7. Cold production techniques for the tartaric stabilization of wines

To carry out the tartaric stabilization, the wine is cooled to the temperature previously set according to the system used continuous or discontinuous. The temperature of the wine must be lowered to �5�C in discontinuous systems and from 0 to �2�C in continuous systems, to cause the crystallization and precipitation of potassium bitartrate and calcium tartrate. The required cold is generally supplied by a scraped surface exchanger, with glycol water or with direct expansion of the refrigerant.

An exchanger with scraped surface is constituted by a cylinder of 0.75–1 m of length and 0.15– 0.20 m of diameter, in whose interior a series of pallets is located around an axial axis with a rotation movement between 300 and 600 rpm. Inside the cylinder circulates the wine to be treated. In this way, concentric to the previous one, is arranged a cylinder or cooling jacket through which the refrigerating fluid is counter flow with the wine. The mission of the pallets is to remove and circulate the ice formed and deposited on the wall of the exchanger, around 10–20% according to the treatment temperature. According to McCabe et al. [8] and Geankoplis [9], the energy transferred from the glycolic water or the refrigerating fluid to the wine in a range t is calculated according to the expression (Eq. (28)):

$$\frac{\mathbf{Q}}{S} = 2 \text{ Uls } (Tv - T\alpha) \sqrt{\frac{t}{\pi \alpha}} \tag{28}$$

where

Two of the cases that take place, the refrigerating fluid changes of state for example R-717 evaporated to 5–7�C (in that case the fluid temperature is constant) or there is no change of state (glycol water), producing a heating throughout the cooling process. As approximate data,

For the purpose of calculating the equipment of the cold production facility, a simplified

Since the must/wine inside the tank is in steady state and this heat exchange equation is applied for counter current flowing fluids, the term (ΔT)ml is simplified to (Tinside tank�Tcirculating water). It should be noted that 50% of the refrigerating protein produced by the shirt dissipates in the environment surrounding the tank if it is not well heat-insulated. In case no insulation is available, the required shirt surface must be doubled, or, if appropriate, the cooling capacity

The cooling jackets have very low heat transfer coefficients due to the steady state in which the must/wine is located inside the tank. The effectiveness of the refrigeration decreases proportionally with the diameter of the tank, being considered that for normal temperatures of circulation water limiting diameters of more than 4 m, due to the vertical thermal stratification that occurs [10, 13, 35]. If only large diameter tanks are available, the effectiveness decreases in a high percentage, so to maintain it in appropriate values, very low temperatures are required in the glycolic water, close to 1–2�C. According to Bouton et al. [10, 13], if the diameter doubles the surface of the jacket, it is multiplied by four and the volume of the must/wine is refrigerated by eight, maintaining the ratio surface/volume. As an approximate value and only for approximate calculations, 2 m2 of cooling jacket per 100 hl of must in fermentation is recommended.

The cooling jackets are used in the tartaric stabilization process of the wines by the system of planting nuclei of crystallization, circulating glycolic water at �7�C to reach temperatures

The coolant fluid flows through an exchanger (plaque or coil) submerged in the must/wine.

It is a very economical solution as it does not require special tanks and large accessory installations. It is applied in wine cellars of small and medium capacity that have tanks with volumes never exceeding 300 hl. As an advantage, the possibility of placing them in the zones of maximum thermal generation is at any level and from the beginning of fermentation.

Both plates and coils are constructed of stainless steel and are usually mobile structures that are moved from one tank to another by flexible conducts. Glycol water circulates in its interior at temperatures of 5–7�C or refrigerating fluids with similar evaporating temperature.

The refrigeration is based on convection and conduction phenomena.

Qc ¼ U � S � ðTinside tank–Tcirculating waterÞml (26)

it is considered that water enters between 5 and 7�C and exits between 10 and 12�C.

equation of the above is used (Eq. (26)):

is reduced by half for calculation purposes.

6.3. Cooling plaques or submerged coils

of �1�C.

90 Refrigeration

Q is the dissipated energy (kJ).

S is the exchange surface (m<sup>2</sup> ).

U is the global coefficient of heat transfer (w/m2�C).

K is the wine thermal conductivity (w/m2�C).

Tv is the wine initial temperature (�C).

Tα: is the initial temperature of glycol water or refrigeration liquid (�C).

t is the total contact time (h).

α: is the thermal diffusivity of wine (w/m2�C).

$$\frac{LUD\_a}{k} = 4.9 \left( \frac{D\_a V \rho}{\mu} \right)^{0.57} \left( \frac{\text{C}\_e \mu}{K} \right)^{0.47} \left( \frac{D\_a n}{V} \right)^{0.17} \left( \frac{D\_a}{L} \right)^{0.37}$$

U is the global coefficient of heat transfer (w/m2�C).

K is the wine thermal conductivity (w/m2�C).

Da is the internal diameter of the exchanger (m).

V is the speed of wine circulating (m/h).

ρ: is the wine density (kg/m<sup>3</sup> ).

Ce is the wine specific heat (kJ/kg�C).

m is the wine viscosity (cP).

n is the stirring speed of the blades (rph).

L is the length of the exchanger (m).

Due to the difficulty of calculation, for practical purposes, the dimensioning of a scraped surface exchanger is performed according to the simplified energy transfer equation (Eq. (29)) [10, 13, 34].

$$Q\_c = \mathcal{U} \times \mathcal{S} \times \left( T\_{\text{wire}} - T\_{\text{glycerol water}} \right)\_{\text{ml}} \tag{29}$$

Q is the dissipated energy (kJ).

U is the global coefficient of heat transfer (w/m2�C).

S is the exchange surface (m<sup>2</sup> ).

(ΔT)ml is the logarithmic mean temperature difference between glycol water/refrigerant and wine (Eq. (30)).

$$\left(\left(\Delta T\right)\_{\text{ml}} = \frac{\left(t\_{\text{em}} - t\_{\text{sa}}\right) - \left(t\_{\text{sur}} - t\_{\text{eu}}\right)}{\ln \frac{\left(t\_{\text{em}} - t\_{\text{sa}}\right)}{\left(t\_{\text{on}} - t\_{\text{au}}\right)}}\tag{30}$$

As values of U, are taken those empirically calculated by different authors. According to López [39], U = 580–870 w/m2�C. For McCabe et al. [40], U = 700–900 w/m2�C. Boulton et al. [10, 13] establish an average value of U = 600–2000 w/m2�C.
