Malolactic Fermentation or what is often referred to simply as MLF, is the process where lactic acid bacteria converts malic acid to lactic acid. For cider makers, MLF can be a very important process because apples are high in malic acid. As a result, MLF can reduce the acidity found in hard cider made from acidic apples known as Sharps. Sharps are apples often used for baking and eating but, can create an overly tart or acidic cider. Cider makers who lack bitter or sweet apples (Bitters and Sweets respectively), can employ MLF to improve the quality of their cider. However, the same lactic acid bacteria that converts malic to lactic acid can also convert other acids. One acid that poses the biggest concern is citric acid.
While malic is the dominant acid in apples and, therefore, cider, other acids are also present. Citric acid is one of those other acids. Citric acid is a stronger acid than malic as it has three acid groups while malic only has two. Given citric acid has three acid groups, it is also less stable. Another acid often found is cider is acetic acid, which is a weaker acid that only has one acid group. It’s also considered an undesirable compound in cider. Citric and malic acid, while tart, can provide a pleasant aroma and taste. Acetic acid creates an aroma and taste of vinegar. So, why is citric acid a concern in hard cider and, especially cider that goes through MLF? Because the lactic acid bacteria (LAB) that performs MLF and converts malic acid to lactic acid, can also convert citric acid to acetic acid(1). This usually happens after all the malic acid has been converted to lactic acid(2). The breakdown of citric acid in cider during MLF usually results in high levels of acetic acid and diacetyl forming. These high levels are because their are two ways citric is turned into acetic acid.
Initially, lactic did bacteria breaks down citric acid directly into acetic acid and oxaloacetic acid, which is the first way acetic acid is created. Oxaloacetic acid can be decarboxylated into pyruvic acid. Like citric acid, pyruvic acid can be processed by lactic acid bacteria into acetic acid and alpha-acetolactic acid. This d the second way acetic acid is created. Finally, the alpha-acetolactic acid can be transformed into diacetyl. Diacetyl is an aromatic compound that produces a buttery taste. In some situations, diacetyl can be reduced to acetoin. This usually only occurs when the pH of the solution is between 4-8. Most ciders have a pH below 4 but, MLF increases the pH as it reduces the total amount of acids in the cider. Both diacetyl and acetoin create buttery aromas, which is why MLF is often associated with this characteristic.
Whether you enjoy the buttery notes or not, the main concern is the creation of acetic acid. This doesn’t come from the malic conversion but the citric acid conversion. While present in some apples, citric acid is generally only naturally present in small amounts so the risk of creating acetic acid during the MLF process is when an acid blend containing citric acid is used or citrus fruit is added to your cider. I definitely enjoy citrus fruits as adjuncts in some ciders but, they come with a level of risk. If you plan to age cider for extended periods or encourage MLF, you should avoid adding citric acid. If you need to add acid, use malic acid. It will require more but, it will give your cider a more natural chemical composition and reduce your risk of accidentally creating acetic acid.
(1) Y. Shima I and associates, Transformation of Citric Acid to Acetic Acid, Acetoin and Diacetyl by Wine Making Lactic Acid Bacteria, Agric. Bill. Chem., 49, 2147, 1985
(2) E.J. Bartowsky and associate, The ‘buttery’ attribute of wine—diacetyl—desirability, spoilage and beyond, International Journal of Food Microbiology 96, 235–252, 2004
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