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The aim of this study was to validate spectrophotometric methods for the measurement of total polyphenol (TP; via the Folin-Ciocalteu method) and total flavonoid (TF) content [via the aluminum chloride (AlCl3) method]. Validation parameters of these methods were determined, including linearity, sensitivity, precision (intra-assay and intermediate), accuracy, LOD, and LOQ. For the validation process, groups of polyphenol standards were used, including phenolic acids (gallic, p-coumaric, caffeic, and chlorogenic acids), flavan-3-ols [(+)-catechin and procyanidins B1 and B2], flavonols (quercetin and quercetin-3-rutinoside), and dihydrochalcones (phloretin and phloretin-2-glucoside). Obtained validation parameters were within acceptable ranges with high determination coefficients, reasonably low LODs and LOQs, and high slopes in the calibration curves for both methods, except for phloretin and phloretin-2-glucoside, for which there were low slopes in the calibration curves for the AlCl3 method. To evaluate differences in polyphenol content, the validated spectrophotometric methods were used to determine TP and TF content in wines (Plavac, Graševina, and Vranac) and juices (blueberry, strawberry, and blackcurrant juice) according to the polyphenol calibration curves. Polyphenol contents were different for both methods in all wines and juices.

Interactions between polyphenols and fibers are important for polyphenol bioactivities, and have been studied in vitro with adsorption process and isotherms. However, the theoretical interpretations of adsorption potentially can be affected by the method of isotherm modeling. The aim was to study the interactions between β-glucan and quercetin derivatives (quercetin-3-glucoside, quercetin-3-galactoside, quercetin-3-rhamnoside) by studying adsorption, and to potentially improve the modeling of adsorption isotherms. Quercetin derivatives were determined by using spectrophotometric method. Experimental results were modeled with Langmuir, Dubinin-Radushkevich, and Hill isotherms using non-linear regression, linear regression, and improved non-linear regression. For improved non-linear regression, code in the R programming language was developed. All quercetin derivatives adsorbed onto the surface of β-glucan. Improved non-linear regression gave somewhat lower errors and may be the most appropriate for adsorption interpretation. According to isotherms obtained with improved regression, it may be suggested that adsorption is higher for rhamnoside and glucoside of quercetin than for quercetin-3-galactoside which agrees with experimental results. Adsorption could be a physical process. The spatial arrangement of hydroxyl (OH) groups on the glycoside part of quercetin could affect the adsorption. In conclusion, a novel approach using improved non-linear regression has been shown to be a useful, novel tool for adsorption interpretation.

Beneficial effects of aronia phenolics are determined by their interactions with dietary fibers, such as β-glucan. The aim of this research was to study interactions between aronia phenolics and β-glucan by investigating the adsorption process. Phenolic compounds were extracted from aronia, analyzed using high-performance liquid chromatography, and adsorbed onto β-glucan at pH 1.5. The adsorption data were modeled by using Langmuir, Dubinin-Radushkevich, and Hill isotherms with a novel non-linear regression developed especially for adsorption isotherms. Aronia phenolics adsorbed onto β-glucan in amounts 31-250 mg/g (individual anthocyanins), 44-123 mg/g (individual flavonols), and 51 mg/g (neochlorogenic acid). The correlation between adsorption capacities and phenolic content was high (r 2 =0.94), which suggested that the adsorption might be concentration dependent. Modeling with a novel non-linear regression allowed more precise determination of adsorption isotherm parameters. Furthermore, there was a correlation between maximum adsorption capacities predicted by models and measured adsorption capacities (r 2 =0.76, r 2 =0.81 and r 2 =0.34 for Langmuir, Dubinin-Radushevich, Hill isotherms, respectively). The suggested bonds involved in interactions are non-covalent bonds (H bonds, Van der Waals forces). Principal component analysis showed that anthocyanins, flavonols, and phenolic acids could differently behave in the adsorption process, which could be due to differences in the chemical structures (ionic nature of anthocyanins, nonionic nature of flavonols and phenolic acids at low pH). In conclusion, aronia phenolics interacted with β-glucan by adsorbing onto its surface, and the novel modeling developed by our team was helpful in the interpretation of this process. Interactions should be further studied due to their importance for the beneficial effects of aronia.

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