Umeboshi

The tasty Umeboshi is a Japanese pickled plum that adds extra flavor to sushi rolls, onigiri (rice balls), and many other dishes. It has medicinal properties and retards bacterial growth, so that

"an umeboshi is usually included in a bento lunchbox, as a "garnish," to prevent food-poisoning, especially during the summer months when bacterial growth is greatest and hydrochloric acid levels in the stomach are lowest."