Microstructure formation by sodium alginate in absence of salts as function of concentration, temperature and storage time

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1 Available online at Journal of Agroalimentary Processes and Technologies 14 (2008) Journal of Agroalimentary Processes and Technologies Microstructure formation by sodium alginate in absence of salts as function of concentration, temperature and storage time Monica Mironescu, Vionela Mironescu, Cecilia Georgescu Faculty of Agricultural Sciences, Food Industry and Environmental Protection, Lucian Blaga University of Sibiu, Romania Abstract In this paper, the formation of microstructure by a food polysaccharide, sodium alginate, is investigated. Optical microscopy is used to analyse the microstructural changes under the influence of concentration, temperature and storage time. The results reveal the formation of structure after a bottom-up strategy, in three phases, from small clusters to elongated aggregates and to biofilms. The structure formation is strongly influenced by the polysaccharide concentration, temperature and storage time. Keywords: microstructure, sodium alginate, bottom-up 1. Introduction Biopolymers are industrially used in the food industry as thickeners, stabilisers, fat substitutes, carriers of nutritional and taste compounds, structural components (Loren and Hermansson, 2003). Examples from such biopolymers are polysaccharides from vegetal, algal or microbial origin, as starch, alginates or xanthan, which are used in various food products as thickening, stabilising and gel-forming agents (Linden and Lorient, 1999) (Belitz and Grosch, 1999). These characteristics are due to the abilities of biopolymers to form specific structures under certain conditions. Structure formation in food can be approached in two ways: top-down and bottom-up (Leser et al., 2003). The first structuring approach refers to the brokendown of large (infinite) structures into smaller ones mostly on the micrometer length scale.the bottom-up strategy refers to the spontaneous formation of selfassembly nanostructures or higher hierarchy structures; this approach was adopted from biological systems by nonfood materials fabrication and recently by the food nano-technologists (Leser et al., 2003).The main process that occurs in the bottom-up process is the colloidal aggregation. Aggregation is the generic term describing the process whereby two or more colloidal particles become joined together under the influence of interparticle colloidal interaction (Dickinson, 2003) in the presence of ions like pectin and alginates (Braccini and Perez, 2001) or gellan (Morris, 1991) or at the thermal treatment (Lee and Brant, 2002). In this paper the bottom-up mechanism for the structure formation in samples with different concentrations during heating and in time is studied. The polysaccharide chosen for this analysis was sodium alginate (NAA). NAA is obtained from algae and forms gels normally in the presence of divalent ions (Braccini and Perez, 2001). Because not all food products contain calcium or magnesium to form gels with alginates, it was considered important to analyse the structure formation and the structural changes during the thermal treatment of aqueous solutions of NAA without ions. Corresponding author: address: monica.mironescu@ulbsibiu.ro

2 2. Materials and methods For the analysis of the influence of heating on structure formation, aqueous solutions of NAA with three concentrations (0.25%, 0.5% and 1%) were used. The samples were heated in flasks under continuous agitation using a heating rate of 2 C/min. The structure formation in various conditions was analysed through optical microscopy using basic fuxine for staining. The digital images obtained were processed with the image manipulation program GIMP 2, as presented elsewhere (Mironescu and Mironescu, 2006). 3. Results and discussions The structures obtained at the thermal treatment of NAA solutions with three concentrations are presented in Figure 1. The results show both the influence of temperature and concentration on the microstructure formation. At room temperature (20 C), NAA is found mostly in solubilised form. It can be observed that with the increase of alginate concentration, some small aggregates are formed. It is also possible that the aggregates existence to be due to the inefficient solubilisation of biopolymer in water. The bottom-up mechanism is proven during the thermal treatment of NAA. In aqueous solutions of NAA, the polysaccharide passes from the soluble state in a solid state. The increase of temperature gives the NAA particles the possibility to interact and to form clusters (or aggregates). The formation of colloidal aggregates is probably due to the decrease of viscosity at the increase of temperature, combined with continuous agitation. The aqueous solutions with 0.25% NAA give linear clusters which aggregate at the increase of temperature. In the samples with higher polysaccharide concentration (0.5%), a fibrillar structure which includes aggregates is obtained, whereas the aqueous solutions with 1% NAA heated at 70 C gives a stable biofilm. Every collision between two clusters results in the formation of a new aggregate. The number and the size of aggregates increase during the resting time, which offer to the aggregates enough time to form bounds. As Figure 1 shows, NAA particles go through three phases: - In the first phase small clusters are formed; - In the second phase clusters interact to form elongated aggregates; - In the third phase the elongated aggregates are connected to form biofilms. Figure 2 presents the evolution of the microstructure in time in samples with 0.5% NAA. The maintenance of samples favours the connection of aggregates produced at the thermal treatment and the obtaining of bigger aggregates. 4. Conclusions Sodium alginate forms structure using the bottom-up mechanism during the thermal treatment, with the formation of colloidal aggregates with different shapes and dimensions, depending on all the analysed parameters. The aqueous solutions with 0.25% NAA form clusters which form aggregates at the increase of temperature. In the samples with higher polysaccharide concentration (0.5%), a fibrillar structure which includes aggregates is obtained, whereas the aqueous solutions with 1% NAA heated at 70 C give biofilms. 35

