The mysterious moods of the wine grape
Pessimism and optimism are personality traits usually assigned to humans. Generally it’s people who can have either a negative or positive outlook on life, but one Washington State University graduate student is studying the outlooks of an unlikely subject: the wine grape.
Joelle Bou Harb travelled from Lebanon to join the WSU Irrigated Agriculture Research and Extension Center (IAREC) viticulture team two months ago to pursue her doctorate in horticulture. Bou Harb has spent the summer studying what she describes as the “personalities” of 30 different varieties of wine grapes based on their isohydric or anisohydric behaviors, that is, their response to changing water availability or water potential.
During a drought the soil and air around a grapevine are dry. This causes a moisture deficit, activating drought-protection, or isohydric, behavior in the vine to prevent damaging water loss, Bou Harb explained. This means the vine’s stomata – tiny pores on the leaves and stem of the plant – will act like valves and close to decrease water loss.
But, not every grapevine responds the same way to a potential drought. Some are much less hopeful about the future than others. When an isohydric variety of grape vine is faced with increasingly dry conditions – in other words, a drought – the plant will actively store water, even to its own detriment. These “pessimistic” varieties act as if circumstances will never improve.
“These varieties have the most sensitive stomata and will keep their water potential above a certain threshold,” Bou Harb said, “even if this means a decrease in their photosynthesis.”
On the other side of the spectrum are anisohydric varieties. These “optimistic” varieties will allow the amount of water in their leaves to drop as environmental conditions dry out, depleting their own internal resources. These varieties act as if more resources will come in the future and thus, they will sacrifice water to maintain photosynthesis levels.
But, just like humans, grapevines can have good or bad days and their “mood,” or response, can change based on the conditions around them, such as soil moisture or temperature, said Bou Harb. This variation is what Bou Harb is hoping to map in the 30 grape varieties she is monitoring.
“My hypothesis is that the boundaries between different groups are fragile,” she said, “and that it is a question of leniency and degree of adaptation rather than the nature of the variety when it comes to ‘hydricity’ or behavior in the face of water loss.”
To categorize the varieties, Bou Harb is monitoring each variety’s stomatal and water storing behaviors, she explained. The dry, semi-arid climate of Eastern Washington is ideal for Bou Harb’s research, allowing her to monitor the vines’ behavior during the transition from cool nighttime hours to hotter daylight hours.
“My research includes a pre-dawn water potential measurement, when the water content of the vine should match the soil moisture levels because the stomata are closed,” she said. “I also measure the midday water potential at noon, when the atmospheric demand is at its peak.”
These two measurements allow Bou Harb to see the stomatal behavior of each variety in relation to environmental demands and help her categorize each variety into isohydric and anisohydric groups.
As the growing season comes to an end, Bou Harb is feeling the pressure to complete her study. She hopes to perfect her methodology before the season is over and, so far, her preliminary data indicates that vapor pressure deficit is a prominent factor in the stomatal behavior of varieties, which supports her prediction that a vine’s behavior is based more on the environment than on genetic predisposition. When she has finished her current analysis, Bou Harb would like to study leading varieties from each group to dissect the physiological basis for the measured behavior.
“First, we should consider the mechanism of stomatal closure,” she said. “Second, we need to understand the mechanism each variety uses to avoid drought effects.”
Eventually Bou Harb’s research will allow researchers to select the best varieties for specific environments and to assess the success of specific irrigation techniques. Understanding plant-water interactions on a physiological level offers numerous economic benefits, Bou Harb said.
And, finally, the grapevines have someone who understands them.
If you put a nose away, what can a tongue tell
This is the question that has driven doctoral student Charles Diako’s research as he explores the unique characteristics of Washington wines. This month, he’s contributed an “ah ha” moment from his experience as a student and researcher in wine sensory science.
The tongue and the nose are two important organs in sensory perception. And, while they seem to work in tandem, they are different in structure and position in the body, as well as in function. While the tongue perceives tastes (sweet, bitter, sour, salty, and umami), the nose functions to detect and discern a wider range of odors or aromas and flavors.
Taste compounds must be in solubilized in liquid to facilitate their detection by the taste buds on the surface of the tongue. In comparison, the nose detects stimuli in gaseous form. In spite of these differences, synergism between these two organs leads to the detection of food flavors. But, in the absence of this synergism, can the function of the tongue still provide some insight into what the nose helps perceive?
My recent research on wine fault evaluation using an electronic tongue, or “e-tongue,” through Carolyn Ross’s lab within the Washington State University/University of Idaho School of Food Science, led to intriguing findings that seem to suggest that some quality defects in wines that are primarily detected by the nose also have unique taste profiles (in some cases, subtle) that the e-tongue can capture.
One compound that can alter the sensory properties of wine is produced by the wild yeast Brettanomyces when it grows in wine: 4-ethyl catechol (4-EC), which is primarily sensed as an olfactory component, or aroma. At low levels, some winemakers feel this compound exerts a positive effect on wine. However, at high concentrations, this and other brettanomyces-related compounds generally exert a negative effect.
To test this, different concentrations of 4-EC were spiked into red wine samples and those samples were subjected to sensory threshold testing to determine the levels at which consumers can detect an aroma difference in wines. Those same samples were “tasted” by the e-tongue for taste profiling. The concentration at which consumers began detecting differences among wine samples was 823 µg/L, while the concentration that the e-tongue was able to detect was 493 µg/L, thus showing that the electronic tongue was more sensitive in detecting 4-EC.
While the control sample (with no added 4-EC) displayed a high sensor response for all seven attributes that the e-tongue can evaluate (sweet, bitter, sour, salty, and umami as well as spicy and metallic), there was a progressive attenuation, or loss, of the sensor signals with increasing amounts of the compound in the wines, leading to different taste profiles for the samples.
Perhaps the electronic tongue, a novel piece of equipment, holds a lot more promise for wine research beyond discernment and taste profiling. The detection of unique signatures of wine taste attributes as a result of wine faults could add another use for this instrument in wine research. This was definitely an “Ah ha!” moment for me and my team, and will be the rationale for further exploration of the use of the e-tongue in the quest for even more high quality wines from Washington State.
Students talk wine at WSU Visitor’s Center
At the first WSU football home game of the year, Devon Griffith, vice president of the Viticulture and Enology club in Pullman, as well as Kaelin Campbell, the club president, wine business management student Ally Stariha poured wine at the Brelsford WSU Visitor’s Center. Keep an eye out for upcoming wine events at the visitor’s center, here.