Yes, sesame paste (including tahini) does go bad, though it takes a while. An unopened jar stored in a cool, dark place typically lasts 18 to 24 months, and often remains fine for one to two years past the best-by date. Once opened, the clock speeds up as air and moisture start breaking down the oils inside.
How Long Sesame Paste Lasts
Unopened sesame paste kept in a pantry or cupboard between 50°F and 70°F will stay fresh for roughly 18 to 24 months from the manufacturing date. Squeeze-style bottles, which let in less air, can push that to 24 to 30 months. Flavored varieties with added spices tend to land on the shorter end of that range.
After opening, sesame paste stays good for several months at room temperature and longer in the refrigerator. Cold storage slows down the chemical reactions that make the oils go rancid. If you live in a hot or humid climate, refrigerating even an unopened jar is a smart move.
Why It Spoils: Fat Oxidation
Sesame paste is roughly 50% fat, and fat is what eventually turns it. When those oils are exposed to air, heat, or light, they undergo lipid oxidation, a chain reaction where the fatty acids break down into smaller compounds that taste and smell off. This same process is what makes old cooking oil or stale nuts unpleasant.
The good news is that sesame paste has a built-in defense. The finished product has very low water activity (a measure of available moisture), typically below 0.35. At that level, microbial growth is essentially stalled and enzymatic breakdown becomes negligible. This is why sesame paste doesn’t mold easily the way wetter foods do. The main threat isn’t bacteria or mold. It’s the slow degradation of fat over time, especially once you’ve broken the seal and introduced fresh air into the jar.
How to Tell It’s Gone Bad
The most reliable indicator is smell. Fresh sesame paste has a nutty, toasty aroma. Rancid sesame paste smells sharp and sour, almost like old paint or crayons. If you’re unsure, taste a tiny amount. Rancid paste has a bitter, astringent bite that’s unmistakable once you know what to look for.
Other signs to watch for:
- Darkened color. A noticeably darker shade than when you bought it suggests oxidation has progressed.
- Mold. Rare in properly stored sesame paste, but possible if water or food particles got into the jar (say, from a dirty spoon).
- Permanent separation. Some oil sitting on top is completely normal and not a sign of spoilage. If you stir it and the paste comes back together smoothly, it’s fine. If the oil and solids refuse to reblend, or the texture feels gritty and dry, the paste has deteriorated past its prime.
Oil Separation Is Normal
This is the most common source of confusion. Nearly every jar of natural sesame paste will develop a layer of oil on top over time. This is just the heavier solids settling while the lighter oil rises, the same thing that happens with natural peanut butter. It says nothing about safety or freshness. Give it a good stir and it should return to a creamy, uniform consistency. Storing the jar upside down between uses can help slow the separation.
Is Rancid Sesame Paste Harmful?
Eating a small amount of slightly rancid sesame paste is unlikely to make you acutely sick, but consuming oxidized fats regularly is a different story. When oils break down, they can produce trans fats and compounds that stress the liver and deplete the body’s antioxidant defenses. Animal research on oxidized sesame oil has shown changes in lipid profiles, liver function markers, and antioxidant enzyme levels. The taste alone will probably stop you from eating much of it, but the health case for tossing rancid paste is real.
Storing It Right
A few simple habits will keep your sesame paste fresh for as long as possible.
Before opening, store the jar in a cool, dark spot away from the stove or any heat source. A pantry shelf works perfectly. After opening, refrigeration is the single best thing you can do. Cold temperatures slow lipid oxidation significantly. The paste will thicken in the fridge, which actually makes it harder for oil to separate, but it loosens up quickly once you scoop some out and let it warm for a minute.
Use clean, dry utensils every time. Introducing moisture or food residue into the jar raises the water activity inside, which can create conditions for microbial growth in a product that otherwise resists it. Keep the lid tirtly sealed between uses to limit air exposure. Glass jars are ideal for long-term storage because they’re nonreactive and don’t absorb odors. Plastic containers work but may subtly affect flavor over time.
If you buy sesame paste in bulk or don’t use it often, consider portioning some into a smaller jar for everyday use and keeping the rest sealed in the fridge. Less frequent opening means less air exposure for the main supply.