Complexation between whey protein and octenyl succinic anhydride (OSA)-modified starch: Formation and characteristics of soluble complexes

Highlights

• Complexation between whey protein and octenyl succinic anhydride (OSA)-modified starch.

• OSA modified starch tended to interact with heated whey protein.

• Soluble complexes were formed between heated whey protein and OSA-modified starch.

• Degree of substitution of OSA-modified starch affects the properties of the complexes.

• MW of OSA-modified starch affects the properties of the complexes.

Abstract

Mixed systems of protein and polysaccharide are widely used in the food industry. It is important for food manufacturers to understand their interactions. In this study, the formation of complexes between whey protein isolate (WPI) and octenyl succinic anhydride (OSA)-modified starch was investigated as a function of pH and protein: starch ratio. OSA-modified starch tended to interact with heated WPI (HWPI) rather than non-heated WPI (NWPI), and the optimum conditions for their complexation were a protein: starch ratio of 1:10 and pH 4.5, probably driven by both electrostatic and hydrophobic interactions. The effects of the degree of substitution (DS) and molecular weight (Mw) of OSA-modified starch on the properties of the complexes formed under the optimum conditions were investigated using absorbance measurements (at 515 nm). Soluble complexes (HWPI-OSA SC) between 0.5% (w/v) HWPI and 5% (w/v) OSA-modified starch with a Mw of 19.24 ± 0.07 × 10⁴ g/mol and a DS of 4.29 ± 0.11% could be formed at pH 4.5. The structure of HWPI-OSA SC was examined using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Characterization of the HWPI-OSA SC revealed that the intermolecular interactions between HWPI and OSA-modified starch led to their different characteristics from HWPI and OSA-modified starch alone.

Introduction

Protein-polysaccharide interactions have a significant influence on the properties of food products, such as texture and stability (de Kruif and Tuinier, 2001, Ye, 2008); their complexes can be utilized to develop effective delivery systems that can encapsulate and protect bioactive compounds such as flavours, essential oils, vitamins, probiotics, and nutraceuticals (Bosnea et al., 2014, Devi et al., 2017, Eratte et al., 2017, Hosseini et al., 2015, Jain et al., 2016, Koupantsis et al., 2014, Xiao et al., 2014). The interactions between proteins and polysaccharides can be either attractive or repulsive. Strong attractive interactions between these macromolecules often occur at pH values below the isoelectric point (pI) of the protein, where the protein carries a positive charge, leading to the formation of complex coacervates (Schmitt, Sanchez, Desobry-Banon, & Hardy, 1998). At pH values above the pI of the protein, anionic polysaccharides can have weak electrostatic interactions with the positively charged patches on the protein, which can lead to the formation of soluble complexes (Dickinson, 1998, Schmitt et al., 1998). It has also been demonstrated that nanoparticles that are stable over a range of pH can be formed under specific conditions (Anal et al., 2008, Ye et al., 2006). The formation of complexes between proteins and polysaccharides is mainly dependent on the physical conditions of the biopolymer solutions, including pH, ionic strength, the ratio and concentration of the biopolymers, and the molecular characteristics of the biopolymers, including charge density, conformation, and molecular weight (Mw).

Whey protein, which accounts for about 20% of bovine milk protein, is now recognized as an important nutritional ingredient in the food industry (de Wit, 1990). Whey protein isolate (WPI, ~95% whey protein) is a commercial whey protein product (Fox, 2003), which can function as an emulsifying, water-binding, gelling, and foaming agent (Singh & Ye, 2014). Octenyl succinic anhydride (OSA)-modified starch is synthesized by an esterification reaction, which involves the partial substitution of hydroxyl groups on the starch with OSA groups (Lin, Liang, Zhong, Ye, & Singh, 2018c), and these hydrophobic substituents impart an emulsifying capability to OSA-modified starch (Torres, Tena, Murray, & Sarkar, 2017). It has been used as a substitute for proteins and gum arabic because of its emulsifying capacity (Sweedman, Tizzotti, Schäfer, & Gilbert, 2013). The presence of free carboxyl groups renders OSA-modified starch anionic at certain pH values, which could lead to interactions with positively charged proteins (Nilsson & Bergenståhl, 2007). As both whey protein and OSA-modified starch are common ingredients in food products, their interactions and their complexation need to be understood in order to develop new food products.

Many studies have mainly focused on the interactions between milk proteins (casein and whey protein), plant proteins (soybean protein and pea protein) or gelatin and non-starch polysaccharides including gum arabic, pectin, and carboxymethylcellulose (Cuevas-Bernardino et al., 2018, Jones et al., 2009, Jones and McClements, 2010, Kim et al., 2006, Liu et al., 2016, Salminen and Weiss, 2014, Santipanichwong et al., 2008, Turgeon et al., 2007, Ye et al., 2006, Zeeb et al., 2018). However, only a few studies on the interactions between protein and OSA-modified starch have been reported. It has been demonstrated that OSA-modified starch can electrostatically interact with protein including gelatin (Wu and McClements, 2015, Zhao et al., 2019), casein (Sun, Liang, Yu, Tan, & Cui, 2016), and egg yolk protein (Magnusson & Nilsson, 2011) at pH values below the pI of the protein. As few studies have examined the interactions between WPI and OSA-modified starch at different pH values, the complexation process between WPI and OSA-modified starch as a function of pH should be investigated. In addition, the earlier studies did not focus on the interactions between the preheated protein and OSA-modified starch. Heat treatment can induce changes in the conformation and aggregation of whey proteins, which has been proven to promote thermodynamic incompatibility and complexation between proteins and polysaccharides at neutral pH (Bryant and McClements, 2000, Chun et al., 2014, Kim et al., 2006). Thus, the interactions between OSA-modified starch and heat-denatured whey protein should be very different from those between OSA-modified starch and native whey protein. Besides the characteristics of the whey protein, the characteristics of OSA-modified starch including Mw and the degree of substitution (DS) may also affect their complexation; the DS of OSA-modified polysaccharides has been proven to influence the number of molecules that interact with the protein (Puerta-Gomez and Castell-Perez, 2017, Puerta-Gomez and Castell-Perez, 2016). Puerta-Gomez and Castell-Perez (2016) investigated the self-assembly interactions between native whey protein and OSA-modified polysaccharides, including depolymerized waxy rice starch and phytoglycogen, with different percentages of OSA modification (3% and 7%) using rheological principles. At 7% OSA modification, critical frequency values were obtained at 0.63 or 0.45 rad/s from the mixture of WPI and 0.184 g/L OSA-modified depolymerized waxy rice starch or 18.4 g/L phytoglycogen, which were higher than those at 3% OSA modification. Based on these results, they suggested that stronger hydrophobic interactions between native WPI and OSA-modified depolymerized waxy rice starch at 7% OSA modification occur at low polysaccharide concentrations; more WPI is required for its association with polysaccharides when the level of OSA modification is lower. Furthermore, Puerta-Gomez and Castell-Perez (2017) used a visual spectroscopy method to investigate the electrostatic precipitates of WPI in combination with OSA-modified depolymerized waxy rice starch or phytoglycogen at 3% or 7% OSA modification. In this study, they found that interactions occurred between native WPI and the polysaccharides with a high level of OSA modification. However, the influence of the Mw of OSA-modified starch on its interactions with whey proteins has not been explored in previous studies.

The objective of this study was to investigate the complexation interactions between WPI or heated WPI and OSA-modified starch as a function of pH (7–3) using absorbance measurements (at 515 nm) and dynamic light scattering and to characterize the structure of the soluble complexes formed under the optimum complexation conditions.