Fungi, especially moulds but also yeasts, are important agents of food spoilage. Penicillium moulds cause soft rot such as of apples, while Aspergillus moulds create patches on the surface of old bread, yoghurt and many other foods. Yeasts spoil sugary foods such as plums and jams, fermenting the sugars to alcohol.[38] Scientific understanding of spoilage began in the 19th century with works such as Louis Pasteur's 1879 Studies on Fermentation, which investigated the spoilage of beer.[39]