Structure and property engineering of α-D-glucans synthesized by dextransucrase mutants.

Seven dextran types, displaying from 3 to 20% α(1→3) glycosidic linkages, were synthesized in vitro from sucrose by mutants of dextransucrase DSR-S from Leuconostoc mesenteroides NRRL B-512F, obtained by combinatorial engineering. The structural and physicochemical properties of these original biopolymers were characterized. When asymmetrical flow field flow fractionation coupled with multiangle laser light scattering was used, it was determined that weight average molar masses and radii of gyration ranged from 0.76 to 6.02 × 10(8) g·mol(-1) and from 55 to 206 nm, respectively. The ν(G) values reveal that dextrans Gcn6 and Gcn7, which contain 15 and 20% α(1→3) linkages, are highly branched and contain long ramifications, while Gcn1 is rather linear with only 3% α(1→3) linkages. Others display intermediate molecular structures. Rheological investigation shows that all of these polymers present a classical non-Newtonian pseudoplastic behavior. However, Gcn_DvΔ4N, Gcn2, Gcn3, and Gcn7 form weak gels, while others display a viscoelastic behavior that is typical of entangled polymer solutions. Finally, glass transition temperature T(g) was measured by differential scanning calorimetry. Interestingly, the T(g) of Gcn1 and Gcn5 are equal to 19.0 and 29.8 °C, respectively. Because of this low T(g), these two original dextrans are able to form rubber and flexible films at ambient temperature without any plasticizer addition. The mechanical parameters determined for Gcn1 films from tensile tests are very promising in comparison to the films obtained with other polysaccharides extracted from plants, algae or microbial fermentation. These results lead the way to using these dextrans as innovative biosourced materials.

Role of vacuum steps added before and after steaming treatment of maize starch. Impact on pasting, morphological and rheological properties.

Standard maize starch (SMS) was hydrothermally treated by three processes; DV-HMT (Direct Vapor-Heat Moisture Treatment), RP-HMT (Reduced-Pressurized Heat Moisture Treatment) and DIC (in French: Détente Instantanée Contrôlée, instantaneous controlled pressure drop). Impact of processes were studied in order to determine the role of added steps of vacuum before and after treatment of starch by live steam at different pressures (1, 1.5, 2, 2.5 and 3 bar) on morphological, pasting (Brabender) and rheological properties of SMS suspension. The three treatments tend to modify the physical properties of starch on the same way, but the extent of these modifications depends on the process and on the level presure applied. The intensity of modifications follows this order: DIC>RP-HMT>DV-HMT. This was attributed to the presence of the initial vacuum step (DIC and RP-HMT) which intensified the transfer phenomena and to the mechanical effect induced by the abrupt decompression towards vacuum (DIC) leading to weakness of starch granules. The treatments changed the shape and distribution size of granules. The sizes were shifted towards higher values after the treatment due both to the improvement of swelling capacity of granules and to the presence of agglomerates particles of different sizes as confirmed by scanning electron microscopy. The agglomerates glued together by gelatinized granules were favored by the direct contact of starch with steam during the treatments. The results showed for all treatments, a reduction of the consistency coefficient (k) and of the yield stress (τ0) of starch suspensions with increasing of process intensity. For severe conditions (3 bar), no difference between the treatments was observed; a complete fluidization of starch suspensions (the consistencies were too weak to be detected), τ0 vanished and the rheological behavior tended to a Newtonian type. Elastic modulus (G'), measured during gelation at 25°C, decreased dramatically (G'<1 Pa), that revealed the loss of rigidity and disappearance of granular integrity of starch.

Structure and mechanical properties of hydroxypropylated starch films.

