The volatile acidity in wines comes from the short-chain acids of the acetic series (acetic, formic, propionic, butyric) and some of their combinations such as ethyl acetate originated during fermentation, which can give the wine the unpleasant odor and “chopped” taste ruining the production. Take into account that the level of sensory detection for these compounds is very low, in the order of 0.6 g / L for acetic and 0.1 g / L for ethyl acetate.
The volatile acidity is a part of the total acidity of the wine, formed by the primary acids that are already present in the grape must (malic and tartaric) and the secondary ones that are those generated during the fermentation processes (acetic, succinic, malic, …). They also add acidity some gases and dissolved substances such as carbon dioxide, sulfur dioxide and sulfites (added as preservatives).
All these acids provide the wine with the necessary acidity for its best preservation, but they must be in the correct amount with the other components to achieve a balanced product.
The pH is one of the parameters to control in wines. Therefore photos are standard equipment in oenological laboratories. The pH can take different values for each type of wine, but its ideal value will normally be between 2.7 and 4.2.
The titer or acid-base titration is the usual way to determine the acidity ( total and volatile ). We will use for this purpose a solution of known concentration (valued base) and an indicator. The turn of the indicator will mark the endpoint, the volume (mL) of base consumed will allow us to calculate the amount of acid present in the sample.
As we have mentioned in the wine, we have a mixture of acids and we want to determine the content of the acetic series ( volatile acidity ) for this reason the available methods resort to a previous separation (distillation). In this post, we will comment on the García-Tena method.
Determination of volatile acids in wine by the García Tena method
The García Tena team allows measuring the volatile acidity of the wine by resorting to a fractional distillation followed by titration with 0.02 N sodium hydroxide and phenolphthalein indicator (1%).
The assembly of the equipment is the one that can be seen in the previous image, in the foreground the pieces for the distillation and the burette and Erlenmeyer for the subsequent evaluation. Take the usual precautions for assemblies with ground parts: apply Vaseline on the joints, join the pieces with the clips. Very important!! Place some pieces of pumice in the spherical distillation flask and never heat up dry, it can break! The water inlet (tap) to the coolant should be placed on the bottom and the outlet to the drain on the top. For the evaluation, prepare the following reagents:
Sodium hydroxide (NaOH) 0.02 N and 1% phenolphthalein
Place in the distillation flask 11 mL of wine without carbon dioxide (*) and connect the distillation apparatus.
Place the 5.1 mL test tube in the coolant outlet and distill to fill (top mark of the test piece).
Immediately replace it with the 3.2 mL test tube. The distillation ends when collecting the 3.2 mL in this second test tube.
Pour the 3.2 ml of the second test tube + a few drops of Fenlftalein (1%) into an Erlenmeyer flask and titrate with 0.02 M sodium hydroxide solution.
The turn of the indicator to a pink color marks the endpoint. Record the volume of the spent base (0.02 M sodium hydroxide) and perform the following calculation:
Volatile acidity (g / L) = 0.366 x V
The volatile acidity is expressed in g / L and will normally be between 0.20 and 0.70 depending on the type of wine and the production process.
V = volume of base spent (mL of 0.002 M sodium hydroxide)
(*) It is important to eliminate carbon dioxide because it provides acidity that can alter the results. For this there are several methods, a vacuum aspiration can degas the wine before placing it in the distillation flask.