The use of fluorescence spectroscopy to develop a variability index and measure grape heterogeneity

AIM: This work aims to investigate fluorescence spectroscopy as a tool to assess grape homogenates to discriminate between samples of varying maturities and to develop an index to objectively characterise the level of grape heterogeneity present in any given vineyard.

METHODS: Cabernet Sauvignon grape bunches were sampled every ten days from veraison through to harvest from the Coonawarra Geographical Indication of South Australia in 2020. After sorting into maturity classes using density baths,¹ berries were homogenised and an Aqualog spectrophotometer was used to record the excitation emission matrix (EEM)² of each maturity class at each sample date. The pre-processed EEM data underwent  parallel factor analysis (PARAFAC) to identify the relevant fluorescence regions that discriminated samples based on maturity. The grape homogenate EEM dataset was then used to formulate a variability index.

RESULTS: Chlorophyll and anthocyanin fluorescence signals were identified from EEM data at excitation wavelengths in the range 250 – 700 nm and emission wavelengths between 400 – 800 nm in grape homogenate samples using PARAFAC. Discrimination between samples depending on maturity was achieved using PARAFAC. The variability index was calculated and levels of grape heterogeneity were quantified.

CONCLUSIONS: This work demonstrated the possibility of using grape homogenate EEM data, particularly in the region of chlorophyll and anthocyanin fluorescence, to objectively measure grape heterogeneity by developing a variability index. Grape heterogeneity has been shown to impact Cabernet Sauvignon wine chemical profile and sensory characteristics.³ Therefore, a tool to analyse grape heterogeneity within a winery could aid viticultural and winemaking decisions to achieve wines of targeted quality and style.