Development of Flash Détente Applications for Impacting Red Wine Style and for Production of Colour Stable Red Wines and Concentrate

Abstract

The basic process flow for making red wines has barely changed over several millennia. Recent advances in technology have led to the development of new or modified processes for red wine production, for example, flash détente (FD). FD processing consists of heating grape must to ~85 C followed by application of vacuum to boil or ‘flash off’ water in grape tissues, resulting in cellular damage that aids extraction of key grape skin compounds into the juice. This technology offers the opportunity to reconfigure the red winemaking process by separating grape colour and phenolic extraction from fermentation. Potential financial benefits include less capital expenditure by using inexpensive white wine fermenters for red wine production, and decoupling fermentation from harvest, through production of red juice concentrate. However, since FD is relatively new, very little research has been undertaken to evaluate the application of this technology to the production of either red grape concentrate or red wine. The research outlined within this thesis therefore aimed to develop an understanding of the impact of FD treatment on red colour extraction, quality, and stability during red juice concentrate production, as well as to assess the impact of fermenting FD treated red juice or must (under different fermentation conditions) on the style, composition and sensory properties of red wine. This work also investigated treatments for stabilising the colour of FD-derived red grape concentrate and red wine. The use of FD in the production of red juice concentrate as a food and beverage colouring has not been well studied. Colour concentration, quality, stability, and filterability were compared and contrasted for Rubired concentrates obtained with traditional conventional must heating (CMH) or FD extraction. FD concentrate had similar levels of red colour and concentrations of C6 (‘green’) aroma compounds, a lower ratio of brown colour, and higher concentrations of caftaric acid and catechin, compared to CMH concentrate. FD processing also generated a significantly higher concentration of grape suspended solids and the juice had four times lower flux compared to CMH juice. Red colour stability was greater for CMH concentrate under normal concentrate storage conditions. The colour stability of Rubired concentrate produced via FD and CMH was compared under accelerated ageing conditions, i.e. by heating at 50, 60 and 70 C, with or without different chemical treatments (i.e. acid addition to lower pH, acetaldehyde or commercial seed tannin addition, applied either individually or in combination) to assess their impact on colour stability. Compositional analyses were performed after 0, 3, 6, and 9 days, to gain insight into colour stability, 5-hydroxymethylfurfual (5-HMF) formation and browning. CMH concentrate had significantly greater 5-HMF formation and red colour stability compared to FD concentrate, as shown by a half-life and activation energy of 233.9 hours and 65.2 kJ/mol versus 203.3 hours and 59.2 kJ/mol respectively, after heating at 50 C. Red colour, malvidin-3,5-O-diglucoside and malvidin-3-O-glucoside loss followed first order reaction kinetics. Acetaldehyde, low pH, and their combination increased red colour stability, as well as violet and brown colour with heating at 50 and 60 °C whereas seed tannin had no significant effects under all treatment conditions. At 70 °C, only acetaldehyde treatment increased red, brown and violet colour, while all treatments involving low pH decreased red, brown and violet colour units, due to acid hydrolysis at the higher temperature. Making red wines from FD derived grape juice or possibly even reconstituted concentrate requires understanding the impact of ‘off-skins’ fermentation conditions, including fermentation temperature and suspended solids content, on wine sensory profile. FD-derived Cabernet Sauvignon juice was fermented ‘off-skins’ with or without suspended grape solids at three different temperatures, i.e. 16, 24 and 32°C. Low fermentation temperature and low suspended solids content increased the concentration of most esters, while high fermentation temperature and high suspended solids content increased the concentration of fusel alcohols, polysaccharides and glycerol. High temperatures also increased linalool concentrations, which were unaffected by solids content. Classic maceration ferment (i.e. the control) gave the highest concentrations of fusel alcohols and 1-hexanol. Descriptive analysis results showed that removing grape solids from FD-derived Cabernet Sauvignon juice prior to fermentation led to wines with increased red fruit (raspberry and strawberry) and confectionery (candied fruit) attributes, while fermenting on 3.5% grape solids increased dark fruit (blueberry and plum jam) notes. Traditional maceration fermentations had significantly higher green and savoury notes compared to all FD treatments. The impact on wine composition and style of fermenting FD treated Merlot must with different levels of skin contact was also investigated. Skin contact ranged from fermentations of juice with no solids contact or zero contact time, to ‘on-skins’ fermentations for which draining and pressing occurred at 17 °Brix, 7 °Brix and 0 °Brix, representing 1, 2 and 5 days of skin contact respectively. On-skin fermentations of FD treated must produced wines with significantly higher intensity ratings for dark fruit, body and astringency, compared to fermentations without skin contact (i.e. ‘off-skin’ fermentations). FD wines received significantly lower green, savoury and dusty ratings compared to control wine from unheated ‘on-skins’ ferment. One of the key challenges with ‘off-skins’ red wine production is colour loss during fermentation. For ‘on-skins’ fermentations, colour loss is mitigated by continuous anthocyanin extraction from skins. Pre-fermentation addition of oenological tannins or toasted oak chips were investigated as methods for stabilising the colour of red wines made from FD-derived juice. Off-skins fermentations, with or without the addition of 0.4 g/L of oenological tannin, produced wines with significantly higher red fruit and confectionery ratings, while fermentations with 4 g/L of medium toasted oak chips gave wines which exhibited increased dark fruit, vanilla, toasty and allspice characters. However, pre-fermentation addition of oenological tannin or toasted oak chips did not improve wine colour stability. This research demonstrated that FD can be used to produce higher quality Rubired concentrate with a lower ratio of brown colour and 5-HMF formation compared to CMH. However, FD had the downside of producing concentrate with poorer filterability and less stable red colour, which could potentially increase production cost and present challenges in some food applications where high colour stability is a key requirement. FD followed by ‘off-skins’ or ‘on-skins’ fermentations under different fermentation temperature, suspended solids concentration, and skin contact time (accompanied by different alcohol strengths) conditions was demonstrated to create differentiated wine styles that can potentially be used as stand-alone wines or blending legs for wine style attainment. Other findings from this research offer insights into the capacity of chemical treatments (pH adjustment, seed tannin and acetaldehyde) and the use of winemaking additives (oak wood and seed tannin) to stabilise the colour of red grape concentrate and FD derived ‘off-skins’ wines respectively. The knowledge gained from this research will help guide winemakers’ use of FD in red winemaking, including ‘off-skins’ fermentations which offer greater logistical flexibility, to deliver differentiated wine styles.