Why does pesto go bad so quick?

Pesto is a delicious basil-based sauce that can transform simple pastas, sandwiches, and more into flavorful dishes. However, pesto also has a reputation for going bad very quickly after being made or opened. Within just a few days in the fridge, pesto can start to darken and develop off flavors. So why does pesto go bad so fast, and is there any way to extend its shelf life?

The ingredients in pesto cause it to spoil quickly

There are a few key ingredients in pesto that contribute to its short shelf life:

  • Basil – Basil leaves contain enzymes that accelerate spoilage. Once the leaves are chopped and exposed to air, the enzymes trigger chemical reactions that degrade the green color and flavor.
  • Garlic – Garlic also contains enzymes that cause it to spoil rapidly. In pesto, the chopped garlic interacts with the basil and other ingredients, leading to faster spoilage.
  • Olive oil – While olive oil is packed with healthy fats, these unsaturated fats can oxidize and go rancid much quicker than saturated fats. The olive oil in pesto provides a breeding ground for bacteria and mold.
  • Pine nuts – Nuts are notorious for harboring molds and bacteria that can contribute to quick spoilage.
  • Cheese – Hard cheeses like parmesan are often added to pesto. While cheese preserves well on its own, when combined with the other ingredients in pesto, it accelerates the rate of spoilage.

The combination of the above raw ingredients, plus the lack of preservatives, makes fresh pesto extremely perishable. Even when kept refrigerated, the pesto only lasts about 3-5 days before going bad.

Signs that pesto has gone bad

Here are some telltale signs that your precious jar of pesto has spoiled and should be discarded:

  • Change in color – Fresh pesto is a vibrant green color. As it starts to spoil, it oxidizes and turns brown or dark grey.
  • Off smells – Rancid, unpleasant odors are a clear sign pesto has gone bad. You may detect sour, rotting, or moldy smells.
  • Mold growth – Mold may appear as fuzzy dots or tendrils on the surface of the pesto.
  • Texture change – Instead of being loose and spoonable, spoiled pesto often gets very thick and grainy.
  • Taste – Bad pesto will simply taste “off” – instead of bright herbal flavors, it tastes dull, bitter, or acidic.

Trust your senses – if pesto smells, looks, or tastes funky, it’s safest to throw it out. Consuming spoiled pesto can cause food poisoning.

How to store pesto properly

To help pesto last a little bit longer, here are some storage tips:

  • Refrigerate in an airtight container. Oxygen speeds up spoilage, so limiting air exposure extends shelf life a few extra days.
  • Pour a thin layer of olive oil over the top of the pesto before sealing. This protective layer minimizes air exposure.
  • Freeze pesto for long term storage. Frozen pesto maintains its flavor for up to 6 months.
  • Store pesto cubes in ice cube trays or muffin tins, then transfer to freezer bags. Thaw only what you need.
  • Add a bit of lemon juice or vitamin E to pesto before freezing. The acidity and antioxidants help prevent oxidation and spoilage.
  • Blanch basil leaves for 1 minute before making pesto. Blanching deactivates the enzymes that accelerate pesto spoilage.

Be aware that even with proper storage methods, homemade pesto only lasts about 1-2 weeks in the fridge and 6 months in the freezer. After that point, quality and safety start to decline.

Ways to use up pesto before it spoils

If you have a tub of pesto that’s nearing its expiration date, here are some delicious ways to use it up:

  • Toss with hot pasta for a quick pesto pasta dish
  • Stir into minestrone or tomato soup
  • Swirl into mashed potatoes or polenta
  • Use as a sandwich spread or burger topping
  • Mix into tuna or chicken salad
  • Dollop over fish, chicken, lamb, or beef
  • Blend into hummus or salad dressings
  • Top pizza, flatbreads, bruschetta
  • Fill an omelet or stuff mushrooms
  • Stir into risotto or vegetable bakes

Freezing pesto in ice cube trays makes it easy to pull out just a tablespoon or two to add big flavor to all kinds of dishes.

