|Abstract or Summary
- Three analytical methods for determination of aroma compounds in alcoholic beverages were developed and validated. These methods are sensitive and reliable that can be applied to analyze volatile phenols, stale aldehydes as well as some highly volatile aromas in alcoholic beverages. In the first study, an ethylene glycol (EG) polydimethylsiloxane (PDMS) copolymer based stir bar sorptive extraction (SBSE)-GC–MS method was developed for the analysis of volatile phenols in alcoholic beverages. Parameters affecting extraction efficiency were studied including ionic strength, pH, extraction time, ethanol content and nonvolatile matrix. Good correlation coefficients with R2 in the range of 0.994–0.999 were obtained for volatile phenol concentration of 5–500 µg L. Recovery for all phenols were from 95.7% to 104.4% in a beer matrix and 81.4% to 97.6% in a wine matrix. The method had a standard deviation less than 5.8% for all volatile phenols. The limit of quantification (LOQs) in beer samples was lower than 3μg L. In the second study, a simple, fast and reliable method was validated, in which headspace sampling was used in combination with gas chromatography-flame ionization detector (GC-FID) for quantifying the most abundant and highly volatile compounds in alcoholic beverages. The linearity for all the compounds covered 2 orders of magnitude with correlation coefficients (R2) between 0.9993-0.9996. The recovery ranged from 92.7%-99.9% in beer samples and 90.9%-117% in wine samples. The repeatability of this method was satisfactory with relative standard deviation of 2.9%-4.7%. The LOQ and LOD were much lower than the reported concentrations in alcoholic beverage including beer and wine.In the third study, a sensitive method for the determination of stale aldehydes in alcoholic beverages has been developed. Aldehydes were derivatized with O-(2, 3, 4, 5, 6-pentafluorobenzyl) hydroxylamine (PFBHA) in solution and the corresponding oximes were extracted using a DVB PDMS SPME fiber and quantified by GC-MS-SIM mode. It is shown that acidic pH and lower temperature would favor the sensitivity of aldehyde derivatives. Meanwhile by using SIM mode, the background can be eliminated a lot and the limit of quantification reached as low as 0.01μg L (0.1ppb) for most compounds, and the linearity is up to 100μg L with R2 in the range 0.993–0.999.In the fourth study, 5 out of 12 yeasts that isolated from a pre-fermentation cold maceration (9°C) of Pinot noir grapes, were confirmed with β-glucosidase activity, namely Metschnikowia pulcherrima, Hanseniaspora uvarum, Lachancea thermotolerans, and two isolates of Saccharomyces cerevisiae. These yeasts were inoculated in Pinot noir grapes sterilized by high hydrostatic pressure, individually or mixed. Pre-fermentation cold maceration (9°C) was performed followed by alcoholic fermentation conducted by Saccharomyces cerevisiae RC212 at 27°C. Aroma analysis of wine by solid-phase microextraction-GC-MS demonstrated that the presence of the different yeast species during the pre-fermentation cold maceration could alter the volatile aroma composition of wine. The yeasts in pre-fermentation cold maceration altered the concentrations of ethyl esters, branch-chained esters, higher alcohols and monoterpenes. In the fifth study, one of the two Saccharomyces cerevisiae isolates with high β-glucosidase activity from previous work was used to further investigate the impact of pre-fermentation cold maceration on Gewürztraminer wine aroma. A commercial yeast was used in comparison where no cold maceration treatment was performed. The quantification results of aroma compounds showed that pre-fermentation cold-maceration did not significantly affect aroma profile of Gewürztraminer wine. OSU yeast isolate produced comparable amount of terpene alcohols as yeast Vin13, a commercial yeast strain known to produce good aromatic white wine. However, OSU yeast isolate produced dramatically higher concentration of 4-vinylphenol and 4-vinylguaiacol, which give spicy, clove-like aroma, a major character of Gewürztraminer wines. The results were highly correlated to the sensory evaluation. OSU yeast isolate is a good candidate in fermenting aromatic white wine. Pre-fermentation cold maceration is not essential in making white wines.In the sixth study, glycosides precursors extracted from pinot noir grape have been reconstituted with a synthetic must. The must is fermented by yeasts belonging to previously isolated different genera. Fermentation was allowed to take place for 48 hours. The fermented solutions were analyzed by headspace solid phase microextraction gas chromatography–mass spectrometry (HS-SPME-GC-MS) to determine the aroma composition. The results shows that the yeast genus exerts a critical influence on the levels of most varietal aroma compounds. The aroma produced from precursors such as norisoprenoids and terpenols has been affected.