Recovery of anthocyanins from Eugenia spp. fruit peels: a comparison between heat- and ultrasound-assisted extraction

Natural colorants have gained increased popularity among consumers and food producers due to their reputation as safer and healthier alternatives to commonly used artificial analogues. These natural pigments can be obtained from by-products resulting from food processing, such as the fruit peels of the Brazilian species Eugenia brasiliensis and Eugenia involucrata, thus contributing to the valorisation and circularity of these undervalued raw materials. Therefore, since these fruit peels present anthocyanin concentrations that justify their exploitation, this study aimed to optimize and compare the recovery of these pigments from these plant by-products using heat- and ultrasound-assisted extraction (HAE and UAE, respectively) methods. For process optimization, a central composite rotatable design coupled with response surface methodology was implemented, considering time, ethanol/water ratio, and temperature (for HAE) or ultrasonic power (for UAE) as relevant independent variables. While UAE resulted in higher extraction yields (40–42%, w/w), HAE led to higher anthocyanin contents (18 mg g−1 from E. involucrata and 323 mg g−1 from E. brasiliensis). Furthermore, the HAE global optimum involved only 2 min of processing. Both theoretical models were experimentally validated by applying the model-predicted extraction conditions, and the obtained anthocyanin-rich extracts were analysed for colour and in vitro bioactive properties. In general, the extraction method did not greatly affect the colour or the antimicrobial and cytotoxic activities of the extracts. However, only E. brasiliensis extracts showed cytotoxicity on human tumour cell lines, which also stood out for their antioxidant activity, possibly due to the higher anthocyanin content. Thus, Eugenia spp. fruit peels could be an alternative renewable source of natural food colourants with bioactive properties. Nonetheless, since E. brasiliensis extracts displayed moderate toxicity towards normal cells, the toxicity threshold should be further investigated to ensure the safe exploitation of this raw material as a possible source of natural food colourants.