Pressing-Clarification of White Grape Must
Drainers (juice separator) are used to carry out the collection of the free juice that has already been largely expelled from the berry after crushing before pressing the must. There are some advantages to a separate draining step (i.e., dejuicing), such as a reduction in the required press capacity. An additional benefit of drainer use is that it allows for a smaller-capacity press, as free run is diverted from the press and only the de-juiced solids will be present in the press. There are many designs of drainers but, generally, they are categorized as static or continuous. Drainers are particularly valuable when dealing with large volumes of must. By removing most of the free-run, press capacity can be used most efficiently and economically to extract the remainder (pressings).
A static drainer is a vertical tank with a conical retort bottom (i.e., pointed to the side toward a press) that contains an inner screen (central or on its side), which the clear juice can flow through, leaving the skins behind. This separation is essential for the production of distinctive varietal white wines and may take place either immediately after crushing or after a period of juice and skin contact. The drainer is filled with must directly from the crusher via a hopper or through the heat exchanger, and the must is left in for a certain period, according to the desired level of skin contact.
In this process, the grapes that are crushed fall by gravity into a continuous inclined dejuicer containing a helical screw and are transferred into the continuous press placed below. This device principally consists of an inclined screw conveyor encased by a cylindrical screen, through which juice drains. Continuous drainers are capable of treating large amounts of grape crops (up to around 50 tons/hr) and liberating a high proportion of free run. However, inclined drainers are reported to produce higher solids juice compared to static drainers and elevated turbidity (1,000-10,000NTU).
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