If I asked you to identify one of the most important compounds found in apple juice that impacts the quality of your hard cider, what would you answer? Would you say the sugar? Maybe you would advocate for the yeast. However, I would argue that the key compounds in your juice are the amino acids it contains. Amino acids are compounds formed by amino and carboxyl groups chained or linked to other compounds. The amino groups contains nitrogen and hydrogen while the carboxyl groups contain carbon, oxygen, and hydrogen. The other linked compounds, known as side chains, contain numerous other atoms. The side chains are how amino acids are generally classified. It’s also good to remember that proteins are really just multiple strains of amino acids linked together in very long chains. Through various pathways and enzymes, yeast process proteins and amino acids into several key elements.
The first key element is nutrients, especially nitrogen. Amino acids are a key source for nitrogen, which is needed for yeast growth and fermentation. Without nitrogen and other nutrients, yeast won’t ferment sugar into ethanol and CO2. The amino group on the amino acids provides that nitrogen source. Research has found that different yeast strains prefer different amino acids. While much research has been done on the preference of nitrogen in Saccharomyces yeast, K. Prior and associates have recently explored the use of nitrogen in non-Saccharomyces strains. They have found that some yeast process amino acids similarly to Saccharomyces spp. but others have different preferences(1). This is mostly likely the result of unique pathways the yeast have for creating enzymes and processing the amino acids. Some yeasts have actually been found to add amino acids to apple juice.
The second key element that amino acids provide are acids. Specifically, they form carboxyl acids from the carboxyl group of the amino acid. These acids are the key building blocks or what are often called precursors to aroma and flavor. Through what is called the Ehrlich Pathway, yeast create fusel alcohol by breaking down the amino acids. This is a decarboxylation process where the amino acids release carbon atoms. However, these carboxyl acids can also form esters, another key aroma element in hard cider. This generally occurs through an esterification reaction between the carboxyl acid and ethanol. These types of reactions create the volatile compounds that give hard cider it’s aroma and flavor profile. Ultimately, they are what can make hard cider great or not.
What is the source of amino acids on your juice? The apple variety is the biggest impact to the amino acids that are present in your juice. This is followed by the orchard practices like fertilization. It’s why I keep trying every apple variety I can find. It’s also why I try different yeasts because while different apple varieties have different amino acids, different yeasts have different preferences for those amino acids and create different flavors. It highlights there are many important compounds and factors involved n making great cider. However, without amino acids, your ability to ferment would be limited and the aromas and flavors would be further reduced. This makes amino acids the most important compound in your juice.
(1) K. Prior and associates, The utilisation of nitrogenous compounds by commercial non-Saccharomyces yeasts associated with wine, Food Microbiology 79, 75–84, 2019
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