Cracking the Code

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By Robert L. Wolke
Wednesday, January 10, 2007

I like to eat pistachio nuts, and I prepare dishes with walnuts and almonds throughout the year. In the holiday season I also use pecans and hazelnuts. How should nuts be stored? Why do they become rancid? When shopping for nuts, either in a store or from a catalogue, what are signs that they are rancid?

Before I get to your question, I must address the nomenclatural nonsense that pervades the world of nuts.

First of all, botanists have their definition of a nut and gastronomes have theirs. To a botanist, a nut is a whole, hard-surfaced fruit containing a seed. To a gastronome, the seed alone may be called a nut. Peanuts are neither; they're legumes. But before I drive you nuts, let's just agree that anything you think is a nut is a nut.

Second, a filbert is a hazelnut is a filbert. They come from two very closely related trees, and you have to be an expert to tell their nuts apart.

Now about rancidity and storage.

The oils in most nuts are chiefly the more healthful, unsaturated kind. But unsaturated fats turn rancid more readily than saturated ones, because the (double) bonds in their molecules are easily broken by oxidation. The resulting molecular fragments, short-chain fatty acids, are likely to taste and smell bad.

Roughly, the more polyunsaturated fatty acids a nut contains, the faster it can turn rancid. On that basis, we can expect the most common nuts to line up this way, in order of fastest to slowest: English walnuts, pecans, Brazil nuts, peanuts, almonds and pistachios, and cashews and hazelnuts.

Most nuts are harvested in the fall, so they're still fresh for our holiday party bowls or all those pecan pies. But containing as much oil as they do (40 percent to more than 70 percent), they eventually will turn rancid.

Unshelled nuts, protected by nature's armor, will keep for a year or even two, especially if refrigerated. Squirrels are able to successfully store acorns over the winter because the cold slows the two chemical reactions responsible for rancidity: oxidation and hydrolysis, which are caused by air and moisture, respectively. So if your nuts were fresh to begin with -- that is, bought from a reputable, high- volume supplier -- their shells might keep them fresh until next Christmas if the nuts are stored in a cool place. (Cashews, by the way, are never sold in their shells, which contain corrosive and poisonous resins.)

Shelled nuts, especially those that have been chopped or roasted, are more prone to rancidity. Secured in an airtight container, they can be stored for four or five months in the refrigerator and for up to a year in the freezer.

How can you tell if a nut is rancid? In the store, sneak a sample if possible. Smell is not reliable; taste is the best test. If it tastes sour or rank, shop somewhere else. When buying from a catalogue, you just have to trust your supplier.

I'm a fan of almond milk and enjoy it occasionally as an alternative to rice and soy. I also enjoy almond butter and toasted, unsalted almonds as a snack. I just saw something on TV about the dangers of making your own, due to the potential for a cyanide byproduct from bitter almonds. I do not make my own, but this raised questions. How do the makers avoid this problem? How can you tell bitter almonds from regular almonds?

Fear not. Bitter almonds and regular, or "sweet," almonds come from different varieties of trees. You will never encounter bitter almonds being marketed for food use in the United States. Edible almonds, grown mostly in California, are exclusively the sweet, nontoxic variety. But sweet almonds grown elsewhere might contain an occasional bitter almond mixed in.

Interestingly, it's the toxic bitter almonds from which we must obtain the almonds' aromatic essential oil. Only they contain amygdalin, a sugar derivative that in the presence of water allows the nut's enzymes to turn it into glucose plus benzaldehyde plus highly poisonous hydrogen cyanide.

The amygdalin and cyanide can be removed from the nuts by crushing, soaking and washing in water, leaving pure benzaldehyde, also known as oil of bitter almonds. That's what is used in almond flavorings, even though it is toxic in large amounts. You'd have to drink several hundred bottles of almond extract to get a lethal dose.

LABELINGO: Peanuts are the cheapest of all nuts in the United States, a fact that has spurred the packers of Planters Mixed Nuts to note on the can that it "Contains less than 50 percent peanuts." But that's by weight. If you examine the contents, you will find that two-thirds of all the nuts are peanuts.

Robert L. Wolke (http://www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh. He can be reached atwolke@pitt.edu.


© 2007 The Washington Post Company

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