Application of response surface methodology to acidified water extraction of black soybeans for improving anthocyanin content, total phenols content and antioxidant activity

Highlights

• The conditions for the maximum yield of anthocyanins from black soybeans were optimized.

• A five-level, three-factor central composite design was employed.

• HCl concentration, solid-liquid ratio and extraction temperature affected the yields.

• The differences between the predicted and experimental values were <3%.

• These results confirm the validity and adequacy of the predicted model.

Abstract

Acidified water extraction of total anthocyanin content, total phenol content and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) scavenging activity from black soybeans (Glycine max [L.] Merr. Cheongja4ho) was optimized using response surface methodology (RSM). A five-level, three-factor central composite design was employed to optimize the conditions for the maximum yields of these three components using19 different experiments. The HCl concentration (0.3–0.5%), solid-liquid ratio (1/30–1/50 g/mL) and extraction temperature (30–50 °C) significantly affected three components. The optimized conditions were HCl concentration of 0.359%, solid-liquid ratio of 1/54.2 g/mL and extraction temperature of 56.8 °C for total anthocyanin content (136.68 mg/100 g), total phenol content (1197.09 mg/100 g) and ABTS⁺ scavenging activity (211.65 mg/100 g). These experimental values fit well with the predicted values. Among three anthocyanins found in the black soybean extracts, cyanidin-3-O-glucoside was the major one (65–73% of the total), followed by petunidin-3-O-glucoside (17–23%) and delphinidin-3-O-glucoside (10–12%).

Introduction

Anthocyanins are water-soluble pigments that are responsible for the pink, red, blue and purple colours in plants. Black soybeans have recently drawn more attention due to their high amounts of anthocyanins that have anti-oxidant, anti-diabetic and anti-cancer effects (Xu and Chang, 2008, Mojica et al., 2017, Kim et al., 2008). The majority of the black soybean anthocyanins are found in their seed coat (Xu and Chang, 2008, Choung et al., 2001, Žilic et al., 2013). Choung et al. (2001) found that the anthocyanin content in the seed coats of ten black soybean varieties ranged from 1.58 mg/g to 20.18 mg/g dry weight. The composition of the anthocyanins, which includes malvidin 3-glucoside, petunidin 3-glucoside, delphinidin 3-glucoside and cyanidin 3-glucoside, has been found to vary depending on the black soybean cultivar (Takeoka et al., 1997, Xu and Chang, 2008, Yoshida et al., 1996).

Earlier investigations have developed a method to extract anthocyanins from black soybeans using organic solvents and ultrasound-assisted extraction (Xu and Chang, 2008, Mojica et al., 2017, Lai et al., 2013). The most common solvent is an acidified methanol (Sancho et al., 2015, Choung et al., 2001, Takeoka et al., 1997). However, there is a concern that organic solvents, due to their toxicity, may be left in the extract for functional ingredients and dietary supplements. Fortunately, the anthocyanins found in black soybeans are soluble in water since they are present as glycosylated compounds with glucose (Takeoka et al., 1997, Choung et al., 2001, Yoshida et al., 1996, Xu and Chang, 2008). Many variables, such as extraction temperature, time, pH, solid-liquid ratio, solvent concentration and particle size, influence the extraction yield of anthocyanins (Cacace and Mazza, 2003, Ng et al., 2012). For these reasons, a reliable extraction method that minimizes the use of organic solvents needs to be developed, which ultimately can be used to produce functional ingredients for the food and pharmaceutical industries.

Response surface methodology (RSM) is a statistical and mathematical technique that has been used to evaluate the effects of independent variables as well as their possible interactions. It has several advantages, such as a relatively short time period and lower costs during the optimization process (Bezerra et al., 2008, Said and Amin, 2015). Anthocyanins are susceptible to degradation by pH, temperature, light and the presence of oxygen (Castañeda-Ovando, Pacheco-Hernández, Páez-Hernández, Rodríguez & Galán-Vidal, 2009). The extraction of anthocyanins from black soybeans, therefore, should be optimized to maximize the diffusion of anthocyanins from their matrices while preserving stability during the extraction process.

To the best of our knowledge, this is the first study on the optimization of acidified water extraction from black soybeans for the purpose of improving anthocyanin content and antioxidant activity using RSM. The objectives of this study were i) to establish the optimal extraction conditions of independent variables such as extraction temperature, extraction time, hydrochloric acid (HCl) concentration and solid-liquid ratio; and ii) to compare the experimental and predicted data at the optimal conditions.