Acidic Foods. Many fruits that are known for their health benefits can actually cause or worsen the condition of canker sores owing to the fact they are highly acidic. Apples, berries, figs, grapes, lemons, limes, pineapples, etc. should be avoided if your gums are already irritated or cut.nything from brushing too hard, biting your gums, harsh mouthwashes, or eating something that causes bruising can lead to the tiny ulcers. If you are susceptible to canker sores, also try avoiding toothpastes and mouthwashes that contain sodium lauryl sulfate.