of cream of tartar, may be reduced to 62 hours (including 24 hours without agitation before filtration), instead of six days for the standard treatment. Under these conditions, the wine was found to be perfectly stabilized (TSat = 7°C).
1.7.3 Rapid Cold Stabilization: Static Contact Process
This technique has the major advantage of reducing the artificial cold treatment of wine to four hours, and sometimes less for white wines. Furthermore, the wine no longer has to be maintained at negative temperatures, but only at 0°C, which minimizes not only energy consumption but also frost accumulation on the equipment. A heat‐insulated, conical‐bottomed tank known as a crystallizer is used. It is equipped with a drain to remove excess crystals at the end of the cycle.
Such high‐performance levels can only be achieved with this type of rapid stabilization treatment by seeding with large quantities of cream of tartar (400 g/hl). This large mass of crystals, with a small initial particle size, must absolutely be maintained in suspension by an agitator, taking care to avoid any unwanted aeration (Section 1.5.2). It is also advisable to blanket the wine with inert gas, or at least use an airtight crystallizer.
Treatment effectiveness is monitored by the rapid response analysis described in Section 1.6.4. If the results are satisfactory, agitation is stopped in order to allow most of the tartrate to settle in the conical bottom of the crystallizer. Complete clarification is not easy to obtain. Great care must be taken in using centrifugation as the crystals are highly abrasive. Good results are obtained with horizontal plate filters, using the crystals themselves as the filter layer. Of course, all these operations must be carried out at 0°C.
The static contact process is a very flexible system. It is possible to run two to three cycles per day with volumes of 50–100 hl in each batch. This technology is advisable for small‐ and medium‐sized wineries. The weak point of this system is the price of cream of tartar, but costs may be reduced by recycling tartrate.
In the case of white Champagne base wines, it has proved possible to recycle the tartrate four times, with almost constant treatment effectiveness (Table 1.17). The continued effectiveness of the treatment, even when the tartrate has been recycled four times, has been explained (Maujean et al., 1986). It was shown that the smallest particle size after treatment (<50 μm) was larger than the initial size in the commercial product.
TABLE 1.17 Changes in the Physicochemical Parameters of Cold‐Stabilized Wine When the Contact Tartrate Was Recycled (Maujean et al., 1986)
| Number of times used | K+ (mg/l) | Total acidity (g/l H2SO4) | Tartaric acid (g/l H2SO4) | pH | pC × 105 |
|---|---|---|---|---|---|
| 1 | 315 | 4.93 | 1.59 | 3.11 | 6.83 |
| 2 | 325 | 4.92 | 1.54 | 3.12 | 6.88 |
| 3 | 320 | 4.90 | 1.59 | 3.11 | 6.84 |
| 4 | 300 | 4.98 | 1.83 | 3.09 | 7.35 |
| 5 | 320 | 4.94 | 1.55 | 3.08 | 6.57 |
Of course, recycling is not possible when red wines are treated, as the crystals become coated with phenols and coloring matter and rapidly lose their effectiveness.
1.7.4 Rapid Cold Stabilization: Dynamic Continuous Contact Process
Unlike the preceding “batch” technology, the process described in Figure 1.17 is a continuous bitartrate stabilization process, where the length of time the crystals are in contact with the wine, i.e. the treatment time, is defined by the throughput in relation to the volume of the crystallizer. Thus, for example, if the throughput is 60 hl/h and the volume of the crystallizer is 90 hl, the average time the wine spends in the system is 1 hour 30 minutes.
This emphasizes the need for a method of monitoring effectiveness with a very short response time. There is, of course, a system for recycling wine through the crystallizer if the treatment is insufficiently effective, but the results must be determined very rapidly, as the energy required to treat these quantities of wine is expensive, and unnecessary extra treatment will by no means improve quality.
Continuous treatment is understandably more demanding than the other processes, because it requires close monitoring, but it is also more efficient. For example, the particle size of the contact tartrate and the level in the crystallizer must be monitored by sampling after a few hours, using the drain system.
FIGURE 1.17 Schematic diagram of a continuous cold stabilization system: 1, intake of wine to be treated; 2, heat exchanger; 3, refrigeration system (with compressor, condenser, etc.); 4, insulation; 5, mechanical agitator; 6, recycling circuit (optional); 7, outlet of treated wine; 8, filter (earth); 9, drain; 10, overflow.
Agitation is partly provided by a tangential input of wine into the crystallizer. This creates turbulence in the bulk of the liquid and maintains at least the smallest crystals in suspension. The wine may also be mechanically agitated.
The throughput, i.e. the average time in the crystallizer, is defined according to the wine's initial state of supersaturation, as well as the type of preparatory treatment (fining, bentonite, etc.) received prior to artificial cold stabilization. The importance of preparation has already been mentioned (Section 1.6.4).
The effectiveness of the three processes described above is generally satisfactory, although results depend on the type of wine (white or red), its alcohol content, and any previous treatment or fining.
It is true that, in contact treatments involving large‐scale seeding, the wine's background is less important. Indeed, enologists do not always have this information if the wine has been purchased from another winery. In any event, wine must be well prepared and, above all, properly clarified to ensure the effectiveness of rapid artificial cold stabilization treatments.
