Crush Pad Operations
Grape Must Management
Grape Must Cooling
After grapes have been destemmed and crushed, the must is cooled. Cooling the must prior to processing will help to reduce juice oxidation and slow down the growth of spoilage organisms and minimize the growth of oxidase enzymes. Oxygen consumption is three times faster at 30 degrees C (86°F) than at 12 degrees C (59°F) (Ribereau- Gayon et al., 2006a). If the laccase enzyme is present, due to the growth of Botrytis on the fruit, keeping the must cool is especially important as sulfur dioxide is much less effective in controlling this enzyme. Typically, the temperature of the must is reduced to between 10 and 18 degrees C (50-64°F) with a heat exchanger (Figure 2.10) or by a direct-expansion refrigerant (Chapter 24).
Strategies to Manage Dissolved Oxygen
Must can be almost saturated with oxygen from the air during the destemming and crushing of fresh grapes. Oxidation of must in the case of the production of white wine is not recommended due to adverse changes in taste, color, and aroma. These changes result from the degradation of phenolic compounds, precursors of flavors, aromas, and colors generated through the action of polyphenol oxidase (PPO) (Section 17.1). The progressive polymerization of phenolic compounds in must causes the formation of yellow to brown pigments. These reactions, therefore, change the color of young white wine from slightly yellow to dark yellow, and even to an unacceptable brown.
Sulfur Dioxide
One of the methods to protect must against the harmful effects of oxygen is the use of sulfur dioxide (Section 3.9, Chapter 18). It plays two roles: it prevents enzymatic and nonenzymatic oxidation and limits the growth of microorganisms. It inhibits the enzymatic oxidation reaction as a result of the slowdown of polyphenol oxidase or its almost complete inactivation. Sulfur dioxide addition in the amount of 50 mg/L causes a reduction in polyphenol oxidase activity in 75 to 90 percent.
Carbon Dioxide
Another method in protecting the must is gas blanketing with carbon dioxide (Section 17.4). Carbon dioxide is the gas of choice because it is heavier than air and will fall and displace the air from the must or juice surface in a partially filled container. Applications vary from the use of solid carbon dioxide blocks and pellets (i.e., dry ice) in harvesting bins and crusher (Figure 2.11), and press sumps, to the in-line sparging of musts with liquid carbon dioxide, and gas dispersal over the must surface with gas from storage cylinders.
Grape Must Transfer
Transferring must to the press or fermentation tanks requires a pump that can meet the flow and head requirements while handling the larger solids without damaging the product. Typically, must is transferred using a positive displacement pump. The nature of this pump, the flow rate generated, the diameter and length of the must line, and the way in which the must is delivered into the fermentor all influence the level of suspended solids in the resultant juices and wines.
In-Line Addition to Musts
In-line additions (e.g., enzymes, nutrients, sulfur, dioxide, carbon dioxide) can be made when transferring the must, rather than batch additions, resulting in a more uniform distribution of the addition throughout the entire load.
Metering the Quantity of Must
Metering of the quantity of must can best be done volumetrically by using a positive displacement pump such as a rotary vane or progressive cavity pump (Boulton et al., 1999). These can be calibrated in terms of rotational speed and throughput. If the pump is used with a variable speed control, the speed can be monitored by a counting sensor and a sensing mark located on the pump axle.
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