Recognize defected wine

When should we return wine in a restaurant? More importantly, how do you tell if a wine is defected or if it simply tastes horrible?  

We can usually identify a defected bottle before tasting it. Look for the following: 

Appearance

Clarity: Cloudy wine hints bacteria spoilage. The wine should be clear. There might be deposits (known as sediments) which are normal and provide no indication of their quality. Definitely return all cloudy wines!

Color: The color of the wine reflects its development. Oxidized wine (aged too long or exposed to oxygen) has tints of reddish-brown or amber-tawny for white wines. Oxidized wine tastes flat and stale.

Condition of the cork: If the cork is cracked, or penetrated by wine, the wine is most likely damaged.

Sometime, you might observe clear sugar-like crystals in the bottle of a bottle or glass. These are tartrates. They formed when the wines are stored in very cold temperature. They are harmless to drink. If anything, they indicate a good quality bottle - one that has not been commercially-treated during vinification.

Smell

Besides getting a solid sniff at the bottle, check for 3 smells that indicate potential defects:

Smell of moldy, musty smell of mushrooms or damped earth. This is caused by the Trichloronisole (TCA) in a corked wine. The smell might worsen with exposure to air. 

Smell of vinegar or sherry. This would indicate high level of volatile acidity and oxidation.

Smell of rotten-eggs. These are caused by excessive hydrogen sulfide formed during the fermentation process. Though unpleasant, the wine is drinkable. One traditional remedy is to drop a copper coin into the wine to eliminate the odor. 

Often, the wine will smell of burnt-matches when uncorked. This is the smell of sulphur dioxide, which was added in the bottling process to keep the wine fresh. There is no need to return the wine. The smell will go away after a few minutes of aeration. 

Taste

An oxidized wine is flat and tastes like sherry/vinegar.

Definitely smell and taste the wine before going along with it at a restaurant. Statistics has it that one in 30 bottles of wines in the past 10 years are corked. Wine experts believe the statistics can be as high as one in every 10 bottles for aged wines.