Analysis of wine

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Analysis of wine

Wines are dilute solutions of weak organic acids, these being mainly tartaric, malic, lactic, ethanoic and succinic acids. The total acidity of a wine is generally expressed as the sum of the 'fixed' acidity (due mostly to malic and tartaric acids) and the 'volatile' acidity. Determination of total acidity is generally achieved by titration of the wine with a strong base, the end point being detected visually with an indicator, or electronically with a pH meter. Since the acids present in wine are relatively weak, titration with a strong base gives an end point in the alkaline region of the pH scale; the American Society of Oenologists recommends pH 8.2 for the end point.

pH

pH has a well recognised effect on the taste of wine; wines of high pH tend to taste rather flat on the palate and are also much more susceptible to oxidative and biological spoilage than are wines of lower pH. The pH of a red wine is sometimes regarded as its most important single feature.

Acidity

The volatile acidity in wine is due to the fatty acids such as formic, acetic, butyric etc. Wines of high volatile acidity are prone to spoilage and specific regulations exist governing the maximum permissible amount of volatile acidity. The volatile acids may be separated from the wine by steam distillation and the volatile acidity subsequently determined from the distillate. Alternatively, the volatile acids may be removed by repeated evaporation of the wine sample and the residue analysed for the non-volatile (or 'fixed') acids. The volatile acidity may then be readily calculated by subtraction of the fixed acidity from the total acidity. Identification and even semi-quantitative determination of individual non-volatile acids is possible using TLC or paper chromatography. This is particularly useful in industry for following the enzyme catalysed conversion of malic acid into lactic acid (called malolactic fermentation) during the fermentation process. The amount of malic acid in grapes decreases as the grapes mature. In some areas, particularly where the growing season is short, the grapes still retain a high degree of acidity when picked. Since the conversion of malic acid to the weaker lactic acid reduces the acidity of the fermenting wine, it is important that the relative amounts of the two acids be monitored.

Tannins

Tannin is a natural ingredient of wines and originates in the skin. Red wines are normally made by fermentation on the skins which extracts a lot of tannin, so that typical values for Burgundies and clarets will be in the 0.15-0.4% range with levels about a tenth of this for typical white wines. For red wines, tannin is essential since it gives the wine the 'bite' that balances the acid and sugar; it also acts as a preservative. As the wines age so the 'tannins' slowly polymerise to give six- or eight-unit tannins that are less abrasive to the taste, with a consequent mellowing of the wine. There is also some loss of tannin in the form of the characteristic deposit found in most of the great red wines on storage.

Sulphur Dioxide

Sulphur dioxide is an essential additive to all wines since it acts both in a biocidal capacity (preventing attack by unwanted bacteria and yeasts) and as an antioxidant. If too little sulphur dioxide is used the wine may suffer from biocidal attack and it will certainly develop the characteristic taste of an oxidised wine; too much sulphur dioxide, on the other hand, will be very evident on the palate. Most of the added sulphur dioxide combines with various aldehydic and ketonic molecules (especially ethanal) and the remaining uncombined sulphur dioxide is available to protect the wine. There are legal limits to the total S02 that is permitted, the limits varying from country to country, but 250 ppm is a commonly accepted value. There is no limit on the amount of free S02 but 20-40 ppm levels safeguard the wine and will not affect the taste; levels below 10 ppm in a white wine show that it is in imminent danger of 'going over the hill'.

Experimental

pH of wine

  • Place 25.0 cm3 of wine in a conical flask.
  • Measure the pH using a pH meter.
  • Compare the pH of red and white wine.


Determination of Total Acidity – white wine

  • Place 25.0 cm3 of white wine in a conical flask.
  • Add a few drops of phenolphthalein.
  • Titrate with 0.1M NaOH (irritant).
  • Record the end point.


Determination of Total Acidity – red wine

  • In order to see the colour change it is necessary to dilute 25.0 cm3 of red wine with distilled water to 250 cm3.
  • Place 25.0 cm3 of diluted red wine in a conical flask.
  • Add a few drops of thymol blue.
  • Titrate with 0.02M NaOH.
  • Record the end point.


