**2.2 Osmotic distillation**

The English-language literature contains various synonyms for osmotic distillation, such as membrane distillation, transmembrane distillation, capillary distillation, or pervaporation. Other sources also speak of isothermal membrane distillation [28, 36].

In the process of osmotic distillation, two liquids are separated by a microporous, non-wettable membrane. Both fluids are directed along this membrane, with none of the fluids permeating the membrane pores. Only the volatile components present in the respective liquids can pass the membrane by evaporating and permeating through the pores of the membrane. This gas phases then go into solution of the other side of the membrane. Due to the hydrophobic nature of the membrane, water cannot penetrate the pores of the membrane. Thus, ions, colloids, and macromolecules that do not evaporate and diffuse through the membrane are completely retained.

Osmotic distillation is an isothermal membrane process at atmospheric pressure. The driving force for the molecule passage is the vapor pressure difference of a substance between the two sides of the membrane. The volatile components permeate from the membrane side with higher vapor pressure to the side with lower vapor pressure until equilibrium sets [12, 13].

In osmotic distillation for the partial reduction of alcohol in wine, water is used as strip medium. Apart from possible losses of volatile aroma components, the ethanol flux is of considerable interest. The flux is the amount of permeate that pass through the membrane per unit time. In osmotic distillation, it can be described as follows:

$$\mathbf{J}\mathbf{e} = \mathbf{K}\mathbf{v}\mathbf{v}\Delta\mathbf{P}\mathbf{b} \tag{1}$$

In this equation, Je (kg/m<sup>2</sup> h) is the ethanol flux, ΔPb is the vapor pressure difference in terms of ethanol (mmHg) on both sides of the membrane, and Kov is the mass transfer coefficient (m/s). The ethanol flux is influenced by a number of factors. Higher feed and strip media speeds will increase the alcohol transfer through the membrane. Furthermore, the temperature has an influence. As temperatures rise, the flux of volatile components increases. For the efficiency of osmotic distillation, it is important that both sides of the membrane are sufficiently hydrophobic. The pores should not get wet, and no water should penetrate the membrane by capillary action [36, 64].

The gas and vapor passage through the membrane pores takes place by diffusion. The permeation of the volatile molecules through the air space of the membrane pores can be described, depending on the pore radius, by Knudsen and Fick's diffusion. Various references suggest that simultaneous water transfer takes place between both sides of the membrane. The higher the process temperature, the higher the water transfer is. If the so-called stripping water is degassed before treatment in order to avoid an undesirable gas input into the wine, the water transfer is also increased. If the wine temperature is higher than the water temperature, the water transfer increases. In their work, Varavuth et al. [64] proved a water transfer to up to 3 l/m<sup>2</sup> per hour. If the membrane is damaged in its hydrophobic property by improper cleaning and storage, it can be assumed that the transfer of water increases. The water vapor permeating the membrane is relatively more composed of light oxygen atoms. The oxygen isotope ratio (O16/18) is a globally recognized indicator of water addition to wine, according to OIV Resolution OENO 353/2009 [1, 28, 36, 64].

Even if relatively small amounts of water are released into the wine, the osmotic distillation for alcohol reduction could simulate significantly higher levels of water in the wine. The technique of osmotic distillation is widely used in various industries. It can be used both for the degassing of liquids and for the alcohol reduction of beer and wine. The targeted addition of gases or degassing of wine is also summarized as gas management. In contrast to alcohol reduction, a vacuum or a gas is applied to the side opposite the wine. As a result, gas can be specifically added to or removed from the wine. Alcohol reduction of wine by osmotic distillation has been studied by a number of other authors [6, 14, 20, 27, 36–39, 42, 45–48, 51, 56–59, 64, 66].

#### **2.3 Reverse osmosis/nanofiltration and other process**

Reverse osmosis is a process for the concentration of liquids, which have a low content of solid components. The passage through the membrane takes place by diffusion through a semipermeable membrane. Consequently the passage takes place against a concentration gradient. During the treatment by reverse osmosis,

**255**

*Alcohol Reduction by Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.85989*

osmosis are known in the wine industry.

