At the heart of the transformation from traditional mouldy cellars to their spotlessly clean modern counterparts has been a desire by winemakers to obtain much greater control over the processes of vinification and the maturation of wine, primarily through an emphasis on hygiene. While the pace of scientific research on the microbiology and biochemistry of winemaking has quickened appreciably since 1945, its origins lie in the middle of the 19th century with the experimental work of Louis pasteur. Until then, the precise reasons why pressed grapes would ferment into wine, and would then become unpalatable if left open to the air for any length of time, were unknown. Practical experience had convinced Roman winemakers of the need to use chemicals such as sulfur to help prevent spoilage, and by the late 15th century in Germany it was recognized that wine kept in large barrels that were subjected to regular topping up would last longer than wine kept in small barrels which were left on ullage. However, it was Pasteur who first reported in western Europe that wine deteriorated mainly as a result of the actions of micro-organisms, and that these could be killed by heating the wine in the absence of oxygen (although see hungary history).