## 1 Overview of Wine Production
### Definition and Composition of GP  
Fresh grape pomace (GP) can be broadly defined as the solid residue composed of skins, seeds, and occasionally stems, resulting from grape pressing, which may or may not contain unfermented sugars [7].  
This complex and heterogeneous bio-waste consists of approximately 425 kg of grape skins, 225 kg of grape seeds, and 249 kg of grape stalks (GS) per tonne, with a high and variable moisture content ranging from 50% to 72% [7]. Among the structural and bioactive compounds present, lignin, pectin substances, sugars, proteins, minerals, and phenolic compounds were quantified [8; 10; 11; 12; 13; 14] (Table 1).  
Traditionally, GP undergoes two distinct unit operations for recovering valuable substances. The choice of extraction process hinges on the unfermented sugar content and the levels of tartrate and grape pigments.  
Fermented pomace from red wine production, which has low residual sugar, can be directly subjected to steam stripping distillation if ethanol recovery is the only desired outcome. The resulting product is a hydroalcoholic solution containing between 20 and 35% alcohol by volume (\(v/v\)), along with small amounts of ethyl acetate, methanol, and acetic acid. This solution is subsequently redistilled in appropriate columns to yield an alcohol with a concentration of 52-80% (\(v/v\)). This process is known in the wine industry as pomace \"dealcoholization\" (Figure 1).  
Conversely, pomace from white or fortified wine production, still rich in unfermented sugars, is subjected to a countercurrent leaching process. This yields a dilute hydroalcoholic solution (approximately 3% (\(v/v\)) alcohol and 30-50 g/L sugars), which is then fermented to increase its alcohol content before distillation. Similarly, if red pomace contains sufficient grape pigments or tartrates to warrant extraction, the same leaching process is applied, followed by concentration of the heat-sensitive dilute pigment solution at low temperatures using a multiple-effect evaporator (Figure 2).  
In the case of dealcoholized grape pomace (DGP), large-scale industrial applications typically involve partial dehydration by pressing, followed by drying in rotary furnaces to a final moisture content of approximately 5%. This process stabilizes the material microbiologically, enabling sieving to separate grape skins from seeds. Industrially, the recovered seeds can be used for grape seed oil extraction, as a component in animal feed, or as an efficient solid biomass fuel [15].  
\begin{table}
\begin{tabular}{l l l} \hline
**Component** & **Amount** & **References** \\ \hline Moisture content & 50–72 & [7] \\ Lignin & 16.8–24.2 & [10] \\ Pectic substances & \(\sim\)20 & [8] \\ Soluble sugars & 1.5–6.2 & [10; 11] \\ Proteins & 2.7–3.8 & [10; 14] \\ Phenolic compounds & 5–10 & [12; 13] \\ Minerals & 1.8–4.6 & [10; 11] \\ \hline \end{tabular}
\end{table}
Table 1: Representative composition of GP (g/100 g, fresh weight basis).  
If no viable market exists for the seeds, or if the distillery lacks steam generators powered by solid fuels, the DGP may be used as soil mulch, as an organic amendment, or be compost--often in combination with other bio-wastes that offer a complementary C/N ratio [16; 17].  
Figure 1: Schematic diagram of the distillation process for fermented pomace. Adapted from [PERSON] et al. [6].  
Figure 2: Schematic diagram of the countercurrent leaching process of pomace. Adapted from [PERSON] et al. [6].  
While the recovery of specific bioactive compounds is affected by the dealcoholization processes mentioned earlier [18], structural compounds are largely unaffected by the conditions used due to their high thermal stability and low water solubility [19].