Effect of ultrasound and enzyme treatment on wheat and corn starches for making porous starches and iron ion absorption capacity

Porous starch granules are becoming of great interest such as non-toxic absorbents, owing to their great absorption capacity derived from the major specific surface area. Pores can protect sensitive elements as oils, minerals, vitamins, bioactive lipids, food pigments such as beta-carotene and lycopene that are sensitive to light, oxidation or high temperature. Alpha-amylases from Bacillus and glucoamylases from Aspergillus niger have the strongest hydrolytic ability toward starch. Ultrasonic treatments have been reported to produce modified starch. In the last years, the effects of sonication on the starch microstructures and properties have been studied. It was shown that the C–C bonds of starch granules were destroyed, and hollows or pores were formed on the surface and inside the granules. Therefore, the main objective of this study was to identify a suitable starch (corn or wheat) to carry and protect iron ions. The enzyme having a fixed concentration of 0.1% within a fixed period of 36 hour, was added to the starch solutions in three different steps - after, simultaneously and before - the ultrasound processes. The power of the ultrasound was 350 watt for 10 minutes. Iron ions (Iron Ammonium Sulfate (II)) were added to the porous corn and wheat starches in concentrations of 40, 60 and 80 ppm.  

The ability of water and oil adsorptions were measured in the produced corn and wheat porous starches. The microstructures of porous starches were revealed by using Scanning Electron microscopy (SEM). After adding iron ammonium sulfate (II) to the porous starches of corn and wheat, the amount of iron ions absorbed and the type of bonds formed between starch and iron ions were determined by inductively coupled plasma (ICP) and infrared spectroscopy (FTIR), respectively. Statistical analysis was performed by using SPSS software and the mean comparison test at 5% probability level and in the form of factorial test.  

The hydration capacity in processed wheat starch was lower than corn starch due to the differences in granular structure of wheat and corn starch. The hydration capacity in native wheat and corn starches was significantly (p < 0.05) lower than the processed forms. No significant difference (P>0.05) was observed between the three steps of adding enzyme (e after, simultaneously and before the ultrasound processes). According to the results, the oil adsorption capacity in the processed starches was more than that of the native forms. Scanning Electron Microscopy (SEM) shows that the native corn and wheat starch granules appeared without any clear of fissures, fractures and pores. The corn and wheat starches which were treated by the enzyme after (step 1), simultaneously (step 2) and before (step 3) the ultrasound change and lose their smooth surfaces and become uneven. The surface of corn granules in group 2 and 3 have less pores and porosities with more laminated. In simultaneously processes (phase 2) the ultrasound causes the enzyme to be inactivated. In phase 1, it can be concluded that the ultrasound helps the enzyme performance in creating the porosities and cavities. The wheat granules in step 1, 2 and 3 have a lot of damage on the surface and it is likely that the surface of the wheat granule is more resistant to enzyme penetration than the corn. The results of the ICP test show that processed corn starch has significantly (P <0.05) greater ability to absorb iron ions than processed wheat starch. Also, corn starch had a significant difference in iron uptake in the step 1 than steps 2 and 3 (P <0.05), but the data of the second and third steps did not differ significantly (P> 0.05). Hydroxyl groups of D-glucose units in starch granules bond with iron ions and FTIR spectrums give drop at 575 (cm-1) wavelengths. This study showed that enzymatic treatment and ultrasound led to the native corn starches convert to porous starches. The corn starch is more suitable than the wheat because the wheat is more resistant and the cavities were rarely formed. The corn porous starch is a suitable carrier for iron ions.