Quiz - Pre-fermentation Clarification to Finishing Flashcards
What are the four primary forms of must clarification?
- Sedimentation
- Clarifying agents
- Centrifugation
- Flotation
Describe sedimentation.
Sedimentation is the simplest form of clarification. Before it takes place, the grape juice is commonly chilled to around 4°C to reduce the rate of oxidation and microbial spoilage and avoid a spontaneous fermentation. The size and shape of the tank dictates how long this rate of clarifying takes; it takes longer in large, tall tanks than in smaller, shorter tanks due to the depth over which the solids need to fall. It can take 12-36 hours. The clear juice is transferred to the fermentation tank leaving the sediment of solids at the bottom of the tank in a process called racking.
Sedimentation (often referred to as settling) is the cheapest in terms of equipment required and the gentlest way of clarifying juice but it is the most time consuming as it can take up to 3 days for some juice to clarify.
What can be done to speed up the clarification process?
Certain products can be added to juice to speed up the clarification process. Pectolytic enzymes break down pectins in the grape juice and thus make the juice less viscous and easier to settle efficiently; pectins are naturally found in plant cell walls.
Breaking them down in this manner allows a more rapid separation between the liquid juice and solids, but it only aids the clarification of the grape juice, not the wine.
Some fining agents, such as bentonite, gelatine and casein, are commonly used to aid clarification of both juice and wine.
As this is an enzymatic reaction, efficiency of this process is temperature dependent.
Describe flotation.
- Flotation speeds up the rate of clarification compared to sedimentation.
- Flotation involves bubbling gas up through the tank of juice. As the bubbles of gas rise, they bring with them the solid particles. The solid particles are then skimmed off the top of the vat.
- Inert gases such as nitrogen and argon are the gases typically used for this process, but both of these are expensive and specialized equipment is needed to bubble the gas through the liquid.
- If oxygen is used as the gas, this is a method of hyperoxidation.
- Fining agents need to be added to the juice for this technique to be successful.
- Although effective and quick, this is an expensive option.
Describe centrifugation.
- A centrifuge is a machine that comprises a rapidly rotating container, which uses centrifugal force to separate solids from liquids.
- The advantage of centrifuges is that they clarify the juice quickly.
- They can also be used continuously.
- However, they are expensive to buy, and therefore are typically only used in wineries needing to process large volumes of juice quickly.
- They also increase the juice’s exposure to oxygen unless the machine is flushed with inert gas.
- Centrifuges can be used on wine as well as juice.
Over-clarification or under-clarification of must can have an adverse impact on the fermentation and the final wine.
Briefly describe some of the risks of over/under-clarification.
Over-clarification can lead to a stuck fermentation as there may not be enough nutrients, which come from the solids, for the yeast to survive. The fermentation needs to be carefully monitored and yeast nutrients may need to be added if the fermentation starts to struggle.
Under-clarification can add astringency which may not be desirable in lighter wine styles. It can also give reductive sulphur compounds that are negative for wine quality.
Give examples of wine styles with differing percentages of solids left after must clarification.
Inexpensive Pinot Grigio - Low level of solids left
Inexpensive Gewurztraminer - Range of options available
Premium, full-bodied, savory Chardonnay - High level of solids left.
Leaving a higher proportion of solids in the must can add complexity and texture to the final wine. Other attractive aromas are also produced during fermentation. Entry-level wines like some Pinot Grigios would not be suitable though given that the range of aromas present would already be minimal, and the winemaker would want to avoid masking them.
A winemaker may choose to use the technique of hyperoxidation on the juice before fermentation. The compounds in the grape juice that oxidize most readily are targeted.
Which results can usually be observed in the juice post-hyperoxidation?
- Bitter compounds are softened.
- Some volatile aroma compounds are destroyed.
- The juice turns more brown.
Hyperoxidation
1. Can produce more stable wines against oxidation after fermentation. The theory being, as the compounds in the juice oxidize, they turn the juice brown; during fermentation the compounds precipitate, returning the wine to its normal color.
