In the present study, we developed novel β-glucosidase-based nano-biocatalysts for the bioconversion of oleuropein to hydroxytyrosol. Using non-covalent or covalent immobilization approaches, β-glucosidases from almonds and Thermotoga maritima were attached for the first time on oxidized and non-oxidized porous carbon cuboids (PCC). Various methods were used for the characterization of the bio-nanoconjugates, such as Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and fluorescence spectroscopy. The oxidation state of the nanο-support and the immobilization procedure play a key role for the immobilization efficiency or the catalytic activity of the immobilized β-glucosidases. The nano-biocatalysts were successfully used for the hydrolysis of oleuropein, which leads to the formation of its bioactive derivative, hydroxytyrosol (up to 2.4 g L−1), which is a phenolic compound with numerous health benefits. The bio-nanoconjugates exhibited high thermal and operational stability (up to 240 hours of repeated use), which indicated that they are efficient tools for various bio-transformations.

BACKGROUND The effects were studied of different inoculation strategies for selected starters –yeasts and lactic acid bacteria (LAB) – used for the fermentation process of two Greek olive cultivars, Conservolea and Kalamàta. The LAB strains applied were Leuconostoc mesenteroides K T5‐1 and L. plantarum A 135–5; the selected yeast strains were S. cerevisiae KI 30–16 and Debaryomyces hansenii A 15–44 for Kalamàta and Conservolea olives, respectively. RESULTS Table olive fermentation processes were monitored by performing microbiological analyses, and by monitoring changes in pH, titratable acidity and salinity, sugar consumption, and the evolution of volatile compounds. Structural modifications occurring in phenolic compounds of brine were investigated during the fermentation using liquid chromatography / diode array detection / electrospray ion trap tandem mass spectrometry (LC/DAD/ESI‐MSn) and quantified by high‐performance liquid chromatography (HPLC) using a diode array detector. Phenolic compounds in processed Kalamàta olive brines consisted of phenolic acids, verbascoside, caffeoyl‐6‐secologanoside, comselogoside, and the dialdehydic form of decarboxymethylelenolic acid linked to hydroxytyrosol, whereas oleoside and oleoside 11‐methyl ester were identified only in Conservolea olive brines. CONCLUSION Volatile profile and sensory evaluation revealed that the ‘MIX’ (co‐inoculum of yeast and LAB strain) inoculation strategy led to the most aromatic and acceptable Kalamàta olives. For the Conservolea table olives, the ‘YL’ treatment gave the most aromatic and the overall most acceptable product. © 2019 Society of Chemical Industry

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