Ultrasound-Assisted Extraction of Anthocyanins from Grape Pomace Using Acidified Water: Assessing Total Monomeric Anthocyanin and Individual Anthocyanin contents

While optimizing the extraction of anthocyanins, it is crucial to go beyond just focusing on maximizing the total content. Evaluating the changes in the profile of anthocyanins, characterized by types and concentrations of individual compounds, also plays a significant role in determining the applications of the product. Thus, this work aims to optimize the ultrasound-assisted extraction of anthocyanins from grape pomace, evaluating total monomeric anthocyanin and anthocyanins by nuclear magnetic resonance (NMR). To achieve it, dried grape pomace (1.5 g) was suspended in acidified water (30 mL, pH 1.5) and placed in a jacked glass reactor (internal dimensions: diameter = 45 mm; height = 110 mm) coupled to a thermostatic water bath at 25 °C. The optimization of ultrasound parameters, including power density (8.3 W/mL and 16.7 W/mL), extraction time (1 and 5 min), and pulse (0 and 2 s), was conducted using a face-centered composite design. Control samples (without ultrasound) were also prepared. Results show that power density and extraction time were the most significant factors affecting the total anthocyanin content. Extractions carried out with greatest power and time yielded extracts with the highest pigment content, exhibiting up to 48% more monomeric anthocyanin compared to the control. Three anthocyanins, namely Malvidin-3,5-diglucoside, Malvidin-3-glucoside, and Malvidin-3-O-(6-p-coumaroyl)-glucoside were identified and quantified in the extract using RMN. Malvidin-3,5-diglucoside was the most abundant compound found in most of the extracts. However, longer extraction times combined with the higher power densities lead to decreases in Malvidin-3,5-diglucoside content, and an increase in the contents of Malvidin-3-glucoside and Malvidin-3-O-(6-p-coumaroyl)-glucoside. It may be attributed to the deglycosylation and cleavage of anthocyanins induced by ultrasound. The findings of this study contribute to a deeper understanding of anthocyanin extraction, which can guide the selection of specific processing conditions to obtain extracts with desired profiles that align with targeted applications.