2. Can also help to remove bitter compounds.
3. Can destroy some of the most volatile aroma compounds and therefore is typically better suited to neutral grape varieties such as Chardonnay
4. Can reduce the levels of methoxypyrazines found in Sauvignon Blanc, and therefore would not be carried out if a pronounced, herbaceous style of this wine was desired.
Name the main types of fermentation vessels used in white wine production and give a brief description of what effect this vessel may have on a wine and why a winemaker might choose this vessel.
Oak barrels/vats
* Small oak barrels are a popular choice for some styles of white wine, but the barrels are expensive.
* Monitoring fermentation in these small vessels is more labor intensive than monitoring a single large vat.
* However, it is thought that wines which have been fermented in oak have a deeper color and fuller body due to the oxidative environment and more integrated oak-derived aromas due to the action of the yeasts (compared to wines that are fermented in stainless steel or concrete and later transferred to an oak barrel for maturation). This is a popular choice for premium Chardonnay.
Stainless steel tanks
* Stainless steel vats are the most popular choice for fruity, floral whites due to the ability to control temperatures easily, and hence maintain the cool fermentation conditions needed to maximize aromas for these styles of wine e.g. aromatic Sauvignon Blanc.
Concrete
* Concrete is traditional in some cooler regions and suitable for whites that do not need to be fermented too cool e.g., Chablis.
Which form of fermentation vessel is most appropriate for each of the following wines?
- Super-premium Chardonnay
- Mid-priced Chardonnay
- Sauvignon Blanc
Super-premium Chardonnay - Small new oak barrels
It is understood that wines that have been fermented in oak have a deeper color and fuller body due to the oxidative environment. The wine produced has more integrated oak-derived aromas due to the action of the yeasts. It can also give the winemaker some more options for blending i.e the time spent in new oak, barrel fermented part of the juice, etc.
Mid-priced Chardonnay - Small oak barrels and stainless steel
It might be fermented in both to save on cost of only using small new oak barrels and then blended later. It could be fermented in stainless steel and oak chips added to the vessel to give aromas of oak.
Sauvignon Blanc - Stainless steel
Sauvignon Blanc is aromatic and depending on the wine style, the flavors need to be preserved throughout the wine making process. Stainless steel is inert and does not impart flavor into the wine. It is also important that the temperature can be maintained at the desired level which is more easily achieved in stainless steel tanks by either inbuilt temperature control or fitting some cooling device to the tank.
What are the conditions required for malolactic conversion to successfully take place?
Malolactic conversion (MLF) usually happens after alcoholic fermentation, but sometimes it can happen during fermentation. The result of MLF is the conversion of malic acid to lactic acid and carbon dioxide.
Heat is also produced. MLF usually takes place between 18-22°C at a pH level of 3.3-3.5 . The level of SO2 needs to be low.
Malolactic conversion can be inhibited in the following conditions:
* the winemaker keeps the juice below 15 °C.
* the pH is too low and SO2 levels are kept high.
* An enzyme called lysozyme can also be added to stop malolactic conversion taking place.
* Lastly, any wines undergoing malolatic conversion can be moved to another part of the winery to prevent bacterial contamination.
What are the effects of malolactic conversion?
- The wine will have more buttery and nutty aromas
- The wine will have fewer fruity aromas
- The color of the wine gets paler
- The level of acidity in the wine is reduced
- The pH of the wine rises
- The wine is more microbially stable
What are the effects of malolactic conversion on Chardonnay?
- perceived greater complexity.
- loss of primary fruit character
Why are inexpensive Pinot Grigios unlikely to undergo maturation post-fermentation?
A low cost, neutral wine probably wouldn’t undergo extended post-fermentation maturation as the winemaker would want to preserve the crisp, fruity aromas. Also, very little complexity would be achieved by maturation as the aromatic profile is quite simple.
List the major post-fermentation winemaking techniques to clarify or stabilize a white wine.
Clarification:
1. Sedimentation
2. Fining
3. Filtration
Stabilization:
1. Microbiological Stabilization
2. Tartrate Stabilization
3. Protein Stabilization
