Study of the grinding of coffee waste prior to its valorization

Abstract

Coffee is one of the most important food products worldwide, but it also generates substantial amounts of waste, including spent coffee (SC), and coffee silver skin (CSS). Despite these organic wastes represent a major pollution hazard, they are also a potential source of compounds that can be used in different industrial applications such as pyrolysis, fermentation, solid-liquid extraction, or hydrolysis. To increase the efficiency of these operations, a reduced particle size of the wastes is beneficial, mostly below 400 µm. The use of a planetary ball wants to be an innovative and efficient method for reducing grinding times and costs for biomass materials such as residual coffee waste. This work is focused on the analysis of the influence of different grinding conditions of a planetary ball mill on the particle size distribution of SC and CSS wastes. A multiple linear regression analysis was applied to quantify the influence of variables. The results demonstrated that it was easier and higher the reduction in CSS (96% in 2 minutes) than in SC (max. 66% in 5 minutes), both at an optimal speed of rotation of 400 rpm. The speed of rotation and grinding time are the most influential variables, while pause time or reversal of direction do not favour the grinding process or in a less manner. This study highlights the potential of planetary ball mills for grinding organic materials, reducing grinding times and costs while improving the efficiency of waste utilization for subsequent industrial applications.