Ultrasound-assisted pregelatinization: The effect of NaCl and CaCl2 with pre and post ultrasound treatment on textural, freeze-thawing stability and morphology

Introduction: Native starch granules obtained from different kinds of plants have unique and intrinsic characteristics that partially satisfy specific needs. However, those properties are essentially not suitable for most food applications. Hence, the starch must be modified chemically, physically, and enzymatically to improve its functional properties. Due to environmental concerns, strict regulations, and expensive price, physical modifications are receiving increasing attention. Some of the physical modifications applied to starch are pregelatinization, sonication, ball-milling, heat–moisture treatment, and pulsed electric field treatment (Zhu 2015). Among these options, pregelatinization is one of the most popular industrial methods of physically modifying starch. Pregelatinized starch is pre-cooked starch that readily disperses in cold water to form stable suspensions (Nakorn et al. 2009).
Materials and methods: Native tapioca starch, containing 13.64% moisture, 0.94% fat, 11.26% protein, and 18.78% amylose was purchased from Sepahan Co. (Isfahan, Iran). The compositions were determined according to the approved methods of the AACC (2000). Amylose content was established by the iodine method reported by Pourmohammadi, Abedi, Hashemi, and Torri (2018). NaCl and CaCl2 were obtained from Fars Glucosin Co. (Marvdasht, Iran). Before each experiment, 5 g of tapioca starch was mixed with (for IS 0.3, 0.88 g and 0.6 g) and (for IS 0.6, 1.76 and 1.2g) of NaCl and CaCl2, respectively (w/w, starch basis) and then suspended in 50 mL of distilled water, and stirred overnight at room temperature then starch solutions were sonicated. Samples were divided two groups: first groups, pregelatinized by ultrasonication and then salts were added to starch solution (AUPS). Second groups salts were added and then sonicated (BUPS) (Fig. 1).Ultrasound- Assisted Pregelatinized Starch: Ultrasonic process was applied according to Abedi et al. (2019) in order to obtain ultrasound-pregelatinized tapioca (UPTS) starch. An ultrasonic generator type UP400S hielscher (400 W, 20 kHz) using an immersible probe in a 100-mL cylindrical jacket glass vessel (180×180 mm2) with the desired temperature (60 °C) and 10 min maintained by a circulating water bath, which varied with pulse durations of 5 s on and 5 s off. The probe was dipped into the 1-cm liquid at the top of the vessel, emitting the sound vibration into the fifty milliliters of tapioca starch (10% w/w) sample via a titanium alloy rod with a diameter of 20 mm.
Results and discussion: Ca+2 ions improved the textural properties along with the reduction in syneresis (%) and this effects progressively improved with rising the ionic strength from 0.3 to 0.6. On the other hand, textural and syneresis properties improved following the addition of Ca+2 ions after sonication. Meanwhile, Na+ ions induce to decrease in textural characteristics and increase the syneresis (%) of starch paste and these effects increased with enhancing the ionic strengths from 0.3 to 0.6. The addition of Na+ ions had synergetic effect on negative impact on textural and syneresis properties which might due to the re-crystallization effect of Na+ ions that was proved by SEM morphology. Conclusion: ions with structural maker or breaker nature with their Valente can affect on starch polymer, meanwhile, the application of ultrasonication along with salts have synegic effect on starch polymers.