Effects of β-cyclodextrin, whey protein, and soy protein on the thermal and storage stability of anthocyanins obtained from purple-fleshed sweet potatoes

Highlights

• WP prevented thermal degradation but increasd color loss of PFSPAEs.

• WP accelerated the degradation of PFSPAEs during storage.

• Soy protein (25 mg/L) enhanced thermal stability of PFSPEs.

• β-Cyclodextrin (2500 mg/L) improved the storage stability of PFSPAEs.

Abstract

The effects of β-cyclodextrin (β-CD), whey protein (WP), and soy protein (SP) on the color loss and degradation of anthocyanins in purple-fleshed sweet potato anthocyanin extracts (PFSPAEs) during thermal treatment and shelf-life storage in model beverage systems by performing chromaticity, degradation kinetics, and principal component analysis. Results showed that WP and SP improved the thermal stability of the PFSPAE, but WP accelerated the color loss of the extract. However, the addition of 25 mg/L SP improved the color and thermal stability of the anthocyanins when heated at 100 °C for 30 min. With regard to the shelf-life storage, the addition of SP and WP showed non-significant effect on the storage stability of the PFSPAE. However, the addition of 2500 mg/L β-CD significantly improved the storage stability of the PFSPAE. In summary, our findings provide useful information on improving the thermal and storage stability of PFSPAEs in beverage systems using food biopolymers.

Introduction

Currently, the food industry is considering the use of natural pigments as a substitute for synthetic colorants in commercial food and beverage products owing to the consumers’ concerns over the potential side effects of synthetic additives and the demand for clean labels and natural ingredients (Martins et al., 2016, Oplatowska-Stachowiak and Elliott, 2017). Anthocyanins are a group of naturally occurring flavonoids in plants, and they are responsible for the bright red, blue, and purple colorations of the fruits, leaves, and flowers in such plants (Castaneda-Ovando et al., 2009, Francis and Markakis, 1989). Anthocyanins have been widely used as natural ingredients in foods because of their desirable color and potential health benefits (He and Giusti, 2010, Li et al., 2017).

Purple-fleshed sweet potatoes (PFSPs) are important sources of natural anthocyanins, and they are widely cultivated and consumed in many countries (Esatbeyoglu et al., 2017, Xu et al., 2015). According to our previous research, chemically, PFSP anthocyanins primarily consist of polyacylated and polyglycosylated structures derived from peonidin and cyanidin. The main anthocyanins are 3-sophoroside-5-glucoside derivatives from cyanidin and peonidin, acylated with p-hydroxybenzoic acid, ferulic acid, or caffeic acid (He et al., 2015, Quan et al., 2019).

Recently, several studies have shown the application of PFSP anthocyanins in fruit juice, alcoholic drinks, and some ready-to-drink beverages (Gérard et al., 2019, Li et al., 2013). However, anthocyanins are unstable compounds that are highly susceptible to chemical degradation in several conditions such as different pH, light durations, temperature settings, oxygen concentrations, enzymatic concentrations, and ingredient interactions, which lead to color fading and loss of bioactivity (Bordenave et al., 2014, Gérard et al., 2019, He et al., 2016a, He et al., 2016b). Food processing (especially thermal processing) and shelf-life storage are two important factors in the production and distribution of beverages, and they can easily result in significant loss of anthocyanins (Eiro and Heinonen, 2002, Patras et al., 2010). As opposed to the extensive applications of synthetic colorants, natural PFSP anthocyanins have had limited applications in commercial beverage productions because of the color loss, off-flavor formation during thermal processing, and short shelf life (He & Giusti, 2010). Therefore, some stabilization strategies such as microencapsulation or structural modifications have been developed to improve the stability of anthocyanins (Wang, Jung, & Zhao, 2017). However, these approaches also have some disadvantages, because altering the structure of the anthocyanins or microencapsulation can affect the bioavailability and the biological activity of anthocyanins (Cortez, Luna-Vital, Margulis, & de Mejia, 2017).

An alternative effective approach to improve the stability of anthocyanins is the addition of various common food biopolymers such as proteins and cyclodextrins (Cortez et al., 2017). Whey protein (WP), soy protein (SP), and β-cyclodextrin (β-CD) are widely used in the food industry for improving the stability of anthocyanins (Chung et al., 2015, Fernandes et al., 2018, Sui et al., 2018). The protective effect and the mechanism of action of these food biopolymers on anthocyanins are different from each other. In our previous study, we demonstrated that preheated WP improved the thermal stability of grape anthocyanins in model beverage systems (He et al., 2016a, He et al., 2016b) because the anthocyanins can form complexes with WP through hydrogen bonding that enhances their stability. The β-CD molecules are oriented in a cyclic manner to form a hydrophobic hollow cavity, which easily induces the blackberry and the chokeberry anthocyanins to bind with β-CD to form inclusion complexes, thereby protecting the anthocyanins from hydration and polymerization reactions (Fernandes et al., 2018, Howard et al., 2013). Further, the interaction between anthocyanins and β-CD depends on the anthocyanin structure, environmental pH, and the β-CD concentration (Fernandes, Sousa, Azevedo, Mateus, & de Freitas, 2013). SP has been reported to bind to anthocyanins via covalent bonds, hydrophobic interactions, and static quenching, and SP has been reported to improve the thermal and oxidation stability of black soybean seed coat and black rice anthocyanin extracts (Chen et al., 2019, Sui et al., 2018, Zhang et al., 2018).

Although numerous studies have been performed on the stabilization methods of anthocyanins, little information is available on the protective effects of proteins and polysaccharides on PFSP anthocyanins. Therefore, in this study, we aimed to evaluate the effects of a polysaccharide (β-CD) and two proteins (WP and SP) on the stability of PFSP anthocyanins in a model beverage system under conditions of simulated shelf-life storage (storage under light exposure for 6 months) and thermal treatment (heating at 100 °C for 30 min). The results of this study may contribute to improving the stability of PFSP anthocyanins that are used in food and beverage systems.