Nuts soften and lose their crispness because they absorb moisture from the air. The most practical fix is to toast them for a few minutes in a low-temperature oven and let them cool; the heat evaporates much of the absorbed moisture and restores their crispness.
Why do nuts become moist?
Nuts such as hazelnuts, almonds, and walnuts have a porous structure. When humidity is high, these pores draw water vapor from the air; nuts left in a humid kitchen or in a container that does not close tightly may become noticeably soft within a few days. Temperature changes also speed up this process because condensation can form on a cold container taken out of the refrigerator as it comes to room temperature. Salted or spice-coated nuts may absorb moisture more quickly because salt attracts moisture, so coated nuts may soften sooner than plain ones.
How to restore crispness
Spread the nuts in a single layer on a baking sheet and toast them at about 150°C / 300°F for 5–10 minutes; the time varies with their size, and smaller ones may brown more quickly, so check them from time to time. Nuts may feel soft while hot from the oven; their true crispness becomes apparent after they cool, so do not judge them too soon. To toast them in a pan, use a dry pan over medium-low heat and stir continuously for a few minutes for a similar result. Pan-toasting is faster than oven-toasting but carries a greater risk of burning, so do not leave the stove unattended.
How to keep them from becoming moist again
After toasting and cooling, store the nuts in a tightly sealed container to minimize exposure to air and delay moisture absorption. Filling the container snugly rather than leaving an air-filled gap can also help. For long-term storage, keep them in a tightly sealed bag in the freezer to preserve freshness and crispness for months. Storing nuts in a cool, dark place away from direct sunlight also helps maintain their crispness and flavor; light and heat are among the main factors that accelerate moisture absorption.
Do all nuts soften at the same rate?
Nuts in their shells, such as hazelnuts and peanuts, are better protected from external moisture as long as they remain in the shell. Shelled nuts absorb moisture much faster because they are directly exposed to the air. High-fat nuts such as walnuts are more prone to becoming moist and turning rancid over time, so keeping them in tightly sealed containers in a cool place is especially important. Small nuts and seeds such as peanuts and pine nuts have more surface area, so they can absorb moisture more quickly and brown faster during toasting. For more reliable results, keep small pieces separate from larger ones when oven-toasting and track their cooking times separately.