*2.3. Experimental Set-Up*

Fermentation tests were carried out in a 5 L batch fermenter (Biostat Biotech B, Sartorius Stedim Biotech, Goettingen, Germany. The fermenter was equipped with one four-bladed Rushton turbine and the usual control systems: temperature, pH, pO<sup>2</sup> and a foam detector. Fish waste and lemon peel (2:1 *w/w*) were homogenized in a blender for 5 min.

The resulting homogenate, with a dry matter content of 40% (*w/w*) was supplemented with 20 mL of *S. cerevisiae* (10<sup>8</sup> cells per mL) and 20 mL of *L. reuteri* culture (10<sup>8</sup> cells per mL), simultaneously. No sterilization procedures were adopted.

Fermentation parameters were 35 ◦C, with pH 5.0 and constant stirring at 200 rpm, and a final working volume of 3.5 L.

All fermentations were carried out for 120 h until no further growth of the selected microorganisms was observed, and the pH value became stable. The pH was not controlled by alkali addition during cultivation.

Medium samples were withdrawn daily from the reaction vessel using a sterile 20 mL syringe and immediately frozen at −20 ◦C until analysis.
