Influence of extraction conditions on ultrasound-assisted recovery of bioactive phenolics from blueberry pomace and their antioxidant activity

Berries such as blueberries (Vaccinium section Cyanococcus spp.) contain abundant phenolic compounds, including anthocyanins (derived from anthocyanidins by glycosylation), flavonols and chlorogenic acids, which are mainly found in berry skin. Some of these compounds are pigments
that impart pleasant and characteristic colours to the fruits. Berry fruits can be processed into juice, wine, jam and marmalade, among other foods. Berry processing generates large quantities of pomace, which consists of skin, seeds and some flesh. Berry flesh contains about 10% of the total polyphenols, while the skin and seeds contain 28–35% and 60–70%, respectively, which makes berry processing by-products an excellent source of polyphenols. According to Struck et al., processing berries into juice leaves approximately 20–30% of the fruit as pomace. Blueberry production in Canada, the second largest producer worldwide after the United States, reached 176,641 tons in 2017, with some consumed fresh and some being processed. Thus, blueberry pomace from food processing results in considerable losses in polyphenols and other valuable bioactive phytochemicals if these are not recovered. Extracting these compounds from the pomace for subsequent use in foods, pharmaceuticals or fine chemicals for healthcare and lifestyle applications is considered the best approach for maximal valorisation of this by-product. For safety, environmental and economical sustainability, green or eco-friendly processes are being developed using various methods such as microwave-assisted extraction, supercritical fluid extraction, accelerated solvent extraction, enzyme-assisted extraction and ultrasound-assisted extraction (USAE). Their main advantages include shorter extraction times, reduced energy consumption, fewer negative environmental impacts, increased safety as well as enhanced innovation and competitiveness, all of which contribute to improving the sustainability of the value chain that supplies the extracts. In this context, USAE is a particularly attractive method due to effective extraction, energy saving and the use of moderate temperatures, which is beneficial for heat-sensitive compounds. It is thus widely used to extract bioactive compounds from plant materials. The main drawback of USAE is the unavoidable use of organic solvents in some applications, yet the equipment is simpler and the overall cost is lower compared to supercritical CO2 extraction which does not use organic solvent. Still, this limitation can be overcome by using ethanol as USAE solvent as it is safe to use in food systems, completely biodegradable, available in high purity form and at low price. Several USAE parameters affect the quality of the extracts. Among them, sonication time, temperature, solvent composition, solid/solvent ratio, particle size of the raw material, matrix parameters as well as ultrasonic irradiations (power, frequency) can affect the quantity, composition and biochemical properties of the extracts. Therefore, USAE methods must be developed to be suitable for use with the plant material considered and the phenolic compounds or fractions of interest. In order to develop an extraction method well tailored for blueberry pomace, the aim of the present study was to assess the effects USAE parameters (sonication time, solid/liquid ratio, solvent composition, pH and extraction temperature) on the total phenolic, flavonoid and anthocyanin contents and total antioxidant activity of extracts prepared from blueberry wine pomace.