Terpenoid analysis in wine

by | Jun 1, 2021 | Oenology research, Winetech Technical

Better, faster, more – for wine aroma analysis.

Terpenoids and wine aroma

Wine aroma is due to a variety of compounds whose origin range from grape to fermentation (alcoholic and/or malolactic) to barrel and bottle ageing. Chemically, aroma compounds belong to a multitude of classes, associated with certain descriptors: esters (fruity/floral), fatty acids (rancid), methoxypyrazines (green pepper), thiols (tropical), lactones (wood), etcetera. Despite their relative low concentration, terpenoids are highly desirable in wines, as they contribute to attributes such as ‘floral’, ‘raspberry’, ‘tobacco’, ‘honey’ and ‘citrus’.1 The classical example of a cultivar where terpenes play an essential role is Muscat. In addition, it was shown that terpenoids can interact with other aroma compounds present in wine and these interactions will produce synergistic and/or masking effects. For example, one terpenoid found in white wine is linalool, described as ‘floral’ and ‘lavender’. In the presence of 3-mercaptohexanol (3MH, the most common thiol in white wine), the increase and decrease in the perception of attributes like ‘peach’, ‘guava’, ‘lemon’ and ‘passion fruit’ cannot be linearly linked to the concentration of the two compounds (linalool and 3MH).2 However, being able to measure the concentrations of the aroma compounds is important when assessing wines, comparing winemaking techniques or evaluating the effect of products such as yeasts.

Terpenoids constitute a large chemical class and are very diverse in terms of molecular structure. This adds complexity, but also variability to their properties, including how they smell. Some of the names are suggestive of their aroma – citronellol and limonene smell citrus-like and are present in high levels in citronella oil and citrus peels, respectively. Nerol was first isolated from neroli oil, hence its name. Damascone, damascenone and ionones (alpha and beta) are also known as ‘rose ketones’ giving roses their typical smell. From a sensory perspective, these compounds are interesting, because small changes in the molecular structure arrangement (i.e. isomers) can lead to the compounds smelling different from each other. Alpha-ionone smells floral, while beta-ionone smells more like raspberry.

Why a new method?

In grapes and wine, analyses range from simple and straightforward to complex. The complexity of the wine matrix accompanied by the inherent low concentrations of certain classes, such as terpenoids, is already an indication that the analysis will be challenging. The isolation of terpenoids often includes a pre-concentration step and the use of highly-sensitive instrumentation. Even though this type of work happens ‘behind closed doors’ (in an analytical lab) and is the domain of the analysts, transparency and communication can lead to better understanding the needs of the producers to create a quality wine.

Recently, the Chemical Analytical Laboratory (CA Lab) of the South African Grape and Wine Research Institute (SAGWRI) and the Department of Viticulture and Oenology (DVO), has added a new terpenoid analysis to its portfolio. The new method shows improved qualitative and quantitative performance compared to the previous one. Twenty terpenoids can now be measured in 50 minutes and the sample volume has been reduced to 10 mL (from 50 mL), without compromising on the sensitivity of the measurements. This could be achieved due to the use of the Mass Spectrometer (MS) for detection. The method can be applied to white and red wines. It has been tested for various parameters (i.e. selectivity, linearity, limits of detection and quantitation, accuracy, and precision) to prove its reliability.3

Who can benefit from the new method?

The method has already been used for a number of applications. Due to its sensitivity, small differences in composition can be measured, making the method useful even for cases when the differences in aroma perception are subtle. As the origin of terpenoids is the grape itself, cultivars and harvest dates have also been shown to influence the terpenoid composition of wines. The method can be a tool for people working on metabolic activity of yeasts and the formation or release of terpenoids during fermentation. A more complete picture of the aroma compounds can also lead to a better understanding of the relationship between chemical composition and sensory perception of wines.

We (the CA Lab) are looking forward to assist researchers and industry alike in their quest for a quality product through the use of analytical tools.

References

  1. Wu, Y., Zhang, W., Yu, W., Zhao, L., Song, S., Xu, W., Zhang, C., Ma, C., Wang, L. & Wang, S., 2019. Study on the volatile composition of table grapes of three aroma types. LWT 115: 108450. Doi: 10.1016/j.lwt.2019.108450.
  2. Wilson, C., Brand, J., Du Toit, W. & Buica, A., 2018. Interaction effects of 3-mercaptohexan-1-ol (3MH), linalool and ethyl hexanoate on the aromatic profile of South African dry Chenin blanc wine by descriptive analysis (DA). South African Journal of Enology and Viticulture Doi: 10.21548/39-2-3165.
  3. Williams, C. & Buica, A., 2020. Comparison of an offline SPE-GC-MS and online HS-SPME-GC-MS method for the analysis of volatile terpenoids in wine. Molecules 25: 657. Doi: 10.3390/molecules25030657.

– For more information, contact Astrid Buica at abuica@sun.ac.za.

 

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