Study of the Functional Properties and the Retrogradation Behavior of Native Wheat Starch in the Presence of Cress Seed Gum and Sucrose

 Wheat starch granules are composed of amylose and amylopectin, which are responsible for the functions of starch such as swelling, gelatinization, pasting, gel formation, and retrogradation. The retrogradation changes the functional properties, it is an undesirable phenomenon in some starch-based foods which reduces the acceptability of food, and shortens the shelf life. Delaying or preventing of starch retrogradation is a major challenge in the food industry. Various ingredients such as carbohydrates imply an important role in improving the functional properties, decreasing amylose leaching and retarding the retrogradation of starch gels. Cress seed gum (CSG), as an emerging galactomannan, has shown the ability to improve the textural and rheological features of food systems based on its unique properties. Addition of CSG to the composite wheat-rice bread increased dough stability and improved the staleness of bread compared to guar gum. Also, the addition of CSG and xanthan gum to gluten-free bread stabilized the texture of bread during storage. Sucrose (SUC) is a common additive in food formulations which is useful as a sweetening agent and textural modification. Sugars have been able to reduce amylose leaching and accelerate or delay starch gel formation. They also delay retrogradation of starch gels during storage. Therefore, the objectives of this study were to determine the impact of different substitution levels of CSG (0, 5, 10 and 15%), SUC (0, 5 and 10%), and their blend on the functional properties (swelling strength and solubility), microstructure features, retrogradation kinetics and synthesis of native wheat starch (NWS) gel (4% w/w).

 Cress seeds were purchased from a local medicinal market. NWS and SUC were supplied from Sigma Aldrich (Spain) and Merck (Germany) respectively. Starch suspensions (0.6 gr powder/20 ml water) substituted with CSG (0, 5, 10, and 15% w/w) and SUC (0, 5, and 10% w/w) were prepared to estimate the swelling power and solubility index. To produce starch gel, the 4% w/w suspensions of NWS-CSG-SUC were prepared by dissolving the appropriate amount of CSG powder and SUC in deionized water. Then, each suspension was poured into a stainless-steel cylindrical container and was heated to 95 oC and held at 95 oC for 3 min and then cooled to 50 oC while stirring continuously at 160 rpm with a mechanical mixer. Finally, the pastes were kept at ambient temperature (25 oC) for 1 h. To assess gel structure, imaging of the gels was performed by scanning electron microscopy (SEM). The retrogradation kinetics and syneresis of gel samples were determined after storage at 4 oC for 0, 1, 7 and 14 days.

 The swelling power and solubility index of NWS increased with increasing the replacement level of CSG. CSG promotes adhesive interactions among the gelatinized granules. This can enhance the forces applied to them, facilitating water entering (increasing swelling), amylose solubilization and its exudation. In contrast, SUC compete with starch molecules for water in the system and thus preventing gelatinization and mobility of starch molecules reduced the swelling power. The starch-gum-sugar mixtures had a higher swelling power compared to the starch and starch-sugar samples. The mixed samples had higher solubility values than each of them individually. SEM images showed that the pore size of starch gel decreased and increased from 33.01 to 29.44 µm and to 45.37 µm with increasing the substitution levels of CSG and SUC, respectively. NWS-15% CSG-5% SUC gel had 31.34 µm pore size. After storage for 14 days at 4 oC, the CSG substitution with NWS reduced the rate of retrogradation and syneresis from 0.101 to 0.52 (s-1) and from 50% to 23%, respectively. It could be related to creating a film around the granules by the leaked amylose and the CSG in the continuous phase, so inhibiting further swelling and polymers leaking out and related to high water holding capacity of CSG. The addition of SUC reduced the rate of the process to 0.096 (s-1) but because of its low water holding capacity, the value of syneresis enhanced to 57%. In the mixed gels, more reduction of the retrogradation rate and syneresis was observed which was clearly at high CSG replacement. The retrogradation rate of the NWS-15%CSG-5%SUC was 0.057 (s-1) and its syneresis was 45%. According to the results, it can be concluded that 15% CSG and 5% SUC replacement levels can effectively improve the functional properties and reduce the rate of retrogradation and syneresis of NWS during storage.