Eco-sustainable recovery of antioxidants from spent coffee grounds by microwave-assisted extraction: Process optimization, kinetic modeling and biological validation

Spent coffee grounds are a rich source of antioxidants and caffeine, which recovery and application in cosmetic field could lead to the formulation of healthier and cheaper products. Indeed, this by-product of food industry is available worldwide in huge amount, already in powder form, reducing the pretreatments required before extraction process. By microwave-assisted extraction is possible to enhance antioxidant extraction yields even when eco-friendly solvents are used. The purposes of this study were the optimization of the antioxidant extraction process, the analysis of its kinetics, and the production of a biocompatible extract to be potentially used in cosmetics. In this work, a first optimization was carried out on the solvent composition, temperature and extraction time by response surface methodology. The extract with the highest bioactive molecule content (32 mg/L of caffeine; 9 mg/L of chlorogenic acid), obtained at 150 °C, 90 min of extraction time and using ethanol/water 54:46 (v/v) as solvent, was tested on human keratinocytes NCTC 2544 in order to assess its biocompatibility. In addition, utilizing the optimal conditions for solvent composition and temperature, the kinetic of extraction was studied and Peleg's model well described experimental results, allowing to observe that an extraction time of 60 min was actually sufficient to reach the maximum polyphenol yield (46 ± 2.9 mgcaffeic acid equivalent/g dried solid).