(110b) Effect of Heating Rate & Shear Stress in Starch Gelatinization

Synthetic polymers based on petrochemicals are useful materials for multiple applications; however, the environmental pollution caused by their resistance to degradation and their durability has turned polymer research into developing ?environmentally friendly? polymers. An alternative of these materials is thermoplastic starch (TPS) because starch is an abundant, non expensive raw material that can be plasticized and used in conventional polymer processing equipment.

TPS production involves two steps: gelatinization, where granules are mixed with a plasticizer to be disrupted, and plasticization, where starch molecular structures (amylose and amylopectin) interact with the plasticizer to form a new polymer matrix. Process conditions in these two steps (temperature and shear rates) are critical in the granular breakdown of starch, since temperature has been proven to affect granule swelling by plasticizer, and in some extend, it has also been shown that shear rate influence the process, but the uncertainties involved in the control of this variables, makes difficult to determine their proper effect in the mechanisms involved.

The aim of this research is to study the relationship between temperature and shear rate in the gelatinization and plasticization processes, for different TPS formulation (low starch concentration and high starch concentration) using native corn starch and glycerol as plasticizer.

Dry native corn starch and glycerol are premixed in a Lighting L1U10F blending system at 150 rpm for approximately 1 hour at room temperature, at two different starch concentration 12% (w/w) and 50% (w/w). Next, viscosity evolution is assessed varying shear and temperature rates in a controlled stress rheometer ARG2 (TA instruments, USA), using a parallel plate geometry with a 20 mm plate and a 1 mm-gap. Samples were stirred at a constant shear rate of 10 s-1, 20 s-1 and 30 s-1. During stirring, samples were heated at 2&10ºC/min from 100°C to 180°C, maintained at 180°C for 15 minutes, and then cooled at 2&10ºC/min to 100°C. While heating, a sudden increase in viscosity indicates that gelatinization temperature is reached. Then, viscosity diminishes due to granular breakdown until it reaches a minimum (complete granular disruption). Finally, when cooling, there is a constant viscosity increase which corresponds to the plasticization stage. Samples were also observed with an optical microscope during viscosity evolution assessment using a Lugol dye. Optical microscopy shows that viscosity increase in the gelatinization process is caused by the starch granules swelling, followed by a disappearance of granular structure caused by granular breakdown.

Results show that a slow temperature rate can change the granule's internal structure. On the other hand, shear rate seems to affect process kinetics and contribute to mixing homogeneity.