In modern distillation, except for the invention of the continuous still in the 19th century, the basic process of distillation has not changed since the 8th century. There have been many changes in the methods used to prepare organic material for the still, and the ways the distilled beverage is finished and marketed. Knowledge of the principles of sanitation and access to standardised yeast strains have improved the quality of the base ingredient; larger, more efficient stills produce more product per square foot and reduce waste; ingredients such as corn, rice, and potatoes have been called into service as inexpensive replacements for traditional grains and fruit. For tequila, the blue agave plant is used. Chemists have discovered the scientific principles behind aging, and have devised ways to accelerate aging without introducing harsh flavors. Modern filters have allowed distillers to remove unwanted residue and produce smoother finished products. Most of all, marketing has developed a worldwide market for distilled beverages among populations that previously did not drink spirits.