Films of acid-hydrolyzed hydroxypropylated pea starch with average molecular weight M w ranging from 3.3 x 10 (4) g/mol to 1.6 x 10 (6) g/mol were prepared from 25% (w/w) solution by casting. The structure of the films was investigated by means X-ray diffraction and calorimetry, evidencing a B-type crystalline structure. In similar drying conditions, 25 degrees C and 40% of relative humidity, the crystallinity varied from 24% for the low molecular weight (A5) to almost none for the highest molecular weight (A160). The influence of the drying temperature was also investigated. A reduction of the crystallinity from 16% to almost none was found when increasing temperature from 25 to 65 degrees C. The glass transition temperature ( T g) at different water contents was determined. The difference of T g between the first and the second scan was interpreted by changes in the water distribution between phases into the B-type crystalline structure. Mechanical properties of the films determined by tensile tests and by DMTA in the glassy state showed no effect of the average molecular weight or of crystallinity. In contrast, thermomechanical experiments by DMTA showed that the average molecular weight of the sample influenced the mechanical relaxation and the moduli in the rubbery state.

Impact of destroying the structure of model gels on volatile release.

The release of a strawberry aroma from different composite gels taken as models of fruit preparations and from a sucrose solution was investigated. The composition of the model systems differed with regard to the gelling agent, either pectin or carrageenan, and to the rigidity of the gel. With the use of atmospheric pressure chemical ionization-mass spectrometry, the release profiles of the aroma compounds were determined under stirring. At the same time, purge and trap measurements were performed to determine the release profiles of the aroma compounds without stirring. The comparison of the patterns obtained using these two complementary methods made it possible to determine how the structure of the matrix, the mechanical treatment, and the properties of the aroma compound affect aroma release. A far greater proportion of the aroma compounds was retained in the fruit preparation systems than in the sucrose solution. The different release profiles could be interpreted in terms of the volatility of the aroma compounds and of their diffusion through the gels.

Influence of ingredients on the self-diffusion of aroma compounds in a model fruit preparation: an nuclear magnetic resonance-diffusion-ordered spectroscopy investigation.

Diffusion-ordered spectroscopy (DOSY)-pulsed field gradient (PGF) nuclear magnetic resonance (NMR) spectroscopy was used to measure self-diffusion coefficients of aroma molecules in model fruit preparations. The impact of the sucrose content on aroma diffusion was specifically investigated, and the relationship with viscosity, water activity, and dry matter parameters was evidenced. DOSY-PGF NMR spectroscopy was found to be a relevant and accurate technique to follow self-diffusion of aroma compounds at low concentrations in a complex food matrix and to obtain information on diffusion of the sucrose and of the water molecules. We showed that aroma self-diffusion was strongly decreased in fruit preparation because of the high sucrose content, which induces the formation of a network through hydrogen bonds with water. Self-diffusion coefficients were determined for aroma molecules of different natures, and values are related to the physicochemical properties of the molecule.

Influence of the NaCl or CaCl2 concentration on the structure of heat-set bovine serum albumin gels at pH 7.

The structure of heat-set systems of the globular protein bovine serum albumin (BSA) was investigated at pH 7 in different salt conditions (NaCl or CaCl(2)) using light scattering. Cross-correlation dynamic light scattering was used to correct for multiple scattering from turbid samples. After heat treatment, aggregates are formed whose size increases as the protein concentration increases. Beyond a critical concentration that decreases with increasing salt concentration, gels are formed. The heterogeneity and the reduced turbidity of the gels were found to increase with increasing salt concentration and to decrease with increasing protein concentration. The structure of the gels is determined by the strength of the repulsive electrostatic interactions between the aggregated proteins. The results obtained in NaCl are similar to those reported in previous studies for other globular proteins. CaCl(2) was found to be much more efficient in reducing electrostatic interactions than NaCl at the same ionic strength.

Arabinoxylan gels: impact of the feruloylation degree on their structure and properties.

Arabinoxylan (AX) samples of decreasing ferulic acid (FA) contents were chemically prepared from water-extractable wheat arabinoxylans without affecting their other structural properties. Gels were obtained from these partially feruloylated WEAX (PF-WEAX) by enzymatic covalent cross-linking of FA leading to the formation of diferulic (di-FA) and tri-ferulic acid (tri-FA). WEAX gelling ability was found related to the WEAX FA content whereas the gel structure and properties depended on the density of newly formed covalent cross-links. FA content of WEAX ranging from 1.4 to 2.3 microg/mg AX gave gels with di-FA cross-links contents from 0.20 to 0.43 microg/mg AX and G' values from 5 to 44 Pa. For WEAX gels with initial FA contents from 1.6 to 2.3 microg/mg AX, average mesh size ranging from 331 to 263 nm were calculated from swelling experiments. Cross-linking densities of gels, determined from swelling experiments, were higher than those that could be theoretically estimated from the di-FA and tri-FA content of WEAX gels. This result suggests that, in addition to di-FA and tri-FA, higher ferulate cross-linking and physical entanglements would contribute to the final WEAX gel structure.

