Q: What’s the difference between “extra virgin” and “extra light” olive oil? They seem to have the same number of calories.

A: Yes, they would. They’re both oils – 100 percent fat – which means each carries 9 calories per gram, or about 120 calories per tablespoon. And they both have about the same amount of healthful monounsaturated fats compared to other fatty acids in their composition. Where you’ll notice the difference is in the flavor.

“Light” olive oils are finely filtered, removing much of the flavor and even some color of the oil. Without the strong flavor often associated with olive oil, light versions would be good choices for baking or other cooking where you don’t want a strong olive oil taste competing with other flavors.

“Extra-virgin” olive oil is on the opposite end of the spectrum. It’s oil from olives that have been pressed – pit and all – just once, and that pressing has resulted in an oil with an acidity level of less than 1 percent. It’s considered the creme-de-la-creme of olive oils – the most flavorful and fruitiest of the bunch.

There’s also “pure” olive oil, which is really just a mixture of extra virgin and light olive oil. There’s usually much more of the light olive oil in these blends – anywhere from 70 percent to 95 percent of the total.

Olive oils are one of the best sources of monounsaturated fat – more than 75 percent of the fatty acids in olive oil are monounsaturated, while about 9 percent are polyunsaturated and about 14 percent are saturated. Consuming either monounsaturated or polyunsaturated fats is linked with lower blood cholesterol levels, which reduces the risk of heart disease.

However, that benefit could be offset if you simply add more fat to your diet and gain too much weight, which would increase heart disease risk. Rather, nutritionists recommend replacing some of the saturated fat and trans fat in your lower fat diet with oils high in monounsaturated fat, like olive or canola oil, or those high in polyunsaturated fat, like corn, soybean or safflower oil.

When making vinaigrette salad dressings, the general rule is one part acid (vinegar or lemon juice) to three parts oil. For grain and rice salads, use one part acid and one to two parts oil.

Here is a Basic Vinaigrette Salad Dressing that uses half extra-virgin olive oil and half mild oil, such as “extra light” olive or canola oil, so the olive oil flavor isn’t as strong. Feel free to use all extra-virgin olive oil or all canola oil, depending on your taste.

Basic vinaigrette salad dressing

2 tablespoons red wine vinegar

3 tablespoons extra-virgin olive oil

3 tablespoons canola oil

1/2 shallot, finely minced

1/2 teaspoon salt

1/2 teaspoon Dijon mustard

1/4 teaspoon ground pepper

Combine all of the ingredients in a jar and shake well. Refrigerate for up to two weeks.

Q: Should vegetable oils be stored in the refrigerator?

A: The ideal storage conditions for most vegetable oils – such as corn, canola, soybean, olive and similar oils – would be in airtight containers in a darkened location that’s generally cooler than room temperature but warmer than in a refrigerator. A wine cellar would do the trick, but not too many of us have one of those. That’s why it’s easy to find different storage recommendations from different experts.

The problem stems from oil’s susceptibility to air, light and heat, any one of which could cause it, over time, to become rancid. Usually caused by oxygen, and boosted by heat and light, rancidity is the process of molecular reactions in fatty acids. Unsaturated fats are more susceptible to rancidity because of their chemical structure. Indeed, the term “unsaturated,” chemically, means that not all of the carbon atoms on the fatty acid chain are paired with hydrogen atoms. (Saturated fats are more shelf-stable precisely because all of the available carbon atoms are “saturated” with hydrogen atoms.) The unpaired carbon atoms in unsaturated fatty acids are vulnerable to oxygen and other reactive molecules, which literally can start a chain reaction that, eventually, deteriorates the fatty acid chain and causes off-flavors to develop. That’s what we know as rancidity.

Generally, unopened vegetable oils should store just fine at room temperature for six to 12 months, or perhaps longer. Once they’re opened, try to use them up quickly, within one to six months. When you use oils that have been stored for a long time, take a whiff. If you don’t like the smell, you won’t like the flavor, either. Time to buy new.

If you don’t use oils very much and want them to last as long as possible, by all means store them in the refrigerator. You probably will see some cloudiness develop (don’t worry – that’s normal), and some oils, such as olive oil, may become too thick to pour until they get back up to room temperature.

If you don’t want to go that far, you can help oil last longer if you make sure it’s in a tightly sealed container (protecting it from air) and stored inside a cupboard (protecting it from light) that’s not near the stove, dishwasher or refrigerator (protecting it from the heat those appliances generate).