Extraction of grape juice: impact of laboratory-scale pressing methods on the chemical composition

The monitoring of grape maturity is essential for determining the ideal harvest time as well as for obtaining the expected characteristics of grape juice and, consequently, of wine. This study aimed to examine the impact of various laboratory-scale pressing methods on key chemical parameters (sugars, pH, and titratable acidity), as well as on phenolic-related indexes and antioxidant activity, in juices from Chardonnay (six grape samples) and Pinot blanc (two grape samples) across two vintages (2022 and 2023). The grape samples were characterized in terms of total and extractable flavonoids and extractability (%). Four different methods (manual pressing, vacuum pressing, small screw press, and juicer) were applied for producing grape juice. The results showed relevant differences in the extractability among the grape samples up to 64.1% and 43.8% for harvests 2022 and 2023, respectively. Comparable sugar content, pH, and titratable acidity were found in the juice samples independently from the method used. On the contrary, notable variability among the different pressing methods was revealed for phenolic-related indexes and antioxidant activity. The small screw press led to lower total phenol index values across grape batches compared to other methods, while the juice samples obtained with the juicer revealed a composition consistently differing in comparison to the other methods. Raman spectroscopy allowed to clearly classify the juice samples based on the pressing method. Good predictive models were obtained due to the composition of juice samples being clearly distinct among methods. This data suggests that an appropriate pressing method should be adopted for monitoring the grape ripening as well as for simulating the pressing under industrial scale.