Agriculture Reference
In-Depth Information
Horizontal screw presses. These presses work on the same principle as the screw-
operated basket press except they are horizontal and more commonly manufactured
from slotted stainless steel, with revolution of the basket the cake to be loosened
between pressure applications.
Air bag presses. Marc is loaded through doors in the side of the basket, distrib-
uted evenly by rotating the basket and then pressed by inflating the bag. Because of
the relatively short distance the juice has to travel to the basket only low pressures
are required and the pressing action is gentler with less extraction of hard, bitter
phenolics and oxidation can also be limited. Therefore these presses are excellent
for white juice pressing, but are also quite suitable for reds.
Continuous screw presses. Marc is fed in at one end of a rotating screw which
forces it through a slotted stainless cylinder against a weighted door. Continuous
screw presses are mainly used in large, high throughput wineries for producing
relatively low-priced wine.
Clarification of White Juice
It is common practice to partially or completely clarify white juice prior to fermen-
tation. The benefits of juice clarification include a more pure fruit flavour expres-
sion that is free from cardboard-like, 'solids' character. However highly clarified
juice may cause slow and/or incomplete fermentation due to removal of certain
yeast nutrients with the solids fraction. Thus, the desirable degree of clarification
depends on the required balance of these factors. Often, full-bodied wines are made
from less-clarified juice (e.g. Chardonnay, as its full flavour is not as easily domi-
nated by 'solids' character), while highly-clarified juice is used to make light, clean,
aromatic wines.
Following are brief descriptions of the main methods of juice clarification:
Settling. Over a period of approximately 24-72 h, the majority of grape solids
will fall out of suspension due to gravity. The clarified juice can then be taken (or
'racked') from the above the settled solids, Juice is held in normal closed tanks at
low temperature (desirably, less than 10 °C), with an addition of sulphur dioxide,
to minimise wild yeast growth and the effects of oxidation. Pectolytic enzymes are
also often used to degrade natural grape pectins that delay settling.
Centrifugation. Continuous centrifuges are often used in large wineries where
their main advantage is continuous running with low labour requirements. Juice or
wine is fed into the outside of a rapidly-spinning rotor and then 'up' the gravity gra-
dient to the centre of the rotor. Because of their greater density relative to the juice,
the solids 'settle' to the outside of the rotor and discharged.
Flotation. Flotation utilises the injection of gas bubbles (nitrogen is most com-
monly used) to the bottom of a tank containing juice. The bubbles carry suspended
solids to the top of the tank where a foamy layer of suspended solids is formed. A
gelatin fining agent is also usually used to facilitate this process. The clarified juice
underneath is then be pumped into fermentation tanks.
