The Grape Health Index: Validation of a Chemometric Model for Quantifying the Wine Grape Infection Status

Accurately evaluating and quantifying microbial spoilage on wine grapes is a major challenge, especially in machine-harvested fruit that is no longer intact as a cluster when it arrives at the winery and cannot be visually inspected. The goal of the study was, using an infrared spectroscopy-based analytical system, to establish a unitless quantitative number that would reflect complex microbial spoilage and could be applied for all types of grapes with only one calibration model. Grapes (cultivars Riesling, Chenin Blanc, Chardonnay, Zinfandel and Petite Sirah) were hand-harvested in two consecutive vintages and separated in the vineyard into visually healthy clusters and infected grapes. Grapes were blended with increasing infection levels between 0 and 20% and models were created using known spoilage indicators. The resulting formula can be used to calculate a weighted index that reflects the microbial infection status of the grape material. Using a common spectroscopy instrument that is already present in larger wineries, the Grape Health Index allows for a quantitative quality assessment within two minutes at the test stand before the grape material is accepted. When visual inspection is not an option, this can help to make data-based quality and blending decisions at a very early stage.