I recently discovered a case of British Columbia sauvignon blanc in my basement after eight years (don't ask). It has turned from pale yellow to brown but it tastes okay to me. Why did it turn brown, and is it safe to drink?
I won't ask how you forgot about it. Heck, I've been known to forget about half-filled wine glasses that have been sitting on my kitchen counter for days. That brown colour was caused by oxidation, or excessive exposure to air. Have you ever sliced an apple and left it on the counter? You get bruised fruit, which similarly turns brown once the skin has been breached.
This will inevitably happen over time with white wines but occurs more rapidly when the bottle seal is defective, as in a faulty cork. Cork is microscopically porous and lets in air over time. Because it's a natural product, made of tree bark, it also tends to be uneven in composition, often with cracks or larger-than-desired pores. It will also dry out excessively if the bottles are stored upright rather than horizontally, which I'm assuming was the case with your sauvignon blanc. If the cork shrinks in a dry environment, boom, too much air gets in, fouling up the wine's freshness.
This does not normally result in risk to human health. Most wine lovers, however, would likely find the flavour unpleasant, as though the sauvignon blanc had been mixed with sherry to give it a tangy, nutty quality.
If you'd rather not throw the wine out, I'd say you've got yourself a pretty good pairing for a bruised apple.
The Flavour Principle by Lucy Waverman and Beppi Crosariol took home top prize last year for best general English cookbook at the Taste Canada Food Writing Awards. Published by HarperCollins.