Heat-induced gelation of bovine serum albumin/low-methoxyl pectin systems and the effect of calcium ions.

Influence of low-methoxyl pectin (LM pectin) and calcium ions (3 mM) on mechanical behavior and microstructure of bovine serum albumin (BSA) gels (pH 6.8, in 0.1 M NaCl) was evaluated. Protein and LM pectin concentrations were fixed at 2, 4, and 8 wt % and 0.21, 0.43, and 0.85 wt %, respectively. Rheological measurements and confocal laser scanning microscopy coupled with texture image analysis by use of the co-occurrence method were performed. Heat treatment of BSA/LM pectin mixtures induced protein gelation and a phase separation process between the two biopolymers, which was kinetically trapped. Calcium ions induced pectin gelation and modified BSA gel properties. Depending on biopolymer concentrations, a balance between pectin and/or protein gel contribution on final gel strength exists. The microstructures of the mixed systems in the presence of calcium can be interpreted as interpenetrated structures. Texture image analysis allowed one to classify more precisely the different microstructures observed in relation with mechanical properties.

Release of isoamyl acetate from starch pastes of various structures: thermodynamic and kinetic parameters.

The release of isoamyl acetate from starch-based matrices was studied on the basis of a cumulative dynamic headspace analysis. Two corn starches were compared, which are known to yield pastes and gels differing in their structures and properties. These properties were assessed by viscometry and viscoelastic measurements. Aroma release was discussed as a function of the structure and texture parameters of the matrix. The release curves obtained from water and from the various starch-based matrices at 25 degrees C showed similar patterns but differed in their initial slopes and in the final plateau values. The lowest initial slopes were obtained for the normal starch dispersions that formed gels due to amylose gelation. The aroma compound was entirely released from water and from the waxy starch pastes. A significant amount of isoamyl acetate remained trapped in the normal starch dispersions.

Flavor release from iota-carrageenan matrices: a kinetic approach.

Release of aroma compounds in selected iota-carrageenan systems was studied by static headspace analysis. By varying the sodium chloride content, different rheological behaviors were obtained ranging from solution to gel. From the release curves, mass transfer (h(D)) and partition coefficients (K(ga)) of ethyl butanoate, ethyl hexanoate, and linalool were extracted using a mathematical model based on the penetration theory. This model, previously developed for flavor release from stirred solutions, was found to fit well the data obtained from structured systems (nonstirred conditions) at the beginning and at the end of the release curves: this allowed the determination of h(D) and K(ga). Matrix effects appeared to be dependent on the chemical class of the compounds. For the alcohol, the main effect on both equilibrium partitioning and mass transfer across the interface was ascribed to a salting effect. In the opposite, for esters, iota-carrageenan addition induced an increase of aroma retention and also a slower transfer across the interface. The respective effects of an increasing viscosity of the medium and of the formation of a tridimensionnal network are discussed.

Rheological properties of sulfoacetate derivatives of cellulose.

Water-soluble cellulose acetate sulfate derivatives (CAS) have been prepared through chemical reaction involving sulfuric acid as a catalyst. These CAS have been obtained from cellulosic materials of different origins (pure cellulose, wheat bran, maize bran) and their rheological behavior in salt-free aqueous solution has been estimated in dilute and semi-dilute regime using dynamic viscoelastic and viscosity measurements. Influence of concentration, temperature of solubilization and temperature of measurement has been investigated. Weak gel-like properties were exhibited at elevated concentration (typically above 7-8 g/L). These systems also exhibited thixotropic properties: the structure was partly broken down upon shearing and recovered at rest. They also displayed thermoreversibility with large hysteresis, the melting temperature being approximately 15 degrees C higher than the temperature at which gelation took place. These overall observations clearly indicate that these distinctive properties arise from intermolecular association of the macromolecular chains of the cellulose derivative.