Extraction of Polyphenols from Unripened Coffee (Coffea Arabica) Residues and Use as a Natural Coagulant for Removing Turbidity

Abstract

The coffee agribusiness generates significant amounts of waste that becomes an environmental problem in producing countries. For example, synthetic coagulants have sustainability disadvantages. Immature coffee beans are collected together with mature beans, and their high polyphenol content makes them unsuitable for coffee production and commercialization. This paper aims to test the coagulant activity of polyphenols extracted from Coffea arabica residues in synthetic water samples to use them as raw material for producing a natural coagulant based on bioeconomy. It would thus allow immature coffee beans to recover, avoiding their inadequate disposition. An extract was obtained from residual green coffee beans using the ultrasound-assisted separation technique with a mixture of ethanol and water in a 1:1 ratio. The Folin–Ciocalteu method was applied for the total polyphenols quantification, resulting in a concentration of 73.54 ± 0.05 mg GAE (Gallic Acid Equivalent) per gram on a dry coffee basis (GAE/gDB). The synthetic water for the study was prepared with kaolin, showing initial turbidity of 520.90 ± 0.1 NTU (Nephelometric Turbidity Units). First, the effect of pH was determined on the coagulant activity at a fixed dose of polyphenols 2.6 mg GAE/L. Second, the dose and pH results were evaluated using a multilevel factorial design with 5.20, 3.90, 2.60, and 1.30 mg GAE/L doses and pH at 2.5, 3.0, 3.5, and 4.0. Third, the turbidity removal achieved was 99.94% at a dose of 3.9 ± 0.05 mg GAE/L and a pH of 2.5. Fourth, the result was compared with the turbidity removal of the aluminum sulfate dosed at a concentration of 3 mg/L on the same water type, with a pH variation between 5.5 and 8, obtaining 98.69% of turbidity removed. Finally, the research demonstrated that the polyphenols extracted from the residues of the Coffea arabica species possess a high electrochemical affinity that would allow removing turbidity by coagulation in waters at specific pH levels with similar removals to those obtained with aluminum sulfate.