Flavors from the devil

My final, considered judgment is that the hardy bulb [garlic] blesses and ennobles everything it touches - with the possible exception of ice cream and pie.” Angelo Pellegrini, 'The Unprejudiced Palate' (1948)

Sulphur compounds do not only keep in check the reduction potential of our solutions in the lab: Nature brings them to our noses and palate as well, leaving their stamp on our cuisine. The pungent smell given by crushed garlic, the tears flowing while cutting onions and the aftertaste of Sauvignon Blanc all share their origins in them. In many foods, the potential of sulphur for redox chemistry makes it subject to the enzymatic transformation of its organic derivatives in flavorful components.
In the cells of the Alliaceae family (which include garlic, onion, shallot, and chive) the enzyme alliinase is sequestered from the cytoplasm. When a bulb of garlic (Allium sativum) is crushed or cut, the cells are damaged and alliinase is released in the cytoplasm. Once there, it comes in contact with alliin, a cysteine derivative without particular smell. Alliinase catalyze oxidize it to give allicin, which gives its… pungent... smell to garlic. Apparently, somewhere along its evolution, the Alliaceae family found the trick handy when animals decided to grab a bite at their expense.
This might help the resourceful biochemist cooking with garlic. For garlic with the flavor but without the bad breath, wrap your bulb in the aluminium foil with olive oil and put in the oven at 325 °F for an hour. The heat will easily inactivate the alliinase.
The alliinase released in onions has a different effect. It allows ultimately the formation of syn-propanethiol-S-oxide from sulfenic acid. The molecule is not only volatile: it reacts with water to give sulfuric acid, giving their lachrymal properties to onions. The good news is that, as this sulfinyl is combustible, lighting a candle next to your cutting board should eliminate most of the annoying molecule.
Sulfur compounds can mean a more enjoyable experience too. A characteristic element of the taste of the Sauvignon Blanc wines rests on certain thiols. Interestingly, those thiols are absent from the wine itself. They originate from S-(R/S)-3-(1-hexanol)-L-cysteine, a component of the grape, which is transformed by salivas’ microflora in volatile thiols. Sulfur can have its upside too after all.

Further readings:
Jones MG et al. 2004 Biosynthesis of the flavour precursors of onion and garlic, J. Exp. Bot., 55 ( 404), 1903
Srarkenman C. 2008 Olfactory Perception of Cysteine-S-Conjugates from Fruits and Vegetables J. Agric. Food Chem., 56, 9575-80