A marble mortar of bright green basil pesto beside a small glass jar of darker pesto with a thin layer of olive oil on top, loose basil leaves and pine nuts on a wooden board

Does Pesto Go Bad? How Long a Jar Keeps, and Why It Turns Brown

Published 16 Sep 202610 min read

A jar of pesto opened on Monday is a different color by Tuesday. The bright green has gone to olive drab, the top has darkened further than the middle, and the question that follows is whether it has turned. Almost always it has not. The color change is an enzyme in the basil meeting oxygen, and it starts in fresh pesto within hours of pounding. Spoilage is a separate event with separate signs. This page tells the two apart, gives storage times for fresh, refrigerated, jarred and frozen pesto, explains why the garlic and oil in the jar make home canning unsafe, weighs the health case, and lists what the Genoa consortium says belongs in a real pesto.

Does pesto go bad?

Yes. Fresh or opened pesto keeps about three days in the refrigerator, per the National Center for Home Food Preservation, and Oregon State Extension allows up to four. A sealed shelf-stable jar keeps in the cupboard to its date, but once opened it should be treated as fresh. Browning on top is an enzyme in the basil meeting oxygen, not spoilage.

Why pesto turns brown

Basil, like apples, avocados and potatoes, carries an enzyme family called polyphenol oxidases. A 2017 review in the International Journal of Molecular Sciences puts it plainly: browning in plants is "usually caused by polyphenol oxidases (PPOs), following cell damage caused by senescence, wounding and the attack of pests and pathogens." Pounding or blending basil is wounding on an industrial scale. Every ruptured cell releases the enzyme, the enzyme meets the leaf's own phenolic compounds and the oxygen in the air, and the products of that reaction are brown pigments. Nothing has fermented and nothing has grown.

The reaction needs oxygen, which is why the surface goes darkest first, and why the oldest trick works: a film of olive oil poured over the top of the jar seals the sauce from the air. Heat also stops it. The enzyme is a protein and heat inactivates it, which is why some cooks dunk the leaves in boiling water and then ice water before pounding, trading a little raw brightness for a pesto that holds its green longer. Industry solves the same problem with chemistry and cold. A 2022 study from the University of Genoa on the basil pastes exported to pesto manufacturers found that ascorbic acid together with blast chilling "proved to be the best solution to better preserve the color of these products during their shelf-life," and ascorbic or citric acid on a jar's ingredient list is there partly for this reason.

	Browning	Spoilage
Looks like	Green fading to olive, khaki or brown, darkest at the surface, evenly	Fuzzy spots of white, gray, blue or pink mold; a slimy or fizzing surface
Smells like	Basil, slightly flatter	Sour, yeasty, fermented, or like wet cardboard from rancid oil
Cause	Polyphenol oxidase in the basil meeting oxygen	Yeast, mold or bacteria growing in the sauce
Safe	Yes	No; discard the jar, do not scrape the top and keep the rest
Prevention	Oil film on the surface, small jars, blanched basil, cold storage	Refrigeration, clean spoon, use within the times below

How long is pesto good in the fridge?

It depends on which pesto. Fresh pesto is an uncooked sauce of leaves, garlic, cheese, nuts and oil with nothing preserving it but cold. The National Center for Home Food Preservation, the USDA-funded program at the University of Georgia that publishes the national home preservation guidance, says it should be "made fresh and stored in the refrigerator no more than 3 days." An Oregon State University extension agent, answering the same question, allows up to four. A shelf-stable jar is a different object: it has been processed and sealed under validated commercial conditions, which is why it keeps in the cupboard to its date. Once opened, that protection is gone, and the jar should be treated as fresh pesto.

