Storing Nuts and Seeds: Rancidity, Mold and Aflatoxin

Nuts and seeds are rich in oil, and that oil is both why they are good for us and why they spoil. This page explains three separate problems in plain words: rancid oil (a quality problem), mold and aflatoxin (a toxin problem) and Salmonella (a germ problem). It is general food-safety information, not medical advice.

Read this first

• Rancid nuts taste bad, but rancid is mainly a quality problem. Mold on nuts is different: WHO advises discarding nuts that look moldy, discolored or shriveled. [5]
• Cold does not kill Salmonella. Refrigerating or freezing nuts slows rancidity but does not make a contaminated nut safe. [1][9]
• Check FDA's recalls page when you buy nuts, seeds or nut butters, and throw away recalled products. [10]
• If you have trouble breathing or swelling of the face or throat after eating nuts or seeds, call 911 (or your local emergency number). That is an allergy, not a storage problem; see nut and seed allergy basics.
• General food-safety basics (fridge at 40°F or below, the two-hour rule) are on our page on food storage and leftovers.

What "rancid" means and how you notice it

Nuts and seeds are high in unsaturated fat, and University of California extension writers say proper storage is needed to keep the oil from going rancid. [1][9] A study of tree nut handling explains that the shelf life of unsaturated fat gets shorter as temperature goes up, which is why nuts do best in a cool place. [9] Illinois Extension says rancid fat may show up as an "off" taste, and recommends checking best-by dates and not buying more than you will use in good time. [2]

Rancid nuts are unpleasant, and the sources we read treat this as a quality issue. We did not find a source that gives a medical risk for eating a small amount of rancid nuts, so we make no claim either way. A simple, cautious rule: if a nut tastes bitter, sharp or "off", spit it out and do not eat the rest of the batch.

How long nuts keep

University of California Agriculture and Natural Resources (2010, written for home gardeners but a general reference for tree nuts) gives these typical times. Storage life is shorter at room temperature. [1]

The same publication adds that shelled nuts absorb moisture and outside smells, so they belong in moisture-free, odor-tight containers such as plastic or glass. [1] The National Center for Home Food Preservation says refrigerated nuts (32 to 45°F) keep quality about a year, and frozen nuts (0°F) one to three years depending on the type. [3] Penn State Extension says the best way to store nuts is to freeze them with little exposure to light and oxygen. [4]

These are quality times for dry, well-handled nuts, not safety guarantees. For room-temperature storage, Illinois Extension says "several weeks and months" in airtight containers, and says to follow the package directions. [2] A survey of 279 California consumers found that most kept almonds, pecans and walnuts at room or refrigerator temperature for up to 6 months and froze them for longer. [9]

Not found in the sources we read: a storage time for ground flaxseed, seeds such as sunflower or pumpkin, or shelled walnuts compared with other nuts beyond the table above. We give no number we did not read. For flax, see flaxseed.

Mold and aflatoxin

Aflatoxins are poisons made by the molds Aspergillus flavus and A. parasiticus. WHO calls them among the most poisonous mycotoxins. [5] WHO lists pistachio, almond, walnut, coconut and Brazil nut among the tree nuts often affected, and says peanuts and hazelnuts are also regularly contaminated. [5] Large doses can cause acute liver damage. Aflatoxins damage DNA, cause cancer in animals, and there is evidence they can cause liver cancer in people. [5] That is a long-term risk from repeated exposure, which is why regulators set limits instead of waiting for symptoms.

Mold is more likely when nuts are not dried well before storage. UC extension says undried or poorly dried nuts are more likely to develop molds that produce aflatoxin, and that walnuts that are not dried enough mold and go rancid quickly. [1]

What regulators do. FDA's action levels for human food list 20 parts per billion (ppb) of aflatoxin for Brazil nuts, peanuts and peanut products, pistachio nuts and "foods" in general (0.5 ppb of aflatoxin M1 for milk). [6] An action level is the point at which FDA may take action against a food; it is not a promise that anything below it is risk-free. The Codex Alimentarius Commission, which works with WHO and FAO, sets its own maximum levels for aflatoxins between 0.5 and 15 micrograms per kilogram, depending on the food. [5] We did not read the European Union limits, so we do not quote them.

