Effects of deodorization on the physicochemical index and volatile compounds of purple sweet potato anthocyanins (PSPAs)

Highlights

• Purple sweet potato anthocyanins (PSPAs) crude extract were deodorized.

• Deodorization was conducted by AB-8 resin purification and chitosan adsorption.

• Impurities of sugars and proteins in PSPAs had been effectively removed.

• Volatile substances in PSPAs were changed and unpleasant odors were decreased.

Abstract

To produce high quality purple sweet potato anthocaynins (PSPAs) extract, a deodorization process was developed and its effects on the physicochemical index including total anthocyanins, total sugar and protein, soluble solids, color values, degradation index (DI), brown index (BI) and clarity were investigated. The product sensory quality was also evaluated. The volatile compounds were analysed by headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC/MS). The results indicated that using AB-8 macroporous resin purification coupled with chitosan adsorption could be an effective method in deodorization of the anthocyanin crude extract from purple sweet potato. Some byproducts in the anthocyanin crude extracts, such as sugars and proteins, had been removed by about 85% and 76% respectively. Compared with the raw anthocyanin extract, the volatile substances in the final product of PSPAs were changed and the unpleasant characteristic odors, such as some alcohols, aldehydes, organic acids and alkanes, were decreased. However, the product yield was also decreased by about 26% after the deodorization process.

Introduction

Recently, owing to both legislative regulations and consumer concerns of synthetic dyes/pigments that may cause various diseases including cancer or heart disease, interests in natural colorant foods, cosmetics and pharmaceutical products have been increased dramatically (Giusti and Wrolstad, 2003, Mazza and Kay, 2008). The significantly increased demand for natural colorants may also due to their apparent attractive color attributes and positive physiological functionalities (Fossen et al., 2001, Gülçin et al., 2005, Lila, 2004). Anthocyanins, a group of phenolic compounds, present a spectrum from orange to blue, satisfying consumers’ desire for a range of food colors. Anthocyanins are water soluble and widely distributed in fruits, vegetables and red wines but most of anthocyanins from conventional sources have limited stability against hydration and pH changes (Cevallos-Casals and Cisneros-Zevallos, 2004, Francis, 1989, Mazza and Miniati, 1993). Exceptionally, athocyanins from purple sweet potatoes (PSPAs) are mono-acylated or di-acylated forms of cyanidin and peonidin, which show good stability (Francis, 1989). Moreover, PSPAs possess potential beneficial physiological and pharmacological properties, such as potent antioxidant capacity, enhancement of sight acuteness, treatment of various blood circulation disorders, as well as vaso-protective and anti-inflammatory properties (Giusti and Wrolstad, 2003, Karlsen et al., 2007, Lila, 2004, Shih et al., 2005, Shih et al., 2007). Hence, the application of cost effective technology to produce high quality PSPAs may add value to food and pharmaceutical industries by providing natural stable colorants with potential health benefits.

However, current PSPAs products have some drawbacks such as relatively high impurities, unpleasant flavor and undesirable odor, which have restricted their application (Chandrasekhar et al., 2012, Liu et al., 2004). The formation of off-flavor in natural colorants may come from the microbial contamination, oxygen degradation, and loss of key odorants or changes in the concentrations of individual aroma substances during the preparation processes (Barbara and Barbara, 2006, Giusti and Wrolstad, 2003, Kensuke et al., 2011, Xu et al., 2014). It has been proposed that the off-flavors in the anthocyanin extracts is mainly derived from hydrochloric acid hydrolysis during acidic aqueous extraction with overheating treatment and during the purification operations such as adsorption and desorption by macroporous resin (Giusti & Wrolstad, 2003). Conventional methods of extraction anthocyanins from plant materials are non-selective and generally yield pigment solutions with large amounts of byproducts such as sugars, sugar alcohols, organic acids, amino acids and proteins (Liu et al., 2013, Liu et al., 2004). Some of these compounds are detrimental to the stability of pigments, e.g. sugars accelerate anthocyanin degradation during storage (Liu et al., 2004). The free sugars and their degradation products in the anthocyanins may lead to the Maillard reaction and form brown compounds (Xu et al., 2014). Therefore, exploring suitable anthocyanin extraction and purification processes to minimize the byproducts and reduce the unpleasant flavor compounds is desirable for the stability of these pigments and will facilitate their applications in a wide range of industries.

Currently, there are several methods of purifying anthocyanins, such as liquid–liquid extraction, high-speed countercurrent chromatography, ion exchange, macroporous resin adsorption and membrane filtration (Cao et al., 2010, Chang et al., 2012, Cissé et al., 2011, Cristina et al., 2012, Gilewicz-Łukasik et al., 2007, Timothy et al., 2014). However, it is very expensive to use chromatographic separation for industrial scale production of anthocyanins (Cao et al., 2010). The tendency of the oligomeric anthocyanins to aggregate may cause them to be retained on the membrane, which restricts the continuous flow of the membrane filtration process (Cissé et al., 2011). Macroporous resin (e.g. Amberlite XAD-4) adsorption seems to be the most suitable method due to its low cost, high efficiency and simple procedure, particularly in extracting anthocyanins from grape wastes (Sandhu and Gu, 2013, Tamura and Yamagami, 1994, Timothy et al., 2014). Different types of resins were reported in purification of anthocyanins from crude extracts of mulberry and red cabbage (Chandrasekhar et al., 2012). Chitosan is a linear biopolymer of N-acetyl-d-glucosamine and d-glucosamine residues with a positive charge in acidic environments due to the presence of amine groups (Jing et al., 2011, Peter, 2005). Utilization of chitosan to remove byproducts from radish anthocyanin extracts has been conducted by Gao et al. (2014). Odorant compounds from radish extracts were decreased by 70% after the chitosan treatment, suggesting it is effective in removing glucosinolates as well as other volatile compounds. The mechanisms of removal of impurities using chitosan include charge neutralisation, adsorption, precipitative coagulation, bridge formation or electrostatic patch formation (Gao et al., 2014).

To our knowledge, research on macroporous resin separation coupled with chitosan adsorption in purification of anthocyanins and odor removal has not been reported. The objective of this work was to develop a deodorization process by employing macroporous resin separation coupled with chitosan adsorption for preparing PSPAs with high purity and color value, and minimum unpleasant odor.