Pesto	Where	How long	Notes
Homemade, fresh	Refrigerator	Three days (NCHFP); up to four (Oregon State Extension)	Oil film on top slows browning; does not extend safety
Chilled tub from the refrigerated case	Refrigerator	Unopened: to the date on the tub. Opened: treat as fresh	These are minimally processed and the date is short for a reason
Shelf-stable jar, sealed	Cool, dark cupboard	To the date on the jar	Heat treated and sealed; the lid should be concave and click when pressed
Shelf-stable jar, opened	Refrigerator, oil film on top	Per the label	The label is the authority; when in doubt, treat as fresh
Frozen, any kind	Freezer, half-inch headspace	Months; NCHFP calls it "long term storage"	Thaw in the refrigerator and use within four days (Oregon State Extension)

How to tell if pesto has gone bad

Throw it out if you see fuzzy mold, if it fizzes or smells sour, yeasty or fermented, or if it has been open in the refrigerator longer than three or four days; do not scrape off the top and keep the rest. Browning alone is oxidation, and brown pesto that smells of basil is fine.

Never leave an open jar of pesto on the counter overnight. The reason is in the next two sections.

Can you freeze pesto?

Yes, and it is the only long-term method the preservation authorities endorse. The NCHFP instruction is short: "package in glass freezer jars or plastic freezer boxes, leaving 1/2-inch headspace," then label and freeze. The Oregon State agent recommends ice cube trays: freeze the pesto in cubes, pop them into a freezer bag, and take out one or two per portion. The cheese question divides cooks. Some leave the cheese out and stir it in after thawing; the NCHFP method freezes the finished sauce as it is. What matters more is speed, both in and out. A shallow tray freezes faster than a deep jar, and the thawed pesto should go straight from freezer to refrigerator, never to the counter, and be used within four days.

Can you can pesto at home?

No. This is the one hard rule on the page. The NCHFP's canning FAQ states it flatly: pesto "may be frozen for long term storage; there are no home canning recommendations." The reason is the pairing of garlic and oil. Penn State Extension explains that "herbs, garlic, and other vegetables used to flavor oils are low acid foods and can be a source of Clostridium botulinum spores," and that "when these spores are surrounded by oil an oxygen free environment is created that encourages the growth of the spores at room temperature." The CDC's rule is blunt: you cannot see, smell or taste the toxin. The danger is not theoretical. In the summer of 1985 a restaurant in Vancouver served commercially chopped garlic in soybean oil that was suspected of having sat unrefrigerated for months; the CDC counted 34 cases of botulism across two clusters, with patients in Canada, the United States and the Netherlands, and the FDA's finding was that the product was "safe if it is kept refrigerated as the label directs." Commercial jarred pesto is shelf-stable because a food plant processes it under validated conditions. There is no validated home process for pesto, in a water bath or a pressure canner, which is why the answer from every extension office is the same. Freeze it.

Is pesto healthy?

Pesto is an uncooked sauce built on olive oil, hard cheese and nuts, so it is calorie dense and salty, and a spoonful goes a long way. The case for it rests on the oil. In November 2018 the FDA allowed a qualified health claim that "daily consumption of about 1½ tablespoons (20 grams) of oils containing high levels of oleic acid, when replaced for fats and oils higher in saturated fat, may reduce the risk of coronary heart disease." The claim is for oils with at least 70 percent oleic acid, and olive oil is on the FDA's list. The FDA's caveat applies to pesto as well: the oil "should not increase the total number of calories you eat in a day." The sodium comes from two aged cheeses and the salt in the mortar. Two caveats. First, a jar made on sunflower or other generic seed oil is not the same argument; the FDA claim covers high-oleic oils, and standard sunflower oil is not one. Second, browning does not change any of this; a brown pesto has the same fat, salt and cheese as a green one.

What is in a real pesto Genovese?

