Enzymatic synthesis of short-chain esters (flavours) might enable their labelling as natural, increasing their value. Covalently immobilised Rhizopus oryzae lipase (EO-proROL) was used to synthesise isoamyl butyrate and acetate. In cyclohexane, the best performer reaction solvent, 1.8 times higher yield of isoamyl butyrate (ca. 100%) than isoamyl acetate (ca. 55%) was obtained. Optimum initial acid concentration (410 mM) and acid:alcohol mole ratio (0.5) were established by a central composite rotatable design to maximise isoamyl butyrate single-batch and cumulative production with reused enzyme. These conditions were used to scale up the esterification (150 mL) and to assess yield, initial esterification rate, productivity and enzyme operational stability. Commercial isoamyl alcohol and fusel oil results were found to be similar as regards yield (91% vs. 84%), initial reaction rate (5.4 µM min⁻¹ with both substrates), operational stability (40% activity loss after five runs with both) and productivity (31.09 vs. 28.7 mM h⁻¹). EO-proROL specificity for the structural isomers of isoamyl alcohol was also evaluated. Thus, a successful biocatalyst and product conditions ready to be used for isoamyl ester industrial production are here proposed.

Lemon (Citrus × limon) residues from agro-industrial processes represent a significant environmental challenge due to their high organic load and acidic pH. This study explored different extraction techniques and processing methods to obtain a stable flavonoid- and mineral-rich extract from lemon peels and evaluated its biostimulant potential on spinach (Spinacia oleracea L.). Vacuum-dried and ultrasound-assisted aqueous extract (V-H) demonstrated stability, high levels of Fe, Zn, Mn, Ca, and phenolic compounds. Foliar application of V-H extract enhanced spinach growth, relative water content, and mineral accumulation, while modulating aquaporin gene expression (PIP2;1), suggesting improved water transport and hydraulic conductivity. Zn-primed extract did not induce into strong physiological effects, highlighting the importance of the extract matrix for full biostimulant activity. Overall, V-H lemon peel extracts combine mineral enrichment and bioactive regulation, offering a sustainable approach to enhance crop performance. These findings also support the valorization of citrus residues within circular economy strategies.