With the invention of the steam engine in 1765, industrialization of beer became a reality. Further innovations in the brewing process came about with the introduction of the thermometer and hydrometer in the 19th century, which allowed brewmasters to increase efficiency and attenuation. Prior to the late 18th century, malt was primarily dried over fires made from wood, charcoal, or straw, and after 1600, from coke. In general, none of these early malts would have been well shielded from the smoke involved in the kilning process, and consequently, early beers would have had a smoky component to their flavors; evidence indicates that maltsters and brewers constantly tried to minimize the smokiness of the finished beer. The invention of the drum roaster in 1817 by Daniel Wheeler allowed for the creation of very dark, roasted malts, contributing to the flavour of porters and stouts. The discovery of yeast's role in fermentation in 1857 by Louis Pasteur gave brewmasters methods to prevent the souring of beer by undesirable microorganisms.