3 (a) 20 C, 0.25% (b) 20 C, 0.5% (c) 20 C, 1% (d) 40 C, 0.25% (e) 40 C, 0.5% (f) 40 C, 1% (g) 50 C, 0.25% (h) 50 C, 0.5% (i) 50 C, 1% (j) 60 C, 0.25% (k) 60 C, 0.5% (l) 60 C, 1% (m) 70 C, 0.25% (n) 70 C, 0.5% (o) 70 C, 1% Figure 1: Structure formation in an aqueous NAA solution with three concentrations during heating 36

4 80 C, initial 80 C, after 24 h 70 C, initial 70 C, after 24 h 60 C, initial 60 C, after 24 h 50 C, initial 50 C, after 24 h 20 C, initial 20 C, after 24 h Figure 2. Structure formation at the thermal treatment of 0.5% NAA solutions and after rest for 24 hours 37

5 References Belitz, H.D., Grosch, W. (1999). Food chemistry, Springer Verlag Berlin Heidelberg. Braccini, I., Perez, S. (2001). Molecular basis of Ca 2+ induced gelation in alginates and pectins: the egg-box model revisited, Biomacromolecules, 2, p Dickinson E. (2003). Colloidal aggregation: Mechanisms and implications, in Food Colloids, Biopolymers and Materials. E. Dickinson and T. Van Vliet, eds. Cambridge, England: Royal Society of Chemistry, p Lee, H-C, Brant, D. (2002). Rheology of concentrated isotropic and anisotropic xanthan solutions. 3. Temperature dependence, Biomacromolecules, 3, p Leser, M.E., Michel, M., Watzke, H.J. (2003). Food Goes Nano -New horizons for food structure research, in Food Colloids, Biopolymers and Materials. E. Dickinson and T. Van Vliet, eds. Cambridge, England: Royal Society of Chemistry, p Linden, G., Lorient, D. (1999). New ingredients in food processing, Woodhead Publishung Ltd., Cambridge, England. Loren N., Hermansson, A-M. (2003). Structure evolution during phase separation and gelation of biopolymer mixtures, in Food Colloids, Biopolymers and Materials. E. Dickinson and T. Van Vliet, eds. Cambridge, England: Royal Society of Chemistry, p Mironescu M., Mironescu V. (2006) New concept for the obtention of biopolymers-based food biofilms, Journal of agroalimentary processes and technologies, XII, no. 1, Morris, E.R. (1991). Pourable gels: polysaccharides that stabilise emulsions and dispersions by physical trapping, IFI, 1, p

3 rd Food Emulsions Short Course November 13 th & 14 th, 2008 University of Massachusetts. David Julian McClements University of Massachusetts

3 rd Food Emulsions Short Course November 13 th & 14 th, 2008 University of Massachusetts David Julian McClements University of Massachusetts Program Objectives Review Basic Principles of Emulsion Science