Shelled walnuts, almonds, pistachios and hazelnuts in separate small piles on a wooden board, one walnut half split to show the pale interior, a few whole in-shell nuts at the edge

Do Nuts Go Bad? Why They Turn Rancid and Where to Keep Them

Published 9 min read

Filed under Nuts

Yes, nuts go bad, but slowly: they do not spoil the way milk does; their oil turns rancid, faster in warmth and light. A bag of walnuts opened after a summer on the shelf smells faintly of paint, and the almonds that have lived in a jar by the stove since spring have lost their crunch and taste of very little. Neither has spoiled the way milk spoils. The oil inside the nut, about half the weight of an almond, has reacted with the air, and the products of that reaction are what the nose picks up. How fast that happens depends on which nut it is and whether it has been roasted, and those two facts decide where the bag belongs. For the salted, spiced bowl at the bar, the Bar Nuts complete guide goes further.

Do nuts go bad?

Yes, mostly by going stale and rancid rather than by spoiling. Nuts are a low-moisture food, around 3 to 6 percent water in the case of almonds. The Almond Board of California's technical summary on shelf life notes that most bacteria and molds will not grow below a water activity of 0.8, and almonds kept cool and dry typically sit around 0.3 to 0.6. What a nut has instead is oil, more of it in a pecan or macadamia than in an almond, and that oil is mostly unsaturated fat. Unsaturated fat reacts spontaneously with oxygen, first into peroxides and then, as oxidation goes on, into volatile aldehydes and ketones. Hexanal is a major one of those secondary products, and together they carry the cardboard, painty and rancid flavors of a stale nut. Heat, light, extra moisture and metals such as iron can all speed the reaction. Almonds are also rich in vitamin E, a natural antioxidant that the Almond Board says helps protect the fat from reacting with oxygen when the nuts are stored cool and dry. Cold, dark and airtight storage works against heat, light and oxygen, and a sealed container keeps out extra moisture.

How to tell when a nut has turned

Smell first. A fresh walnut smells mildly nutty and tastes sweet. The California Walnut Commission's own test is blunt: if the walnuts smell like paint thinner, they are rancid. The Almond Board's sensory vocabulary for oxidized almonds runs to the same words, cardboard, painty and rancid. A nut that smells of paint is past use in any dish. Then bite: a nut that has picked up moisture can lose some of its crunch.

Which nuts turn fastest

A 2018 peer-reviewed review of tree nut quality and shelf life states that nuts high in monounsaturated fat are more stable and less susceptible to oxidative rancidity than nuts high in polyunsaturated fat. The table ranks nine common nuts by polyunsaturated fat per one-ounce serving, using the USDA figures compiled in the Almond Board's nutrient comparison chart, which gives a rough guide to how carefully each one needs to be stored.

Nut Polyunsaturated fat per oz Monounsaturated fat per oz Calories per oz Storage risk
Walnut 13 g 2.5 g 190 Highest; the most polyunsaturated fat of the nine
Pine nut 10 g 5.5 g 190 High; second in polyunsaturated fat
Brazil nut 7 g 7 g 190 High; polyunsaturated and monounsaturated fat in equal measure
Pecan 6 g 12 g 200 Medium-high; twice as much monounsaturated as polyunsaturated fat
Pistachio 4 g 7 g 160 Medium; the University of California gives pistachios the longest freezer life of the nuts it lists
Almond 3.5 g 9 g 160 Medium-low; rich in vitamin E, which helps protect the fat
Cashew 2 g 8 g 160 Low
Hazelnut 2 g 13 g 180 Low
Macadamia 0.5 g 17 g 200 Lowest; the least polyunsaturated fat of the nine

The chart's cashew, macadamia and pistachio figures are for dry-roasted kernels; the rest are raw. The practical reading: walnuts and pine nuts make the strongest case for cold storage from the day the bag opens, and the further down the table a nut sits, the more forgiving it is of a lapse.

Where to keep them, and for how long

The Almond Board's recommendation for every form of almond is below 50°F and below 65 percent relative humidity, with moisture-barrier packaging, and it notes that almonds held that way can have more than two years of shelf life. A kitchen at room temperature runs warmer than 50°F, which makes the refrigerator the closest home equivalent. The University of California's food preservation guidance gives walnuts, almonds, pecans and pistachios up to a year in the refrigerator, and in the freezer a year or more for almonds, two years or more for walnuts and pecans, and three years for pistachios. That guidance sets the refrigerator at 40°F or lower. Whichever cold box is used, the container matters: an airtight, odor-tight glass or plastic jar, because walnuts can absorb the flavors of other foods, and the Walnut Commission specifically warns against storing them near fish, cabbage or onions.

