Vicinal diketones (VDKs) are naturally produced compounds in the fermentation process. The monitoring of VDK levels is of great importance as the concentration of these compounds can greatly alter the flavor of a beverage.
Vicinal diketones (VDKs) are naturally produced compounds in the fermentation process. The monitoring of VDK levels is of great importance as the concentration of these compounds can greatly alter the flavor of a beverage. VDKs produce a butter-like flavor in the beverage so they are undesirable in lighter beers that want a clean crisp taste, but are actually wanted at higher levels in wines to give a smoother taste and feel. The most commonly monitored VDKs are 2,3-butanedione also known as diacetyl and 2,3-pentanedione also known as acetyl propionyl. VDKs are typically found in the 0–100 ppb range in beers, and can be anywhere from 100 to 5000 ppb in wine.
The rise in popularity of craft beer has seen a dramatic rise in the number of smaller independent breweries, many of which cannot justify they cost and space of large expensive GC systems. A method has been developed using the Ellutia 200 series gas chromatograph and the Ellutia EL-2100 Head-Space autosampler to provide an offering that can address the analysis at a lower entry price.
The EL-2100 Head-Space autosampler was coupled with a 200 series GC fitted with an ECD. The column used for the analysis was a 1.5 m glass packed column, 2 mm I.D packed with 80/20 Carbopack BAW / 5% Carbowax 20M. The GC method was as per Table I.
Table I: GC method
A number of standards of different strengths ranging from 1000 ppb down to 5 ppb of 2,3-butanedione, 2,3-pentanedione, and 2,3-hexanedione where made up in a 5% ethanol/water mix. These were then used to perform a calibration for the analysis of real samples.
Before any analysis can take place the sample must first be degassed, the simplest way to achieve this is to repeatedly pass the sample between two beakers until the sample stops foaming. This is to help prevent pressure forming when the sample is heated in the headspace vial and from the CO2 eluting during the chromatography. Once degassed 2 mL of the sample is then placed into 20 mL headspace vial that is then purged with nitrogen. This is to help prevent oxygen in the air interfering with the chromatography.
Figure 1: 25 ppb VDK standard.
The system was able to analyze the compounds of interest down to the required low ppb levels. Figure 1 shows a chromatogram of the 25 ppb standard mix used in the calibration. Figure 2 shows an actual sample of a pale ale that contained just under 10 ppb diacetyl.
Figure 2: Sample of a pale ale showing 10 ppb diacetyl.
The Ellutia 200 series GC and EL-2100 autosampler can provide a simple lower cost solution to VDK analysis allowing breweries with smaller budgets the ability to perform this useful analysis.
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