The effect of freezing time on the efficiency yield of potato starch hydrolysis by alpha-amylase

In order to achieve high-efficiency hydrolysis by α-amylase, freezing pretreatment of potato starch was performed at -25°C for 4 days (PF4) and 8 days (PF8). For this purpose, potato starch (NPS) and freeze pretreatment were suspended in water by exposure to alpha-amylase (0.15% w/v) for 10 hours at 25°C. Morphological properties, percentage of hydrolysis, degree of crystallinity by X-ray diffraction (X-ray) and pasting properties by rapid visco analyzer (RVA) were studied in native, freezed and hydrolyzed starches. Freezing caused surface damage of starch granule and according to the morphology results, scratches, dents and cracks were observed on the surface of the granule, and these changes were much more obvious after 8 days of freezing than after 4 days of freezing, and they were easily hydrolyzed. It was placed by α-amylase and caused the disruption of starch granules. The results of the hydrolysis percentage showed that freezing produced a trace percentage in the production of hydrolyzed starch. Hydrolysis percentage (%) of natural starch increased to 16.8%, 24.1% and 40.8% after hydrolysis in P, PF4 and PF8, respectively. In addition, the relative crystallinity after freezing decreased from natural starch (37.7%) to 34.8% in PF4 and 33.2% in PF8, and this decrease was very substantial (p˂ 0.05) after treatment with α-amylase which decreased to 34.3%, 29.4% and 25.5% in P, 4 days and 8 days of freezing, respectively. Gelatinization temperature and pick viscosity after amylase treatment in all natural starches under freezing treatment decreased significantly (p˂ 0.05) and the decreasing trend was observed in the order of PF8 > PF4 > P, which granular and intramolecular structures disruption as well as low swelling power could be the reason behind these results.