How Barrier Sprays Can Better Protect Grapes From Smoke Damage

As wildfire events become more frequent across California, Washington, and other Pacific Northwest states, smoke damage to high-value crops is an increasingly more common problem. Wine grapes are particularly susceptible to such damage from smoke exposure.

Barrier sprays are showing potential in reducing fruit absorption of volatile phenols, according to Tom Collins, Associate Professor at Washington State University (WSU) and leading scientist on smoke exposure and wine research. He shared some important research outcomes during a webinar in January.

ADSORBENT DOUBLE ACT

Barrier sprays are materials that can be applied before, and in some cases, during early stages of smoke exposure to form a type of barrier to prevent compounds in smoke from impacting grape quality and wine taste, he said.

Collins and his team started their initial trials with barrier sprays in 2018, focusing on kaolin and bentonite. “We make slurries of these two clay compounds and spray them on the vines,” he explained. “They are strong adsorbents and may be capable of capturing some of the smoke compounds before they impact the fruit.”

One study that informed his team’s work was Australian research involving kaolin. Its results showed that within a day of the end of smoke exposure, most of the guaiacol had been converted to glycosides in the exposed fruit. As far as kaolin application effectiveness is concerned, it was dependent on application rate, how much was applied, and whether good coverage was achieved on the fruit and on the vine. One thing not clear from the study was whether kaolin was removed before researchers did any analysis or winemaking with the fruit. This is where the WSU research began, Collins explained.

After some inconclusive studies in three prior seasons, a new study of kaolin and bentonite applications was launched in 2021. Kaolin or bentonite was applied as a slurry the day prior to a scheduled smoke exposure. After seven days, researchers went in with a backpack sprayer to wash off as much of the product as possible on target vines.

Two controls were part of the study. One, where nothing was applied, and the other, where the products were applied but not rinsed off.

Collins’ team returned at normal maturity, harvested the fruit, produced bucket scale wines and micro scale ferments with the fruit, and analyzed them for free and total volatile phenols, he said.

In the controls, researchers rinsed some of the vines and others were left unrinsed. In this case there was no difference in the level of guaiacol between rinsed and unrinsed grapes. On the other hand, there was a substantial difference between vines that were applied with kaolin or bentonite and rinsed, and those that were not rinsed.

“When kaolin and bentonite are not rinsed off, they both have higher numbers than the vines where we didn’t do anything,” said Collins. “And I think this shows we are adsorbing compounds from the smoke, and if you don’t wash off the application afterwards, the kaolin releases them, and they are absorbed into the fruit. We end up with higher numbers than if we hadn’t done anything at all.”

He added that their use of backpack sprayers may have limited the potential benefit of rinsing. Using a more thorough method of washing the fruit would probably result in a further reduction.

KAOLIN TRIAL

Although results are not in, a barrier spray trial using kaolin was conducted last year. In this trial kaolin was applied to either the entire canopy or only the fruiting zone. After smoke exposure, kaolin was washed off half of the vines using backpack sprayers.

“We wanted to understand if we need to apply kaolin to the entire canopy to get an effect,” Collins explained. “As we did in the previous study, we went in a couple days after the smoke event and washed the kaolin off half the vines using backpack sprayers. Once these results are available, we’ll have four years of this study completed and will be able to put a paper together.

“One of the key lessons here,” he continued, “is that if you’re going to use kaolin or bentonite, you have to go back in and wash it off after the smoke is cleared, which may mean you have to reapply it if there is a risk of second exposure. But I don’t think you can leave it on the vine for very long once the smoke is cleared.”