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
page 253
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 vol- ume
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)
Control Pressing 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 65 22.0 67 23.2 60 23.4
Residual musts
Volume (hl)
19 21.5 33 37 33 40
selected must/
Volume ratios
Potential
blended must alcohol
(% vol.)
11.3
86 12.3 79 9.8 69 13.1 64 10.7 67 12.6 60454 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 oxi- dation 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 fermenta- tion. 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.