Katerina Alba

Engineering of acidic O/W emulsions with pectin

Pectins with distinct molecular design were isolated by aqueous extraction at pH 2.0 or 6.0 and were examined in terms of their formation and stabilisation capacity of model n-alkane-in-water emulsions at acidic pH (pH 2.0). The properties and stability of the resulting emulsions were examined by means of droplet size distribution analysis, Lifshitz-Slyozov-Wagner modelling, bulk rheology, interfacial composition analysis, large-amplitude oscillatory surface dilatational rheology, electrokinetic analysis and fluorescence microscopy. Both pectin preparations were able to emulsify alkanes in water but exhibited distinct ageing characteristics. Emulsions prepared using pectin isolated at pH 6.0 were remarkably stable with respect to droplet growth after thirty days of ageing, while those prepared with pectin isolated at pH 2.0 destabilised rapidly. Examination of chemical composition of interfacial layers indicated multi-layered adsorption of pectins at the oil-water interface. The higher long-term stability of emulsions prepared with pectin isolated at high pH is attributed to mechanically stronger interfaces, the highly branched nature and the low hydrodynamic volume of the chains that result in effective steric stabilisation whereas acetyl and methyl contents do not contribute to the long-term stability. The present work shows that it is possible by tailoring the fine structure of pectin to engineer emulsions that operate in acidic environments.

Influence of pH on mechanical relaxations in high solids LM-pectin preparations

The influence of pH on the mechanical relaxation of LM-pectin in the presence of co-solute has been investigated by means of differential scanning calorimetry, ζ-potential measurements and small deformation dynamic oscillation in shear. pH was found to affect the conformational properties of the polyelectrolyte altering its structural behavior. Cooling scans in the vicinity of the glass transition region revealed a remarkable change in the viscoelastic functions as the polyelectrolyte rearranges from extended (neutral pH) to compact conformations (acidic pH). This conformational rearrangement was experimentally observed to result in early vitrification at neutral pH values where dissociation of galacturonic acid residues takes place. Time-temperature superposition of the mechanical shift factors and theoretical modeling utilizing WLF kinetics confirmed the accelerated kinetics of glass transition in the extended pectin conformation at neutral pH. Determination of the relaxation spectra of the samples using spectral analysis of the master curves revealed that the relaxation of macromolecules occurs within ∼ 0.1s regardless of the solvent pH.

Mesoscopic structure of pectin in solution.

Mesoscopic structure of pectin with different molecular characteristics was investigated by means of small angle X‐ray scattering (SAXS), electrokinetic measurements and data modelling. The influence of a broad range of pH (2–7) on chain conformation in the dilute and semi‐diluted regime was investigated. Scattering data and concomitant analysis revealed two length scales at all environmental conditions studied. pH showed greater influence at acidic values (pH 2.0) enhancing the globular component of the structure due to association of galacturonic acid residues. Double logarithmic scattering intensity plots revealed fractal dimensions of 1.9 ± 0.2 in the low‐q regime and 1.5 ± 0.2 in the high q‐region, irrespectively of the specific environment. Increase in branching of RG‐I regions of the polysaccharide chains enhanced the compact conformation irrespectively of the pH or concentration. The present work shows that radical changes in pectin conformation can be induced only under strongly acidic conditions a finding that has important consequences in tailoring the technological performance of these biopolymers.

Seaweed Polysaccharides (Agar, Alginate Carrageenan)

Seaweed polysaccharides are widely utilized in the food industry and are of particular technological importance due to their broad spectrum of functionality with particular emphasis on gelation and viscosity enhancement. The functionality is adjusted with great precision by modifying the fine structure of polysaccharides or changing solvent conditions (e.g., pH, ionic strength, etc.). The most industrially relevant types of seaweed polysaccharides are carrageenan, agar, and alginate. Carrageenan and agar are galactans whereas alginate consists of mannuronic and guluronic acid residues. Seaweed polysaccharide backbones are frequently functionalised with various groups (e.g., sulphate, methoxy, etc.) and all three carry negative charges. Consequently, they have the ability to interact to a variable extend with cations resulting in formation of gels with properties that cover the entire range of the industrially relevant thermo-mechanical spectrum. Seaweed polysaccharides are essential tools for texture modification, colloidal stabilization, fat reduction, and shelf-life extension as well as creation of new products and find applications across food industry.

Pectin Conformation in Solution

The interplay of degree of methylesterification (DM), pH, temperature, and concentration on the macromolecular interactions of pectin in solution has been explored. Small-angle X-ray scattering complemented by atomic force microscopy and molecular dynamics was employed to probe chain dimensions and solution structure. Two length scales have been observed with the first level of structure characterising chain clusters with sizes ranging between 100-200 nm. The second level of structure arises from single biopolymer chains with a radius of gyration between ∼6 and 42 nm. The development of a range of macromolecular dimensions in vitro and in silico shows that the chain flexibility increases with DM and at acidic pH, whereas hydrogen bonding is the responsible thermodynamic driving force for cluster formation. High methyl pectins create structures of lower fractal dimension with less efficient packing. This work unveils pectin conformations covering most of its industrially and biologically relevant environments, enabling rational design of advanced biomaterials based on pectin.

Coarsening Mechanisms of Alkane-in-Water Okra Pectin Stabilized Emulsions

The influence of emulsifier type (okra pectin extracted at low and high pH values) on coarsening mechanisms in acidic oil-in-water emulsions was studied. Ultrasound-assisted emulsification at pH 2.0 was employed in order to fabricate emulsions of smaller droplet sizes and improved stability on ageing. Emulsions were characterized by means of static light scattering, rheology and interfacial composition (e.g., adsorbed protein and pectin). Okra pectin extracted at higher pH showed greater emulsion stabilizing capacity than those extracted at lower pH values, as revealed by droplet size distribution analysis and the rate of destabilisation kinetics. Ostwald ripening was assigned as the governing destabilisation mechanism for emulsions formulated with pectin extracted at high pH. Emulsions stabilized with okra pectin extracted at low pH exhibited a rapid droplet growth, due to Ostwald ripening-induced coalescence as evidenced by the development of the DSD curves on storage. These findings show that okra polysaccharides demonstrate remarkable differences in their stabilizing properties and magnitudes of Ostwald ripening and coalescence events depending on the employed isolation protocol.