Once the stemmed/crushed grapes are separated from the vines, the grapes are heated to about 180 °F (82.2 °C) to extract pigment from the skins, after which the heated material is subjected to mechanical pressure while enclosed in cotton press cloths. Hot pressing is appropriate for deeply pigmented grapes where maximum color extraction is desired.
Whereas, the immediate or cold press procedure is necessary to maintain the initial color of light colored grapes.
The juice is then filtered pasteurized by heating to 170 °F (76.7 °C) and stored in bulk in covered tanks at about 40 °F (4.4 °C). This provides for the separation of tartaric acid salts.
Tartrates must be precipitated. Otherwise, it will settle out upon cooling or even when filtered juice is refrigerated.
This juice is then siphoned off from the tartrate and treated with enzymes, which break down pectins, or with casein for purposes of clarification. Typically, 50-100 ppm of pectinase enzymes is sufficient for the de-pectinization prices at this stage.
The juice is then filtered and bottled. The bottles are capped and then pasteurized by heating in water at 170 °F (76.7 °C) for 30 min.
Manufacturing of grape juice