Reasons homemade pesto goes bad so quickly

There are a few reasons why homemade pesto tends to go bad faster than commercial pesto products:

  1. No preservatives – Commercial pestos contain preservatives like potassium sorbate to prevent mold growth and extend shelf life. Home cooks don’t add preservatives.
  2. Processing methods – Commercial pesto undergoes high pressure processing which deactivates spoilage enzymes. Homemade pesto doesn’t go through the same processing.
  3. Pasteurization – Store-bought pesto is pasteurized to kill microbes. Home cooks typically don’t pasteurize homemade pestos.
  4. Acidity – Commercial pestos are often more acidic (around pH 4.2), which better prevents bacterial growth. Home cooks don’t adjust pH.
  5. Storage – Open containers and improper storage (warm temps, too much air exposure) also affect homemade pesto’s shelf life.

Without the processing methods and preservatives used by manufacturers, it’s challenging for home chefs to make pesto last more than a week or two.

Tips for making pesto last longer

If you want your homemade pesto to keep for more than just a few days, here are some tips:

  • Add lemon juice or vinegar – The extra acidity helps inhibit bacterial growth. Start with 1 tsp per cup of pesto.
  • Use all olive oil instead of half olive/half vegetable oil – Olive oil’s antioxidant content preserves better.
  • Blanch basil first – Scalding basil for 1 minute stops enzymatic activity that causes spoilage.
  • Replace pine nuts with walnuts or almonds – Pine nuts turn rancid fastest. Other nuts have longer shelf lives.
  • Omit or reduce garlic – Garlic’s enzymes speed up pesto spoilage.
  • Immediately refrigerate or freeze – Don’t leave pesto out at room temp.
  • Store in airtight containers – Minimize air exposure as much as possible.

Keep in mind that even with these tweaks, homemade pesto stored in the fridge won’t last much more than 2 weeks. For long term storage up to 6 months, freezing is best.

Can you eat spoiled pesto?

It’s not recommended to eat pesto after it has spoiled. Consuming spoiled pesto can potentially lead to food poisoning with symptoms like:

  • Nausea and vomiting
  • Abdominal cramping
  • Diarrhea
  • Fever and chills
  • Headache

The bacteria, molds, and yeasts that grow on pesto produce toxins that can cause illness. Some common culprits include salmonella, E. coli, listeria, and staphylococcus aureus.

It’s better to be safe than sorry and discard any pesto that is past its prime. The taste and texture will be unappealing anyway. Accidentally eating a small amount likely won’t cause illness, but it’s wise not to tempt fate by knowingly eating spoiled pesto. Trust your nose – if it smells funky, throw it out.

Should you refrigerate store-bought pesto?

Unlike homemade pesto, commercially prepared pesto is shelf-stable and does not require refrigeration until after opening. Unopened pesto can be stored in the pantry for up to 2 years. Once a jar is opened, refrigeration extends the shelf life up to 4-6 months.

Refrigeration inhibits mold growth and slows down enzymatic activity. Keep the pesto toward the back of the fridge where the temperature is coldest. And ensure the lid has a tight seal to prevent air exposure.

If you won’t use up an opened jar within a couple months, consider freezing half of it in ice cube trays for longer storage. Thaw cubes as needed.

Also keep in mind that refrigerating pesto affects texture and taste. The olive oil in pesto can solidify and separate when cold. Take out portions to let come to room temp before eating for best flavor and texture.

Can you freeze pesto without blanching the basil?

Freezing is an excellent way to preserve fresh pesto for several months. However, without blanching the basil first, frozen pesto can suffer more deterioration in color, flavor, and texture over time.

Blanching helps deactivate enzymes that break down pesto’s color and flavor during freezing. Dropping basil leaves in boiling water for just 1 minute prevents enzymatic activity without cooking the leaves.

After blanching and shocking in ice water, the basil is ready to use in pesto recipes as normal before freezing.

