Standard Reference Method

Abstract: Color is an important sensory parameter required for the quality control of beers. A new multivariate image analysis method for the color determination of beers was proposed and validated. Reference color values were determined using the SRM (standard reference method) system, which is based on absorbance measurements at 430 nm. Digital images were obtained with an iPhone 7 smartphone. The obtained RGB histograms were used for building partial least squares (PLS) models. The developed method is direct, simple, and rapid, not requiring sample pretreatment steps as the reference method. Beer samples of different styles, brands, and brewery companies were obtained in a large variety, totalizing 128 samples and comprising a range from 3 to 130 SRM units. A global PLS model built with all the beer samples presented too large prediction errors for some samples in the lower part of the SRM scale (below 12 units). Thus, considering the requirement of dilution prescribed by the reference method for samples with absorbances higher than 1.0, two local calibration models were built: for high SRM range (above 12 units) and low SRM range (equal or below 12 units) samples. A previous PLS discriminant analysis (PLS-DA) model was used to assign the beer samples to these two classes, resulting in 78 and 50 samples in the high- and low-range models, respectively. These two models were validated according to the Brazilian and international guidelines, being considered linear, accurate, precise, and unbiased. Uncertainties were also calculated for estimating confidence intervals for the predictions of the validation samples. The developed method could be easily adapted in a mobile platform, spreading its use and opening the possibility for the commercial production of a dedicated equipment.

Abstract: Beer color is an important sensory attribute, the first one that the consumer observes. There are two standard methods accepted for determining the color of these products, one related to the European Brewery Convention (EBC) and the other is the Standard Reference Method (SRM). Both methods are based on absorbance, but in case of the more and more popular fruit beers these methods give false result since these products appear in varied colors and have different spectra than regular beers. In this study 39 different types of beers were investigated, including fruit beers and beer based mixed drinks to compare their color in CIE 1976 L*a*b* color space, absorption-based colors and transmission spectra. ΔE*ab values of products with less than 5% EBC difference ranged from 4.5 to 17.4. There were magnitude differences in the transmission spectra of these products, fruit beers showed different tendencies due to the added fruit or fruit juice. The highest ΔE*ab value belonged to two traditional Weissbiers. Absorption-based methods are not able in many cases to differentiate between products which have nearly the same EBC or SRM color but visually are different. A multi-wavelength method would be reasonable to be developed for more objective and accurate beer color determination.