To clean mildew from an L.L. Bean backpack, the company advises first brushing the mildew off the backpack. Spread the backpack open in the sunshine to kill the mold. Then wash as directed on the label. Mildew stains can be removed with a paste of lemon juice (one cup) and 2-3 tablespoons of salt. Leave the paste on the backpack for 15 to 30 minutes. Then rinse and let dry.
To remove the odor of mildew from a backpack, Trails.com suggests first immersing the backpack in a tub of warm water with a gentle detergent and letting it soak for 30 minutes. Then scrub it with a clean brush. Re-submerge the backpack in a tub of clean water and swish it around until there are no more bubbles, then lay it in a well-ventilated space to dry. It's important to let the backpack dry thoroughly to kill any remaining mold spores.
Exposure to sunlight will help disinfect the backpack. To avoid future problems, it's necessary to ensure that the backpack dries thoroughly after each use, as mildew thrives in a damp environment. For more stubborn cases of mildew, a solution of water (one quart) and 2 tablespoons of bleach is effective, although it could affect the color of the fabric, according to online magazine Outside.