Amido de trigo Mourisco (Fagopyrum esculentum): caracterização e efeitos das modificações físicas nas propriedades térmicas, estruturais e morfológicas

Abstract

Buckwheat (Fagopyrum esculentum) is a pseudo cereal and it nutritionally stands out for its high quality of protein and for containing more vitamins and minerals when compared to other cereals. Its seed presents a high content of starch, being an alternative source to supply the global demand for this polymer. The aim of this work was to evaluate the thermal, structural and morphological properties of the common buckwheat seed starch in its native form, modified by milling; ultrasonic treatment and double modification; extracted by the aqueous method. Initially, the native starch was subjected to the treatment with a ball mill and it was observed by the thermogravimetry (TG-DTG) that the ground starch has presented a similar profile of decomposition, dehydration and oxidation of the native sample. By the differential scanning calorimetry (DSC) it was observed that the ground starch was pre-gelatinized. The diffraction pattern wasn’t changed, its relative crystallinity decreased significantly, and there was no change in the functional groups present in the starch, according to X-ray powder diffraction (XRD) and Fourier transformed infrared spectroscopy (FTIR), respectively. The granule morphology observed by field emission gun-scanning electron microscopy (FEG-SEM) showed roughness on the surface and decreased mean diameter of the ground starch. Subsequently, the native starch was subjected to an ultrasound treatment at different times. Through TG analyzes, a significant increase in the period of thermal stability could be observed, especially after a longer sonication period. The sonicated samples have showed a decrease in the temperatures and enthalpy gelatinization compared to native starch; while the type A crystalline pattern and the starch functional groups remained unchanged, as observed by the DRX and FTIR analysis, respectively. A substantial change in the morphology of the samples was observed, with formation of pores, depressions and cracks, and they were intensified with the increase of the treatment time. For the double modification, the sonicated samples were submitted to the milling process and it was possible to observe a significant increase in thermal stability. The starch was pre-gelatinized and the relative crystallinity decreased significantly compared to native and sonicated starch. Changes in the surface of the granules, such as cracks, depressions and pores, have been intensified. On the whole, changes in all properties were more intense for the doubly modified starches. The kinetic results provided the kinetic triplet for each stage of mass loss observed in native and ground starch; the particle size distribution has affected the dehydration mechanism, thus, without affecting the activation energy and the similarity of kinetic models and activation energy obtained in the second decomposition step that has confirmed that the physical process used hasn’t affected the chemical structure of the starch; in the oxidation of native starch, high activation energy was required to obtain degradation core followed by its branching; while in the ground starch, there was the formation of decomposition cores in the second stage capable of accelerating the oxidation process with less activation energy, indicating that the particles are more exposed to thermal degradation due to decreased particle size.