Garrett Morales, Sonet Van Zyl,* Jeremiah Loyd, and Stephen Sommer
*CSU Fresno, 2360 E. Barstow Avenue, Fresno, CA 93740 (svanzyl@mail.fresnostate.edu)

Within the San Joaquin Valley, daytime temperatures during the peak of the wine- grape harvest season can reach well over 37°C. As temperatures increase, biological and chemical reactions also tend to increase. Because of this, winegrapes are typically harvested in the early morning to combat the effects of these biological and chemical reactions, i.e., spoilage of the winegrapes by microorganisms. As many vineyards and processing facilities are spread out over large distances in the state, longer wait times at winery test stands give grapes in the gondolas the opportunity to be exposed to elevated temperatures. This in turn allows microbial spoilage to occur within the gondola before reaching the test site. With no standard method of measuring the impact of microbial spoilage within the gondola, other than visual observations, absorbance by FT-MIR spectroscopy is being used to create a standard. Fresno State has worked for 10 years on a calibration method using clean winegrapes inoculated with the most common forms of spoilage found in the Central Valley: Aspergillus niger, Rhizopus stolonifera, and Penicillium italicum. These molds are dosed by weight to create a linear trend that can measure the amount of microbial spoilage. Thirty-six samples were taken from nine different varieties during the 2019 harvest as well as 30 samples taken from 10 different varieties during the 2020 harvest to build upon this model. Analytical attributes that were tracked  throughout this study, which aid in monitoring the amounts of spoilage, include total soluble solids, tritratable acidity, ethanol, volatile acidity, tartaric acid, gluconic acid, and glucose. Currently, the most predominant indicators of spoilage appear to be ethanol, volatile acidity, and gluconic acid.

Funding Support: California Winegrape Inspection Advisory Board, Agriculture Research Institute