CuSO4 h2s CuS h2so4

Certain inorganic precipitates can be thrown in wine, with tartrate being a key problem. This is avoided by cold treatment of the wine. Protein hazes are avoided by the use of chilling and bentonite.

Maintaining wine in an anaerobic state and with 20-30 mgL-1 SO2 is generally sufficient to prevent spoilage by most bacteria and yeast. Furthermore, when fermented to dryness, most white wines are relatively resistant to spoilage.

The use of other micro organisms in wine production

Red wines usually undergo a malolactic fermentation, effected by the lactic acid bacteria Pediococcus (homofermentative), Leuconostoc (heterofermen-tative), Oenococcus (heterofermentative) and Lactobacillus (either). In this process, malic acid is degraded to lactic acid with an attendant decrease in total acidity and a net increase in pH. The bacteria concerned prefer a relatively high pH and tend to be inhibited by SO2. They also do not perform well at too low a temperature. For an effective malolactic fermentation, the wine should have a pH of 3.25-3.5, a total SO2 level below 30ppm and zero free SO2. The malolactic fermentation formerly depended on the microflora native to the process, but in most instances nowadays the specific bacterial strains required are seeded into the vessel.

Grapes from warm climates tend to contain less malic acid and therefore benefit less from such a fermentation than do grapes from relatively cold areas.

A further type of natural fermentation effected in the production of some wines is the application of certain yeasts (formerly believed to be Torulaspora delbrueckii but likelier to be S. cerevisiae) growing as a film on the surface in the production of 'flor' sherry. The main impact is the production of acetaldehyde.

Champagne/sparkling wine

The best such wines are produced from the juice of Pinot noir or Chardonnay grapes. There must be rigorous avoidance of colour development, hence the extensive use of SO2, bentonite and PVPP.

Fermentation in bottle is effected by a culture of S. bayanus that is floc-culent and able to perform at high alcohol concentrations. The parent wine, invert sugar and yeast are delivered into pressure-resistant bottles sporting a lip for the application of a crown cork. A 2.5-cm headspace will be left in the bottle before it is laid on its side and held at 12°C. The wine will ferment to dryness over a period of several weeks but may be left for more than a year for the achievement of best quality.

There follows the process of 'riddling' in which the yeast is worked into the neck of the bottle. The yeast is loosened by hitting the bottom of the bottle with a rubber mallet or by using a shaking device. Then the bottle is put neck down into a rack at an angle of 45°. The bottle is rotated a quarter turn daily until the yeast sediment has all arrived at the cap. Then the inverted bottle is chilled to 0°C and carried through a brine bath cold enough to yield a frozen plug of wine about 3.5 cm long. The cap is removed and as the ice plug is forced out, it scrapes the yeast with it. The bottle is immediately turned upright again, refilled with wine containing sugar and some SO2, corked and labelled.

In an alternative approach, very cold riddled wine is completely removed from bottles, pooled and cold stabilised under pressure. It is filtered and returned to bottles for corking and labelling as 'sparking wine'.

Certain wines are carbonated simply by bubbling with carbon dioxide prior to packaging (cf. beer).

Ageing

Contrary to most beers, wines tend to benefit from ageing, which is performed either in tank, barrel or bottle. The extent of ageing is likely to be less for white

Table 3.5 Examples of compounds developing in alcoholic beverages aged in oak.

Cyclotene Dihydromaltol Ellagic acid 4-Ethylguaiacol Ethyl maltol

4-Ethylphenol Eugenol Furaneol Furfural Gallic acid

Hydroxymethyl furfural

P-Ionone

Maltol

5-Methylfurfural

P -Methyl-y -octalactone Norisoprenoids Syringaldehyde Vanillin

Flavour changes occurring during ageing are not solely due to extraction of substances from the wood. Other significant events include oxidation, evaporation and chemical reactions leading to the production of new compounds.

Flavour changes occurring during ageing are not solely due to extraction of substances from the wood. Other significant events include oxidation, evaporation and chemical reactions leading to the production of new compounds.

Norisoprenoids

wines than for reds. During the ageing of wines, there is careful monitoring of colour, aroma, taste and the level of SO2.

