Before the advent of refrigerators, fresh foods were subject to rapid spoilage, which put narrow constraints on trade. As a result, highly perishable items such as fish had to be rendered shelf stable for trade through mass production by means of fermentation, drying, smoking, and pickling (with smoke, salt, or acid) (Foodstuffs, conservation of). Herring and cod became internationally traded commodities and fed the populations of Europe (Food).