3. Refrigeration requirements in prefermentative operations

The fermentation of sugar by the yeast glycolytic allows the cells to transform glucose and fructose into pyruvic acid and this, through an enzymatic complex of carboxylase activity, will be transformed into acetaldehyde, which is finally reduced by the alcohol dehydrogenase into ethanol. This transformation is an exergonic reaction, releasing heat which, when accrues in the must, causes a thermic elevation.

The metabolic activity of the yeasts increases in proportion with the temperature with maximum rates between 25 and 28C [16]. Temperatures above 32–35C imply high risks of fermentation stops, as well as further proliferation of acetic and lactic bacteria. Fermentations below 18C are distinguished by delayed onset (longer latency phase) and very slow fermentation development. In years of warm harvests, in large deposits or cellars in the middle of the season with several fermentation tanks, it is easy for the microbial activity itself to pass through the 35C barrier, negatively affecting both cellular viability and the sensorial characteristics of the wine [1–3, 16, 17]. On the other hand, fermentations at moderate or even low temperature (below 18C) allow preserving the aromatic precursors of the grape varieties and stimulating the formation of secondary compounds by yeasts. All this underlines the importance of having a temperature control of fermentation in the wine cellar. It is considered suitable for fermentation of red wines 28–30C (aid to maceration), while for fermentation of white and rose, temperatures below 22C are recommended. The cooling of the must or the crushed-grapes, as indicated above, allows the fermentation process to start at the desired temperature in the case of warm harvests. It is an added advantage in the case of a cold debourbage.

The low temperature fermentations (13C or less) have a great interest for the production of white and rose wines, especially for musts arising from varieties with a great aromatic potential. In addition to avoiding their evaporation, the low temperatures considerably condition the bacteria development, allowing the use of less doses of sulfur dioxide.

It does not happen the same with the yeasts. The low temperatures influence in a different way on the different species which concur in the must. Some yeasts such as Kloeckera apiculata dominate fermentation at 13C, according to Heard and Fleet [18].The survival of non-Saccharomyces species affects the production of certain undesirable volatile substances such as acetic acid and ethyl acetate [19]. The temperature affects the biochemical activities of fermentative yeasts, which in turn affects the wine and its composition. The most notable effect of yeast adaptation at low temperatures is the increase in the degree of unsaturation of the fatty acids [20] and the reduction of the synthesis of sterols. Both changes are an important determinant of the membrane fluidity and reduce the transit of nutrients, resulting in an inhibition of fermentative activity.

According to Suárez and Iñigo [1], not all sugar molecules will follow the equation of Gay-Lussac, obtaining two molecules of ethanol and two of CO2 for each mole of glucose, but depending on the metabolism of the yeast, a certain number of molecules are going to be intended to glycerine and pyruvic acid, which will be the origin of secondary products in wine. As a consequence, the thermal flow originated during fermentation will depend on the importance of these secondary reactions.

Bouffards [21] using a sealed calorimetric chamber determines the heat of fermentation reaction between 83.7 and 100.5 kJ/mol. Subsequently, several authors have established a thermal flow ranking from 71 [22] to 106 kJ/mole [1], according to the purity of the fermentative process and the derivation of the glucose molecules toward other secondary metabolic routes. Assuming the most thermodynamically adverse case and the known parallel reactions always occurring to the alcoholic fermentation, the average mean value of heat flow most widely accepted by the various experts is 100.32 kJ/mole [2, 3, 10, 19, 23].

This heat released by mole of transferred sugar corresponds to the theoretical case of an instant fermentation process. In fact, the process lasts several days in which heat dissipation takes place by contact of the tank walls with the outside and by the release of volatile products released during the transformation. According to this, the data referred to by the authors mentioned above as energy released during the process are modified by a factor corresponding to the rate of consumption of sugars throughout the fermentation process with units of [mole/m3 .h]. This aspect, directly related to the concentration of ethanol and CO2 produced per unit of time, depends on the physical and chemical conditions of the fermenting must (content in sugars, temperature, acidity, pH and richness in nitrogen sources), operating conditions (yeast morphology population, temperature, agitation and oxygenation) and fermentation time.

To identify and quantify the influence of these factors on the amount of heat energy released throughout the fermentation time, several authors have developed simulations and mathematical models.

