Apple and pear juice contain a variety of organic acids. Even sweet apples with high pH like Red Delicious and Ambrosia are still acidic. They won’t contain the same quantity as a tart and sharp Granny Smith or bittersharp cider apple, but they contribute to the hard cider flavors. When I assess apples for hard cider, I often find sweet apples have 3-4 grams per liter of titratable acids and a pH reading over 4.0. Other apples can be as high as 20 grams per liter with pH values below 3.0. However, they all have various organic acids. It’s just the magnitude of how much. Let’s explore what these acids are and how they contribute to your hard cider.
Acetic Acid: At first glance, you might think that acetic acid is bad and you wouldn’t want any in your hard cider. This is because it is the main acid in vinegar and we generally don’t want our hard cider becoming vinegar. However, acetic acid is quite prevalent and desirable in hard cider. It is the main volatile acid in wine and cider. It is also critical in the formation of ethyl acetate, which is an ester known to create fruity aromas. You may recall from my post about esters that esters are formed from alcohol and organic acids. Ethyl acetate is ethanol and acetic acid. The main concern with acetic acid and ethyl acetate is the amount. Small amounts are good while large amounts are not. If you get too much acetic acid, you get vinegar. If you get too much ethyl acetate, you will have a solvent-like aroma or nail polish.
Citric Acid: Often associated with citrus fruits like oranges, grapefruit, and such, it is also found in apple juice and hard cider. Many Saccharomyces Cerevisiae yeasts strains will produce critic acid as part of the fermentation process. Like all things in cider, a little is great but a lot can lead to other issues. For example, citric acid is a leading producer of diacetyl. Diacetyl is the compound that gives chardonnay its buttery flavor. Also, citric acid is more susceptible to microbial degradation during malolactic fermentation by lactic acid bacteria. This leads to increased level of acetic acid. If you are like me and enjoy adjunct ciders with citrus (see my recipe section), you might want to avoid long aging or try to avoid MLF to reduce the risk of acetic acid and vinegar. Just for reference, this isn’t a problem that I noticed in my citrus hard ciders so it’s not a guaranteed event but is something you should consider.
Lactic Acid: This is generally derived from the breakdown of pyruvic acid and the transformation of malic acid. Lactic acid is not generally found in apple juice so it is formed through the fermentation processes. Some yeast have a trait that will create lactic acid may convert malic acid to various compounds but the most common way malic acid is transformed to lactic acid is from lactic acid bacteria, LAB, after fermentation. This is the malolactic fermentation process often call MLF. Transformation of malic acid to lactic acid is often desirable if you have dry hard cider with a low pH (high total acid).
Malic Acid: Malic is the most prevalent acid in apples and hard cider. Dry hard ciders made from culinary apples, will often be harsh because they lack tannins and sugar to balance the malic acid. Converting malic acid to other compounds by using yeast that have this trait (an example is Lalvin 71B) or MLF will help to smooth these types of hard ciders but you can also use adjuncts to provide more balance. The fermentation process naturally creates and transforms malic acid. Research shows that the level of malic acid changes at different time through the fermentation cycle (1).
Pyruvic Acid: While apple juice naturally has some pyruvic acid, it is a precursor to many other substances. It can be excreted in large quantities by yeast during fermentation. It is also used by yeast later in the process to create these other compounds. This is why it tends to increase dramatically early in the fermentation process before dropping down. While not as high as malic, it is usually one of the higher secondary acids, along with citric acid. It normally finishes the fermentation process as the second most significant acid in hard cider. Research has shown that a deficiency in thiamine increases the amount of pyruvic acid produced. One of the main reason thiamine is reduced is by adding sulfites to your juice. Nitrogen also has a significant impact on pyruvic acid creation along with niacinamide.
Succinic Acid: Initially low, succinic acid increases throughout the fermentation process. Formed along with glycerol during the metabolism of pyruvic acid. Succinic acid is considered to have positive traits for hard cider. It makes sense that nutrients like nitrogen and thiamine that increase or decrease pyruvic acid would have the same impact on succinic acid. Nitrogen was found to cause a reduction in pyruvic and succinic acid when it was either too low or too high. Succinic acid can be converted to a variety of esters and volatiles, which is why it is considered to have positive traits for hard cider production. As I noted in my research on hard cider color, this could be another reason to avoid treatment of juice with Campden tablets. Potassium sulfite can reduce thiamine. Thiamine is an important compound in the creation of pyruvic and succinic acid.
These are only the most common organic acids found in apples and hard cider. Also, not all apples have the same acids or levels. However, these major acids play important roles in the making and stabilization of your hard cider. Cider lacking acid can be insipid and cider with too much acid and no balancing sweetness, tannins, or aromas can be too harsh. Along with the apple varieties, yeast and bacteria play a big part on how the acids in your hard cider will evolve and ultimately create the flavors and sensory elements you enjoy. This is where understanding some of the science can help you with the art. Tasting and measuring allows you to better blend apple varieties and your hard cider. It will help you create that crisp taste you desire without being overwhelming. Understanding more of the science behind acids and your fruit will ultimately help you in the art of creating better hard cider.
(1) H. Zhang and associates, Determination of organic acids evolution during apple cider fermentation using an improved HPLC analysis method, Eur Food Res Technol (2008) 227:1183–1190
(2) Wine Science, Fourth Edition (2014)
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