Dr. Elizabeth Tomasino, Assistant Professor
You may often wonder how one determines the complex composition of wine. There are various technologies developed to allow researchers to break up the various compounds and investigate each individually. One of the common techniques to determine aroma composition of wine is known as head space solid phase micro extraction gas chromatography mass spectrometry (HS-SPME-GCMS).
Aroma compounds that can volatilize are absorbed onto a fiber and then injected into a gas chromatography mass spectrometer (GCMS). High temperatures are applied to the fiber and remove the volatile compounds which move through a column that separates out each individual compound based on temperature, molecular weight, polarity, and other factors. Once the compounds reach the mass spectrometer, a unique spectrum is produced for each compound. This is similar to an person’s fingerprint (Figure 1).
Depending on the research question, it is possible to obtain both qualitative and quantitative information using a GCMS. However, there are limitations to this equipment as some compounds cannot be properly identified because they come out at the same time and do not separate, requiring other separation techniques. A technology that has emerged to provide greater separation is the multidimensional gas chromatography (MDGC). This technology was first developed in 1989 and has been used extensively in the petrochemical industry, and only recently has this been applied to wine science. When comparing the two methods, GC can identify about 150-200 compounds with one dimension of separation while up to 400 compounds can be identified and measured using MDGC with two dimensions of separation.
Multidimensional gas chromatography allows researchers to fine-tune compound separation by “cutting” areas that may consist of multiple compounds. The instrument consists of a GC connected to a GCMS by a heated transfer line (Figure 2).
Within my research lab at Oregon State University, I have a MDGC that can perform “heart-cutting,” where only specific portions of the compound spectrum (or chromatogram) are cut and transferred to a second GC. Flavor and fragrance analysis is commonly done using “heart-cut” MDGC. I used this technology during my PhD studies in New Zealand, and I am excited to apply it to a number of projects here. I will be focusing on correlating the new analytical information of specific compounds generated from MDGC to wine sensory data. Despite significant advancements in the determining of wine composition, our understanding of how individual compounds impact the sensory properties of a wine is still limited.
Currently we are using MDGC to measure chiral terpenes present in aromatic white wines. Terpenes are a class of aroma compounds responsible for floral, pine, and citrus-fruit aromas that are found in many plant essential oils. Terpenes can have significant impact on wine aroma, but they are difficult to measure since the various terpenes are closely related. The main issue in identification is due to the fact that these are chiral compounds that have the same atomic formula but a different three-dimensional arrangement of atoms that form mirror images that are not superimposable. Your left and right hands are examples of non-superimposable mirror images. Why do we care about chiral compounds? Well, these compounds may smell differently and be perceived at different concentrations. For example, limonene is a terpene that is found in the rind of citrus fruit. The isomers of limonene have different aroma activities; R-(+)-limonene, smells like fresh oranges and the odor threshold is 200 ppb. S-(-)-limonene, smells like turpentine and lemon with an odor threshold of 500 ppb (Figure 3). (Boelens et. al 1993, Friedman & Miller, 1971).
Depending on the amount and type of different isomers present, the wine may smell very different. A study is being conducted to measure a range of different chiral terpenes in wine to determine if different varieties, place of origin, or other winemaking processes impact the ratio of chiral terpenes. These data will be paired with sensory trials to determine concentration thresholds for compounds impacting aroma.
This MDGC technology is being used in a number of studies measuring wine volatile compounds and linking them to sensory impacts. I collaborated with Dr. Laurent DeLuc’s lab to determine the effects of berry variability at harvest on Merlot wine quality. The MDGC was also used in collaboration with an entomology project with Dr. Vaughn Walton to measure the volatile compounds associated with Brown Marmorated Stink Bug taint in wine. This method is being used in conjunction with winemaking and sensory research to determine threshold levels of Brown Marmorated Stink Bug taint. We will also look at the processing steps in winemaking that impact the taint expression.
Another study that is being conducted involves understanding the role of important volatile aroma compounds in Pinot noir. The MDGC technology is well-suited for this project, as Pinot noir aroma is difficult to characterize due to many closely-related compounds which impart specific aromas but are present at very low concentrations. In spring 2014, we will investigate the impact of two key norisoprenioids, ß-ionone and ß-damascenone, on Pinot noir aroma in Oregon wines. Future work will attempt to tie Oregon’s regional Pinot noir wine styles to chemical composition and sensory data. This equipment, combined with the already extensive analytical equipment available in various labs at the OWRI, will serve as another tool to increase the knowledge of wine science for the Oregon winegrape industry.