The defining component of Fino sherries is the strain of yeast known as flor that floats in a layer on top of sherry in the wine barrel. Until the mid-19th century most sherry winemakers did not understand what this yellowish foam that randomly appeared in some of their barrels was. They would mark these barrels as "sick" and relegate them to their lowest bottlings of wine. It turned out that this strain of Saccharomyces yeast thrived in air, and the more "head room" there was in the barrel the more likely it was to develop. Over time winemakers noticed that these wines were lighter and fresher than their other sherries, with the flor acting as a protective blanket over the wine that shielded it from excessive oxidation.