Nut Refrigerator, 40°F or lower Freezer Rank by polyunsaturated fat (of nine)
Walnut Up to a year Two years or more 1
Pecan Up to a year Two years or more 4
Pistachio Up to a year Three years 5
Almond Up to a year A year or more; more than two years at the Almond Board's cool, dry conditions 6

The storage figures are the University of California's and the Almond Board's, and the rank column comes from the fat table above. The University of California's figures treat all four nuts alike in the refrigerator, so the fat ranking is the better guide to which belongs in cold storage first.

Do roasted and seasoned nuts keep differently?

A roasted nut keeps less well than a raw one, and the almond shows why. Inside a raw almond the oil is held in many tiny globules called oleosomes, walled off from each other by a membrane network, and that structure plus the nut's own antioxidants protect the oil from oxygen. Roasting bursts those globules. The Almond Board's summary is explicit that roasted almonds have a shorter shelf life than raw or blanched ones, that oxidation can begin immediately after roasting, and that roasted nuts should be cooled fast and sealed away from air. Chopping and slicing do the same thing by a different route: more surface exposed to air. Roasted almonds typically sit below 3 percent moisture, and the Board notes that at very low moisture lipid oxidation also increases. A seasoned nut that began as a roasted one carries all of this with it. The working rule for seasoned nuts: buy in small amounts, keep the jar sealed, and lean on the refrigerator in summer. The Spiced Nuts Three Ways recipe makes the case for roasting small batches at home.

Are raw almonds actually raw?

Not in the strict sense, if they were grown in California. After Salmonella outbreaks traced to raw almonds in 2001 and 2004, a rule under the federal marketing order for California almonds required a treatment step that reduces Salmonella by at least 4 log, and compliance became mandatory on September 1, 2007. The treatment is a separate step from roasting, so the roasting damage described above should not apply in the same way to a treated almond that has not been roasted.

Are nuts good for you, and how much is a serving?

The 1.5-ounce figure comes from a regulatory decision. On July 14, 2003, the FDA issued a letter of enforcement discretion allowing a qualified health claim on nut labels: that scientific evidence suggests but does not prove that eating 1.5 ounces per day of most nuts, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. Walnuts received their own version of the claim on March 9, 2004, and macadamias on July 24, 2017. The phrase "suggests but does not prove" is the point of the word qualified: the evidence is supportive rather than conclusive. The composition is not in dispute. By USDA figures, a one-ounce handful runs 160 to 200 calories, 13 to 22 grams of fat that is mostly unsaturated, 2 to 6 grams of protein and 1 to 4 grams of fiber, with almonds among the leanest and the most fiber-rich, and macadamias and pecans the richest. An ounce is roughly 23 almonds. That mostly unsaturated fat is the same fat that turns rancid in a jar left by the stove.

Are peanuts nuts?

Botanically no. The Peanut Institute calls them legumes. The University of Florida's extension service groups them with peas and beans as legume pods: the plant flowers above ground, then a structure called a peg, formed from the fertilized ovary, grows down into the soil, where the pod forms and matures. The distinction has one consequence that matters on a label. Under United States allergen law, tree nuts and peanuts are two separate major allergens, and a label must declare which tree nut is present by name, almond or pecan or walnut, rather than saying "nuts". A seasoned-nut mix that contains both has to declare peanuts and each tree nut it contains. The nut butter storage and shelf life guide covers what happens once any of these nuts is ground into a jar. Big Spoon Roasters makes butters from both sides of the line, peanut and tree nut, among them a vanilla caramel almond cashew butter (shop Big Spoon Roasters Vanilla Caramel Almond Cashew Butter).

Mold, aflatoxin and when to throw them out

One safety question regulators track closely for nuts is aflatoxin, a class of toxins produced by Aspergillus molds. The FDA's action level for total aflatoxins in human food is 20 parts per billion. Under a USDA and FDA agreement, every lot of raw peanuts, domestic and imported, is sampled and tested for aflatoxin, pistachio shipments must carry an aflatoxin certificate, and imported Brazil nuts can be inspected under a voluntary program. In the kitchen, moisture is the trigger to watch: the Almond Board notes that when almonds pick up moisture, mold may start to grow. A nut that shows mold belongs in the bin, and the same dry, cold, sealed storage that slows rancidity keeps the moisture out.

Short answers to the rest

What about the green Sicilian pistachios? Pistacchio Verde di Bronte, grown on volcanic soils on the slopes of Etna and registered as a DOP in 2010, is harvested only every other year, and the production rules allow the crop to be stored up to 24 months after harvest. Brontedolci, founded in Bronte in 1997 as a small nougat workshop, makes Brontoncini, small soft nougats in assorted flavors (shop Brontedolci Brontoncini), and the Brontedolci producer guide tells the workshop's story. Which salt for seasoned nuts? The flaky finishing salt guide covers the choice. Where do nuts fit in the rest of the pantry? The shelf-by-shelf pantry storage guide places nuts beside everything else in the kitchen.

The shop is at 263 Josephine Street in Cherry Creek, Denver, where visitors can ask staff questions, and products ship across the continental U.S. from theambrosianpantry.com.

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