Effect of moist (Hydrothermal) and dry heating process on different particle size of waxy rice flour on the rheology of batter and staling of gluten-free bread

Celiac disease is a type of immune disorder of the small intestine caused by gluten, and other non-wheat grains such as corn, rice, millet, etc. can be used to prevent the occurrence of disease symptoms in susceptible people (Torobika et al 2021). Waxy rice flour is a suitable and widely used raw material as a starch module to compare the effect of different types of honey and heat treatment on the rheological properties of flour gels (Seow et al. 2019). Among gluten-free products, bread is considered the most widely consumed food, because it is easy to eat and convenient for mass production. Due to the removal of gluten, many gluten-free breads are prone to becoming stale. Gluten, as a protein that inhibits the staleness of bread, leads to a decrease in the rate of moisture transfer from the core of the bread to the crust (Haghighat‐Kharazi et al 2018). Therefore, research on delaying staleness and reducing bread waste is one of the concerns of researchers, and they try to delay staleness by adding additives, changing cooking and storage methods. Particle size distribution is the most widely used technique for classifying solid particles, which is effective in improving rheological properties by affecting the physicochemical properties of flour during hydration, such as water absorption, solvent retention, sedimentation, and adhesion properties. Heat treatments, depending on the intensity of temperature and storage time, by modifying starch granules, denaturing proteins, deactivating enzymes, reducing microbial load, and even modifying taste and aroma are suggested as a suitable method to change the performance of weak and gluten-free flour (Gomez and Martinez 2016). Due to the fact that the effect of rice flour particle size on the rheological properties of dough and gluten-free bread has not been studied so far, in this research, by dividing rice flour with different particle sizes (180, 150, 125 microns) and using heat treatment (moist and dry heat) were investigated to improve the functional characteristics of rice flour and improve the rheological characteristics of dough and gluten-free bread.

The content of moisture, pH, ash, protein of rice flour was measured using AACC standard method (2000) and the total amount of starch was measured by alkaline extraction method and the amount of starch damage was measured by non-enzymatic rapid method. In order to classify the size of the particles, waxy rice flour was divided by a shaker sieve with different sizes of 180, 150 and 125 microns, then under the influence of dry heat treatment for 2 hours and moist heat treatment with humidity, 25% for 5 hours at a temperature of 110 degrees. Celsius was placed. In order to produce gluten-free bread, the formulation used by Haghighat‐Kharazi in 2020 was used with a slight change. Ingredients for gluten-free bread formulation for 100 grams of rice flour included 125 ml of water, 4.5 grams of sugar, 2 grams of salt, 6 grams of vegetable oil, 3 grams of yeast and 2 grams of xanthan gum. The rheological parameters of the pastes included strain, frequency and temperature. In order to check the staleness of bread during the storage period, moisture tests of core and shell, blue activity of core to shell, analysis of bread texture and DSC were used. Finally, the factorial test was used to investigate the effects of particle size and heat treatment of rice flour, and Duncan's multiple range test was used to compare the means at the 5% probability level.

The effect of particle size and heat treatment and their interaction showed a significant effect on the degree of starch damage (p<0.05). The samples of dry heat treatment and wet heat treatment had less starch damage than the control sample. Changing the particle size and dry and wet heat treatment led to a change in the rheological behavior of the samples. By reducing the size of particles, the viscous and elastic modulus of the samples increased. Wet heat treatment samples had the lowest G* modulus. The effect of particle size and dry heat treatment on tissue analysis was not significant (p>0.05). While the wet heat treatment significantly had the lowest hardness and Chewiness during the storage period (p<0.05). Although the amount of skin moisture and water activity increased during the storage time, the effect of particle size and heat treatment on the moisture level of the skin was not significant, while the wet heat treatment samples had more brain moisture and brain water activity during the storage time. were (p<0.05). Wet heat treatment samples compared to dry heat treatment and the change in particle size had the lowest enthalpy among the samples on the first day of cooking and during the storage period.

In general, the results of the present study showed that the degree of starch damage and the rheology of batters were affected by particle size and heat treatment. Changing the particle size and heat treatment led to an increase and decrease in starch damage, respectively. Rice flour with smaller particle size and wet heat treatment sample obtained with particle size less than 125 microns showed better rheological behavior. The results of the evaluations related to the staleness tests of bread in this research showed that during the storage time of the sample obtained with a particle size of less than 125 microns and samples of wet heat treatment due to the lower enthalpy of retrogradation and the high capacity of maintaining moisture, also The effect of wet heat treatment samples on the reduction of hardness and Chewiness led to the improvement of bread staleness. In total, the measured characteristics of using flour obtained from wet heat treatment compared to flour from dry heat treatment, as well as flour with a particle size of less than 125, led to the improvement of the rheological characteristics of gluten-free bread dough.