[10, 15, 18, 65].

pursuing this approach.

ing more permeate per m2

fires can be remedied.

membrane process [8].

some applications also at 40 bar [44].

pressure must be applied that exceeds the osmotic pressure of the solution to be concentrated. The separation of various substances is due to retention in terms of molecular size and by the solution-diffusion mechanism. Originally, reverse osmosis was developed for water treatment or desalination, but nowadays a number

of other applications in the food and beverage industry are possible [43, 67].

Common applications of reverse osmosis in the food and beverage industry are the use in dairies or the concentration of juice. Various processes based on reverse

For must concentration, the reverse osmosis is carried out without further process step. Other enological applications based on reverse osmosis require a second process. Depending on the purpose of the application, various other procedures are used for this subsequent step. When reverse osmosis is used to lower the alcohol content of wine, a permeate is separated in the first step. In addition to alcohol, this aqueous solution contains only a few volatile aroma components. Then, in a second step, this fraction is reduced in its alcohol content by another technology. This is done either by further membrane process, e.g., the osmotic distillation or by a common distillation at atmospheric pressure. Another approach could be replacing the permeate of the reverse osmosis by water, but that so-called diafiltration would mean the addition of water. In many countries the dilution of water is not allowed

Another approach for alcohol reduction is described by Bui et al. [7]. In experiments, they couple two reverse osmosis treatments by differentiating membrane cutoff. In the first step, a permeate with an alcohol content of about 6 vol.% separated. In a second step, this permeate is reduced by a second reverse osmosis treatment to an alcohol content of only 2 vol.%. This fraction is blended back to the initial retentate of the first treatment step to give a reduced-alcohol wine. To date, this approach has not been successful in practice, or there is no plant manufacturer

Nanofiltration is a process similar to reverse osmosis. The separation limit of the membranes is usually between 100 and 500 Da. The pore size is between 1 and 10 μm depending on the membrane, and the usual working pressure is 10–30 bar, in

Compared to reverse osmosis, nanofiltration operates at lower pressure, produc-

and the pore size of the membranes. However, other wine components permeate in a higher extent through the nanofiltration membrane. Due to that higher losses of aroma components could occur during the alcohol reduction of the permeate.

Reverse osmosis can also be used in winemaking to reduce volatile acidity [63]. Here, a permeate is separated in the first process step. In addition to ethanol and water, this also contains proportionally more volatile acid. This solution is then passed through an ion exchanger in the second process step. The volatile acid content is thus reduced, and sensory errors can be remedied to a certain extent [68, 69]. Other problems in wines can also be reduced by using reverse osmosis. Fudge et al. [24] describe a method in which off-flavors caused by smoke from larger forest

This treatment requires the separation of a permeate first. Then this fraction passes in a second process step: a column with adsorber resins. This significantly reduces volatile phenols such as guaiacol and 4-methylguaiacol. A similar approach was used by Ugarte et al. [65] to remove off-flavors caused by volatile phenols formed by *Brettanomyces* yeasts. Generally speaking, reverse osmosis offers a barrier so that the desired wine constituents are not that widely lost in further treatment steps. Consequently, reverse osmosis in winemaking can be described as a universal

of membrane area. This is due to the membrane structure

#### *Alcohol Reduction by Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.85989*

*Advances in Grape and Wine Biotechnology*

pressure until equilibrium sets [12, 13].

In this equation, Je (kg/m2

capillary action [36, 64].

to up to 3 l/m<sup>2</sup>

[1, 28, 36, 64].

follows:

Osmotic distillation is an isothermal membrane process at atmospheric pressure.

