The flash release (FR) process, consisting of rapidly heating the grapes and then applying strong vacuum, has been proposed to increase the polyphenol content of red wines. Its impact on polyphenol extraction kinetics and on the polyphenol composition of red juice and wines was studied over two seasons on different grape varieties (Grenache, Mourvedre, Carignan). The FR process allows fast extraction of all phenolic compounds (hydroxycinnamic acids, flavonols, anthocyanins, catechins, proanthocyanidins) and can be used to produce polyphenol-enriched grape juices. However, the concentration of all polyphenols dramatically decreased throughout fermentation when pressing was achieved immediately after FR. The FR wines made with pomace maceration were also enriched in polyphenols compared to the corresponding control wines. Increasing the duration of high-temperature exposure in the FR treatment further increased extraction of phenolic compounds but also accelerated their conversion to derived species. The tannin-to-anthocyanin ratio was particularly low in the wine fermented in the liquid phase, higher after FR than in the control, and even higher after longer heating. FR resulted in an increased tannin-to-anthocyanin ratio and an increased conversion of anthocyanins to tannin-anthocyanin adducts showing the same color properties as anthocyanins. The tannin-to-anthocyanin ratio was particularly low in the wine fermented in the liquid phase that also contained larger amounts of orange sulfite bleaching-resistant pigments.
<p style="text-align: justify;"><strong>Aims</strong>: Thermo-treatment of grapes, followed by pressing and fermentation in liquid phase, is a growing practice in red winemaking to obtain light and fruity wines. Must clarification before fermentation, a key step to get the expected wine profile, is hardly controlled and strongly varies between different musts. To better understand this variability and its potential impact on quality, suspended solids in several red musts were characterized and the performances of different clarification techniques compared.</p><p style="text-align: justify;"><strong>Methods and results</strong>: Results show a large variability in turbidity and total wet suspended solids between different raw and clarified musts, and a lack of correlation between these values. Clarification is always higher for vacuum filtration than for disk-stack or decanter centrifugation, with strong differences between musts for a given process. Despite a large size distribution, most of suspended particles are micronic and sub-micronic. TEM observations and analyses indicate that they are mostly membrane and organelle fragments along with (macro)molecular aggregates formed during juice extraction. Their overall composition differs from that found in white musts.</p><p style="text-align: justify;"><strong>Conclusion</strong>: Particle heterogeneity and size distribution account for the difficulties encountered in red must clarification. Results also raise the question of the relationship between must turbidity and content in compounds likely to affect wine quality. </p><strong>Significance and impact of the study</strong>: This study constitutes a first characterization of suspended solids in thermo-treated red musts. It provides elements to (i) reason their clarification and (ii) identify the technological and qualitative impact of must suspended solids.
Membrane contactors are proposed as a new technology to control mass transfer of volatile compounds in separation processes. This technology consists in the use of a porous membrane which permits to immobilise the interface of transfer. Applications presented in this study concern the partial dealcoholisation of wine and the aroma recovery from agro-industrial processing waters. For these two applications, the stake is the selective transfer of the volatile compounds: 1) to maximise the ethanol transfer and minimise the aroma compounds in the case of partial dealcoholisation of wine and 2) to control the selective extraction of odorous compounds in the case of the deodorisation treatment of agro-industrial processing waters. Using the same technology, a hollow fibre membrane contactor, different configurations have been studied: Membrane-based solvent extraction (MBSE), Membrane Air-Stripping (MAS) and Isothermal Membrane Distillation (IMD). Based on systematic experimental studies on synthetic aqueous medium containing fifteen aroma compounds with a large range of physico-chemical properties an analysis of mass transfer in each configuration has been performed. The relation between mass transfer coefficients and partition coefficients in MAS and MBSE has been established (figure 1). Limiting steps of mass transfer have been identified and a resistance-inseries model to predict mass transfer has been proposed. The feasibility of a partial dealcoholisation of wine using hollow fibre membrane contactors has been studied according to the isothermal membrane distillation (IMD) principle. The selective transfer between ethanol and volatile aroma compounds has been analyzed according to thermodynamic properties of a set of key wine aroma compounds. The global analysis of the three configurations (MBSE, MAS and IMD) also allowed identifying which process is the most adequate regarding the nature of the volatile compounds involved. Figure 1. Relation between the overall aqueous phase-based mass transfer coefficient (Kf, m.s-1) of volatile compounds and their partition coefficient in MAS (left side) and MBSE (right side).
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