Analysis of polyfunctional thiols in beer : Developing a novel method of extraction and analysis using GC-FID

Abstract

In this project, attempts were made to identify and quantify two polyfunctional thiols – 4-mercapto-4-methyl-pentanone and 3-mercaptohexyl acetate – and three monoterpene alcohols – linalool, geraniol, and β-citronellol – in beer samples. In order to avoid the use of hazardous compounds such as dichloromethane and hydroxymercuriobenzoate, common in traditional methods, two novel techniques were tested: a pure liquid-liquid extraction of the beer samples with cyclohexane, and a liquid-liquid extraction with cyclohexane followed by solid phase extraction on silver ion columns. Analyses were then performed with GC-FID. Evaluation of the methods showed both to be insufficiently sensitive to measure the low concentrations of thiols in the beer samples. The complexity of the beer matrix also proved cumbersome, as the analyte peaks of the more abundant monoterpene alcohols in the chromatograms were often blotted out by non-analyte peaks. To improve this method towards thiol analysis, it is recommended to further enrich the extracts, such as by allowing the volatile solvent to evaporate and thereby increase the concentration of analyte in the remaining solvent, or to use a different detector such as a mass spectrometer or a sulfur chemiluminescence detector. To improve the methods towards monoterpene alcohols, it is recommended to use a column that more specifically retains the compounds of interest or use a slower temperature programme to improve resolution between peaks.

Popular Abstract

Understanding the flavour chemistry of beer is one of the most effective ways of improving it. In this study, attempts were made to analyse the content of polyfunctional thiols in beers treated with products intended to increase the content of thiols. The analyses were performed using two novel methods, developed to avoid the hazardous chemicals used in traditional analysis.

Over the last decade, the popularity of hazy beers with fruity aroma and low bitterness has spiked massively. As a consequence of this much effort has been put into understanding the key to producing these delicate beer styles, and by applying methods and previous findings from the wine industry to beer and hops, scientists have been able to isolate so-called polyfunctional thiols as an explanatory factor. Thanks to their incredibly low detection threshold, some of these substances – although only present in nanogram per litre levels – give beer or wine a pleasant smell of tropical fruit, box tree or black currant. In order to evaluate whether a process, a product, or a hop variety contributes to the overall thiol content in a beer, a reliable method of analysis is required. This is challenging as the concentration of thiols is so low, and traditional methods have had to rely on using hazardous compounds to achieve good results. In this study, beers treated with products meant to increase their thiol content were analysed using two novel methods involving less hazardous chemicals. The thiols were extracted using either cyclohexane – a relatively common organic solvent popular for its low toxicity – or using solid phase extraction on silver ions – chosen because silver ions have a similar affinity to thiols as do the more traditionally used mercury compounds but are much less toxic.
After extraction the extracts were analysed in a gas chromatograph, which separates volatile compounds based on their volatility and their affinity to the column on which the separation is performed. As the compounds leave the gas chromatograph at different times, they are detected in a flame ionization detector, and an amount of each substance can be determined.
Unfortunately for this study, the results proved to be inconclusive: while some hop compounds, such as the common monoterpene alcohol linalool, could be detected in many of the studied samples, no thiols were reliably detected. It is likely that this is because of the low concentration in the beer samples, and thus the method would need to be revised to include some concentrating step in order to be effective. This could be done by including an evaporation step, where a large amount of the solvent is evaporated and the concentration of analyte is thus increased, however this would risk losing or altering some of the analytes, which would require further evaluation.

• author: Lindmark, Arvid
• supervisor: Lars Nilsson
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLGM10 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Agriculture and Food Sciences
• keywords: Beer, Hops, Thiols, 3MHA, 4MMP, Food Technology
• language: English
• id: 9093085
• date added to LUP: 2022-06-27 09:33:19
• date last changed: 2022-06-27 09:33:19