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The effect of pH on gel structures produced using protein–polysaccharide phase separation and network inversion

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Abstract

Forming heat-induced gels through combined effects of micro-phase separation of whey protein isolate (WPI; 5%, w/v, 100 mm NaCl) by pH change (5.5, 6.0, and 6.5), and addition of κ-carrageenan (0–0.3%, w/w), were evaluated. The microstructure of WPI gels was homogeneous at pH 6.0 and 6.5 and micro-phase separated at pH 5.5. Addition of 0.075% κ-carrageenan to WPI solutions caused the microstructure of the gel to switch from homogeneous (pH 6.0 and 6.5) to micro-phase separated; and higher concentrations led to inversion of the continuous network from protein to κ-carrageenan. Protein solutions containing 0.075% (w/w) κ-carrageenan produced gels with increased storage modulus (G′) at pH 6.5 and decreased G′ at pH 5.5. All gels containing 0.3% (w/w) κ-carrageenan had κ-carrageenan-continuous networks. It was shown that microstructural and rheological changes were different in WPI and κ-carrageenan mixed gels when micro-phase separation was caused by pH rather than ionic strength.

Introduction

The ability to design food structures is important to producing healthy foods (Norton & Norton, 2010). One of the most common ways to adjust food structure and produce a wide range of textures is by changing the amount and types of proteins and polysaccharides (van den Berg, Rosenberg, van Boekel, Rosenberg, & van de Velde, 2009). This is because biopolymer mixtures can be formulated to produce single-phase or phase separated conditions (Tolstogouzov & Braudo, 1983).

Solution conditions determine the degree and type of phase separation and rheological behavior of whey proteins–κ-carrageenan mixed gels (de Jong, Klok, & van de Velde, 2009; de Jong & van de Velde, 2007). In our previous investigation, we increased ionic strength to cause protein micro-phase separation at pH 7.0 (Çakır & Foegeding, 2011). Increasing κ-carrageenan concentration in WPI solutions under single-phase conditions caused progressive microstructural transitions from homogeneous to protein continuous to bicontinuous to carrageenan continuous. Alternatively, microstructure moves from particulate to coarse stranded to carrageenan continuous under micro-phase separated conditions (Çakır & Foegeding, 2011). The microstructures were associated with distinctive sensory texture properties (Çakır et al., 2012).

Micro-phase separation of whey proteins is also caused when the pH approaches the isoelectric point (Ako, Nicolai, Durand, & Brotons, 2009). Ould Eleya and Turgeon (2000) investigated rheological properties of heat-set β-lactoglobulin and κ-carrageenan mixed gels between pH 4.0 and 7.0; suggesting similarity between pH 5.0 and 7.0, but a different gel at pH 4.0. Their study was limited to one polymer combination (10% β-lactoglobulin + 1% κ-carrageenan) and did not measure changes in microstructure.

The objective of this study was to determine how pH-induced micro-phase separation of whey proteins alters the effect of κ-carrageenan on gel structure and rheological properties.

Section snippets

Materials

Whey protein isolate (WPI) Bipro™ (93.37% protein, dry basis, nitrogen × 6.38) was obtained from Davisco Foods International, Inc. (Le Sueur, MN, USA). GENUGEL® κ-carrageenan (CHP-2) was donated by CP Kelco Inc. (Lille Skensved, Denmark). Respective WPI and κ-carrageenan mineral contents (w/w), determined by inductively coupled plasma atomic emission spectroscopy, were (i) 0.08% P, <0.005% K, 0.07% Ca, <0.005% Mg, 1.70% S, and 0.0008% Na (w/w), and (ii) 0.002% P, 16.93% K, 0.06% Ca, 0.12% Mg,

Microstructure

Whey protein gels at pH 6.0 and 6.5 formed homogenous networks (i.e., network strands were not visible at this length scale) (Fig. 1). At pH 5.5, under micro-phase separation (Ako et al., 2009), a particulate network formed. The addition of 0.075% κ-carrageenan to WPI caused a mild degree of phase separation in the gels, resulting in a coarsening of the protein network, with structures similar to a particulate network when ionic strength is used to induce micro-phase separation (Çakır &

Conclusions

Micro-phase separation of whey proteins is caused by decreasing electrostatic repulsion among molecules due to solution pH and ionic strength (Ako et al., 2009). This investigation showed that microstructural and rheological properties are different if pH, rather than ionic strength (Çakır & Foegeding, 2011), is used to induce micro-phase separation when combined with effects of κ-carrageenan. Two mechanisms were involved in reinforcement of gel rigidity: a segregative phase separation at high

Acknowledgments

Support from the North Carolina Agricultural Research Service and USDA-NRI competitive grant program (grant number: 2008-35503-18682) is gratefully acknowledged. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named nor criticism of similar ones not mentioned. The Cellular and Molecular Imaging Facility at North Carolina State University and the assistance provided by Dr. Eva Johannes is acknowledged.

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Present address: Fonterra Co-operative Group Limited, Private Bag 11029, Palmerston North 4442, New Zealand.

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