Alkalis are the opposite of acids. Alkalis are solutions with a pH greater than neutrality or water, i.e. above pH 7.0. In food science, alkalis are "any substance which neutralizes or effervesces with acids and forms a caustic or corrosive solution in water". An alkali can, also, be defined as a soluble base; bases are chemicals that neutralise acids to form salts.
Common alkalis used in the kitchen are bicarbonate of soda and egg albumen. Some other foods are closer to neutral than other more acidic foods, so can reduce the acidity of a mixture, e.g. milk or yoghurt.
Foods are generally not alkaline, because this results in a "soapy" mouthfeel when alkalis react with fats; this is because potassium and sodium carbonates are used to make soaps, hence the soapy taste. While generally unpopular, alkaline tastes that are liked include: soda bread, scones and pancakes, which have some residual taste of bicarbonate of soda, and the ash on bonfire-toasted potatoes or breads.
Bicarbonate of soda can soften skins of seeds, so is often added to water for boiling beans or chickpeas. Bicarbonate of soda can, also, reduce the tannins in tea, so may be added to iced tea recipes, which are remarkably refreshing in warm weather.