In osmotic distillation for the partial reduction of alcohol in wine, water is used

Je = Kov ΔPb (1)

ference in terms of ethanol (mmHg) on both sides of the membrane, and Kov is the mass transfer coefficient (m/s). The ethanol flux is influenced by a number of factors. Higher feed and strip media speeds will increase the alcohol transfer through the membrane. Furthermore, the temperature has an influence. As temperatures rise, the flux of volatile components increases. For the efficiency of osmotic distillation, it is important that both sides of the membrane are sufficiently hydrophobic. The pores should not get wet, and no water should penetrate the membrane by

The gas and vapor passage through the membrane pores takes place by diffusion. The permeation of the volatile molecules through the air space of the membrane pores can be described, depending on the pore radius, by Knudsen and Fick's diffusion. Various references suggest that simultaneous water transfer takes place between both sides of the membrane. The higher the process temperature, the higher the water transfer is. If the so-called stripping water is degassed before treatment in order to avoid an undesirable gas input into the wine, the water transfer is also increased. If the wine temperature is higher than the water temperature, the water transfer increases. In their work, Varavuth et al. [64] proved a water transfer

per hour. If the membrane is damaged in its hydrophobic property

by improper cleaning and storage, it can be assumed that the transfer of water increases. The water vapor permeating the membrane is relatively more composed of light oxygen atoms. The oxygen isotope ratio (O16/18) is a globally recognized indicator of water addition to wine, according to OIV Resolution OENO 353/2009

**2.3 Reverse osmosis/nanofiltration and other process**

Even if relatively small amounts of water are released into the wine, the osmotic distillation for alcohol reduction could simulate significantly higher levels of water in the wine. The technique of osmotic distillation is widely used in various industries. It can be used both for the degassing of liquids and for the alcohol reduction of beer and wine. The targeted addition of gases or degassing of wine is also summarized as gas management. In contrast to alcohol reduction, a vacuum or a gas is applied to the side opposite the wine. As a result, gas can be specifically added to or removed from the wine. Alcohol reduction of wine by osmotic distillation has been studied by a number of other authors [6, 14, 20, 27, 36–39, 42, 45–48, 51, 56–59, 64, 66].

Reverse osmosis is a process for the concentration of liquids, which have a low content of solid components. The passage through the membrane takes place by diffusion through a semipermeable membrane. Consequently the passage takes place against a concentration gradient. During the treatment by reverse osmosis,

h) is the ethanol flux, ΔPb is the vapor pressure dif-

The driving force for the molecule passage is the vapor pressure difference of a substance between the two sides of the membrane. The volatile components permeate from the membrane side with higher vapor pressure to the side with lower vapor

as strip medium. Apart from possible losses of volatile aroma components, the ethanol flux is of considerable interest. The flux is the amount of permeate that pass through the membrane per unit time. In osmotic distillation, it can be described as

**254**

pressure must be applied that exceeds the osmotic pressure of the solution to be concentrated. The separation of various substances is due to retention in terms of molecular size and by the solution-diffusion mechanism. Originally, reverse osmosis was developed for water treatment or desalination, but nowadays a number of other applications in the food and beverage industry are possible [43, 67].

Common applications of reverse osmosis in the food and beverage industry are the use in dairies or the concentration of juice. Various processes based on reverse osmosis are known in the wine industry.

For must concentration, the reverse osmosis is carried out without further process step. Other enological applications based on reverse osmosis require a second process. Depending on the purpose of the application, various other procedures are used for this subsequent step. When reverse osmosis is used to lower the alcohol content of wine, a permeate is separated in the first step. In addition to alcohol, this aqueous solution contains only a few volatile aroma components. Then, in a second step, this fraction is reduced in its alcohol content by another technology. This is done either by further membrane process, e.g., the osmotic distillation or by a common distillation at atmospheric pressure. Another approach could be replacing the permeate of the reverse osmosis by water, but that so-called diafiltration would mean the addition of water. In many countries the dilution of water is not allowed [10, 15, 18, 65].

