Elsevier

Food Hydrocolloids

Volume 51, October 2015, Pages 193-199
Food Hydrocolloids

Heat treatment of calcium alginate films obtained by ultrasonic atomizing: Physicochemical characterization

https://doi.org/10.1016/j.foodhyd.2015.04.037Get rights and content

Highlights

  • Planar films of calcium alginate were obtained using an ultrasonic atomizing device.

  • Calcium gluconolactate was used as gelling agent.

  • Optimum dry films were subjected to heat treatment.

  • Heat treatment enhanced brittleness and developed an ochre color on the films.

  • Heat treatment promoted dehydration of the matrix and alginate chemical degradation.

Abstract

Planar films of calcium alginate were obtained using an ultrasonic atomizing device. Sodium alginate solutions of 0.6% and 0.9% (w/v) were nebulized with calcium gluconolactate solutions (gelling agent) of 0, 1, 2 and 3% (w/v) at a flow rate of 0.3 mL min−1 for 20 min. After drying, thickness and mechanical properties were determined. In view of the results of mechanical properties, manageability and flexibility, calcium alginate films obtained using 0.9% sodium alginate and 2% calcium gluconolactate were selected as “optimum dry film” samples. These samples were cut into rectangular pieces and heated at 180 °C for 0, 4, 8, 12, 20 and 24 min. Thickness, mechanical and optical properties, differential scanning calorimetry (DSC) thermograms, Fourier transform infrared spectroscopy (FTIR) spectra, and scanning electron microscopy (SEM) micrographs were analyzed in order to characterize the physicochemical properties of heat-treated samples. The heat treatment produced thickness reduction, a yellow ochre color development and an increase in the brittleness of the films. DSC, FTIR and SEM studies suggested that heat treatment produced further dehydration of dry films and thermal dehydration–degradation of alginate macromolecules.

Introduction

Sodium alginate is one of the most important polysaccharides used for hydrogels preparation (Draget, 2000). The hydrogel properties are typically controlled by alginate chemical microstructure determined by α-L-guluronic and β-D-mannuronic present in varying proportions and sequences, type of gelling ions and gelling conditions. Alginate gelation occurs when divalent cations (usually Ca2+) interact with blocks of guluronic residues. According to the “egg-box” model (Grant, Morris, Rees, & Smith, 1973), two contiguous, diaxially linked guluronic residues form a cavity that acts as a binding site for calcium ions. This binding induces chain–chain associations forming stable junction zones of dimers and lateral interactions between these dimers. As a result, the gel is formed and mechanical properties are directly related to the number of “egg-box” sites. Thus, the increase in network crosslink density results in a higher fracture stress.

The procedure of introducing gelling ions is an additional parameter influencing the properties of alginate hydrogels (Draget, 2000). The external gelling method consists in exposing alginate solution directly to the gelling ions solution and alginate hydrogel is irreversible formed due to ion diffusion. The second method, called internal gelling, is based on mixing an insoluble source of gelling ions with the alginate solution followed by releasing the gelling ions by lowering the pH value after addition of organic acids or by hydrolyzing lactones (Papajová, Bujdoš, Chorvát, Stach, & Lacík, 2012). When the external gelling method is used for preparation of planar alginate hydrogels, the almost instantly gelation of alginate produces a heterogeneous dispersion of gel lumps. In view of this problem, preparation of planar alginate hydrogels by external gelling requires slow rate of exposure of alginate solution to gelling ions in order to control gelling and hydrogel properties. This issue was tackled by exposing solution of sodium alginate to an aerosolized spray of Ca2+ solution (Cathell and Schauer, 2007, Papajová et al., 2012). However, these authors obtained small planar alginate hydrogels. One objective of this research was to develop an ultrasonic atomizing device that allows the preparation of calcium alginate films of larger surface areas, suitable for being used as edible films.

On the other hand, although gelation kinetics is altered by the source of calcium, neither the final alginate gel strength nor the resistance to calcium diffusion are modified (Lee & Rogers, 2012). CaCl2 reaches a gel strength plateau fastest, followed by calcium lactate and calcium gluconate. CaCl2 is the most usual source of calcium when the bitter taste can be masked and a fast throughput is required, while calcium organic salts may have an advantage when the membrane thickness/hardness needs to be manipulated. Calcium gluconolactate, a commonly food additive, is calcium gluconate mixed with calcium lactate. Other objective of the present work was to use calcium gluconolactate as the gelling agent for the formation of dry calcium alginate edible films.

In contrast to most gelling polysaccharides, alginate gels have the particular feature of being cold setting and are heat stable. In practice, this means alginate gels can be heat treated without melting. This is the reason why alginates are used in baking creams (Smidsrød & Draget, 2004) as an edible barrier to reduce fat uptake in fried foods (Albert & Mittal, 2002) and as an edible coating that improve the quality of microwaveable chicken nuggets (Albert, Salvador, & Fiszman, 2012). The last objective of this research was to study the physicochemical characteristics of dry calcium alginate films subjected to heat treatment.

Section snippets

Materials

Sodium alginate (SA) from brown algae (medium viscosity), calcium lactate hydrate (CLH) and calcium gluconate anhydrous (CGA) were purchased from Sigma–Aldrich (St. Louis, MO, USA). SA had an approximate mannuronic/guluronic ratio of 1.56, a degree of polymerization range of 400–600, and a molecular weight of 80,000–120,000. Solid CLH and CGA were mixed at a weight ratio of 4:1. This mixture was called calcium gluconolactate, CG. All other reagents were of analytical grade.

Preparation of dry alginate films by external gelling method

Solid SA and

Appearance and thickness of the films

Calcium alginate films obtained by the methodology described in Section 2.2. were visually homogeneous without brittle areas or bubbles. In addition, the films were easily manageable and flexible.

Fig. 2 shows the thickness of the films prepared from SA solutions of 0.6 and 0.9% (w/v) in presence of different CG concentrations in the nebulizer reservoir. Film thickness increased as the concentration of SA increased for all the CG concentrations assayed (p < 0.05), Fig. 2. As can also be seen in

Conclusions

Dry calcium alginate films of adequate size and mechanical properties, suitable to be used as edible films, were obtained using a novel device in which a sodium alginate solution was nebulised with a calcium gluconolactate solution as gelling agent. The dry films obtained were then subjected to heat treatment at 180 °C for different times and physicochemical characteristics of the resultant products were analyzed. Heat treatment produced thickness reduction, a yellow ochre color development and

Acknowledgments

This work was supported by grants from Universidad Nacional de Rosario (UNR) (1BIO 313), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIP 2013-2015 GI), Secretaría de Estado de Ciencia, Tecnología e Innovación de la Provincia de Santa Fe (SECTeI) (2010-061-12) and Agencia Nacional de Promoción Científica y Tecnológica de la República Argentina (ANPCyT) (PICT-2008-1308).

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