With more than 600 chemical compounds potentially occurring in wine, there are some complex reactions vintners just can’t control.

A little sniff of something that seems odd to you can mean big things have gone wrong in the wine. Your nose is your first line of defence in detecting wine flaws and, worse, wine faults.

Little departures from normal are flaws, which sometimes can actually add to a wine’s complexity. But big things are faults, and can lead to rejection by amateur tasters and wine judges alike.

Often folks notice something is off with the smell or taste, but drink the wine, anyway. Lots of bottles with serious cork taint, for instance, are never returned to the LCBO, which has an enlightened policy of returning your money for such bad bottles.

For example, two years ago U.K. wine critic Jane MacQuitty stated in The Times of London that half the South African wines she tasted had a “peculiar, savage, and burnt rubber” odour. Winemakers in the Cape region have been driven batty looking for the source.

I call it the aroma of rusty nails and pond weed. It is indigenous to, and characteristic of, South African reds. After visiting South Africa and talking to its winemakers, I actually think it is bacterial in origin, something endemic to the terroir of that country.

As for MacQuitty, not long ago she tasted a raft of premium-level sauvignon blancs from New Zealand and wrote they were not only “grassy” and “watery,” but “evil.”

All of this made me think Star readers could use a primer on common wine flaws and faults. Call it close encounters of the wine kind. The LCBO will refund your money for badly flawed wine. Just don’t try to return a bottle that’s less than half empty – that’s just dumb – or a bottle that’s died from living too long in your cellar. If it is bad, you will not get past the first glass.

Rancid cheese and wet cardboard are just two indicators that your wine is off:

Rancid cheese

Wine ages slowly in the bottle, but when it has grown far too old prematurely, this is the scent. It’s pretty nasty and may cause your head to jerk backward in disgust. Such a wine is DOA (dead on arrival) and you should summon, or text message, your local CIS team to do an autopsy. But it will not help. So save your calories. You may have also noticed the browning edges or rims of this wine. Pour the culprit down the kitchen sink and watch for subsequent fish death in Lake Ontario. You may want to alert local health authorities.

Wet cardboard

Also described as mouldy newspaper, dank basement, or wet dog. Known as cork taint, or TCA (2,4,6-trichloroanisole), even though screwcaps are taking over the world, about 7 per cent of conventionally corked wines show this. It varies in intensity and can grow more noticeable as a wine breathes in the glass after pouring. On the palate, badly corked wines taste bitter and their normal flavours are muted. TCA has multiple causes, usually related to chlorophenols found in pesticides and wood preservatives, and can evenbe traced to chlorine used to bleach natural corks and to clean wineries. It’s nasty to smell and taste, but not harmful to your health.

Barnyard

Referred to politely in British wine circles as “animal” odours, but more candidly as horse sweat, cow dung, sheep shit, mousiness or Band-Aid odours in North America. Long regarded as a virtue by French winemakers, this smell has been traced to a wine storage yeast called Brettanomyces which naturally lives on the skin of fruit. One winery, Château de Beaucastel, relies on this to give their cult red its distinctiveness. A little “Brett” is palatable, but its overwhelming presence can be repugnant, reminding one of fresh manure spread on an organic vineyard.

Stale peanut butter

Ontario wine lovers well recall the harvest that included an infestation of oriental orange ladybugs. When riled, these ladybugs released high quantities of unpleasant, volatile compounds called pyrazines, which made the wines taste like rancid peanut butter. A little went too long a way and it masked the natural great scents and flavours of hundreds of wines that year. Pyrazines are high in sauvignon blancs, not always from ladybugs, giving a green bell pepper aroma. It may bug you that insects wind up in wine, but vineyards are full of good and bad insects, and some get crushed with the grapes. I have seen thousands of earwigs trying to crawl up stainless steel crusher sides after night harvests by giant combines. Very rarely I will taste stink bug in a wine, which is a truly nasty odour, likened to rancid almonds.

Match sticks

Also can be perceived as burnt rubber, rotten eggs and mothballs. This comes from sulphur dioxide, which is used normally in moderation to preserve wine flavour and colour. When overdone, it can be very pungent, even prickling your nostrils. It most often occurs in recently bottled wines, and tends to settle down after aging, either in the bottle or in your glass. If you cup your hand over your glass and shake the wine up, it often will release the odours from the wine.

Dirty dishrag

I’ve been smelling this a lot recently, in whites and reds. To other noses, it can be reminiscent of furniture or floor polish. The wine is often slightly bitter and metallic tasting. Experts have been working on discovering what causes this, which is called atypical aging or ATA (the wine smells fine at bottling but develops this after a few months). Some scientists think it’s 2-aminoacetophenone, where amino acids have gone wonky. So far there are more theories than solutions.

Rubbery, skunky

This is from mercaptans, caused by the reaction of hydrogen sulphide with other elements in the wine, including amino acids or ethanol. Mercaptans form sometimes if a finished wine is allowed to remain too long in contact with the lees, or dead yeast cells. Some tasters find this reminds them of the smell of onions, as well. It is usually quite subtle, more a background irritation than something that makes you stop drinking your wine and call for a support group.

Nail polish

Also reminiscent of paint stripper, vinegar, dill pickles. This is volatile acidity, or VA. It comes from the formation of acetic acid in wine and is quite common, especially in amateur winemaking. Such vinegary traits result from the growth of acetic acid bacteria. When it shifts into the solvent spectrum or ethyl acetate, the smell is like nail polish remover (acetone), and can be quite off-putting.

Cooked cabbage

Also asparagus, canned corn or truffles. These scents often derive from dimethyl sulphide, which is natural in wines, but, over a sensory threshold, becomes offensive. It is thought these stem from the breakdown of sulphur that contains amino acids. Fortunately, it is rarely encountered.

Geranium

This is caused by malolactic bacteria acting on potassium sorbate in the winemaking. The result is ethoxy hexadiene, and even minuscule levels are nasty. In wine judging, this aroma usually results in the wine being removed as undrinkable.

Butterscotch

Also rancid butter. This is known as diacetyl (2,3 butane dione) and, in most wines, it is not regarded as a flaw. For instance, chardonnays that have undergone malolactic fermentation and wood aging often show buttery traits, which can be very pleasant. Sometimes such winemaking is overdone, and over-the-top butterscotch or rancid butter aromas result. Such buttery characters should not overwhelm the natural fruit in the wine.