Alec Sobotka, Yanping Qian, Ye Feng, Taylor Krueger, Chong Fang, Steven Mayer, and Michael Qian*
*Oregon State University, 3051 SW Campus Way, Lab 244, Corvallis, OR, 97331 (michael.qian@oregonstate.edu)

Smoke exposure in grapes has become a major concern for the United States wine industry. Smoke exposure can lead to off-aromas with a smoky, ashy, and medicinal characteristic. When grapes are exposed to heavy smoke, volatile phenols and other volatile compounds are absorbed by the grapes and subsequently bind to the sugars to form glycosides. When the grapes are fermented, the off-aroma compounds are released from their bonded glycosides, contaminating the produced wine. The volatile phenols, including guaiacol; 4-methylguaiacol; and m-, o-, and p-cresol, are the most studied compounds related to smoke exposure. These compounds are typically analyzed by gas chromatography-mass spectrometry (GC-MS), providing information on smoke exposure and potential smoke taints. Due to the costly and time-consuming nature of GC-MS, we strove to find a more efficient method to quickly assess smoke exposure based on surface-enhanced Raman spectroscopy (SERS).

A Thermo Scientific Nicolet 6700 FT-IR / NXR FT-Raman spectrometer with a 785 nm laser was used to collect spectra. Raman spectra of volatile phenols were compared in synthetic wine. Silver nanoparticles were prepared in the lab and various silver and gold nanoparticles were obtained commercially. These nanoparticles were evaluated by SERS to enhance detection sensitivity. It was found that the commercially prepared gold nanoparticle strips can significantly enhance the absorption of guaiacol at 576 cm-1. Using gold nanoparticle strip SERS, guaiacol can be detected at 100 µg/L in synthetic wine. Further research is being conducted on authentic wines.

Funding Support: American Vineyard Foundation and Oregon Wine Board