Determination of Fixed or non-volatile Acidity

  • Carefully evaporated three 25.0 cm3 samples of wine on a hot plate until the volume of each had reduced to 5-10 cm3. Avoid overheating.
  • Add about 25 cm3 of hot distilled water and evaporate the solutions to a final volume of 5-10 cm3. Repeat the process several times
  • After the residues have cooled, dilute to about 50 cm3 with distilled water.
  • Perform the same titrations as for the ‘determination of total acidity’.


Qualitative detection of non-volatile acids by TLC

  • Line a chromatography tank with filter paper and add about 100 cm3 of a freshly prepared chromatography solventnote 1 (flammable).
  • Cover the tank and allow to equilibrate.
  • Place spots of the following on to a TLC plate:
  • Succinic (butandioic acid) acid solution
  • Citric acid solution
  • Lactic acid solution
  • Malic acid solution
  • Tartaric acid solution
  • Mixture of the above acids
  • Wine

All solution are as 0.8% aqueous ethanol solutions (flammable).

  • Develop the plates until the solvent front is about 1 cm from the top of the plate. Mark the position of the solvent front.
  • Transfer the plate to an oven at 110°C and leave for about two hours (or overnight) in order to remove completely the formic acid left from the developing solvent.
  • Spray the cool plate with 0.04% aqueous solution of bromocresol green.
  • Record the Rf values.


Determination of' ‘Tannin’

A standard solution of potassium permanganate is used to oxidise the tannin and colouring matters, and the end point is determined using indigo carmine which is itself oxidised by excess of permanganate; as the indicator uses up some permanganate it must be measured out carefully and allowed for by a blank determination. The alcohol would also be oxidised so it is first removed by gently boiling the wine.

(a) Titre A
  • Pipette 5 cm3 of wine into a conical flask and add 10 cm3 of distilled water.
  • Place a funnel into neck of conical flask (to reduce losses by spilling) and heat the flask gently until the volume of wine and water has been reduced to 5-7 cm3 (By now the alcohol will have boiled off).
  • Add about 250 cm3 of cold distilled water and pipette in 2 cm3 of the indigo carminenote 2 indicator.
  • Titrate with 0.004M KMnO4 until a golden yellow colour appears.
(b) Titre B
  • Pipette 5 cmnote 3 of decolourised wine3 into the conical flask and continue as in (a).
  • This blank determination allows for the indicator and any oxidisable compounds (other than tannins, anthrocyanins etc) in the wine.
(c) Calculation

The amount of KMnO4 used up by the tannins and colouring pigments equals A - B = C. Percentage tannin and pigments equals C x 0.0166.  

Determination of Sulphur Dioxide

This makes use of the conventional iodine titration, using starch as an indicator: For free, or uncombined sulphur dioxide, the wine is acidified and titrated; for total sulphur dioxide wine is first made alkaline with sodium hydroxide to break down the bisulphite compounds and then acidified and titrated. The methods are quick and sufficiently accurate for the required purpose. With red wines there is some masking of the end point but it can normally be seen without too much difficulty if a comparison solution of un-titrated wine is available.

(a) Free S02
  • Pipette 50 cm3 of wine into a conical flask.
  • Add approximately 5 cm3 of 25% H2SO4 (corrosive) and 2-3 cm3 of starch solution.
  • Titrate with 0.01M I2 until the first blue colour appears.
(b) Total (free and combined) S02
  • Place about 25 cm3 of 1M NaOH (corrosive) in the conical flask and pipette in 50 cm3 of wine.
  • Shake and leave for 15 min.
  • Add 10 cm3 of 25% H2SO4 (corrosive) and 2-3 cm3 of starch solution and titrate with 0.01M I2.
(c) Calculation

Amount of S02 present (in parts per million) = 12.8 x No. of cm3 of I2 used

Notes

  1. 16:2:5 mixture of butanol, formic acid and water (flammable)
  2. Indigo carmine indicator method – Dissolve 0.5g of the dyestuff in 60cm3 of warm distilled water. Cool the solution and add 4cm3 of conc. H2SO4. Make the solution up to 100cm3 with distilled water. Filter.
  3. Decolourised wine method – Take 20-25cm3 of the wine and add 1g of activated charcoal. Stir thoroughly. Leave for a few hours. Filter.

Reference

Haddad, PR and Sterns, M and Wardlaw, J (1978) Analysis of wine: an undergraduate project. Australian Journal of Education in Chemistry, 15 (3). pp. 87-89. ISSN 1445-9698