Another approach for alcohol reduction is described by Bui et al. [7]. In experiments, they couple two reverse osmosis treatments by differentiating membrane cutoff. In the first step, a permeate with an alcohol content of about 6 vol.% separated. In a second step, this permeate is reduced by a second reverse osmosis treatment to an alcohol content of only 2 vol.%. This fraction is blended back to the initial retentate of the first treatment step to give a reduced-alcohol wine. To date, this approach has not been successful in practice, or there is no plant manufacturer pursuing this approach.

Nanofiltration is a process similar to reverse osmosis. The separation limit of the membranes is usually between 100 and 500 Da. The pore size is between 1 and 10 μm depending on the membrane, and the usual working pressure is 10–30 bar, in some applications also at 40 bar [44].

Compared to reverse osmosis, nanofiltration operates at lower pressure, producing more permeate per m<sup>2</sup> of membrane area. This is due to the membrane structure and the pore size of the membranes. However, other wine components permeate in a higher extent through the nanofiltration membrane. Due to that higher losses of aroma components could occur during the alcohol reduction of the permeate.

Reverse osmosis can also be used in winemaking to reduce volatile acidity [63]. Here, a permeate is separated in the first process step. In addition to ethanol and water, this also contains proportionally more volatile acid. This solution is then passed through an ion exchanger in the second process step. The volatile acid content is thus reduced, and sensory errors can be remedied to a certain extent [68, 69].

Other problems in wines can also be reduced by using reverse osmosis. Fudge et al. [24] describe a method in which off-flavors caused by smoke from larger forest fires can be remedied.

This treatment requires the separation of a permeate first. Then this fraction passes in a second process step: a column with adsorber resins. This significantly reduces volatile phenols such as guaiacol and 4-methylguaiacol. A similar approach was used by Ugarte et al. [65] to remove off-flavors caused by volatile phenols formed by *Brettanomyces* yeasts. Generally speaking, reverse osmosis offers a barrier so that the desired wine constituents are not that widely lost in further treatment steps. Consequently, reverse osmosis in winemaking can be described as a universal membrane process [8].

#### **2.4 Vacuum rectification**

Distillation is a thermal separation process in which liquids are vaporized and the vapor then condensed. Generally, distillation is a process that separates substances according to their relative volatility. The relative volatility is a measure of the separability of a distillation with respect to two components to be separated. The relative volatility of two components (α) is calculated from the quotient of the K values of the respective substances [32, 34]:

$$\begin{array}{l} \text{values of the respective substances [32, 34]:}\\\\ \infty \, i, j = \frac{\text{K} - \text{Value Substitance i}}{\text{K} - \text{Value Substitance j}} \end{array} \tag{2}$$

The volatility of a substance, in turn, depends on the K value. The K value of a substance describes the tendency of a substance to volatilize [32]:

$$\mathbf{K}\_{i} = \begin{pmatrix} \text{mole fraction substance i in vapor phase} \end{pmatrix} \\ \text{(mole fraction substance i in liquid phase)}$$

The higher the *K* value, the higher the amount of the respective substance in the vapor phase. The *K* value depends on the temperature, pressure, and composition of the liquid [32].

Higher temperatures greatly increase the vapor pressure, so the K value of the substance increases as well. If the vapor pressure of the liquid mixture is equal to the ambient pressure in the distillation unit, the liquid begins to boil. The vapor pressure of the liquid mixture is composed according to Dalton's law from the vapor pressures of the individual components, also called partial pressures together. Depending on the nature of the composition of the liquid mixture, the boiling point shifts [34].

The alcohol content of the rising vapors during distillation increases when the boiling liquid contains more alcohol. In addition, the boiling point is lower with increasing alcohol content of the liquid. On the other hand, it can be seen that the gain factor decreases as the alcohol content of the solution increases. The gain factor describes the amount in which the alcohol content increases from the starting liquid until the distillate. The vacuum distillation achieves lower boiling points by applying a vacuum in the column. By lowering the pressure inside the plant, the volatility of the components is increased, and thus the boiling point of the ethanol is reduced. Consequently, the energy required to boil from the ethanol decreases. As a result, the thermal load on the ingredients of the treated liquid is minimized. Alcohol reduction of wine takes place at around 26–35°C [14].

