Extraction and pre-treatments.
Grape crushing is also similar to that used by Sherry manufacturers, i.e. grapes are conveyed via a screw to the crusher, although destemming does occur, to prevent unwanted bitter flavors occurring in the wine. Separation of must with differing polphenol contents does not occur post crushing (as with Sherry), as pressing (juice extraction from skins) does not occur at this stage. The juice is pumped into open fermentation tanks for initial fermentation. The Must has a low total acidity (6 g l, as tartaric acid) and a pH of approximately 4, although correction to pH 3.6 occurs, as does the addition of sodium metabisulphate (100mg/l) to reduce microbial load and aid the extraction of grape pigments.
Base Wine Fermentation
As is the case with Sherry manufacture, inoculation with specific yeast strains may occur in the fermentation vessel, otherwise spontaneous fermentations occur from strains in the winery/grape micro flora, which are Sachharomyces cerevisiae type yeasts. Alcoholic fermentation and the products of (acetaldehyde, esters, etc) are the same as that described for Sherry production. Glycerol is formed during alcoholic fermentation, levels reaching 3 – 8 g l. Other important port flavor compounds produced during this period (by the yeast) are butanol, isobutanol, hexanal
(a volatile aldehyde produced from the oxidation of unsaturated fatty acids and acetate.
Red Port Vinification: This process is significantly different than the process utilized for Sherry production, as a red base wine is produced (rather than white) which is fortified mid fermentation.
Fermentation of this port style occurs at temperatures slightly higher than that of Sherry ~ 28°C, primarily to influence the microbial population in the must, produce fruity flavors and ensure efficient extraction of required pigments. Fermentation progresses for a duration of 2 – 3 days, during which time maximum extraction of phenolic compounds and anthocyanin pigments must occur. Therefore, there must be maximum contact between the juice and the grape skins, which is facilitated by various autovinifiers, which spray juice onto the crust of skins that forms over the juice during fermentation, due to the action of evolved CO2.
Fermentation over this period results in a decrease in must specific gravity, from 1.090 – 1.100 to 1.045 – 1.050. Fermentation does not go to completion as with Sherry production, as at this point the fermenting must is run off the solid material and into a suitable vessel (wood or steel), where fortification occurs with a brandy spirit of 75% volume, to increase alcohol by volume to 19 – 20%, thus terminating the fermentation and creating port. The brandy contributes 20% of the total wine volume and is non-neutral in character, making a significant contribution to the character of the wine, adding esters, higher alcohols and acetaldehydes states that fortification at this point serves to retain large amounts of acetaldehyde (a product of yeast fermentation) in the wine, stabilize color by forming anthocyanin – tannin polymers and to mask (typical levels of 500mg/l-1 bitterness by retaining sweetness.
The must solids are often pressed and juices with various levels of tannins are produced (similar to those described in Sherry), some of which are blended with the port, and that with high tannin used to produce spirit. Port pH is corrected to 3.6 with tartaric acid, to correct the rise in must pH due to the addition of the spirit.
White Port Vinification: Juices for white styles may contain grape skins up to the point of fermentation initiation, before racking off the solids occurs and fermentation continues. This produces a harder style wine with increased browning potential, due to the extraction of phenolic compounds. Fermentation temperatures are often lower than that of red port, around 18 – 20°C, to retain required fruity aromas that may be driven off at higher fermentation temperatures. White ports are combined with juice from the pressed solid materials, to achieve tannin levels desired by the manufacturer, however fashionable, lighter style ports may not include such juice to preserve delicate flavours.
Typically, both red and white ports experience fermentation to consume half of the fermentable sugars, leaving between 80 – 120 g sugars per liter, however sweeter styles are produced (geropigas), with residual sugars up to 150 g/l. Dry styles are also produced for blending purposes, which contain sugars up to 50 g/l.
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