Integrated profiling, fingerprinting, and chemometrics as a tool for distinguishing the impact of Pulsed Electric Fields (PEF) pre-treatment, winemaking, and storage on Merlot grape juice and wines

Abstract

The thesis mainly aims to evaluate the impact of Pulsed Electric Fields treatment of high electric field strengths (33.1 and 41.5 kV/cm) with specific total energy levels of 16.47 and 49.40 kJ/L on Merlot grapes, PEF-processed at commercial scale (500 kg/hr). The Merlot juice obtained immediately after PEF processing was examined at pre-maceration (PM). The fermenting must and the resulting wine were assessed on the completion of maceration - alcoholic (MAF) and malolactic fermentations (MLF), respectively. The wine stability was further tested during bottle storage at different temperatures (4°C, 25°C, and 45°C) for 150, 120, and 56 days, respectively. Comprehensive assessment of these samples was achieved through combined targeted profiling of phenolic compounds, colour, and oenological properties, and untargeted volatile fingerprinting coupled with multivariate data analysis in order to confidently identify discriminant markers that are most affected by winemaking, storage, or PEF treatment. These markers were further linked to relevant (bio)-chemical reactions.

Upon winemaking, grape musts at PM typically contained the highest concentrations of C6 compounds (compounds composed of six carbons and other elements) which are green odourants. After MAF, more phenolic compounds and aroma precursors were extracted and transformed by yeast metabolism, which developed the colour and aroma of the fermenting musts. At the end of MLF, the extracted phenolic and aroma compounds decreased and transformed into the derivative compounds as a result of lactic acid bacteria metabolism. It was important to note that grape samples at each winemaking stage were consistently distinguished according to the intensity of specific energy applied before maceration. High specific energy PEF treatments of 49.40 kJ/L produced grape juice with the highest concentrations of anthocyanins, flavonols, stilbenes, hydroxycinnamic acids, flavanonol, and flavanols. Interestingly, a PEF pre-treatment on grapes was found to immediately reduce C6 compounds in juices at PM. After MAF, the young wine made from PEF-pre-treated-grapes contained more anthocyanins, stilbenes, and flavanols, more volatile esters, and a lower green odourant 1-hexanol than the untreated (no-PEF treated) counterpart. After MLF, wines produced from grapes treated with PEF at high specific energy retained higher levels of anthocyanins, stilbenes, flavonols, flavanols, hydroxycinnamic acids, and hydroxybenzoic acids. Overall, application of high specific energy and high electric field strength of PEF on Merlot grapes led to production of finished wines with a distinct phenolic composition and volatile profile.

As a function of storage time, an increment in the phenolic acids in conjunction with a decrement in the monomeric anthocyanins resulted in dulling the colour for all wines. Esters and acetates were the most reduced compounds in wines at 4°C over time. Degradation reactions including Maillard, anthocyanin cleavage, and hydrolysis were particularly accelerated at 45°C over time. Interestingly, only the wine made from grapes treated at high specific energy and high electric field strength had a decrease in anthocyanins when stored at 4°C. Storage at 25°C increased the phenolic acids in the wines made from PEF-treated grapes. At 45 °C, citronellol and 2-phenylethyl acetate in control wines were significantly reduced as a function of storage time. Meanwhile, only citronellol was lost at higher rates in wines made from grapes treated at high specific energy; and, only 2-phenylethyl acetate was diminished at faster rates in wines from grapes treated at low specific energy. More furan compounds were formed in control and wine obtained from grapes treated at low PEF intensity. Overall, PEF treatment applied to grapes at the initial stage of winemaking appeared to impact wine composition evident even after bottle storage.

These findings clearly evidenced the potential of pre-treating grapes with PEF to produce wines with unique characteristics at an industrial scale. Moreover, integrated targeted profiling and untargeted fingerprinting coupled with multivariate data analysis and discriminant marker selection was shown successfully, for the first time in the literature, as a useful tool to optimise electric field strength and specific energy of PEF treatments for winemaking application, as well as in describing the distinct properties of the wine produced.