From lactose to alkyl galactoside fatty acid esters as non-ionic biosurfactants: a two-step enzymatic approach to cheese whey valorization

Cheese whey is the main wastestream of dairy industry. After protein recovery, the resulting whey permeate (WP) may contain up to 160 g/L of lactose, thus providing a pool of carbohydrates (lactose, glucose and galactose) as feedstock for the synthesis of fine and commodity chemicals. Exploitation of WP as starting material is a solution for its valorization and the reduction of the environmental burden associated to its disposal. Specifically, lactose was used for the synthesis of Sugar Fatty Acid Esters (SFAE), which are non-ionic surfactants characterized by emulsifying, stabilizing, and detergency properties. [1] A library of alkyl-β-D-galactosides was synthesized by reacting lactose with naturally occurring alcohols 2a-2g through a transglycosylation reaction catalyzed by the covalently immobilized β-galactosidase from Aspergillus oryzae in a ternary system composed of alcohol/acetone/McIlvane buffer (Scheme). [2] All the alkyl galactosides were fully characterized by 1H NMR, 13C NMR and MS analyses. n-Butyl β-D-galactopyranoside (3d) was submitted to the esterification step with palmitic acid in a solvent-free system at 80 °C by using Novozym 435 as the biocatalyst, affording pure 6-O-palmitoyl-1-O-butyl galactopyranoside (4d). In parallel, the same alkyl galactosides were synthesized by the chemical glycosylation of galactose using Amberlyst® 15 as catalyst. In this case, alkyl galactosides were obtained as isomeric mixtures of - and -pyranosides and - and -furanosides that were submitted, without further purification, to the enzymatic esterification as reported above. [3] Evaluation of the emulsifying properties of the synthesized alkyl galactoside fatty acid esters is ongoing according to the methods applied to 6-O-palmitoyl-1-O-butyl glucosides. [3] Tuning the reaction conditions for the esterification between fatty acids and sugars to obtain SFAE is a challenging task. Derivatization of the sugar-based “polar head” of SFAE into less polar derivatives (i.e. alkyl galactosides) before the esterification step played a key role to overcome the striking different solubility of sugars and fatty acids. Acknowledgements This work was financially supported by Cariplo Foundation (Italy) (call: “Circular Economy for a sustainable future 2020”, project BioSurf, ID 2020-1094). References [1] N.S. Neta et al., Crit. Rev. Food Sci. Nutr. 2015, 5, 595 [2] D. Ahumada et al., Front. Bioeng. Biotechnol. 2020, 8, 859 [3] S. Sangiorgio et al., under review