H. Carrillo-Navas

Acid hydrolysis of native corn starch: Morphology, crystallinity, rheological and thermal properties

The acid hydrolysis of native corn starch at 35 °C was monitored during 15 days. After this time, the residual solids were about 37.0 ± 3.0%. First-order kinetics described the hydrolysis data, giving a constant rate of kH = 0.18 ± 0.012 days(-1). Amylose content presented a sharp decrement of about 85% and X-ray diffraction results indicated a gradual increase in crystallinity during the first 3 days. SEM micrographs showed that hydrolysis disrupted granule morphology from an initial regular shape to increasingly irregular shapes. Fractal analysis of SEM images revealed an increase in surface roughness. Fast changes in the thermal effects were caused by molecular rearrangements after fast hydrolysis of amylose in the amorphous regions in the first day. Steady shear rate and oscillatory tests showed a sharp decrease of the apparent viscosity and an increase of the damping factor (tan(δ)) caused by amylose degradation.

In vitro digestibility, physicochemical, thermal and rheological properties of banana starches.

Banana starches (BS) were isolated from Enano, Morado, Valery and Macho cultivars. The BS possessed B-type crystallinity and an amylose content varying from 19.32 to 26.35%. Granules had an oval morphology with different major-to-minor axis ratios, exhibiting both mono- and bi-modal distributions and mean particle sizes varying from 32.5 to 45 μm. BS displayed zeta-potential values ranging between -32.25 and -17.32 mV, and formed gels of incipient to moderate stability. The enthalpy of gelatinization of BS affected the crystalline order stability within the granules. In-vitro digestibility tests showed fractions as high as 68% of resistant starch. Rheological oscillatory tests at 1 Hz showed that BS dispersions (7.0%, w/w) exhibited Type III behaviour, attributed to the formation of a continuous phase complex three-dimensional amylose gel reinforced by swollen starch granules acting as fillers. Amylose content and granules morphology were the main factors influencing the BS properties.

Sorption isotherms, thermodynamic properties and glass transition temperature of mucilage extracted from chia seeds (Salvia hispanica L.)

Freeze-dried chia mucilage adsorption isotherms were determined at 25, 35 and 40°C and fitted with the Guggenheim-Anderson-de Boer model. The integral thermodynamic properties (enthalpy and entropy) were estimated with the Clausius-Clapeyron equation. Pore radius of the mucilage, calculated with the Kelvin equation, varied from 0.87 to 6.44 nm in the temperature range studied. The point of maximum stability (minimum integral entropy) ranged between 7.56 and 7.63kg H2O per 100 kg of dry solids (d.s.) (water activity of 0.34-0.53). Enthalpy-entropy compensation for the mucilage showed two isokinetic temperatures: (i) one occurring at low moisture contents (0-7.56 kg H2O per 100 kg d.s.), controlled by changes in water entropy; and (ii) another happening in the moisture interval of 7.56-24 kg H2O per 100 kg d.s. and was enthalpy driven. The glass transition temperature Tg of the mucilage fluctuated between 42.93 and 57.93°C.

Impact of ghosts on the viscoelastic response of gelatinized corn starch dispersions subjected to small strain deformations

Corn starch dispersions (5.0% w/w) were gelatinized by heating at 90°C for 20 min using gentle stirring. Under these conditions, ghosts, which are insoluble material with high amylopectin content, were detected by optical microscopy. Strain sweep tests showed that the gelatinized starch dispersions (GSD) exhibited a loss modulus (G″) overshoot at relatively low strains (∼1%). In order to achieve a greater understanding as to the mechanisms giving rise to this uncharacteristic nonlinear response at low strains, very small constant torques (from 0.05 to 0.5 μN m) were applied in the bulk of the GSD with a rotating biconical disc. This resulted in small deformations exhibiting torque-dependent inertio-elastic damped oscillations which were subjected to phenomenological modelling. Inertial effects played an important role in the starch mechanical response. The model parameters varied with the magnitude of constant small applied torque and could be related to microstructural changes of ghosts and to the viscoelastic response of GSD.

Effect of gelatinized flour fraction on thermal and rheological properties of wheat-based dough and bread

This work considered gelatinized wheat flour fraction with properties similar to hydrocolloid to enhance the strength of dough network by improving water retention and rheological characteristics. The gelatinized (90 °C) fraction of the wheat flour was incorporated in the dough formulation at different levels (5, 10, and 20% w/w). The effects of the gelatinized flour (GF) fraction on the dough rheology and thermal properties were studied. The incorporation of GF induced a moderate increase of dough viscoelasticity and reduced the freezing and melting enthalpies. On the other hand, the changes in bread textural properties brought by incorporation of GF were insignificant, indicating that the gelatinized fraction acted as a binder that enhanced water trapping in the structure. SEM images showed a more heterogeneous crumb microstructure (e.g., gas cells, porous, etc.) bread prepared using GF. Drying kinetics obtained from TGA indicated that the water diffusivity decreased with the incorporation of GF, which suggested that the bread had a compact microstructure.