Use of chemical indicators of beer aging for ex-post checking of storage conditions and prediction of the sensory stability of beer.

The rate of beer aging is affected by storage conditions including largely time and temperature. Although bottled beer is commonly stored for up to 1 year, sensorial damage of it is quite frequent. Therefore, a method for retrospective determination of temperature of stored beer was developed. The method is based on the determination of selected carbonyl compounds called as "aging indicators", which are formed during beer aging. The aging indicators were determined using GC-MS after precolumn derivatization with O-(2,3,4,5,6-pentaflourobenzyl)hydroxylamine hydrochloride, and their profile was correlated with the development of old flavor evolving under defined conditions (temperature, time) using both a mathematical and statistical apparatus. Three approaches, including calculation from regression graph, multiple linear regression, and neural networks, were employed. The ultimate uncertainty of the method ranged from 3.0 to 11.0 °C depending on the approach used. Furthermore, the assay was extended to include prediction of beer tendency to sensory aging from freshly bottled beer.

Analysis of free fatty acids in beer: comparison of solid-phase extraction, solid-phase microextraction, and stir bar sorptive extraction.

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.

Determination of free medium-chain fatty acids in beer by stir bar sorptive extraction.

Free medium-chain fatty acids in beer originate from raw materials, mainly from the fermentation activity of yeasts, and can influence beer taste, vitality of yeasts and also the foam stability of beer. This study presents the development of the method for the determination of free fatty medium-chain acids including caproic acid, caprylic acid, capric acid and lauric acid in beer or wort using stir bar sorptive extraction (SBSE). The combination of this extraction technique with solvent back extraction of the extracted analytes and subsequent gas chromatographic analysis with flame ionization detection was used for the determination of these compounds. The influences of different solvent back solutions, sampling time, solvent back extraction times and different contents of ethanol were studied. The method had high repeatability (RSD <6.7%), good linearity (the correlation coefficients were higher than 0.9963 for quadratic curves over the concentration range 0.5-8.0mg/l) and recoveries 57-89%.