In this work, biodiesel was produced from spent coffee grounds (SCG) oil extracted using n-hexane as a non-polar solvent, and acetone and 1-butanol as polar solvents, in order to simultaneously obtain SCG oil and antioxidant component. Moreover, oxidation stability and antioxidant capacity of SCG oil and their biodiesel production process were evaluated using the Rancimat method and oxygen radical absorbance capacity (ORAC) method, respectively. ORAC values of SCG oil from polar solvents and a non-polar solvent were 97–98 and 93–386 μmol-TE/g, respectively. Antioxidant activities of SCG oils from polar solvents were higher than those of SCG oil from non-polar solvent. However, these ORAC values were decreased by one third on raw oils during biodiesel production model experimental. These characteristics were utilized to develop an improved method of producing waste rapeseed biodiesel: (1) blending the SCG oil with rapeseed oil; (2) soaking powdered SCG in the reaction mixture during biodiesel production; and (3) adding active components extracted using methanol. In method (1), oxidation stability was improved such that 10 wt% of polar solvent extracted SCG oils increased the stability from 1 to 3 h. Also, oxidation stabilities in methods (2) and (3) were improved by soaking 40 wt% SCG and adding 1 wt% SCG extract, respectively.
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