Technical Yearbook 2023

Differences in transport between amino acids, proteins and ammonium

limited to producing aroma compounds arising only from sugar metabolism. These aromas include acetic acid (vinegar off-aroma), medium-chain fatty acids, and certain higher alcohols and esters. The most common ester present in wine is ethyl acetate (nail polish remover off-aroma) which is formed through the reaction of acetic acid with ethanol, but other more sought-after esters include phenyl-ethyl acetate (rose/honey) and ethyl isovalerate (fruit). In a study of amino acids added to a wine fermentation, it was noted that the yeasts produced a more diverse range of aroma compounds. Indeed, important aromatics are produced from certain amino acids in the so-called Ehrlich pathway (figure 1). This pathway is linked with sugar metabolism to produce certain amino acids, which result in the production of a corresponding aroma compound. This means that some aroma compounds are produced from both sugar and amino acid metabolism, while others can only be produced via the breakdown of specific amino acids, as in the Ehrlich pathway. The Ehrlich pathway (figure 1) is responsible for breaking down certain amino acids into aromatic compounds that can have unpleasant odours. Depending on the redox status of the cell, these compounds can be short-chain fatty acids formed by the oxidation of an aldehyde. Examples of fatty acids produced through this pathway include isovaleric acid, which can cause rancid cheese odour, and propionic acid, which can create a pungent aroma. Higher alcohols, such as isobutanol with a savoury/wine-like aroma or isoamyl alcohol that is pungent, are also produced. Higher levels of amino acids in grape juice have been observed to result in an increased production of volatile fatty acids and higher alcohols, as these are substrates for aroma compounds through the Ehrlich pathway. The yeasts can then metabolise these higher alcohols and fatty acids further into acetate esters or fatty acid ethyl esters, which impart desirable aromas to the wine. Common acetate esters produced from amino acids are isoamyl acetate (banana aroma), isobutyl acetate (fruity/

Amino acids are the building blocks of peptides and proteins that are essential as enzymes, transporters and structural components, amongst other functions. One key difference between amino acids, proteins and ammonium is their assimilability by the yeast. Under fermentation conditions, proteins do not contribute to the YAN in the main wine yeast, Saccharomyces cerevisiae , as it lacks transporters able to take in such large molecules, whereas amino acids are already in a form that can be incorporated by various transporters. Some of these transporters can take up all amino acids, while others are much more specific and only facilitate the uptake of one or a limited number of amino acids. Once in the cell, amino acids can be stored or used directly to produce proteins, but are most often broken down to liberate the nitrogen that may be used similarly to ammonium (see below). The remaining skeleton may then be used to produce higher alcohols, fatty acids and esters. Ammonium is transported into the cell via permeases, where it must be converted into organic forms, such as amino acids, via the process of nitrogen assimilation. These organic forms can then be used by the cell to produce any of the other amino acids. The uptake of nitrogenous compounds from the grape juice differs for each amino acid and ammonium, with each requiring different regulatory mechanisms for incorporation. The mechanisms for nitrogen metabolism are complex as its intermediates are also shared between other metabolic pathways, including carbon metabolism. Effect on aroma production Amino acids and ammonium differ in their impact on the production of specific aromatic compounds. Studies have found that supplementing certain amino acids enhances the production of aroma compounds, including esters, which can contribute to fruity and floral aromas in wine. Most aroma compounds in wine arise from sugar metabolism. In studies where yeasts were provided with ammonium only, they were

FIGURE 1. The Ehrlich pathway leading to the production of fermentative aroma compounds.

67

TECHNICAL YEARBOOK 2023