Efeito dos parâmetros de extrusão termoplástica na microestrutura e propriedades físicas do grits de milho

Abstract

This work aimed to study the different changes that may occur with the extrusion process such as the structural part of the processed material, the effects on the protein part through the study in electrophoresis and even with carotenoids by chromatography, as well as the physical properties expansion as a function of changing parameters such as humidity and processing temperature. In chapter 1, a brief review related to the theme of the Thesis is presented, in which topics of the extrusion process are addressed, such as physical and chemical phenomena due to changes in thermomechanical treatment, characteristics of determinations by electrophoresis in the observations obtained in the band’s protein. Aspects related to observations by optical microscopy and barred electron microscopy, among other aspects. In chapter 2, the objective was to verify the effect of humidity and temperature on the microstructure of expanded corn grits. Normal corn grits were processed at 15, 18 and 21% moisture at temperatures ranging from 120, 140 and 160 °C, following a 32 factorial experimental design; using a 5:1 compression ratio single screw Brabender extruder, die 3mm diameter, 14.3 mm length; 150 rpm screw speed and 17 rpm feed speed. Sectional, longitudinal, volumetric expansion index and water absorption index were analyzed. Then the raw and processed samples were submitted to optical microscopy and scanning electron microscopy and electrophoresis analyses. The results showed that moisture had a greater impact on the physical and microstructural properties of grits expanded. At 15%, they had higher expansion values. Regarding water absorption, it was higher at 18% and 120 °C of temperature. It was observed in the SEM and electrophoresis results that there was a decrease in the size/weight of the protein component. Thus, the work concludes that lower processing humidity (> 15%) and high temperatures < 160 ºC cause higher shear rates and mechanical effort, deeply transforming the starch-protein complex. In chapter 3, the objective was to evaluate the effects of inlet moisture (13 – 17%) and barrel temperature (120 – 160 °C) on the physicochemical properties and changes in the carotenoid profile of extruded grits puffs. of corn. The extrudates were prepared in a single-thread extruder, according to a 32 factorial design with two replications. The linear coefficients of feed moisture and barrel temperature mainly affected the physicochemical properties. On the other hand, the β112 interaction coefficient predominated changes in total carotenoids, lutein, zeaxanthin and β-carotene. The quadratic coefficients were also important for changes in total color (in relation to inlet moisture) and for β-cryptoxanthin, specific mechanical energy and volumetric expansion index (in relation to barrel temperature). β-cryptoxanthin and β-carotene increased, while lutein and zeaxanthin decreased. The mathematical models developed from the answers revealed two viable operating regions in the explored domain. For a satisfactory process, from a technological and nutritional point of view, extrusion under operating conditions ranging from 13.2 – 13.7% of inlet moisture to 120 – 132 °C at barrel temperature is suggested. Under these conditions, the required specific mechanical energy input was 410 – 450 kJ/kg and crunchy extruded with a volumetric expansion index greater than 12, a mechanic work less than 0.4 N.mm, observing moderate increments in the β-carotene and β-cryptoxanthin levels. The use of cultivars richer in carotenoids can contribute to the production of healthier snacks.