Why Do Nuts Go Rancid Over Time?

Nuts go rancid over time because the fat they contain comes into contact with oxygen in the air and slowly oxidizes. This reaction is called rancidity; oxidized fat molecules turn into sharp, bitter compounds that spoil the characteristic flavor of nuts. Nuts with a high fat content, such as walnuts and hazelnuts, go rancid much faster than varieties with relatively less fat, such as peanuts; they do not all spoil at the same rate. How exactly does oxidation (rancidity) happen? As the fat molecules in nuts come into contact with oxygen in the air, their structure begins to break down; light and heat make this process much faster. Over time, the compounds that form turn the familiar smell of nuts into a sharp, waxy odor. This reaction is slower in in-shell nuts because the shell protects them, and much faster in shelled and chopped nuts because their surface area is greater. Which nuts go rancid faster? Nuts with a very high fat content, such as walnuts, can become noticeably rancid within a few months, while relatively lower-fat varieties such as almonds can last for months even at room temperature. Hazelnuts fall between the two. Roasted nuts go rancid a little faster than their raw form because some of the oil reaches the surface during roasting. How does storage temperature affect rancidity? A walnut left in an open container at room temperature may start to lose flavor within a few weeks, while the same walnut can last for a few months in the refrigerator and up to a year in the freezer. Low temperatures noticeably slow the oxidation reaction. A jar of nuts left on a sunlit counter goes rancid much faster than the same jar in a dark cupboard. How can you tell if nuts have gone rancid? Rancid nuts usually give off a sharp, waxy, or paint-like odor; it is clearly different from the mild, natural smell of raw nuts. Their flavor also leaves a slightly sharp, sometimes metallic bitterness. Nuts that look normal but have changed in taste and smell are also considered rancid; in that case, the nuts should not be eaten, and one should always be tasted to check before adding them to a recipe. Ways to keep nuts fresh longer. Store nuts in a tightly sealed, airtight container in a cool, dark cupboard to keep them fresh many times longer than in an open bowl at room temperature. Storing a large quantity of nuts in the freezer can preserve a flavor close to that of the first day even after months. Leaving nuts in their shells until cracking them, and planning to eat roasted nuts sooner than raw ones, are practical ways to preserve freshness as efficiently as possible. Buying nuts in small amounts that will be eaten soon instead of in a large package is another practical measure; tightly resealing an opened package each time and keeping nuts transferred to a jar in an airtight container shortens their exposure to air and delays rancidity. Keeping nuts on a cool cupboard’s back shelf rather than near the oven or stove, the warmest places in the kitchen, also delays rancidity caused by sunlight and heat. Cleaning out old residue before adding each new batch to a jar also prevents nuts with a changed smell from being mixed unnoticed with fresh ones.