# Acid and Volatiles of Commercially-Available Lambic Beers
## Results and Discussion
### Volatile and Semivolatile Compounds  
A total of 50 aroma compounds were identified by SPME-GCMS using a combination of retention index and mass spectral matching against library standards (Table 4). Compounds that could not be identified by comparing their retention index values were marked as tentatively identified. The compounds identified belonged to a number of different chemical groups (ketones, acids, alcohols, and phenols). Thirty-three of the 50 compounds identified have not been previously reported in gueuze lambic beer. Seventeen compounds have been reported by both [PERSON] et al. [12] and [PERSON] et al. [18]. The compounds previously reported by [PERSON] et al. [12] were acetic acid, lactic acid, butyric acid, propionic acid, isobutyric acid, propanol, butanol, isobutanol, isoamyl alcohol, amyl alcohol, amyl alcohol, ethyl acetate, and ethyl lactate. [PERSON] and his colleagues [18] reported finding caproic (hexanoic) acid, caprylic (octanoic) acid, capric (decanoic) acid, isoamyl acetate, ethyl caproate (hexanoate), ethyl caprylate (octanoate), ethyl caprate (decanoate), and phenethyl acetate. [PERSON] et al. [17] used SPME to characterize the volatiles in different types of beer (lambic was not analysed) and reported that volatile fingerprints could be characterized based on the type of fermentation (top versus bottom) and style. The major chemical classes that account for gueuze lambic beer were alcohols, acids, esters, phenols, aldehydes, and sulfur compounds. The production of alcohols in beer is a result of yeast metabolism [19]. Of the 50 compounds identified, eight were alcohols. Phenethyl alcohol, isoamyl alcohol, and isobutanol have been previously reported [12] in lambic beer. The compounds 2-methyl-1-butanol [20], 1-hexanol [21], heptyl alcohol [22], 1-octanol [22], 2-nonanol [21], and 1-decanol [22] have been previously reported in beer, but not lambic beer. Twenty-three esters were detected using SPME GC-MS. In prior research, only seven have been previously reported and they are ethyl acetate, lactate, butyrate, caproate, caprylate, caprate, and phenethyl acetate [12; 18; 23]. An additional fifteen ethyl esters were detected using SPME. These compounds are shown in Table 4.  
Acids play a vital role in the aroma and flavor profiles of lambic beer. A total of seven acids were identified using SPME GC-MS. The acids identified were acetic, lactic, isovaleric hexanoic, valeric, octanoic, and decanoic acid [12; 18]. With the exception of isovaleric and valeric acids, all have been previously reported in gueuze lambic beer, often associated with the aged hops used. Isovaleric and valeric acid, however, have been reported in other styles of beer [19]. External standard calibration curves prepared in distilled water were used to quantify isovaleric acid (IVA), ethyl octanoate, 4-ethylphenol (4 EP), 4-ethylguaiacol (4 EG), ethyl caprylate, octanol, ethyl undecanoate, and ethyl acetate (Table 5). Isovaleric acid has been previously reported in beer, but not in lambic beers [19]. Isovaleric acid, 4-ethylphenol and 4-ethylguaiacol are key components in the overall aroma of _Brettanomyces_[24]. The concentration of isovaleric (3-methylbutyric acid) acid for gueuze lambic beer ranged from 1.92 mg/L for Oude Gueuze Vieille-3.01 mg/L for Cuvee Renee. Isovaleric acid was found in six of the nine commercial beers (Cuvee Renee, Oude Gueuze Vieille, Cantillion, Cantillion Bio, Girardin, and Oude Boon). When comparing the means for all the brands, no difference was found for IVA. Both 4-ethylphenol and 4-ethylguaiacol are known by-products of the yeast species _Brettanomyces_. Neither compound, however, has been quantified for lambic beers. The concentration of 4-ethylphenol ranged from 0.28 mg/L to 1.13 mg/L. Cuvee Renee had the highest concentration of 4-ethylphenol at 1.13 mg/L and it was found at 0.28 mg/L for both Girardin and Oude Boon. Table 5 includes a comparison of 4-ethylphenol levels in commercial brands.