Optimization of heat- and ultrasound-assisted extraction of anthocyanins from Hibiscus sabdariffa calyces for natural food colorants
Introduction
The globalization of the industrial food sector, together with consumer’s awareness about the existence of bio-based alternatives to the artificial additives, nowadays massively used, and with potential toxic effects in humans, has promoted the demand for food products formulated with natural ingredients recovered from plant materials (Carocho et al., 2015, Martins et al., 2016). The scientific research in this area has gained international prominence (Almeida et al., 2018, Carocho et al., 2016, Pinela et al., 2017), as it is still necessary to expand the range of natural options and find new sources (e.g., plants, algae and insects), as well as to develop sustainable processes for an efficient recovery of the target compounds (e.g., anthocyanins, carotenoids, and beet derivatives).
The global food colouring market has grown rapidly in recent years and it is expected to continue growing by 10% to 15% annually (Carle & Schweiggert, 2016). The colour, in addition to be an important food sensory attribute, often related to flavour, safety and overall quality, also greatly influences product’s marketing success. At the same time, there is a growing interest in replacing the artificially obtained colorants by natural counterparts, since the former have been associated with adverse health effects, including hyperkinesis, skin rashes, tumours, kidney damage and migraine, among others (Ramesh & Muthuraman, 2018). Natural colorants can also provide an extensive range of colours, with the advantage of being innocuous and can provide beneficial health effects (Castañeda-Ovando, Pacheco-Hernández, Páez-Hernández, Rodríguez, & Galán-Vidal, 2009). However, the high stability and low cost of the synthetic food colorants have limited the use of the natural counterparts by the industrial sector (Carocho et al., 2015). Moreover, there are only few natural alternatives approved by federal authorities (Martins et al., 2016).
Plants are an interesting source of natural pigments endowed with colouring potential and bioactivities (Jabeur et al., 2017). Among them, Hibiscus sabdariffa L. (Fam. Malvaceae), also known as roselle, is an annual medicinal shrub relatively easy to grow and used worldwide by the food and pharmaceutical industries (Da-Costa-Rocha, Bonnlaender, Sievers, Pischel, & Heinrich, 2014). It comprises two main varieties, the altissima Wester, cultivated for the jute-like fibre, and the sabdariffa, generally pigmented and cultivated for the edible calyces used in the preparation of herbal teas and beverages, and a number of pastry products (Sharma et al., 2016). In folk medicine, H. sabdariffa calyx infusions are used for their diuretic, febrifugal and hypotensive effects, and for helping to lower body temperature; while other preparations are used for treating sore throats and coughs, liver, cardiac and nerve diseases, and genital problems (Da-Costa-Rocha et al., 2014). Some of these traditional uses have been validated by scientific studies, which have shown that calyx extracts have strong antioxidant and antihypertensive capacities, together with antihypercholesterolaemic, antinociceptive, and antipyretic effects, among others (Ali et al., 2005, Da-Costa-Rocha et al., 2014). Therefore, this plant has high potential to be used in the development of new functional and therapeutic products.
Most of the phytochemical studies on the H. sabdariffa constituents have been directed towards the characterization of pigments, namely anthocyanins (Ali et al., 2005, Beye et al., 2017, Jabeur et al., 2017). Delphinidin-3-glucoside, cyanidin-3-glucoside, and in particular delphinidin-3-sambubioside (hibiscin) and cyanidin-3-sambubioside (gossypicyanin) have been identified in calyx extracts (Alarcón-Alonso et al., 2012, Beye et al., 2017, Salazar-González et al., 2012). These anthocyanins are responsible for the characteristic red colour of the H. sabdariffa calyces and can be recovered for subsequent use as colorants in different industrial sectors.
In order to turn bio-based colorants into real and efficient alternatives to the widely used artificial analogues, it is necessary to find promising sources for their extraction and develop sustainable recovery processes. Today, several technologies are available to enhance extraction, including ultrasounds (López et al., 2018), microwaves (Liazid, Guerrero, Cantos, Palma, & Barroso, 2011), pulsed electric fields, and pressurized and supercritical fluids (Corrales et al., 2008, Garcia-Mendoza et al., 2017). Among them, ultrasound-assisted extraction (UAE) brings significant benefits over conventional heating methods in terms of time and solvent consumption and extraction yield (Chemat et al., 2017a, Marić et al., 2018). This “green” processing technique also reduces energy and water consumption, allows recycling of by-products through bio-refining, and ensures a safe and high quality product (Chemat et al., 2017a). In addition, UAE has been recognised as suitable for industrial applications (Vilkhu, Mawson, Simons, & Bates, 2008). However, the efficiency of these processes is affected by process variables (e.g., time, temperature, ultrasonic power and solvent). Therefore, it is necessary to use appropriate experimental designs and optimization tools to determine the optimal extraction conditions leading to the best responses in terms of recovering of target compounds.
This study was performed aiming at optimizing the recovery of the two major anthocyanins found in H. sabdariffa calyces by heat- and ultrasound-assisted extraction processes to serve as natural colorants (a workflow scheme is presented in Fig. A1). The three most relevant independent variables for each process were combined in a circumscribed central composite design, and response surface methodology (RSM) was used for process optimization. It is thus intended to identify which method and extraction conditions are the most suitable to extract these colouring compounds.
Section snippets
Plant material
Dried flowers of H. sabdariffa were supplied by a local company (Pragmático Aroma Lda, Alfândega da Fé, Bragança, Portugal) that produces medicinal and aromatic plants with organic certification. According to the producers, the plant material was dehydrated in a drying chamber with controlled conditions of temperature, relative humidity and air velocity, in order to ensure the quality of the final product. The red flower calyx consisting of 5 large sepals with a collar (epicalyx) of 8 to 12
Experimental data for RSM optimization
Although some previous studies on the extraction of anthocyanins from H. sabdariffa calyces can be found in literature, no reports detailing the optimal conditions maximizing their extraction are presently available. In addition, the compositional diversity of anthocyanins’ natural sources (e.g., fruits, flowers, leaves, stems and roots) does not allow to directly extrapolate the extraction conditions of these pigments from previously studied sources. Therefore, it is important to conduct
Conclusions
Nowadays, consumers are increasingly choosing food products formulated with natural additives due to the understanding of the strong relation between health and diet. Therefore, it is important for the industrial food sector to find novel sources and efficient extraction methods to support the production of bio-based ingredients, including colorants. In this study, two extraction methods were applied, and optimized by combining the effects of three relevant independent variables, to maximize
Acknowledgements
The authors are grateful to the Foundation for Science and Technology (FCT, Portugal) and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2013), FEDER through POCI-COMPETE2020 and FCT for financial support to LA LSRE-LCM (POCI-01-0145-FEDER-006984), J. Pinela (UID/AGR/00690/2013_DNAABN) and L. Barros contract. This work is funded by the European Regional Development Fund (ERDF) through the Regional Operational Program North 2020, within the scope of Project
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