Pressing-Clarification of White Grape Must
Grape Must Clarification
Must clarification is an integral part of white wine production and involves the removal of turbidity causing particles following pressing. Wines made from juices containing too many suspended solids have heavy, green aromas and bitter tastes (Ribereau-Gayon et al., 2006a). They are also more colored, richer in phenolic compounds, and their color is less stable to oxidation. At the end of fermentation, they often contain reduction odors, more or less difficult to eliminate by aeration and racking. The fruity character of white wines may be masked by the excessive production of higher alcohols, associated with juice containing high levels of suspended solids.
Must clarification can be achieved by natural settling or forced methods like flotation or centrifugation which require investment in equipment, but can save much time, energy, and capacity in the long run. Filtration is seldom used to clarify white juice prior to alcoholic fermentation since it often lacks the capacity and is prone to blockage due to the turbid juice.
The most simple and effective juice clarification method is natural settling. The natural settling process can also only take place when there is no movement in the juice. Typically, free-run and the first pressing and subsequent pressings undergo natural settling separately in wide shallow tanks. In order to minimize the settling distance, horizontal tanks are usually used for the clarification of white juice (Figure 4.6). The juice is cooled to slow the initiation of alcoholic fermentation and limit oxidation.
Flotation is an alternative way of clarifying must. Rather than let the solids slowly settle to the bottom of a tank, nitrogen or air is introduced to the must. The small gas bubbles that form adhere to particulates in the juice, enabling them to float to the top of the tank where a foam cap forms that can then be separated from the clarified juice below. The bubbles that combine with solids are called flocs. Various flotation gases such as nitrogen, carbon dioxide, oxygen, argon, and mixtures of oxygen and nitrogen are utilized.
A centrifuge uses centrifugal force to separate the solids from juice (Figure 4.9). It is done by rotating the turbid juice or wine at a high speed in the drum of the centrifuge. The solids are moved to the side of the drum where it is removed and the clarified juice or wine move out of the centrifuge.
To achieve clarification, must is often treated with pectic enzymes, also known as pectinases or pectinolytic enzymes, which hydrolyze pectins leading to increased settling rate, juice yield, filtration rate, and ultimately wine clarity (Chapter 20). Pectins are naturally occurring compounds which occur in the grape cell wall, which increase the viscosity of the must. The pectin content of grapes depends on factors such as grape ripeness, vintage conditions, grape variety, and grape health.
Bentonite can be added to facilitate settling and subsequent protein removal. Bentonite is negatively charged and so attracts positively charged particles such as proteins, thus removing them from the must. When used in the settling process bentonite must be used after the addition of pectolytic enzymes (3-5hrs), otherwise the bentonite will strip out the enzyme and so make it ineffective (Schneider, 2019). Although effective in removing sediment bentonite can also remove desirable aromas and flavors.
Clarification Affect on Fermentation
Although large amounts of suspended solids are generally undesirable, excessive clarification can lead to slower than usual fermentation rates, and even incomplete fermentation, to the production of unusually high levels of such byproducts as acetic acid, pyruvic acid, and hydrogen sulfide. Grape solids provide nutrients for yeast cell development, particularly lipids and nitrogen. Lipids are integral components of cell membranes, which allow the cell to tolerate the ethanol concentration found late in fermentation.
Degree of Juice Clarification
The quantity of solids in white juice is most commonly expressed as NTUs using a turbidimeter. In a practical sense, NTU is the most useful way to assess, express and compare the relative quantity of solids in individual juices. The suggested range for white grape juices is between 100 and 250 NTU (Butzke, 2010a), which corresponds to 0.3 to 0.5 percent of suspended solids; the optimal value can vary with the variety and the wine style. For example, values between 80 to 120 NTU's may be recommended for cleaner, more aromatic, fruit-driven wine styles (e.g., Sauvignon Blanc, Riesling) whereas for more complex wine styles with greater mouthfeel and body (e.g., Chardonnay), values between 160 to 200 NTU's may be necessary.
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