Article by Becca Yeamans of ‘The Academic Wino’
How a wine tastes is dependent upon many factors, including (but not limited to) the variety, the vintage, where the grapes are grown (soil, climate, etc), as well as the viticultural and winemaking techniques employed during processing. The compounds responsible for how wine tastes are known as free volatile compounds, as well as aromatic precursors, the latter of which are present at much higher concentrations. Non-volatile sugar-bound conjugates (a.k.a. “glycosidic compounds) have been well studied and have been shown to be released over time during wine aging or by using specific winemaking techniques. Specific glycosidic compounds known to be released over time, thus affecting how a wine develops and tastes, include terpenes, C13 norisoprenoids, benzenic derivatives, volatile phenols, and C6 compounds. All of these glycosidic compounds have low odor thresholds, thus requiring very little to elicit a sensory response.
While wine aromatics have been extensively studied, it is not well known exactly how compounds responsible for aromatic character in wines interact with the physiological make-up of the human mouth. In addition to environmental and chemical sources, it is possible that the perception of different wine aromas can be altered by physiological factors like mouth temperature, saliva composition, or the oral microbial community present in each individuals’ mouths. Studies focusing on onions, bell peppers, and grapes found that the microbial community in the human mouth hydrolyzed odorless compounds into their corresponding volatile aromatic compounds, giving reason to believe something similar could potentially happen with wine. Perhaps the microbiota living in the human mouth can hydrolyze these odorless precursors and convert them into their corresponding aromatic compounds, just like it’s been shown with other foods.
A 2015 study in the journal Food Chemistry aimed to evaluate whether or not human oral microbiota can convert odorless aromatic precursor compounds in wine into their corresponding aromatic glycosidic compounds. The results could potentially have a profound impact on our understanding of how we taste and evaluate wines.
Experiment 1: In vitro
For the in vitro experiments, three microbes commonly found in the human mouth were cultured on sterile growth media. Different concentrations of grape extract were added to the microbes, with bacteria growth measured after 24-48 hours, depending upon the specific microbe. From these growth measurements, inhibition of growth was also calculated.
Experiment 2: Ex vivo:
For the ex vivo experiments, fresh saliva was collected from three volunteers (ages 28-31). Prior to collection, the volunteers had not taken any antibiotics or other medications, and were non-smokers. Volunteers did not consume any food or beverages within two hours of the saliva collection time.
The saliva from each volunteer was divided up into four different treatments: 1) fresh saliva in an aerobic culture, 2) fresh saliva in an anaerobic culture, 3) sterilized saliva (pasteurized), and 4) non-enzymatic saliva (heated). Microbe counts were measured after 24-48 hours at 37oC.
Grape extract (comparable to 40g of grapes) was added to the saliva cultures and bacterial growth was monitored. Glycoside hydrolysis by the microbes was measured by monitoring and analyzing the volatile compounds released after four time periods (0hr, 2hr, 24hr, and 72hr).
To test the ability of human oral microbiota to hydrolyze glycosidic compounds in general, a standard solution of octyl-β-D-glucopyranoside was cultured with the saliva samples and monitored over time.
- No human oral microbiota was found in the sterile or non-enzymatic saliva treatments (as expected).
- Adding octyl-β-D-glucopyranoside to human saliva resulted in hydrolysis and the release of the volatile compound aglycone.
- None of the oral microbes were inhibited by the glycosidic extract.
- Every oral microbe tested was able to hydrolyze the glycosidic compounds in the grape extract, resulting in the release of terpenes, benzenic derivatives, and C6 alcohols.
- Note: Many of these compounds can be associated with various aromatic characteristics in wines (e.g. terpenes can produce flowery or citrus aromas and certain benzenic compounds such as β-phenylethanol can produce rose aromas.)
- While some aromatic compounds were found as a result of the oral microbes hydrolyzing the glycosidic compounds in the grape extract, other common compounds were not present (i.e. C-13 norisoprenoids, vanillins, and volatile phenols).
- The ability to hydrolyze and the resulting aromatic compounds produced from this hydrolysis depended upon the type of oral microbe present.
- A. naeslundii was the producer of the most linalool and its oxides, which is associated with floral notes in grapes and wine.
- E. faecalis, A. naeslundii, and S. mutans produced the most aroma-causing aglycones.
- G. adiascens, V. dispar, and F. nucleatum produced the fewest aroma-causing aglycones.
- NOTE: Some of these bacteria have difficulty growing in culture media, so it isn’t clear whether their lack of glycosidic hydrolysis is real or a function of being unable to grow under the experimental conditions.
- There were significant differences between the three volunteers in terms of the species make up of their mouths. No person had the same species in the same amounts.
- While the microbial counts were statistically the same for each volunteer, the aromatic aglycones produced were significantly different, which is likely due to the different species present in each individual’s mouth.