Without blanching, you’ll still get decent results freezing pesto. But expect somewhat dulled color and flavor by the 6 month mark. The pesto may also darken and become a bit soggy after thawing.

For best quality frozen pesto with vivid green color and pronounced basil flavor even after months in the freezer, take the extra minute to blanch the basil first. But in a pinch, pesto can be frozen without blanching and remain safe to eat.

Which nuts work best in pesto for flavor and shelf life?

Nut Flavor Shelf Life
Pine nuts Delicate, buttery Shortest – go rancid quickest
Walnut Heartier, earthier Medium – can last a few weeks in pesto
Almonds Nutty and mild Longest – naturally resist rancidity

Pine nuts offer the most iconic pesto flavor, but unfortunately also spoil the fastest. Walnuts and almonds have longer shelf lives, so they are better options for pesto you want to keep for more than just a few days.

Almonds are the best choice for prolonging homemade pesto’s shelf life. They bring a mild nutty flavor that works nicely with the basil and cheese.

Walnuts also pair well with pesto ingredients. They contribute an earthier, bolder nut flavor compared to almonds. Walnut pesto keeps a bit longer than pine nut pesto too.

Should you make pesto with all olive oil or half olive and half vegetable oil?

Traditionally, pesto recipes call for a 50/50 mix of olive oil and neutral vegetable oil like canola or grapeseed. However, using 100% extra virgin olive oil results in a fresher, brighter tasting pesto that keeps slightly longer.

Here’s a comparison of the two oil options:

  • Olive/Vegetable Oil Blend
    • Milder, more subtle flavor
    • Lower cost
    • Prevents bitterness from large amount of olive oil
    • Shorter shelf life – around 5 days
  • 100% Olive Oil
    • Bolder, fruity olive flavor
    • Higher cost
    • More antioxidants help preserve freshness
    • Slightly longer shelf life – about 1 week

For the freshest, most vibrant taste, go with all extra virgin olive oil. The naturally occurring antioxidants help slow oxidation and rancidity. Refrigeration also helps mitigate any bitterness.

But the vegetable oil blend works well too, especially if you want to stretch your olive oil budget. Just be sure to refrigerate and use within 5 days.

Should you make pesto in a food processor vs. mortar and pestle?

You can make pesto either in a food processor or with a mortar and pestle. Here’s how the two methods compare:

  • Food processor
    • Much faster and easier
    • Yields smooth, emulsified texture
    • Brighter green color retained
    • Less oxidation since made quickly
    • Let’s you make larger batches
  • Mortar and pestle
    • More traditional method
    • Slower and more labor intensive
    • Often yields grittier, less emulsified texture
    • Tends to darken with extended pounding
    • Limited to small batches

For easy pesto prep that lends itself well to freezing, the food processor is ideal. It quickly pulverizes ingredients into a smooth, creamy paste while retaining the signature green color.

However, traditionalists argue hand pounding with a mortar and pestle results in superior flavor and aroma. The long pounding extracts essential oils and melds the flavors. But be prepared for some elbow grease.

For most home cooks, the food processor streamlines pesto making with outstanding results. Yet the mortar and pestle ritual offers unmatched heritage and pride of creation. Choose whichever method best fits your needs and values.


Pesto is an incredibly flavorful sauce, but its lack of preservatives coupled with ingredients prone to spoilage means it has a very short shelf life. With proper refrigerated storage, pesto lasts 3-5 days after being made. For longer term storage, freeze pesto in ice cube trays. frozen pesto maintains quality for up to 6 months. Look for signs of spoilage like color changes, odor, mold, and texture changes. Consuming spoiled pesto can cause food poisoning, so it’s best to err on the side of caution and discard any expired pesto. To prolong homemade pesto’s shelf life, blanch the basil first and opt for ingredients less prone to spoilage like lemon juice, almonds, and all olive oil. But ultimately, pesto’s perishability is the tradeoff for its incredible flavor and texture when fresh. Just make sure to enjoy it quickly and freeze any leftovers.

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