Sauces: homemade or from a jar
Knowing when to give up on that sauce you didn't finish with Sunday's dinner comes down to a few factors: its base -- broth, water or dairy -- texture, smell and flavour.

Fat and salt are preservatives, so the fattier or saltier a sauce, the longer the shelf life. That's how and why traditional dishes such as duck confit and salt cod were developed -- as a means of preservation in a time before refrigeration.

Same with sauces; fat and salt content will affect their life span. So will exposure to air, though cream- or milk-based sauces tend to form a "skin," which acts like a lid of sorts. A skin doesn't mean it's necessarily bad; scrape the skin off and test what's underneath. Another sign that a cream or cheese sauce is past its prime is when water leaks out and forms little pools.

Sweet, alcoholic and butter- or oil-based sauces will last longer than dairy sauces but never, ever take chances with fish or shellfish sauces.

Leftover sauce will go bad faster through cross-contamination: don't use the same utensil to serve the meatloaf then dip it into the cheese sauce.

We asked cookbook author and expert on tasty and healthy home cooking Norene Gilletz how many days she keeps a sauce. "It depends on whether it's tomato- or dairy-based, but generally, leftover sauces -- as with most leftovers -- will only keep about two or three days, safely." Gilletz also points out that certain contaminants, such as deadly botulism, are odourless, so the old sniff test won't protect you.

Toss homemade and store-bought sauce when:
• it's older than three days
• you spot the first signs of the fuzzies -- that's mould
• it no longer smells like it did when you first made it or opened the jar
• it's changing colour: a streak of pink in a white cream sauce suggests the beginnings of a bacteria colony, as does a spot or two of black

Condiments
Based in vinegar with heavy doses of sugar, salt and oils, condiments, including ketchup, mustard, relish and bottled vinaigrettes, can last for ages -- especially pickles and olives. Still, they're not indestructible and are still vulnerable to cross-contamination, so don't stick your mayo knife into the pickle jar!

A vinegary environment is a double-edged sword. While it may prohibit bacteria growth in the food, it's very corrosive and, with time, can eat away at metal lids. Get in the habit of wiping away drips after every use and running the inside of the lid under warm water every now and then.

Photography, iStockphoto.com.