Nanocellulose-sakacin A: conjugates for food packaging purposes

Introduction: the project aims at exploring the use of deproteinized cheese whey as cheap substrate for the production of sakacin A by Lactobacillus sakei. The bacteriocin is then purified employing food-grade protocols and subsequently incorporated in nanocellulose films and coatings, to obtain active packaging devices. Methods: Lactobacillus sakei DSMZ 6333 was used as sakacin-A producer. The strain was grown in a 14 L fermenter employing a low cost medium formulation containing deproteinized cheese whey and added with arginine (0.5 g/L) and Tween 80 (0.5 g/L). Sakacin-A was purified by precipitation with ammonium sulphate from the supernatant obtained after culture centrifugation (10000 rpm, 45 min at 4 °C) and subsequently lyophilized. Activity was evaluated against Listeria innocua DSM 20649 by diffusion assay (AU/mL), and a calibration curve was also set-up by HPLC for quantification. The crude Sakacin-A preparation was used for the preparation of active films by incorporation, esterification on cellulose nanofibers (CNF) or by grafting on TEMPO-CNF suspension comparatively, to avoid the selective oxidation of primary alcohols to the corresponding aldehydes and carboxylic acids. Antimicrobial, chemical and mechanical properties of the obtained materials were finally assessed. Results: sakacin-A reaches the highest titre after 9 h incubation at 26 °C. Precipitation with ammonium sulphate and subsequent lyophilization allows to obtain a 10X-concentrated extract (yield 2.5 g/L of supernatant). Films produced by incorporation of sakacin in CNF evidenced the formation of clear halos of Listeria growth inhibition, confirming its antimicrobial efficacy (Fig. 1). Spectra obtained by Fourier Transfor Infrared spectroscopy show changes in bands related to sakacin immobilization on cellulose nanofibers (Figure 2). Discussion: the possibility of obtaining active packaging materials through the use of conjugates prepared employing sakacin-A and nanofibers represents an interesting opportunity to recycle a by-product stream (deproteinized cheese whey) into added-value products, with a number of industrial applications.