Boulton [24] establishes that the loss of heat throughout the fermentation process is defined by Eq. (2):

$$\mathbf{dQ/dt} = \Delta H \times \mathbf{dS/dt},\tag{2}$$


This quantification system of the energy released during the fermentation process is the most widely accepted by the experts, although there is no consensus regarding the calculation and valuation of the term dS/dt, that is, to say on the system of calculation of the decrease in the concentration of fermentable sugars per unit of time. Several authors propose mathematical models to define this rate of degradation, based on empirical data from different variables.

The model proposed by El Haloui et al. [25] relates the concentration of residual sugars with the volume of CO2 released, according to Eq. (3):

$$S = 3.92 \times V\_{CO\_2} + 0.1463 \times S\_o - 117\tag{3}$$

where

The metabolic activity of the yeasts increases in proportion with the temperature with maximum rates between 25 and 28C [16]. Temperatures above 32–35C imply high risks of fermentation stops, as well as further proliferation of acetic and lactic bacteria. Fermentations below 18C are distinguished by delayed onset (longer latency phase) and very slow fermentation development. In years of warm harvests, in large deposits or cellars in the middle of the season with several fermentation tanks, it is easy for the microbial activity itself to pass through the 35C barrier, negatively affecting both cellular viability and the sensorial characteristics of the wine [1–3, 16, 17]. On the other hand, fermentations at moderate or even low temperature (below 18C) allow preserving the aromatic precursors of the grape varieties and stimulating the formation of secondary compounds by yeasts. All this underlines the importance of having a temperature control of fermentation in the wine cellar. It is considered suitable for fermentation of red wines 28–30C (aid to maceration), while for fermentation of white and rose, temperatures below 22C are recommended. The cooling of the must or the crushed-grapes, as indicated above, allows the fermentation process to start at the desired temperature in the case of warm harvests. It is an added advantage in the case of a cold

The low temperature fermentations (13C or less) have a great interest for the production of white and rose wines, especially for musts arising from varieties with a great aromatic potential. In addition to avoiding their evaporation, the low temperatures considerably condition

It does not happen the same with the yeasts. The low temperatures influence in a different way on the different species which concur in the must. Some yeasts such as Kloeckera apiculata dominate fermentation at 13C, according to Heard and Fleet [18].The survival of non-Saccharomyces species affects the production of certain undesirable volatile substances such as acetic acid and ethyl acetate [19]. The temperature affects the biochemical activities of fermentative yeasts, which in turn affects the wine and its composition. The most notable effect of yeast adaptation at low temperatures is the increase in the degree of unsaturation of the fatty acids [20] and the reduction of the synthesis of sterols. Both changes are an important determinant of the membrane fluidity and reduce the transit of nutrients, resulting in an inhibition

According to Suárez and Iñigo [1], not all sugar molecules will follow the equation of Gay-Lussac, obtaining two molecules of ethanol and two of CO2 for each mole of glucose, but depending on the metabolism of the yeast, a certain number of molecules are going to be intended to glycerine and pyruvic acid, which will be the origin of secondary products in wine. As a consequence, the thermal flow originated during fermentation will depend on the

Bouffards [21] using a sealed calorimetric chamber determines the heat of fermentation reaction between 83.7 and 100.5 kJ/mol. Subsequently, several authors have established a thermal flow ranking from 71 [22] to 106 kJ/mole [1], according to the purity of the fermentative process and the derivation of the glucose molecules toward other secondary metabolic routes. Assuming the most thermodynamically adverse case and the known parallel reactions always occurring to the alcoholic fermentation, the average mean value of heat flow most widely accepted

the bacteria development, allowing the use of less doses of sulfur dioxide.

debourbage.

78 Refrigeration

of fermentative activity.

importance of these secondary reactions.

by the various experts is 100.32 kJ/mole [2, 3, 10, 19, 23].

S is the sugars consumed in a precise moment (g/L).

VCO2 is the volume of CO2 produced until that moment (L).

So is the initial concentration of sugars (g/L).

The volume of CO2 released can be calculated by empirical analysis in laboratory scale tests with Muller valve occluded flasks [1] or at the level of experimental microvinifications using CO2 flowmeters [26, 27].

Afterwards, and based on empirical data, the same authors [28] build a model which relates fermentation curve with the density and initial concentration of sugars in the initial must (Eq. (4)):

$$S = 0.99109 \times S\_o - 2.096 \times 10^3 \times d\_{30^\circ} \text{ relative must density} - \text{wire at } 30^\circ \text{C} \tag{4}$$

The concentration of residual sugars in each moment of the fermentative process can be calculated, establishing the concentration of glucose-fructose by enzymatic methods or by detection systems based on measurements of radiation in that infrared spectrum.

