Troubleshooting Your Wine
The faults and flaws and their analysis and resolution presented below are from my latest book, Modern Home Winemaking, where you can find additional information should you wish to explore these further.
REFERMENTATION IN BOTTLES
DESCRIPTION
A previously still wine — that is, wine that had no perceptible fizz on the palate — now tastes slightly fizzy or even carbonated, and there may be visible sediment. In the worst case, corks can pop or bottles explode due to pressure build-up. The powdery sediment is often mistaken for tartrates by those not familiar with tartrates — the latter are crystals, not powder-like.
CAUSES
This problem is due to refermentation in bottles; this happens when the wine has not fermented to complete dryness, i.e., there is an appreciable amount of residual fermentable sugars, and which has not been properly stabilized. This is a common occurrence in sweetened wines, or with novice winemakers rushing to bottle wine but not understanding the implications of residual sugar and incomplete or improper stabilization. Unless wine is sterile filtered at 0.45 micron (absolute) to remove yeast and bacteria that may cause refermentation or other unwanted reactions, these microorganisms are always present — or should be assumed to be always present — and waiting for an opportune moment to strike if conditions are good, for example, if there is residual sugar.
ASSESSMENT
Do a visual inspection followed by a taste test.
If you suspect that wine has started refermenting in bottles, look for any visible fine sediment — these are the fine lees that form during AF — making sure not to confuse these with tartrates, which are bigger and more shiny having a crystal-like appearance. Hold the bottle by the neck and turn it upside down; if the lees make the wine cloudy, it is a sure sign of refermentation. Open the bottle and taste the wine; if it tastes fizzy, then it is definitely a refermentation problem.
REMEDIAL ACTIONS
To avoid corks popping out or bottles exploding, uncork all bottles and pour the wine back into a carboy or appropriate holding vessel, let the wine complete AF to dryness, i.e., SG below 0.995 (Brix below –1.5), fine with a fining agent, stabilize with KMS, optionally filter, and re-bottle.
If you have no means of measuring the amount of residual sugar (RS), add sulfite and sorbate before re-bottling. If you can measure RS, add sulfite and sorbate if RS is greater than 2 g/L; if RS is less than 2 g/L, you only need to add KMS but sorbate is recommended as a safeguard.
IMPORTANT: Sorbate cannot be used to stop an active AF; you must stop AF or let it complete before adding sorbate.
CAUTION: Never use sorbate in wine that has undergone MLF; lactic acid bacteria can metabolize sorbic acid and cause a series of reactions resulting in an off-putting geranium odor.
PREVENTIVE ACTIONS
To avoid refermentation problems, always ferment to dryness to a SG below 0.995 (Brix below –1.5). At SG 0.995 (–1.5 Brix), wine can still have up to 7–8 g/L or more of residual sugar, sufficient for yeast to happily restart fermentation when conditions improve, primarily when FSO2 decreases, especially if temperature increases.
If you can measure RS and it is greater than 2 g/L or if SG is 0.995 or greater (Brix greater than –1.5), add KMS and sorbate; if RS is less than 2 g/L, only KMS is needed but sorbate is recommended as a safeguard. In all cases, sterile filter the wine down to 0.45 micron (absolute), if so equipped, to reduce yeast and bacterium populations to insignificant levels.