Production of Starch Nanocrystals from Agricultural Materials Using Mild Acid Hydrolysis Method: Optimization and Characterization

Due to an insufficiency of fossil fuels and environmental pollution, sustainable and efficient material utilization has become the greatest importance. This work aimed to produce nanosized filler for biobased materials from renewable resource by an efficient production. Banana and tapioca starch nanocrystals (SNCs) were prepared from mild acid hydrolysis method. The effects of acid type, acid concentration, reaction time, and temperature on percent yield and degree of crystallinity were investigated in order to find the suitable condition for SNCs preparation. The chemical structure, degree of crystallinity, and morphology of the obtained SNCs were examined by Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), and transmission electron microscopy (TEM), respectively. Thermal properties were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results revealed that the suitable conditions for preparing SNCs were by using 3.5 M H2SO4 at 40°C for 7 hrs or 10 hrs depending on starch type, which leading to high degree of crystallinity as 47.13% and 60.06% for banana and tapioca SNCs, respectively. The particle size of both SNCs was less than 200 nm, approximately 30–70 nm. The banana SNC displayed parallelepiped nanoplatelets with C-type crystallinity, while the tapioca SNC showed spherical nanoplatelets with A-type crystallinity. The degradation temperature of banana and tapioca SNCs occurred in ranging between 280 and 310°C, which was lower than their native starches due to the disintegration of the SNC molecular chain during acid hydrolysis reaction. Nevertheless, melting enthalpy (ΔH) of SNC fillers enhanced after hydrolysis indicating that there was an increment of degree of crystallinity owing to the chain rearrangement of starch molecules.