López and Secanell [15] develop a complex model of the heat evolution curve during fermentation based on physical-chemical parameters such as initial sugar content, total acidity, fermentation temperature and duration of this one. They get expressions such as Eq. (5):

$$\text{dQ/dt} = \frac{k\_1}{k\_2 - k\_1} (e^{-k\_2 t} - e^{-k\_1 t}) \tag{5}$$

in which k<sup>1</sup> and k<sup>2</sup> at the same time are constants representing values of activation energy of the fermentative chemical reaction and depend on the temperature and the total acidity of the must. Starting from this equation, the authors calculate the maximum energy evolution during the process according to the following expression (Eq. (6)):

$$Q\_{\text{max}} = 100.32/180 \times \text{S}\_{\text{o}} \times F\_{\text{c}} \tag{6}$$

where

So is the initial sugar concentration (g/L).

Fc is the correction factor depending on initial concentrations.

Avilés [11] states that the heat developed in fermentation of the must is determined by the probable alcoholic strength of the wine according to Eq. (7):

$$Q = \colon K \times^{\circ} A \times L \times P \tag{7}$$

where

K is the heat transfer coefficient for each probable alcoholic strength depending on the material of the fermentation tank (1, 3/2, 2).

�A is the probable alcoholic grade of the wine (%vol/vol).

L is the must volume in fermentation (L).

P is the coefficient of thermal development by the metabolic activity of yeasts.

Starting from a wide series of empirical data, and from the equation raised in 1978, Boulton et al. [10] established that with a fermentation ratio of 2 Brix/day the energy released is approximately 0.46 kJ/L h for white wines, while for red wine with a ratio reaching 6 �Brix/ day of sugar consumption, the energy released can be set at 1.36 kJ/L h.

According to the Federation Départamentale des Centres d'Etudes et d'Informations Oenologiques of the Gironde-France, FD-CEIOG [12], the heat released in fermentation is defined by Eq. (8):

Refrigeration in Winemaking Industry http://dx.doi.org/10.5772/intechopen.69481 81

$$Q = \rho \times \mathbb{C}\_{\epsilon} \times D\_t \times VF \times V/100.32 \tag{8}$$

ρ is the density of the must-wine (kg/m<sup>3</sup> ).

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

Dt is the elevation of the temperature by %vol (K%vol). The mean value of Dt is 2.8 K/%vol.

VF is the fermentation rate (%vol/J).

V is the total volume (m3 ).

<sup>S</sup> <sup>¼</sup> <sup>0</sup>:<sup>99109</sup> � So � <sup>2</sup>:<sup>096</sup> � 103 � <sup>d</sup><sup>30</sup>� relative must density � wine at 30�

The concentration of residual sugars in each moment of the fermentative process can be calculated, establishing the concentration of glucose-fructose by enzymatic methods or by

López and Secanell [15] develop a complex model of the heat evolution curve during fermentation based on physical-chemical parameters such as initial sugar content, total acidity, fer-

> ðe �k2<sup>t</sup> � <sup>e</sup>

�k1t

Qmax ¼ 100:32=180 � So � Fc (6)

Q ¼: K �� A � L � P (7)

detection systems based on measurements of radiation in that infrared spectrum.

<sup>d</sup>Q=d<sup>t</sup> <sup>¼</sup> <sup>k</sup><sup>1</sup>

the process according to the following expression (Eq. (6)):

Fc is the correction factor depending on initial concentrations.

probable alcoholic strength of the wine according to Eq. (7):

�A is the probable alcoholic grade of the wine (%vol/vol).

So is the initial sugar concentration (g/L).

of the fermentation tank (1, 3/2, 2).

L is the must volume in fermentation (L).

where

80 Refrigeration

where

defined by Eq. (8):

mentation temperature and duration of this one. They get expressions such as Eq. (5):

k<sup>2</sup> � k<sup>1</sup>

in which k<sup>1</sup> and k<sup>2</sup> at the same time are constants representing values of activation energy of the fermentative chemical reaction and depend on the temperature and the total acidity of the must. Starting from this equation, the authors calculate the maximum energy evolution during

Avilés [11] states that the heat developed in fermentation of the must is determined by the

K is the heat transfer coefficient for each probable alcoholic strength depending on the material

Starting from a wide series of empirical data, and from the equation raised in 1978, Boulton et al. [10] established that with a fermentation ratio of 2 Brix/day the energy released is approximately 0.46 kJ/L h for white wines, while for red wine with a ratio reaching 6 �Brix/

According to the Federation Départamentale des Centres d'Etudes et d'Informations Oenologiques of the Gironde-France, FD-CEIOG [12], the heat released in fermentation is

P is the coefficient of thermal development by the metabolic activity of yeasts.

day of sugar consumption, the energy released can be set at 1.36 kJ/L h.

