Bacterial cellulose as stabilizer of o/w emulsions
Graphical abstract
Introduction
Cellulose is the most abundant natural polysaccharide, being the major structural component of plants. It is a linear homopolymer of β(1-4)-d-glucose residues linked together by glycosidic bridges. Chemically modified cellulose is a food additive, which is mainly used for its gelling and thickening properties (McClements, 2005). The most common cellulose derivatives used in food are: carbomethyl cellulose (CMC) and hydroxypropyl methylcellulose (HPMC). Many studies have proven the stabilizing properties both of CMC (Arancibia et al., 2013, Ghanbarzadeh and Almasi, 2011, Hayati et al., 2011) and HPMC (Camino and Pilosof, 2011b, Camino et al., 2011a, Futamura and Kawaguchi, 2011, Schulz and Daniels, 2000, Wollenweber et al., 2000).
Nevertheless, it has been well known that bacteria (Komagataeibacter sucrofermentans DSM 15973) could synthesize cellulose (Martinez-Sanz et al., 2011, Okiyarna et al., 1992, Ougiya et al., 1997). Specifically, when being fermented in a culture rich in polysaccharides, these bacteria produce pellicles of bacterial cellulose (BC). The cellulose pellicle consists of random assembled fibrils, less than 100 nm wide (Okiyarna, Shirae, Kana, & Yamanaka, 1993). BC has distinctive advantages over traditional sources of cellulose such as plants even though they have the same chemical structure. In particular, BC has lower density, higher crystallinity, higher water holding capacity, higher mechanical strength due to its web-like network structure and higher purity as it is pure cellulose and does not associate with lignin or hemicelluloses (Iguchi, Yamanaka, & Budhiono, 2000). Thanks to these properties, bacterial cellulose fibrils are increasingly being used in various areas, such as biomedicine (Fu et al., 2013, Meftahi et al., 2010), paper industry (Chawla, Bajaj, Survase, & Singhal, 2009) and many others. Its applications in the food industry are recently investigated. BC could be used to improve rheology of food as a thickening, stabilizing or gelling agent. Also BC could produce low-calorie and low-cholesterol foods (Shi, Zhang, Phillips, & Yang, 2014).
Solid colloidal particles have been shown to accumulate at the interface between two immiscible liquids and stabilize the emulsion drops against coalescence by forming a mechanically robust monolayer at the liquid–liquid interface. These emulsions are typically referred to as Pickering emulsions (Pickering, 1907). Good mechanical properties and high resistance to coalescence are only a few of Pickering emulsion properties (Chevalier & Bolzinger, 2013). Cellulose fibrils have recently being used as particles to produce Pickering emulsions (Kalashnikova, Bizot, Cathala, & Capron, 2011). However, little research has been done on the stabilizing properties of bacterial cellulose (Ougiya et al., 1997). Nevertheless, there is no report on the effect of preparation conditions of the emulsions stabilized by bacterial cellulose. Most studies investigated the mechanism of the stabilizing effect of microfibrillated cellulose (Andresen and Stenius, 2007, Winuprasith and Suphantharika, 2013, Xhanari et al., 2011) or microcrystalline cellulose (Adeyeye et al., 2002, Oza and Frank, 1986).
Hence, the objective of the present study was to investigate the preparation and characterization of olive oil-in-water (o/w) emulsions stabilized by bacterial cellulose (BC) taking into account the factors that influence the properties of these emulsions, such as cellulose concentration and emulsification method (high shear or ultrasonication), pH, temperature and ionic strength. Also, the emulsion stabilizing properties of BC was compared with that of commercial celluloses (HPMC and CMC). The emulsion characterization was carried out by optical microscopy, dynamic light scattering, multiple light scattering and rheology in an attempt to investigate the stabilization mechanisms which took place.
Section snippets
Materials
HPMC Tylopur SE-15 and SE-4000 (food grade) were obtained from SE Tylose GmbH & Co. KG (Wiesbaden, Germany). Cellulose derivatives differed in viscosity which was 15 and 4000 mPas respectively at 20 °C of 2% wt aqueous solutions. They were then coded as HPMC L and HPMC H referring to their low (L) and high (H) viscosity. The molecular weight was 30.000 g/mol, and 300.000 g/mol respectively, whereas the degree of substitution was MeO = 28.0–30.0 M % and HPO = 7.0–12.0 M % for both of them
Phase separation
To evaluate the stability of the different emulsions upon storage, phase separation for emulsions of various cellulose concentrations and types was recorded for a 7 days period, and is presented in Fig. 1a–d.
From this figure it can be conducted that the higher the concentration of cellulose used to stabilize the emulsions was, the lower the phase separation was. Specifically, by increasing cellulose concentration from 0.1 to 1% wt, all the emulsions became more stable against coalescence. As
Conclusions
In the present work, the stabilizing properties of bacterial cellulose in o/w emulsions and its differences between emulsions stabilized by commercial celluloses (HPMC, CMC) were studied. BC showed better emulsifying capability compared to HPMC and CMC as its emulsions separated with a slower rate. This higher stability is due to the flocs of BC fibrils which adsorbed to the surface of the oil droplet and formed a strong network. This strong network could be evidenced by the high yield stress
Acknowledgments
This work is part of the “Nonastru” project (11SYN-2-718), implemented within the National Strategic Reference Framework (NSRF) 2007–2013 and co-financed by National (Greek Ministry - General Secretariat of Research and Technology) and Community Funds (E.U.-European Social Fund).
Authors would like to thank F Gutkind& Co Ltd (Oxon, United Kingdom) for donating CMC samples as well as Stamatios Tsimas and Anastasia Georgiadou from National Technical University of Athens (NTUA) for the
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