The flavour of white wine is very largely determined by the esters produced during fermentation. Some chardonnays are aged in oak barrels, from which some characteristics are derived (Table 3.5). Diverse oaks may be used in ageing, with relevant compounds increasing in level being guaiacol, eugenol and furfuryl alcohol (Fig. 3.13). Burgundy and Loire whites are left on the lees for up to 2 years ('sur lies').

Red wines, having undergone their malolactic fermentation are then aged. Bordeaux wines are held 2 years in barrel. By comparison, zinfandel ageing should not be excessively prolonged in order to retain the raspberry character.

Packaging

Residual oxygen in wine is removed by sparging with nitrogen gas. Careful control of oxygen levels is effected during the bottling operation per se. Some

Tartaric Acid Effecting Malic Acid

Table 3.6 The major components of table wine.

Component

Ethanol Methanol Propanol Isobutyl alcohol Active amyl alcohol Isoamyl alcohol

1-Hexanol

2-Phenylethanol 2,3-Butanediols Sorbitol Mannitol Erythritol Arabitol Glycerol Malic acid Tartaric acid Succinic acid Citric acid Acetaldehyde Acetoin Diacetyl Ethyl acetate Isoamyl acetate Mono-caffeoyl tartrate Mono-p-coumaroyl tartrate Mono-feruloyl tartrate Various other esters Total amino acids Protein

Tannins

Histamine

Tyramine

Potassium

Sodium

Nitrate

0.007-0.07 0.007-0.17 0.019-0.1 0.08-0.35 0.001-0.012 0.005-0.07 0.015-1.6 0.005-0.39 0.08-1.4 0.03-0.27 0.013-0.33 1.1-23 0-6.0 0.5-4.0 0.5-1.3 0-0.3

0.003-0.49

0.0007-0.138

0.0001-0.0075

0.001-0.23

0-0.009

0.008-0.03

0.001-0.016

Various, but low

0-0.012

Data from various sources.

winemakers add sorbic acid as an antimicrobial preservative for sweet table wines. If such an additive is to be avoided, then more attention must be paid to cold filling and sterility.

Taints and gushing

Cork taints on wine can come from several sources. Trichloroanisole affords a musty or mouldy character, geosmin an earthy note and 2-methylisoborneol a chlorophenolic aroma (Fig. 3.14). They are due to chlorine treatment of corks with subsequent methylation by bacteria and moulds. It is advisable to keep corks at very low moisture content (5-7%) in order to minimise this problem. Of course metal- or plastic-lined caps do not present this risk - but are widely unfavoured in view of their lesser aesthetic appeal. Taints may also arise from wooden vessels employed in the winery.

Gushing in wine may arise due to microscopic mould growth. As for beer, the shelf life of wine is greatly enhanced by cool temperature of storage.

The composition of wine

Table 3.6 presents an approximate summary of the main chemical components of wine.

Bibliography

Amerine, M.A. & Roessler, E.B. (1983) Wines: Their Sensory Evaluation.

San Francisco: WH Freeman. Amerine, M.A. & Singleton, V.L. (1977). Wine: An Introduction, 2nd edn. Berkeley:

University of California. Amerine, M.A., Berg, H.W., Kunkee, R.E., Ough, C.S., Singleton, V.L. & Webb, A.D.

(1980) The Technology of Wine Making, 4th edn. Westport, CT: AVI. Boulton, R.B., Singleton, V.L., Bisson, L.F. & Kunkee, R.E. (1996) The Principles and Practices of Winemaking. New York: Aspen. Dutruc-Rosset, G. (2000) The state of vitiviniculture in the world and the statistical information in 1998. Bulletin de l'Office International de la Vigne et du Vin, 73,1-94. Fleet, H., ed. (1993) Wine Microbiology and Biotechnology. Chur: Harwood. Jackson, R.S. (2000) Wine Science: Principles, Practice, Perception, 2nd edn.

San Diego: Academic Press. Waterhouse, A.L. & Ebeler, S., eds (1998) Chemistry of Wine Flavor. ACS Symposium Series No. 714. Washington, DC: American Chemical Society.

Making Your Own Wine

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