Dried beans are one of the most durable of all pantry staples, a reliable source of protein and carbohydrates that requires only a cool, dark, well-ventilated storage place. When properly stored, pintos and other dried beans can remain edible for years. But they're not indestructible. Water can make them susceptible to mold and spoilage, and otherwise-perfect beans can be damaged by worms.
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Almost invariably, the pest that's taken up residence in your pinto beans is a variety of bean weevil. These are a small beetle, rather than a true weevil, though that distinction holds little importance once you've found the larvae in your pantry. The beetles make tiny, perfect holes in your beans; hollowing them out as they grow, pupate and produce new beetles. Left to their own devices, they'll quickly spread through the entire bag, reproducing briskly for as long as the beans can feed them. Throw out any infested bags, and any other beans that shared the same storage area should be carefully segregated and monitored for unwelcome guests.
Baby, It's Cold Inside
One way to ensure your pinto beans are safe from the bean beetle's domestic instincts is to freeze the beans. If you grow your own, you can bag them and freeze them as soon as they're dried. Four to seven days in a chest freezer is usually adequate to kill any larvae, eggs and adults. Once you've done that, you can open the beans and let them return to room temperature. If you live in a humid climate, they might display some condensation, but you can return them to their original arid condition in a food dehydrator or an oven at its lowest setting. Repackage them in bags, jars or airtight containers for storage.
Take Their Breath Away
The beetles and their larvae require oxygen for survival, just as humans do. A second way to protect your pintos from invasion is to deprive them of that necessity. Many food storage companies offer oxygen absorption strips, which bind up surprisingly large quantities of that gas and make it unavailable to insect pests. To use one, place the pintos in an airtight container and add an oxygen absorption strip or pellet. Seal the jar until it's needed, and any beetles or larvae will suffocate in the interim. Alternatively, place the beans in a container and add a piece of dry ice. The carbon dioxide will sink through the beans, suffocating the pests. When it disperses, package the beans for storage.
Although bean weevils appreciate a nice warm home, they don't tolerate higher temperatures well. If your oven is capable of maintaining a steady 130 degrees Fahrenheit, you can spread your pintos on a baking sheet and keep them at that temperature for 30 minutes or longer. This kills the beetles, larvae and eggs quite effectively. If you've used a food dehydrator to dry your beans, you shouldn't need to do this. Dehydrators typically operate at 140 F, which kills the weevils while the beans dry.
As insect pests go, bean weevils are relatively innocuous because they don't eat other foods. Once you've discarded the infested pintos, remove all other cans or packages from the infected shelf and vacuum carefully, especially in the corners. Wipe down the shelf with a disinfectant, and clean the foods you've removed before you put them back.