is the most consumed alcoholic beverage in the world (Nikolova et al. 2017; Salek et al. 2022). A German consumes on average 100 liters per year (Klein et al. 2012). Mixed beer drinks such as Radler beer are also becoming increasingly popular, thanks to their thirst-quenching effect and low alcohol content of 1 to 2.7 % (Kunze 2016). Radler consists of about 50 % Pilsner beer and 50 % emonade (Radler sweet) or sparkling water (adler sour). Pilsner eer is a bottom-fermented beer that is fermented and stored at low temperatures. The beer is filtered to remove yeast cells and turbidity contained in the beer in order to give it a clear appearance (Kunze 2016). It is a strongly hopped beer, highly carbonated and frothy (Rimbach et al. 2015).
According to the German Purity Law, beer is made exclusively from barley malt, hops, yeast and water (Narziss 1984). The characteristic coloration is due to the molecules extracted from the malt grain and hops. Malt production, which is the germination of the ripened grain, ends with kilning at 80°C. At this high temperature, the Maillard reaction starts, which influences the coloring and aromas (Matissek 2019). A chemical reaction takes place between the carbonyl groups of the reducing sugars and the amino groups of the amino acids, peptides or proteins (Wang et al. 2011). Heterogeneous, nitrogen-containing, aroma intense brown pigments known as melanoids are formed, which absorb light at wavelengths as high as 420 nm (Wang et al. 2011). Besides color, melanoids also contribute to the stabilization of aroma compounds and foam and register anti-oxidative properties (Pieczonka et al. 2021). In addition, reactions of amino compounds with dicarbonyls occur, which are called Strecker aldehydes (Kunze 2016). These are usually very aroma intensive.
Also color defining is the oxidation of polyphenols during storage and ageing (Koren et al. 2020). Flavan-3-ol monomers and proanthocyanidin oligomers are the two main phenolic components which influence the beer color changes as a result of oxidation (Koren et al. 2020). Phenolic compounds are phytochemicals, which are derived from phenylalanine and tyrosine (Khan and Shafiur Rahman 2021). They are secondary metabolites of plants (Khan and Shafiur Rahman 2021). 70–80% of the polyphenols in beer originate from the malt and 20–30% from the hops (Quifer-Rada et al. 2015). In addition to color, polyphenols also influence bitterness, flavor, odor and oxidative stability of food items (Khan and Shafiur Rahman 2021).
For the sensory quality, the color of beer is of great importance, as it gives the consumer the first impression. In many breweries, color determination is still done visually by color comparison with standardized color discs and indication of the color in units of the European Brewery Convention (EBC) scale (Kunze 2016). However, this method is very subjective, as it depends on the examiner's vision and impression (Koren et al. 2020). An alternative method that neglects the subjective impressions of humans is the use of ultraviolet and visible (UV/VIS) spectroscopy as a simple and reliable method to determine the color of beer. The UV-VIS range extends from 200–800 nm and the light absorption spectroscopy in the UV-VIS region is used to analyze a wide range of food and beverage samples (Khan and Shafiur Rahman 2021). The ultraviolet region ranges from 200 nm to 350 nm while wavelengths greater than 350 nm belong to the visual range (Wilson op. 1994). This spectroscopic technique is frequently used in food analysis because it is a simple, accurate, fast and high precision method (Khan and Shafiur Rahman 2021). According to the standard Analytica-EBC color measuring method (European Brewery Convention 1975) the absorbance to determine the color of beer is measured at 430 nm. A factor is used to adjust the absorption of the EBC color scale (Klein et al. 2012).
$$EBC={A}_{430}\bullet 25$$
The EBC scale ranges from 0 to 80. As rather light colored beer, Pilsner beers typically have EBC values of 5.3 to 7.5 (Narziss 2017). In the United States, the color of beer is also determined using a photometric measurement method at 430 nm called Standard reference method (SRM) (Mosher and Trantham 2021). The EBC method is multiplied by a factor of 25, whereas the SRM is multiplied by a factor of 12.7 (Mosher and Trantham 2021). Thus, the color spectrum of the SRM takes values from 2 to 40 (Mosher and Trantham 2021).
In the following, the absorption of sour Radler beer was determined photometrically to calculate their mixing ratio. For this purpose, calibration lines with known mixing ratios were created to be able to determine the dilution of unknown samples. This method could serve as a rapid quantitative quality control of the mixing ratios of sour Radler beer and could be used in breweries for routine control.