While some pantry staples like salt and baking soda last indefinitely, flour isn't one of them. All flours are made from grains, seeds or vegetables, including wheat, corn, rice, soy, amaranth, millet, legumes and potatoes. Like the ingredients the flour is made from, the flour itself can go rancid eventually because all of them contain fats in small amounts. These fats can turn rancid over time, which leads to the spoilage of the flour. Whole grain flours contain more fat than other types of flour, which is why they spoil the most quickly. rancid flour may not only smell musty, but also oily, and will usually become discolored.

Most manufacturers list an expiration date on the flour that they produce. While you can use this as a general guide, you can also rely on your senses when it comes to determining whether your flour is still usable. Spoiled flour develops an unpleasant, off odor and bitter flavor. Flour stored in warm, damp conditions will expire more quickly than flour stored in a cool, dark pantry. The moisture encourages the growth of mold, which is what usually causes the flour to have that musty odor. Only use flour if it has no odor at all, or a sweet, pleasant one, meaning that it's fresh.

The best case scenario that can occur when using musty-smelling flour to create anything from gravies to baked goods is that your finished products will taste strange or have an unusual texture. If pantry pests, like grain beetles, have spoiled and contaminated your flour, they may end up in your finished goods. While these pests may not make you sick, they'll definitely disgust anyone who eats the items you made with flour infested with them. The worst case scenario is that goods made with musty flour could make someone sick because they might contain mycotoxins like Aspergillus species, Penicillium species and even some strains of Salmonella.

Flour can last up to six months past its expiration date if stored properly. To maintain the freshness of your flour, keep it as dry as possible in an air-tight container. When you purchase your flour, immediately put the unopened bag inside of a resealable plastic bag or plastic container to keep air and moisture out. If you live in a hot, humid locale, you can refrigerate or freeze your flour in its air-tight container to extend its shelf life.