Malolactic fermentation (MLF) is a process that usually occurs after primary or alcoholic fermentation completes. Fundamentally, it’s the conversion of malic acid to lactic acid. Malic acid is more acidic compared to lactic acid so MLF reduces the acidity of your cider. Other reactions that impact aroma also occur. Diacetyl creation is one of the most common. Unlike primary or alcoholic fermentation, MLF is generated by bacteria and not yeast and is not technically fermentation because it doesn’t produce alcohol. It does produce CO2, which gives the impression of fermentation. Instead of yeast, MLF is created by lactic acid bacteria (LAB). So, how do you encourage MLF? You can inoculate with LAB, age on lees, and avoid adding sulfites in your cider. Let’s explore in more detail.
Add LAB strains to speed the MLF reaction process.
There are three main types of LAB that can survive the fermentation process. Lactobacillus and Pediococcus species are more active at high pH conditions and Oenococcus species are better for low pH conditions. Most cider where you are seeking to encourage MLF is probably low pH (higher acidity) so inoculating with Oenococcus strains are the better choice for encouraging MLF in hard cider. Adding LAB also helps speed up the process. A natural MLF process can take months to occur.
Aging on Lees
Age your cider on lees to provide more nutrients to the LAB.
One of the reasons natural MLF can take months is because the cider lacks nutrients. Fermentation will often deplete all or almost all of the key nutrients that LAB need to process malic acid into lactic acid. Aging your cider on the lees, which is the sediment that precipitates out of the cider as it ages, can give it the nutrients that are lacking. This is because this sediment is full of yeast and as the yeast die, they go through a process called autolysis. Autolysis is a process where yeast die and release key nutritional compounds. LAB uses these compounds to evolve your cider and one of those evolution processes is MLF. It’s generally better to age cider on the fine lees versus the gross lees. Gross lees are those formed from the primary fermentation, which contain more apple solids. Fines lees are what accumulate after the cider has been siphoned off the gross lees. This is composed mostly by yeast as falls out of suspension when your cider clarifies.
Don’t Use Sulfites
Avoid sulfites, which will kill or inhibit bacteria.
Lastly, avoid using sulfites if you wish MLF to occur, especially after fermentation is complete. Campden (a brand name for sulfites), is used to kill or inhibit LAB and acetic acid bacteria. Adding it will usually prevent MLF, which is one of the reasons it is often added to cider or wine. If you think you might want to encourage MLF, avoid sulfite treatments, at least until after MLF occurs.
If you are using culinary or eating apples to make hard cider, you might find it’s quite tart. Encouraging MLF is a way to reduce that tartness and you can encourage it by adding LAB like Oenococcus, aging on lees (fine preferred), and avoiding sulfite treatments.
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