C (4)

Þ (5)

Flanzy [3] established that the released energy during fermentation can be estimated approximately by knowing the concentration of sugars consumed per hour and liter of must, taking into account that one mole of glucose is equal to 180 g (Eq. (9)):

$$Q: (X/180) \times V \times 100.32 \tag{9}$$

X is the concentration of sugars consumed per liter of must and hour (g/L h). The value of X varies according to the activity of fermentation, with mean values of X = 2g/L h for white wines of slow fermentation at a low temperature and of X = 7 g/L h for red wines. 180 g/mole glucose.

V is the total volume of the must to ferment (L).

100.32 kJ/mole glucose.

These mathematical models are difficult to apply in the cellar, so for practical purposes it is more interesting to consider the quantity of sugar consumed per liter of must and per hour. However, there are no reproducible models of the consumption of sugars during fermentation, but the thermodynamic curve of fermentation must be determined for each specific vinification. As an operational parameter for the calculation of cold storage needs of a wine cellar, the maximum value of energy released must be considered (Eq. (10)).

$$Q = \frac{100.32}{180} \times S\_o \tag{10}$$

This means that for a must of 12� the heat released is 133 kJ/L of must in fermentation, although for the calculation of cold production equipment it is interesting to refer the total energy produced during fermentation to the duration of the same Q/t.

Part of this energy produced in fermentation is absorbed by the alcohol, the H2O and the CO2, which when released into the atmosphere partially or totally cools the system. A total of 180 g of glucose produces 88 g of CO2 and 92 g of ethanol when they ferment. Each mole of CO2 formed drags 13.62 kJ for each mole of glucose metabolized. The heat absorbed by the water and the alcohol is determined by the rate of vaporization of the substances, depending on the temperature reached during fermentation and the exterior of the premises [11]. Due to the difficulty of these calculations and the slight thermal decrease, they mean, most authors [2, 3, 10] consider that the heat lost in form of CO2, ethanol and water equals 10% of the total heat generated.

On the other hand, a certain percentage of the energy released in fermentation is dissipated by the environment, depending on the outside temperature. When the set fermentation temperature is lower than the environment temperature, it produces a thermal transfer from the environment to the tank. The calculation of the energy released or absorbed is based on the conduction/convection heat transfer equations (Eq. (11)) [8–10]:

$$Q = \mathcal{U} \times \mathbb{S} \times \Delta T \tag{11}$$

U is the heat transfer coefficient. It is a function of the material of constitution of the tank as well as the speed of circulation of the air in the exterior and the presence or not of circulating currents inside the same. According to various authors, the mean value for stainless steel tanks, the static regime of outdoor air and must/indoor wine is U = 16.72 kJ/m<sup>2</sup> H �C [3, 8-10] or U = 4.64 w/m2 . �K [3, 18].

S is the outer surface of the tank in contact with the environment.

ΔT is the temperature difference between the must in fermentation with the exterior (�C or �K).

If fermentation tank (or later of storage) is located outside the winery, the solar thermal input is important in regions with high isolation. In order to calculate the energy input, it is necessary to take into account the degree of incidence of the solar rays on the surface of the deposit. Flanzy [3] considers that this thermal contribution varies between 400 w/m2 in winter and 800 w/m2 in summer for northern countries. Generally speaking, in Spain, the values can vary between 700 and 1100 w/m<sup>2</sup> , respectively. On the other hand, during the night, a significant cooling of the fermentation mas takes place. In complex calculations, involving many variables not always known, this heat dissipation is not taken into account when calculating the cold storage needs, being a margin of safety of the calculations made.

According to what has been said so far, the heat produced during the fermentation process to be dissipated by the application of cold is (Eq. (12)):

$$\mathbf{Q\_{total}} = \mathbf{Q\_{formation}} - \mathbf{Q\_{dissipation}} \,\mathrm{by\,CO\_2,\,H\_2O} \,\mathrm{and\,ethanol} \neq \mathbf{Q\_{ambient}} \tag{12}$$
