Martine Le Meste

Towards an improved understanding of glass transition and relaxations in foods: molecular mobility in the glass transition range

Abstract Recent research has contributed to a better understanding of the glass–liquid transition (GLT) and its relationship with relaxation processes in the material. This paper reviews models and theories that are currently used to describe and explain the physical changes in the GLT temperature range ( T g ); ageing below T g , changes in mechanical properties above T g , and the concept of fragility are described. Measurements of the GLT temperature are now practised routinely in many food laboratories, but lack of information on the experimental conditions may lead to discrepancies between results. Several examples from the food domain are reported, illustrating that the GLT has been mainly used to interpret, with more or less success, changes in low moisture foods and biomaterials. Taking the temperature of GLT into consideration alone cannot sufficiently explain changes as a function of temperature or water content, particularly when chemical/biochemical reactions are concerned. The relationship between molecular mobility and the GLT is discussed. More measurements of the various types of molecular motions are necessary, specially in close vicinity to the GLT and in the glassy state.

Effect of Small and Large Wheat Starch Granules on Thermomechanical Behavior of Starch

ABSTRACT The physicochemical properties of small- and large-granule wheat starches were investigated to reveal whether gelatinization properties and rheological behavior differ between size classes of wheat starch. All samples contained 60% water (w/w, wb). The starch granule size and shape were examined by scanning electron microscopy in the separated A- and B-type granule populations and in the whole wheat starch granule population. Differential scanning calorimetry (DSC) and electron spin resonance (ESR) analyses were performed in parallel with rheological measurements using dynamic mechanical thermal analysis (DMTA) to relate the viscoelastic changes to modifications in dynamic properties of aqueous solutions and structural disorganization of starch. The small (B-type) granules had slightly higher gelatinization temperature and lower gelatinization enthalpy than did the large (A-type) granules. Also, B-type granules had higher enthalpy for the amylose-lipid complex transition. Moreover, our results su...

Molecular flexibility in wheat gluten proteins submitted to heating

Prolamin proteins are responsible for the network that gives wheat dough its viscoelastic properties. Non‐prolamin depleted gluten was prepared under conditions that preserve its functionality. Electron Spin Resonance (ESR) was used to provide information about the dynamics of the protein at temperatures between 5 and 90°C by specific spin labelling of its cysteine residues. The spectra were of a composite type, resulting from at least two populations of spin labels largely differing in molecular mobility. The correlation time of the less mobile nitroxide radicals was determined by saturation transfer ESR. Upon heating there was a transfer from the slow to the fast moving population of radicals, and an increase of mobility of this last catagory that followed the Arrhenius law. The effect of temperature on molecular flexibility was reversible. This was not the case for purified, polymerised glutenin subunits extracted from gluten. Urea created similar modifications on gluten as heat.

Rheological properties of soft wheat flour doughs: Effect of salt and triglycerides

ABSTRACT The small deformation rheological properties of wheat flour doughs in relation to their structure and hydration were studied by dynamic mechanical thermal analysis, differential scanning calorimetry, and electron spin resonance. The effect of salt and triglycerides was also examined and compared with results we obtained previously on starch dispersions. Moisture content was adjusted to 48 or 60% (w/w, wb). Samples contained 0–16% NaCl (g/100 g of flour-water) and 0–18% triolein or lard (g/100 g of flour-water). The obtained results suggested that starch has an active role in determining the evolution of dough rheological characteristics during heating. The main factors controlling rheological behavior during thermal treatment are the volume fraction and deformability of starch granules. Gluten changes the viscoelasticity of the continuous phase and competes with starch for water. The addition of sodium chloride to flour dispersions shifted the structural disorganization and rigidity increased dur...

Effect of triglycerides on gelatinisation and rheological properties of concentrated potato starch preparations

Abstract The effect of triglycerides on gelatinisation and rheological properties of potato starch was studied by calorimetric and rheological measurements, microscopy and electron spin resonance. Samples contained 45 or 48% water (w/w, wb) and the lipid content ranged from 0 to 20% (g/100 g starch–water). The addition of triglycerides had no effect on gelatinisation process and rheological behaviour of starch during heating. Triolein seems to form a separated dispersed phase in the system which does not prevent the hydration and swelling of starch granules and the melting of ordered regions, in our studied conditions. A lower value of the storage modulus (G′) of starch preparations at ambient temperature was detected in the presence of triglycerides. Shortenings could diminish bread firmness by a lubricant effect, without modifying retrogradation of starch. Triglycerides do not influence the changes in the water distribution during thermal treatment in the starch system. This behaviour has implications for the use of fat replacers in bakery products with a reduced fat content.

Temperature-induced phase change in a fat. A study by electron spin resonance

Electron Spin Resonance, Differential Scanning Calorimetry and rheological techniques have been used to study the physical changes induced by temperature in lard and in the solid and liquid fractions obtained by fractionation of lard at 15 C. The mobilization process of a C18 fatty acid nitroxide derivative dispersed in the molten fat has been observed in the temperature range −50 to +70 C.The mobilization of the probe seemed to be concomitant with the melting of the low melting point glycerides. Above this temperature, all the probes were in the liquid phase and their mobility reflected the viscosity of their liquid environment, or the viscosity of the bulk fat when crystal was no longer present.Probe mobility was temperature dependent, and it was identical for the three fats at the same temperature, despite their different triglyceride compositions.