Labo VinMethanol
[ Methanol, a very toxic alcohol, is always present, in small amount in a wine. Contrary to what is generally thought, it is not formed during the fermentation, but in the must, well before the fermentation starts, by hydrolysis of the fruits pectin by dint of an enzyme: pectin methylesterase present in the fruits. Very ripe fruits contain already some methanol (as well as ethanol). ]
Methanol detecting in pure alcohol
Two methods can be used for methanol measuring in a solution containing also ethanol. The first one is a colorimetric dosage. The second method necessitates the use of a gas chromatograph, a little available device. We will not speak about it here.
1) Method using fuchsine
- Prepare the following reagents: potassium permanganate (KMnO4) (3.16 g/100mL twice distilled water), phosphoric acid (H3PO4) 25%, oxalic acid (COOH)2 (6.3 g/100 mL), sulfuric acid (H2SO4) 96%, discoloured fuchsine (reduced rosaniline) prepared as follows:
mix 30 mL of a fuchsine solution in water at 0.1% with 200 mL of a saturated SO2 solution; shake and add 3 mL concentrated H2SO4. Let lie for 24 h, filter if any precipitate has appeared. The solution is colourless or light yellow. - Dilute the sample to be analysed to about 10% ethanol and withdraw 5 mL in a test tube.
- Add 3 mL KMnO4 and 2 mL H3PO4.
- Wait for 15 min (a brown precipitate appears).
- Add 2 mL oxalic acid and heat in a water bath (without boiling) up to discoloration.
- Add 1 mL H2SO4 letting it flow along the inner wall.
- Add 3 mL discoloured fuchsine.
- Let lie for 30 min.
- If the solution is pink, some methanol is present. Compare with pure ethanol and with ethanol plus one drop methanol, or prepare a standard scale.
Source: Proost number 8, p. 27.
2) Methanol seeking as an aldehyde
A very detailed method, using chromotropic acid, is given here (in French): Recueil international des méthodes d'analyses OIV Méthanol.
Maximal authorized amount
According to the country and the beverage, the tolerated maximal amount can vary. Nevertheless, European Union has the good idea to match the concerned legislation.
Table of the maximal methanol amounts authorized in the European Union.
| Alcohol | g/hL pure methanol |
|---|---|
| Alcohol from agricultural origin | 50 |
| Wine brandy | 200 |
| Brandy | 200 |
| Brandy from grape marc | 1000 |
| Brandy from fruit marc | 1500 |
| Fruit brandy | 1000 |
| Fruit brandy (plum, mirabelle, quetsche, apple, pear, raspberry, blackberry) | 1200 |
| Fruit brandy (Williams pear, red currant, black currant, rowan berry, elderberry) | 1350 |
| Brandy from cider and perry | 1000 |
Source: Internet site Wikignôle.