Lecture+7.++Presses - UNIVERSITY OF CALIFORNIA Department...

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UNIVERSITY OF CALIFORNIA Department of Viticulture and Enology VEN 135 WINE TECHNOLOGY AND WINERY SYSTEMS 7. PRESSES The purpose of pressing is to recover the juice (or wine) associated with the pulp and skin section of the grapes that is not readily released by natural draining. Presses can be classified into BATCH and CONTINUOUS types and there are many types of batch presses. The composition of the press fractions has long been known to be different from that of the "free-run" fraction. These differences can be positive in terms of varietal character but are often negative in terms of acidity, pH and tannin content. The extent of these differences is determined by the way in which pressure is applied, the nature of the screen employed and the movement of the skins with respect to the screen. In this regard the batch presses are generally less prone to skin tearing (and subsequent phenol and tannin extraction) than are the continuous presses. As with other unit operations, the generation of suspended solids is a concern because it will lead to the need for further clarification of the press fractions to avoid the risk of off-flavor generation. BATCH PRESSES Batch presses operate in a cycle in which they are filled, pressurized, depressurized and emptied. The filling time is determined by the must pump or conveyors and size of the press. The press cycle is made up of one or more steps in which the pressure is increased to some value, held and then released. The pressure is generally increased to a maximum pressure of 4-6 atm (bars) in a period of between 1 to 2 hours. Most batch presses (with the exception of basket presses) are rotated while the pressure is being applied so that a regular shaped cake is developed. There are many types of batch presses. They range from simple basket presses in which wooden slats provide the screen surface and a capstan is used to apply the pressure (Figure 7.1) to bladder and membrane press in which air pressure is used to press the skins against stainless steel screens. Basket Presses Large hydraulic basket presses (Figure 7.2) have enjoyed a resurgence of popularity in recent years. This is most likely for three reasons. First, the technology has improved including: (1) the baskets are now larger and constructed out of stainless steel, (2) the baskets can 1 Figure 7.1. A traditional basket press with wood slats and a manual capstan.
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This note was uploaded on 09/29/2010 for the course VEN 91866 taught by Professor Davidblock during the Spring '09 term at UC Davis.

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Lecture+7.++Presses - UNIVERSITY OF CALIFORNIA Department...

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