Why does the flavor of cider change when it ages? Part of those changes can come from bacteria or yeast. This micro flora can make malolactic fermentation (MLF) occur or a souring by Brettanomyces yeasts. However, one of the biggest impacts can come from the yeast that fermented your hard cider. That’s right, the yeast that converted that sugar to ethanol and carbon dioxide and created all the wonderful esters and organic acids may still have more to offer to you. In the fermentation process, yeast are absorbing and secreting various compounds. Many of the compounds they excrete are toxic to them, like ethanol. Other compounds they absorb actually help them in their task of reproducing. Just like us, they want to live so they try to retain or make many different compounds and use these in their quest to live and reproduce. That means that an individual yeast cell is full of many compounds and many of these compounds add character to a cider. Mannoproteins, lipids, amino acids, and fatty acids are just a few examples of compounds that are found in yeast cells that can positively contribute to the quality of your cider.
When a yeast cell dies, the membrane that contains the cytoplasm and all the yeast organelles breaks down. This releases the compounds found in the yeast cells into the cider. That decomposition process of the yeast cell and release of compounds is known as autolysis. When you age a cider on the lees of the fermentation process, you are allowing autolysis to evolve the flavors of your cider. Gross lees, the sediment at the bottom of your fermenter after primary fermentation completes, contains high quantities of yeast. It also contains a lot of other solids and compounds that generally don’t contribute as positively as yeast autolysis will. This is why many only age ciders on what would be fine lees. Fine lees are those that occur after a cider has been racked off the gross lees following fermentation. Most of what is suspended in a cider after fermentation are yeast cells. These yeast cells precipitate out of the cider and settle at the bottom of your aging container. With time, yeast cells will die and autolysis occurs. It is this process and the aging of cider on these fines lees that will evolve your cider’s flavor. Traditional sparkling ciders often rely on autolysis to impart more robust flavors before they go through a riddling process that ends with this yeast lees being disgorged.
You may have thought the yeast you used only matter during fermentation but autolysis is the opportunity for that yeast to impart a final influence on your cider.
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