Does treatment of grape juice with aspergillopepsin-i influence wine aroma?

Acid aspergillopepsins-i (ap-i) have been suggested for use in winemaking due to their ability to degrade proteins, which reduces haze formation and the necessity for bentonite to achieve protein stability. These endopeptidases cleave non-terminal amino acid bonds of proteins, resulting in their degradation. Consequently, their activity may increase substrate for native exopeptidases resulting in an increased concentration of amino acids in industrial settings. Both effects can potentially improve fermentation kinetics and the concentration of yeast-derived aroma compounds. However, the use of ap-i in winemaking, according to oiv-oeno 541a-2021, requires shock-heating of the must (60°-75°c), which can deplete grape-derived volatiles in the wine. This study investigates the impact of ap-i supplementation in grape juice (gewürztraminer and sauvignon blanc) in accordance with the oiv resolution, on the amino acid composition of the must and aroma compounds in wines after fermentation. Results from amino acids analysis, volatile analysis, and wine tasting (involving 15 expert panellists) were compared among the outcomes of four treatments: heat-shock treatment with and without ap-i, only ap-i supplementation, and untreated control. Heat treatment of grape musts, when accompanied by ap-i addition, increased the concentration of amino acids. This effect was particularly pronounced in sauvignon blanc must, where 12 out of 20 amino acids were enriched by the combined treatment, up to approximately a two-fold increase. Heat-treated wines of gewürztraminer were richer in acetates, with or without ap-i supplementation, respectively, increased by +43% and +85%. Both heat treatments influenced terpene concentration in wines, resulting in a reduction of up to 68% in gewürztraminer compared to the control. Polyfunctional thiols, compounds associated with sauvignon blanc’s typicality, were not influenced either by heating or ap-i addition. Despite the varied profile of wines, expert judges found no significative differences in the main descriptors associated with the analysed aroma compounds, although they often described gewürztraminer treated solely with ap-i as more floral, while the heating diminished its typical characteristics. Meanwhile, sauvignon blanc, when treated with ap-i, regardless of heat treatment, has been recognised as more tropical and fruitier.