Gelatinization and freeze-concentration effects on recrystallization in corn and potato starch gels

Freeze-concentration of starch gels was controlled by temperature and gelatinization with glucose and lactose. The aim of the study was to evaluate the effects of freezing temperature and gel composition on starch recrystallization behaviour of corn and potato starch gels (water content 70%, w/w) in water or glucose or lactose (10%, w/w) solutions. Starch gels were obtained by heating in differential scanning calorimetry (DSC). Samples of starch gels were frozen at -10 degrees C, -20 degrees C and -30 degrees C for 24h and, after thawing, stored at +2 degrees C for 0, 1, 2, 4 and 8 days. The extent of starch recrystallization was taken from the enthalpy of melting of the recrystallized starch by DSC. Freezing temperatures, glucose, lactose and the origin of the starch affected the recrystallization behaviour greatly. The recrystallization of amorphous starch during storage was enhanced by freeze-concentration of gels at temperatures above T'(m). Molecular mobility was enhanced by unfrozen water and consequently molecular rearrangements for nucleation could take place. Further storage at a higher temperature enhanced the growth and the maturation of crystals. In particular, glucose decreased the T'(m) of the gels and consequently lower freezing temperatures were needed to reduce enhanced recrystallization during storage. Freeze-concentration temperatures also showed a significant effect on the size and the perfection of crystals formed in starch recrystallization.     

Starches from different botanical sources I: Contribution of amylopectin fine structure to thermal properties and enzyme digestibility

Fifteen starches from different botanical sources were selected to study the influence of structural features on thermal properties and enzyme digestibility. Morphological appearance, X-ray diffraction pattern, apparent amylose content, unit-chain length distribution of amylopectin, thermal properties and enzyme digestibility of starch varied with botanical source. It was demonstrated that the distribution of unit-chains of amylopectin significantly correlated with functional properties of the starches. Gelatinization temperature of native and retrograded starches decreased and increased with a relative abundance of unit-chains with an approximate degree of polymerization (DP) of 8–12 and 16–26, respectively (P<0.01). Similar unit-chain lengths also affected the enzyme digestibility of starch granules (P<0.01).     

Influence of photocrosslinking on the retrogradation of wheat starch based films

The ageing of wheat starch based materials was followed as a function of time for different systems suitably hydrated to be in the rubbery state. In particular, one aspect of retrogradation, that is to say the development of crystallinity is analysed in order to study the influence of a specific crosslinking treatment of starch films made by UV irradiation (photocrosslinking). Mechanical and thermal properties showed that an important limitation on this particular ageing occurs after photocrosslinking at room temperature. A process of accelerated ageing, monitored by modulated DSC, was also performed at 60 °C in order to increase retrogradation kinetics. The results showed a recrystallisation decrease and it appeared that the presence of the plasticiser enhances retrogradation. For a plasticised sample (17% glycerol), a decrease of 30% of the melting enthalpy was observed compared to 91% for an unplasticised one. A modification in the nature of the melting peaks, as recorded by modulated DSC, allowed us to propose a model for amylopectin retrogradation after crosslinking.     

State diagram of potato starch–water mixtures treated with high hydrostatic pressure

Potato starch–water mixture was treated with high hydrostatic pressure (HHP) of up to 1.2 GPa, and effect of starch content (10–70% (w/w)) on HHP-gelatinization was investigated by differential scanning calorimetry (DSC). Depending on the treatment pressure and potato starch content, DSC thermograms showed decrease in enthalpy change of heat gelatinization reflecting the progress of HHP-gelatinization and increase in enthalpy change of re-gelatinization of retrograded starch. From the viewpoint of the enthalpy changes, physically modified state of HHP-treated potato starch–water mixtures was classified as follows: no change, partial gelatinization, complete gelatinization, partial gelatinization and retrogradation, and complete gelatinization and retrogradation. A state diagram of potato starch–water mixtures (treatment pressure vs. starch content) was presented.     

Gelatinization mechanism of potato starch

The non-Newtonian behavior and dynamic viscoelasticity of potato starch (Jaga kids red ’90, 21.0% amylose content) solutions after storage at 25 and 4°C for 24 h were measured with a rheogoniometer. The flow curves, at 25°C, of potato starch showed plastic behavior >1.0% (w/v) after heating at 100°C for 30 min. A gelatinization of potato starch occurred above 1.0% at room temperature. A very large dynamic viscoelasticity was observed when potato starch solution (3.0%) was stored at 4°C for 24 h and stayed at a constant value with increasing temperature. A small dynamic modulus of potato starch was observed upon addition of urea (4.0 M) at low temperature (0°C) even after storage at 25 and 4°C for 24 h. A small dynamic modulus was also observed in 0.05 M NaOH solution. Possible models of gelatinization and retrogradation mechanism of potato starch were proposed.     

Biocomposites of thermoplastic starch with surfactant

Thermoplastic starch films were prepared by a casting technique. Microfibrillated fibers from husks of corncobs were added as reinforcing agents and glyceryl monostearate (GMS) as surfactant. The films were characterized using X-ray diffraction studies, thermal and mechanical analysis and water uptake experiments. Differential scanning calorimetry (DSC) and X-ray diffraction showed the formation of amylose–GMS complexes. Compared to films without GMS the films with GMS showed significant reductions in water uptake and an increase in tensile strength. Important differences in the DSC measurements in the 160–200 °C range of films with and without GMS were also exhibited. These effects can be related to the reinforcement of the polymer matrix by the web-like network of the microfibrillated fibers, the formation of amylose–GMS complexes and the interactions of the polar groups of the GMS with the hydroxyl groups of the cellulose. Retrogradation tendencies of the TPS films were also changed by these phenomena.