# Analysis of Potent Odour-Active Volatile Thioids in Foods and Beverages with a Focus on Wine
## 1 Introduction--Importance of Thiols to the Aroma of Foods and Beverages  
Aroma is inarguably one of the most important quality aspects for any food or beverage product, with unique and characteristic aromas being attributed to a large range of volatile compounds with various physico-chemical properties. At the time of writing this review, a commercial database, Volatile Compounds in Food, had compiled a total of 9514 volatile components that were identified in natural and processed food products from published literature data [1], and the list continues to grow. Amongst the vast numbers of volatiles in the database, volatile sulfur compounds (VSCs, sulfur-containing volatiles) are the second largest category just after volatile esters, and they represent around 13% of total volatiles (Figure 1a) [1]. VSCs play an important role in the aromas of foods and beverages, not only because of their broad presence, but also for their significant sensory contributions due to concentrations that are well above their low odour detection thresholds (ODT) [2; 3]. Volatile thiols, historically known as mercaptans and consisting of the structure R-SH, are a sub-category of VSCs that are of particular interest, because they have some of the lowest ODTs (ng/L and lower) of any volatile compound identified in nature [6]. Such potent volatile thiols have been in the spotlight of aroma research and they are frequently regarded as \"potent\" [7], \"key aroma\" [8], \"aroma-active\" [9; 10], or \"aroma-impacting\" [11] odorants. Some of the most famous examples in foods and beverages include 3-sulfanylhexan-1-ol (aroma descriptor: grapefruit, ODT: 60 ng/L) and 3-sulfanylhexyl acetate (aroma descriptor: passionfruit, ODT: 4 ng/L) in wine [6], 4-methyl-4-sulfanylpentan-2-one (aroma descriptor: boxwood, ODT: 0.8 ng/L) in beer [12] and wine [6], and 2-furfurylthiol (aroma descriptor: roasted coffee aroma, ODT: 0.4 ng/L) in coffee [13] (Figure 1b). From a total of 395 publications (1990 to 2019, using \"Volatile Thiol\" as the search keyword) retrieved from Web of Science Core Collection and visualised by a network approach, it was obvious that research on volatile thiols had been focusing on \"detection\" and \"reaction\", and the majority  
Figure 1: (a) Doughnut chart showing the relative percentages of volatile sulfur compounds identified in foods according to Volatile Compounds in Foods database [1], where each segment represents one chemical category of volatiles; (b) Examples of chemical structure, aroma descriptor, and ODT of some of the most studied volatile thiols in wine [4]; and, (c) Bibliometric map of volatile thiol research visualised from a total of 395 publications (from 1990–2019) retrieved from Web of Science Core Collection using “Volatile Thiols” as keyword. Literature analysis and graph construction by VOSviewer [5]. Note the abbreviations 3-MH and 3-MHA used in panel (c) are in keeping with much of the earlier literature; however, the IUPAC names (i.e., sulfanyl prefix instead of mercapto) are used in this review and abbreviated as 3-SH and 3-SHA. of publications had emerged from the field of \"wine\" research (Figure 1c). Indeed, much progress on the development of analytical methods for untargeted identification and targeted quantitation for volatile thiols has been achieved in wine [4]. As such, this review emphasises the field of wine research and particularly covers literature that is dedicated to developing analytical methods. The analytical challenges and requirement for volatile thiol analysis in the context of wine, foods, and other beverages are first presented, followed by strategies that were developed to address those analytical challenges. Sample preparation techniques (selective extraction with metal ions, derivatisation), chromatographic separation, mass spectrometric detection, quantitative analysis with stable isotope dilution assay (SIDA), and matrix effects have been reviewed. Finally, future trends and directions for volatile thiol research have been proposed.