Seven things. The Consorzio del Pesto Genovese, the producers' body in Genoa, publishes the official recipe: Basilico Genovese DOP, extra virgin olive oil, Parmigiano Reggiano DOP, Pecorino Sardo DOP, garlic, pine nuts and coarse salt, pounded in a marble mortar with a wooden pestle, rotating rather than striking "so as to tear, not shear, the fragrant leaves." The basil is the protected ingredient. The European Union registered Basilico Genovese as a protected designation of origin under Regulation (EC) 1623/2005, and the production rules confine it to "the seaward side of Liguria (south of the watershed)," grown in natural soil, and the defining requirement of the specification is a leaf with no mint note at all. The consortium's recipe carries its own warning about industrial jars: read the label carefully, because products sold as Pesto Genovese can carry ricotta alongside or instead of the aged cheeses, cashews instead of pine nuts, and cheap seed oil instead of olive oil. The upcoming piece on how to buy pesto walks the label line by line, and our olive oil buying guide explains why the oil matters as much as the basil.

Should pesto be heated?

No. Pesto is a raw sauce, and cooking it does two bad things at once: it drives off the basil's volatile aroma and it splits the oil, cheese and leaf into a greasy pool. The consortium's technique is the one in the earliest securely dated printed recipe, Giovanni Battista Ratto's La Cuciniera Genovese of 1863, which pounded garlic, basil, Dutch and Parmesan cheese in a mortar (no pine nuts yet), dissolved the paste "with fine oil in abundance," and thinned it with "a little hot water without salt" to dress lasagne, tagliolini and gnocchi. The modern instruction is the same. Drain the pasta, keep a cup of its water, and stir the pesto into the pasta off the heat, loosening it with hot pasta water until it coats. The consortium names the shapes it dresses best, trofie, trofiette, trenette and the silk handkerchiefs called mandilli de saea, with boiled potato and green beans as the optional Genoese additions. The pasta shapes piece explains why a twisted or ridged shape holds a thick sauce better than a smooth one.

On quantity, the consortium's recipe gives an answer. Its batch, 50 grams of basil, half a glass of oil, six tablespoons of Parmigiano, two of Pecorino, two cloves of garlic and a tablespoon of pine nuts, dresses 600 grams of dried pasta. By our arithmetic that batch weighs roughly 200 grams, so the working ratio is about one part pesto to three parts dried pasta by weight, or around 30 grams of pesto per 100 gram portion. A 180 gram jar is one dinner for six, or two dinners for three, which is about what its shelf life after opening allows.

Do the same rules apply to jarred tomato sauce?

The principle does, the details differ. A sealed jar of passata or marinara is protected by heat treatment, acidity and a vacuum seal, and keeps in the cupboard to its date. The moment the seal breaks it is a fresh food and belongs in the refrigerator. Tomato sauce has acidity on its side and no raw garlic sitting in oil, so it is a lower-risk jar than pesto, and it freezes well in the same way. Our tinned tomatoes and passata page handles that shelf.

Short answers to the rest

Can you eat brown pesto? Yes, if it smells of basil and shows no mold or fizz. Why is the top of the jar darker than the middle? Oxygen reaches the top first; stir it, then pour oil over it. Does the oil layer stop it spoiling? No, it stops it browning; only cold slows spoilage. Does pesto need refrigeration before opening? A shelf-stable jar does not; a chilled tub does, always. Is separation a bad sign? No, oil rising is normal in a sauce with no emulsifier; stir it back. Does a pine nut pesto taste different from a cashew one? Yes, pine nuts are resinous and buttery, cashews sweeter and blander, which is why the consortium objects. Is the metallic taste from the jar? More often from pine nuts past their best; nut oils go rancid before the basil fades. Can you make pesto in a food processor? Yes, though the consortium notes that mortar and machine "give different results on the palate." The pantry storage guide covers the cupboard the sealed jars live in.

The Ligurian pestos, including Ranise's basil pesto (shop Ranise's basil pesto) from the Riviera and Ritrovo's Ligurian basil pesto (shop Ritrovo's Ligurian basil pesto), sit beside Rustichella's trofie (shop Rustichella's trofie), the shape Genoa built for them. They are at 263 Josephine Street in Cherry Creek, Denver, and ship across the continental U.S. from theambrosianpantry.com. Once the lid comes off, the refrigerator and a spoon of oil do the rest.