What to do at home. WHO advises inspecting nuts and throwing away any that look moldy, discolored or shriveled; buy nuts as fresh as you can, store them dry and not too warm, and eat a varied diet. [5] The sources we read do not describe cutting mold off a nut as a safe step, and the advice is to discard. Because toxins cannot always be seen, do not taste-test a suspicious nut.

Salmonella: a different problem

Salmonella is a germ, not a mold toxin. It does not grow in dry foods, but it can survive in them for a long time. [9] University of California writers note that dry foods like nuts are not a common source, but outbreaks have been linked to raw almonds, peanut butter and sesame seeds. [1] Refrigeration and freezing do not destroy it, and you cannot tell by sight, smell or taste. [1] In a lab study of raw almonds, the Salmonella present did not decline over 1.5 years in the refrigerator or freezer. [9]

Examples you can read: in 2015, CDC and FDA tracked 13 people sick in 10 states with Salmonella linked to JEM Raw sprouted nut butter spreads, which were recalled. CDC noted the products have a long shelf life and could still be in homes, and said any remaining spread should be thrown away. [8] Illinois Extension adds that "activating" or soaking nuts is not supported by research as a way to improve digestion or absorption, and that in some studies long or room-temperature soaking increased Salmonella growth. [2] More on soaking is in our guide on raw, roasted, soaked and sprouted nuts and seeds.

Almond pasteurization

In the U.S., almonds grown in California come under a USDA marketing order. Its text says handlers must treat almonds before shipment with a process shown to give at least a 4-log (99.99 percent) reduction of Salmonella, validated by an approved expert. [7] There are exemptions: for example, untreated almonds may go directly to approved manufacturers who will process them further. [7] (The text we read shows a cross-reference to an amendment published in September 2026, so details may change.) UC extension says these treatments, which include roasting, blanching, steam or other approved methods, are meant to keep a raw almond's flavor and texture, and that almonds not sold through regular market channels may not be treated. [1] This is why many "raw" almonds in stores are in fact pasteurized. Other nuts may or may not be treated. [1] We did not read a rule requiring other nuts to be treated.

Bulk bins, roasted nuts and nut butters

• Bulk bins: Illinois Extension notes that raw nuts and seeds may be sold in bulk bins or in sealed containers. [2] We did not find a source on bulk-bin risk compared with sealed packages. Sensible steps from the sources: buy from a store with fast turnover, check for off smells or visible mold, and take them home to a cool place.
• Roasted nuts: Dry- or oil-roasted nuts are available salted or unsalted. [2] We did not find a source comparing shelf life of roasted and raw nuts for people at home.
• Nut butters: Natural nut butters separate over time and need stirring. Illinois Extension says some natural peanut butters recommend refrigeration after opening, so follow the label. [2] The Salmonella that caused the 2015 outbreak was in a sprouted nut butter, which shows butters can be a source. [8] If you make nut butter at home, use pasteurized nuts. [2]

Heavy metals in seeds

Some seeds can take up metals such as cadmium or arsenic from soil. We did not read a source that measured this for nuts and seeds for this page, so we make no claim here. See heavy metals and contaminants in foods for what we have read.

Where to check recalls

FDA keeps a Recalls, Market Withdrawals and Safety Alerts page, listing recalls from press releases and public notices (not every recall has a press release). It also shows allergen recalls such as undeclared sesame. [10] Recalls older than three years move to an archive. [10] You can also sign up for email alerts there.

Related pages

• Food storage, leftovers and food safety basics
• Nut and seed allergy basics
• Raw, roasted, soaked and sprouted nuts and seeds
• Nut and seed butters
• Pasteurized vs unpasteurized foods
• Heavy metals and contaminants in foods