Insights into the composition, structure-function relationship, and molecular organization of surfactants from spent coffee grounds

Abstract

Macromolecular/polymeric constituents of surfactants and their self-assembly in aqueous solutions play a key role in their technologically relevant application such as stabilization of foams and emulsions. However, such information on coffee-based surfactants is scarce. The present study is first-of-its-kind in elucidating the chemical and molecular fingerprints of surfactants derived from spent coffee grounds (SCG). Initial part of the study involved estimation of the total proteins and sugars, and monosaccharide composition of high (HMW) and low molecular weight (LMW) surfactants derived from dry and defatted SCG, which in turn is the residue obtained after hot water brewing of roasted Robusta and Arabica coffee granules. The HMW surfactants had higher protein and sugar contents that were directly related to their foamability and foam stability, respectively. Mannose and arabinose were the major sugars identified in HMW surfactants. LMW surfactants showed lower protein and sugar contents and absence of arabinose. Thus, only the HMW surfactants were subjected to MALDI-TOF mass spectrometry, LC-MS-MS based protein profiling, circular dichroism spectroscopy, and small-angle X-ray scattering. The presence of quercetin-sugar complexes, protease, transferase, and acylated proteins and a β-turn type protein secondary structure were detected in the SCG-surfactants. The transition from lamellar to rod-shaped micellar structures was observed when the surfactant concentration in aqueous solution was increased from a lower concentration to its critical micelle concentration (0.2%–0.8%, w/v).