There can be hundreds of compounds in your hard cider that create aromas and flavors. Esters are one of those. They are generally described as sweet or fruity. For example hexyl acetate is often thought to smell like apple. However, they can also be described as solvent, which could be ethyl acetate. Other esters can drive other aromas and flavors notes in your cider. As defined by Chris White and Jamil Zainasheff in their book Yeast (2010), esters are volatile compounds formed from an organic acid and an alcohol. Let’s explore in detail what that means.
Volatile: This doesn’t mean it loses it’s temper and gets mad easily. It means it is easy to evaporate. This is why you will tend to notice it in the aroma of your hard cider. It is more likely to evaporate and be picked up when you smell and taste the hard cider.
Organic Acid: Organic acids are generally defined as compounds with carbon and hydrogen bonds that are acidic in nature. Acidic means the pH is below 7.0 but most of the acid you see in cider will be even lower. Examples of familiar organic acids that are often found or used in hard cider, beer, and wine are lactic acid, malic acid, citric acid, acetic acid, and tartaric acid. However, there are a lot of other acids as well.
Alcohol: Generally, we are dealing with ethanol, which is the main alcohol produced during fermentation. However, a lot can be created from fusel alcohols which are also created during hard cider fermentation.
Therefore, esters are compounds formed by acids reacting with alcohol. This can occur during fermentation as well as afterward during the aging or maturation process. Your hard cider can be a living thing that evolves over time. That evolution can be a positive thing, especially if you maintain good sanitary practices, as chemical reactions continue throughout the life of your hard cider. However, it is dynamic and the sensory markers you have when a hard cider is a month old will generally be very different when it’s 6 months old.
I found a great graphic about esters from James Kennedy, a chemistry teacher in Australia. You can find this and other great chemistry information on his site here.
Yeast impact the amount and type of esters that are produced. That means that nutrients and temperature also impacts the esters being produced because they impact the yeast. There are a lot of articles stating how fermenting too hot or too cold or without proper nutrients will result if “off flavors” caused by the production of various esters and other compounds, see my tip on fusel alcohols. However, most of that assumes you are brewing beer or making wine (grapes) and are trying to replicate a specific style for the yeast you are using. For hard cider, there really are not many, if any, yeast that are made specifically for hard cider. Most yeast packaged as hard cider yeast are really wine yeast.
This makes it challenging to define what an “off flavor” is. Generally, it would be a flavor that is outside of the expected results. If you are making an English Ale, there is an expectation for how it will smell and taste. If it doesn’t smell and taste that way, it has “off flavors”. The same would be for a wine. However, most labs making yeast aren’t developing cider specific cultures. There are not hundreds of years of selective cropping occurring like you find with brewer’s yeast. As a result, it’s really difficult to say whether the esters produced are truly “off flavor” or not. I suggest when talking about hard cider and the creation of esters and other aroma compounds, we talk about the types of flavors created and leave the consumer of the hard cider to decide if the aromas and flavors are “off”or not. However, knowing how esters are created does allow you to better target the flavors you desire.
Yeast and the fermentation process have a large impact on the creation of esters. This is because a common acid for ester production is acetyl-coenzyme A or acetyl-CoA. This along with fusel alcohols and various enzymes stimulate the production of esters. If the yeast have sufficient amounts of oxygen, it will use the available acetyl-CoA in the early stages of fermentation to directly produce sterols, which it needs to reproduce. If oxygen is limited in the early stage of fermentation, it will use the available acetyl-CoA to produce esters as it seeks to create the oxygen its needs. Maximizing oxygen in the must or juice, minimizing fusel alcohols, and reducing various enzymes will reduce esters production. Reversing that will maximize their production.
It is also interesting to assess how temperature impacts flavors. Generally, the warmer the fermentation, the more esters will be produced. However, most people who want a lot of fruity aromas in their hard cider or wine will ferment at lower temperatures. This seems opposite of what you should do. However, it is believed that lower fermentation temperatures create more fruity esters (citrus, apple, and such) while higher temperatures create esters with more floral aromas. The result may be that people are more attuned to the fruity aromas and view these positively, which accounts for the perception that cold fermentation is better for ester generation.
At the end of the day, I believe the best way to produce a hard cider that is to your desired flavor and aroma profile is consistency in process. For hard cider, it is less about always using the prescribed amount of nutrient or temperature. It is about consistency. You should seek to use a consistent apple base, nutrient level, temperature, and yeast culture to reproduce a hard cider. Varying these elements outside of ranges prescribed for beer or wine only make “off flavors” if they are flavors you don’t want. For me, that means experiment more and take even better notes.
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