Methods were developed for the analysis of natural antioxidants including phenolic compounds and flavonoids in beverages and plant extracts using gradient HPLC with multi-channel electrochemical coulometric detection. Suitability of various reversed-phase columns for this purpose was compared; pH and mobile phase gradients were optimized with respect to the separation selectivity and sensitivity of detection. Because of different target compounds in various sample types, the overlapping resolution maps and the normalized resolution product approaches described earlier were used to select optimum columns and gradients to suit the analysis of the individual sample types. The methods were applied to the analysis of phenolic compounds and flavonoids in beer, wine, tea, and yacon extracts. 32 phenolic compounds were identified and determined, including derivatives of benzoic and cinnamic acids, flavones, and a few related glycosides. Eight-channel CoulArray detection offers high selectivity and sensitivity with limits of detection in the low microg L(-1) range, at least an order of magnitude lower than single-channel coulometric detection using the Coulochem detector. No special sample pretreatment is necessary and, because of the compatibility of the CoulArray detector with gradient elution, phenolic antioxidants of different polarities can be determined in a single run. In addition to the retention times, the ratios of the areas of the pre-dominant and post-dominant peaks to the area of the dominant peak can be used for improved identification of natural antioxidants.

J. Inst. Brew. 114(2), 150-159, 2008Hot water (45°C) extracts of ten barley varieties and their corresponding malts were analyzed in terms of antioxidant activity. The ferric reducing antioxidant power (FRAP) and radical scavenging activity (ABTS), ranged from 0.23-0.45 mg GAE/g dw for malt and 0.12-0.25 mg GAE/g dw for barley. The hull-less malt KM 1910 was the variety with the best antioxidant properties, whereas the highest antioxidant capacity for barley was detected for the variety Merlin. A significant positive correlation between the methods FRAP, ABTS and ITT was found (p < 0.01). The influence of fertilization (20 kg N/ha) on barley antioxidant capacity was studied. The results obtained suggest that the impact of fertilization was not evident and that it depends significantly on barley genotype. The total polyphenol content, as measured according to Folin-Ciocalteu's method, ranged from 0.6-2.9 mg GAE/g dw and correlated positively with all the antioxidant methods used (p < 0.01). Free phenolic compounds were measured by HPLC with a CoulArray detector. The dominant phenolic compound was ferulic acid and its content ranged from 12.5-21.9 and 7.8-56.1 μg/g dw for barley and malt, respectively. The content of catechin ranged from 11.0-17.0 μg/g dw in barley and 0.9-12.1 μg/g dw in malt.

Solid-phase extraction (SPE), solid-phase microextraction (SPME) using carbowax/divinylbenzen fiber, and stir bar sorptive extraction (SBSE) followed by solvent back extraction have been used for the extraction of free fatty acids (caproic, caprylic, pelargonic, capric, lauric, myristic, palmitic, stearic, oleic, linoleic, and linolenic acids) from beer. Subsequent gas chromatographic analyses with flame ionization detection were used for the determination of these compounds. Medium-chain fatty acids (caproic-lauric acid) were determined as free acids, and long-chain fatty acids (myristic-linolenic acids) were determined as methyl esters after methylation by BF(3)-methanol 14%. Linearity, recovery, and repeatability of all methods have been determined and compared with the SPE method used as a reference (SPME method was used only for medium-chain fatty acid determination). All three procedures provide similar working parameters characterized by high repeatability (2.3-16.3%) and good linearity (correlation coefficient ranging from 0.9919 to 0.9999). Results of beer analyses obtained by using these three methods were highly correlated. Although all methods provide compatible alternatives, for medium-chain fatty acid analysis SPME may be a more appropriate technique due to its operational simplicity, repeatability, and low cost.

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J. Inst. Brew. 113(2), [154][155][156][157][158] 2007 This work presents the development of a simple, rapid and low cost method for the determination of some beer flavours as esters such as isoamyl acetate, ethyl caproate, ethyl caprylate, phenyl acetate, ethyl caprate, phenylethyl acetate, ethyl laurate, ethyl myristate and ethyl palmitate using stir bar sorptive extraction (SBSE). The combination of this extraction technique with solvent back extraction and subsequent gas chromatographic analysis with flame ionisation detection was used for the determination of these compounds. The effects of different mixtures of organic solvents and the influence of sampling time and time of solvent back extraction were studied. An optimum procedure for the SBSE method was developed. The method had high repeatability (RSD 2.1-7.3%), good linearity (the correlation coefficients were higher than 0.9995 for quadratic fit over the concentration range from 0.015 to 30 mg/L) and acceptable recoveries (78-107%) and precision (RSD < 7.3%).