Handbook of Enology Volume 1 the Microbiology of Wine and Vinifications

Handbook of Enology Volume 1 The Microbiology of Wine and Vinifications 2nd Edition Pascal Ribe reau-Gayon Denis Dubourdieu Bernard Done`che Aline Lonvaud Faculty of Enology Victor Segalen University of Bordeaux II, Talence, France Original translation by Jeffrey M. Branco, Jr. Winemaker M.S., Faculty of Enology, University of Bordeaux II Revision translated by Christine Rychlewski Aquitaine Traduction, Bordeaux, FranceCopyright 2006 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England

Cold Pressing (Cryoextraction) Cryoextraction permits the selection of the most sugar-rich and therefore ripest grapes. Chauvet et al. (1986) first proposed this technique. Cryoextraction is based on a law of physics. Raoults’s law states that the freezing point of a solution lowers as the solute concentration increases. When the temperature of a white grape crop is lowered to 0◦C, only the grapes containing the least amount of sugar are frozen. By pressing at this temperature, a selected juice is obtained which represents only part of the total juice volume. The potential alcohol content of this juice, however, is higher. By further lowering the temperature of the grape crop and the pressing, the number of frozen berries is increased. As a result, the selected must volume is further diminished and the potential alcohol content increased. Table 14.4, which gives the results of an experiment, specifies the increase in potential alcohol content for grape crops pressed at different temperatures. The selected must volume represents between 60 and 80% of the total volume, depending on the temperature. The selection of the most sugar-rich and thus ripest grapes by cryoextraction is the primary cause of improved wine quality. Freezing may also concentrate grapes diluted by rainy harvest sea- sons. A phenomenon called supraextraction is also involved. Freezing and thawing grapes results in tissue destruction and a better sugar extraction. In practice, the grape crop is collected in small containers and frozen in a walk-in freezer at vary- ing temperatures as low as −16◦C. The grapes are then pressed in a horizontal moving-head press or in a pneumatic press. The freezing point of the richest grapes is sufficiently low that their juice may be extracted without difficulty. Table 14.5 Table 14.4. Effect of pressing temperature (cryoextraction) on the volume and alcohol potential (% vol.) of selected and residual musts from noble rot grapes (Sauternes) (Chauvet et al., 1986) ControlPressing musts: temperature potential (◦C) alcohol (% vol.) 16.7 −5 16.0 −6 16.0−8.5 16.2 −10 16.0 −11 17.0 −15 Selected musts Volume Potential (hl) alcohol (% vol.) 116 19.4 81.5 19.6 74.5 20.7 6522.0 6723.2 6023.4 Residual musts Volume (hl) 19 21.5 33 37 33 40 selected must/ Volume ratios Potential blended must alcohol (% vol.) 11.3 86 12.379 9.8 69 13.164 10.767 12.660454 Handbook of Enology: The Microbiology of Wine and Vinifications Table 14.5. Improvement of Sauternes must quality in 1985 by using cryoextraction (Chauvet et al., 1986) considerable oxidation of grape phenolic sub- strates during overripening, however, the oxidation risks are less significant than one would think. 2. By blocking fermentation for several hours, it permits a coarse clarification by sedimentation. 3. It is also capable of inhibiting the development of acetic bacteria, often present in large quan- tities on botrytized grapes, as well as lactic bacteria, responsible for malolactic fermentation. Finally, sulfiting restricts the proliferation of some spoilage microorganisms, particularly apiculated yeasts that “waste” sugar and form unwanted by-products in the wine. 4. It destroys antifungal substances and thus facilitates alcoholic fermentation. Sulfiting juice intended for botrytized sweet wine production has often been criticized. This operation leads to increased concentrations of bound sulfur dioxide, which remains definitively in the wine. Subsequent SO2 additions must therefore be limited to remain within legal total SO2 limits, thus compromising the microbiological stabiliza- tion of wine. In practice, this inconvenience of sulfiting is attenuated by the fact that only 40–60% of the SO2 added in juice is found in the bound form in wine. The rest is oxidized into SO3. To conclude, a light sulfiting at 3 – 5 g/hl, for example, is generally recommended at this stage. (d) Juice Clarification, Bentonite Treatment and Juice Corrections Clarification is an indispensable operation in white winemaking, but its advantages are more disputed for botrytized sweet wines. It has been blamed for making wines thinner. This criticism seems excessive—in most cases, clarification improves aromatic finesse and gustatory qualities. However, the natural settling of botrytized musts is difficult to effect: the suspended solids and the highly dense juice have similar specific weights. Must viscosity is also high due to the high sugar concentration and the presence of glucidic colloids produced by Vineyard Grape temperature at potential alcohol (% vol.) A −13 B−9 C−7 D−7 E−9 pressing (◦C) Control must must 15.9 21.5 13.2 19.8 13.0 19.0 13.6 20.3 18.0 23.3 gives the results obtained for five Sauternes vine- yards in 1985. Installing a cryoextraction system is expensive but energy costs are reasonable. In any case, this technique can only be applied for limited grape crop volumes. The improvement in must quality is not only limited to an increase in potential alcohol content. These musts have the same characteristics as their late-harvested counterparts: they are more difficult to ferment; the risk of volatile acidity production is greater; and the sulfur dioxide binding rate is higher. However, these cryoextraction selected wines are clearly preferred over the control wines (Chauvet etal., 1986) due to their increased concentration, finesse and distinctiveness. This technique is applicable to all types of white grape crops but is especially interesting for noble- rotted grape crops. Excessive humidity during overripening can prevent the grapes from obtaining the desired concentration levels. Prolonging the concentration phase would only increase grape degradation and lower crop quality, but, by picking early, cryoextraction can be used to eliminate excess water and select the grapes capable of producing quality botrytized sweet wines. The quantity is diminished but quality is maintained. This method can complement manual sorting during the harvest, but cannot replace it.