General Winemaking Options (2) Flashcards
Describe alcoholic fermentation
The conversion of sugar into ethanol and carbon dioxide carried out by yeast in the absence of oxygen. Also produces heat which has to be managed.
Alcohol fermentation choices
1) yeast
2) temperature
3) fermentation vessels
Yeast
The collective term given to the group of microscopic fungi that convert sugar into alcohol and affect the aroma/flavor characteristics of wines.
Yeast needs
1) oxygen at first to multiply
2) once oxygen is used up they switch to fermentation
3) a viable temperature range
4) access to yeast nutrients (low levels of nitrogen lead to stuck ferment and reductive notes, diammonium phosphate and thiamine are also common additions)
What does the fermentation process produce?
1) ethanol
2) carbon dioxide
3) heat
4) volatile acidity (nail polish remover smell)
5) SO2 (in small amounts)
6) wine aromatics
7) glycerol (increases body)
How does fermentation affect wine aromatics?
1) from aroma precursors
- compounds with no flavor until released by fermentation
- ex: 4MMP giving boxwood/ gooseberry
- ex: terpenes such as linalool and gerniol giving floral/ grapey
2) created by yeast
- ex: esters give many fruit flavors like banana
- ex: some strains produce reductive sulfur compounds giving rotten eggs
- ex: acetaldehyde giving bruised apple/ paint thinner
Most common species of yeast used in winemaking and why
Saccharomyces cerevisiae
1) withstands high acidity
2) withstands increasing alcohol
3) reliably ferments to dryness
4) fairly SO2 resistant
Ambient yeast
- present in vineyards and wineries
- includes a range of yeast species (ex: Kloeckera and Candida)
- most die out as the alcohol raises past 5% thus Saccharomyces cerevisiae quickly becomes the dominate strain anyway
Pros:
1) adds complexity
2) free
3) yeast populations are unique to a place
4) helpful in marketing
Cons:
1) fermentation may start slowly, dangerous for the build-up of unwanted VA, spoilage organisms, and bacteria that cause unwanted flavors
2) risk of a longer or stuck fermentation, making it vulnerable to spoilage organisms
3) a consistent product cannot be guaranteed
Cultured yeast
- yeast strains selected in a laboratory and grown in volumes suitable for sale
- often single strains of Saccharomyces cerevisiae
- to use them the must may be cooled down or dosed with SO2 to prevent ambient yeasts beginning before the yeast is added
- a starter batch of fermenting grape juice and the activated yeast is added to the tank
Pros:
1) reliable, fast ferment to dryness
2) produces low levels of volatile acidity and given its speed and reliability, there is less danger of spoilage organisms
3) consistent
4) large selection of strains to choose from (ex: Saccharomyces bayanus is used for must with high alcohol potential and for re-fermenting sparkling wine
Cons:
1) certain similarity of fruit expression (industrial wine)
2) costs money
How does temperature affect alcoholic fermentation?
- winemakers prefer a warm temp to start to get the yeast population established
- cool (12-16C / 54-61F) promote esters that give fruity aromas and flavors, fruity whites and roses
- midrange (17-25C / 63-77F) retains fruit and gives low tannin extraction / fruity reds and less fruity whites
- warm (26-32C / 79-90F) maximum extraction of color and tannin but loss of fruit / high concentration of flavor and tannin
- above 35C / 95F the fermentation may slow down and kill yeasts
Options for temperature control
1) by the temp of the cellar
2) water to glycol jackets surrounding stainless steel
3) inserts put into vessels
4) pumping over or délestage, which releases heat
Fermentation vessels
1) stainless steel
- easy to clean, temp controlled, large range of sizes, protected from oxygen, high level of mechanization possible, $$$
2) concrete
- $ to $$$, high thermal inertia (maintains even temp), eggs permit natural lees stirring
3) wood
- retains heat well, allows for a small amount of oxygen, hard to clean, expenive at first
4) plastic vessels
- light, versatile, useful for small-batches, permeable to oxygen, no temp control
5) terracotta
- for small-production, includes amphora/ quevri/ tinaja
Malolactic Conversion
The result of lactic acid bacteria converting malic acid into lactic acid, carbon dioxide, and heat. Typically after alcoholic fermentation but occasionally during it.
What conditions are needed to malolactic conversion to occur
1) 18-22C / 64-72F
2) a moderate pH 3.3-3.5
3) low total SO2
Historically happened spontaneously the spring following the harvest but now its often started by inoculating with cultured lactic acid bacteria.
How to avoid malolactic conversion
1) add the enzyme lysozyme that kills lactic acid bacteria
2) move away from batches going though MLF to avoid the spread
3) filter it out
Results of malolactic conversion
1) reduced acid, increased pH (slightly more vulnerable)
2) color loss in red wines
3) greater microbial stability (bc it wont happen spontaneously in bottle)
4) reduced fruit notes and the addition of buttery flavors
5) increased volatile acidity
Malolactic conversion what vessel and when?
- barrels have the advantage stirring the lees at the same time, promoting integration of flavor, but barrel temp must be monitored
- large tanks
- malolactic conversion during alcoholic fermentation is shown to increase fruity characteristics and shortens production time thus saving money
Post-fermentation adjustments
Generally smaller as they will be less integrated.
1) acidity
2) pH
3) tannins
(above same as pre-fermentation)
4) sugar levels
5) removal of alcohol
6) color
Post-fermentation removal of alcohol
1) add water to must (reduces flavor too)
2) reverse osmosis $$$
3) spinning cone $$$
Post-fermentation color adjustment
1) fining to reduce color
2) addition of a grape-derived coloring agent aka MegaPurple (if legal)