To increase the alcohol content in the distillate, the rising vapors in the distillation column are amplified. This is done by allowing the ascending vapors to flow through the so-called caps of the column against an incoming liquid. The vapor is enriched with volatile components such as ethanol, while the incoming liquid is enriched with high-boiling components from the steam. Depending on the field of application, the columns have different numbers of amplifier caps. This countercurrent distillation or rectification mentioned method is cheaper and less expensive apparatus, as multiple repetitions of single-stage distillation [30].

In general, the alcohol content in the distillate can be up to a content of 97.2 vol.% increase. Then a so-called azeotrope occurs. With an aqueous alcohol solution of 97.2 vol.%, the boiling point at atmospheric pressure is 78.15°C and thus below the usual boiling point of ethanol. Since the rising vapors from this mixture have the same composition as the starting liquid, the gain factor is 1.0, and so no further concentration is realized [34].

**257**

countries.

*Alcohol Reduction by Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.85989*

**2.5 Spinning cone column**

aroma recovery [8].

**2.6 Further treatments**

content of wine such as:

• Pervaporation

• Dialysis

• Dilution

distillate are returned to the nonalcoholic wine [4, 33].

between the wine and the so-called strip phase.

slurries with a high solid matter content [9, 35].

• Adsorption of ethanol by organic resins

In industrial vacuum rectification plants, no further reduction in temperature can be detected during evaporation when the pressure is lowered below 1 mbar. The pressure losses caused by the flow in the pipelines between distillation column and condenser are in charge of that. In order to reduce the loss of aroma during distillation, the condensate is passed to the so-called aroma leaching in countercurrent to the nonalcoholic wine following the rectification. Some of the flavors from the

A special form of vacuum rectification is the spinning cone column. This unit is used in the food and beverage industry in various areas for aroma separation and

Unlike conventional columns for vacuum rectification, no static installations are used. Within the cylinder of the spinning cone, there are pairs of a fixed and a movable cone installed. The wine running down the column from the top forms a thin film due to the rotation of the cones. On the underside of the movable cones, there are fins, which swirl the rising vapors and thus allow an increased exchange

The special design of the spinning cone column helps to overcome the disadvantage of conventional columns for vacuum rectification. The mass transfer in the column is reduced by the application of the vacuum that instead of turbulent flow, only a laminar flow of the boiling gases prevails. This general disadvantage of distillation under vacuum is qualified by the fact that rotating inserts are mounted in the column. The liquid running down is transformed by its rotation into a thin liquid film. On average, this liquid film is less than 1 mm thick. This results in a very efficient contact between vapors and liquid, whereby the necessary residence time is reduced in the column. In addition, the construction of the spinning cone column, unlike columns for vacuum rectification, can also work with viscous or

A number of further enological methods are conceivable to reduce the alcohol

Except from dilution, all of these are of a technical nature. However, none of these methods have been really successful so far. The reasons for this can be seen either from an economic point of view or in legal aspects. The dilution with water is probably the oldest form of wine fraud and was formerly often used for volume increase. Nowadays the targeted addition of water to reduce the sugar content in must and so to reduce the alcohol content in wine is not legal in most wine-producing

Nevertheless, water addition is legal under certain requirements in some countries. Article 17,010 of the California Administrative Code has the following

### *Alcohol Reduction by Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.85989*

In industrial vacuum rectification plants, no further reduction in temperature can be detected during evaporation when the pressure is lowered below 1 mbar. The pressure losses caused by the flow in the pipelines between distillation column and condenser are in charge of that. In order to reduce the loss of aroma during distillation, the condensate is passed to the so-called aroma leaching in countercurrent to the nonalcoholic wine following the rectification. Some of the flavors from the distillate are returned to the nonalcoholic wine [4, 33].
