## 4 Discussion
### Gc-Ms
The volatile compounds identified using GC-MS came from a number of different chemical groups: esters (18), ketones (1), acids (5), alcohols (10), aldehydes (2), furans (2), monoterpene alcohol (1) and phenols (2). The largest number of volatile and semi-volatile compounds identified came from the chemical group known as esters. Esters are considered trace compounds in comparison to ethanol and carbon dioxide. Esters are present in a relatively small concentration overall and tend to play an important role overall in the aroma profile of the finished product. The reason for this is due to esters' low odor threshold in beer [13]. Thus, it is crucial that brewers try to balance their ester concentration with other flavor compounds [22]. Esters are primarily formed during the vigorous phase of the fermentation process by the enzymatic condensation of organic acids and alcohols [13; 32]. Esters specifically are synthesized within the intracellular space of the fermenting yeast cell [24]. Esters can be divided into two major groups: acetate esters and medium-chain fatty acid (MCFA) ethyl esters. Acetate esters are produced from acetic acid with ethanol or a higher alcohol [13; 32]. The GC-MS analysis identified a total of six acetate esters; however, only ethyl acetate, isoamyl acetate, and phenyl ethyl acetate were identified in all of the aged samples. Amyl acetate was identified in samples 3-28 months, while 2- methylbutyl acetate was only identified in the 28-month-old sample. Although a dozen or so esters have been identified in beer, only six of them are considered important in having a major contribution to the aromatic profile: ethyl acetate (solvent-like aroma), isoamyl acetate (banana), isobutyl acetate (fruity), phenyl ethyl acetate (rose and honey), ethyl hexanoate (sweet apple), and ethyl octanoate (sour apple) [13; 32]. Lambics, unlike most other beers, can be aged for extended periods of time, allowing for a number of complex chemical reactions to continue [6]. Of the six major acetate esters, only ethyl acetate and phenyl ethyl acetate were identified by the panelists; however, isoamyl acetate, ethyl hexanoate, and ethyl octanoate were also identified by GC-MS. Ethyl acetate can be found in much higher concentrations in lambic beers than any other styles [13]. Ethyl acetate has a solvent-like fruity aroma with an odor detection threshold level of 30 ppm [13; 33]. The average concentration of ethyl acetate is between 8 and 48 ppm for traditional beers, while it is 33.4-67.6 ppm for filtered gueuze and 60.9-167 ppm in unfiltered gueuze [13] Ethyl acetate was only detected by the panelists at 9 months with a ranking of very weak; it is possible that panelists were unable to pick up on the compound because it was coming off the column during the initial two-minute solvent delay by the GC-O panelists despite being detected by the GC-MS. The ester profile of aging beers can undergo a drastic change due to continuous action associated by the yeast during bottle conditioning [34] and prolong storage [34; 35]. As a beer ages, certain esters will decrease, while other ester will increase causing the beer to take on a more of a wine-like aroma [13]. One example is isoamyl acetate; it will start to hydrolyzed over time while the beer is being stored [22]. Although isoamyl acetate was identified in all the aged samples, it was not identified by any of the GC-O participants. Isoamyl acetate is commonly found in lambic beer but the levels that are found in lambs differ from that of traditional beers. In lambic beers, isoamyl acetate is found in much lower concentrations then in traditional beers. In traditional beers, isoamyl acetate can range anywhere from 1.2 to 2.8 ppm in lagers and 0.7 to 3.3 in ales [13; 33]. SPME was used as the extraction technique throughout this experiment and has the capability of extracting compounds at the ppb level [36]. Although isoamyl acetate has a threshold level of 1.1-1.6 mg/L within beer, the concentration itself could have been below the threshold [37].