Ph.D. Dissertation:

Application of extrusion technology for the production and formulation of functional instant porridge powder based on composite flour (Oat and food by-products)

In this study, an extrusion process utilizing composite flour made from oats and selected by-products of agricultural and food processing industries (wheat bran, broken rice, and apple pomace) was employed to develop an instant porridge powder. In the first phase, the extrusion process was optimized using Response Surface Methodology (RSM) with a Central Composite Design (CCD). Three independent variables were examined: the apple pomace-to-wheat bran ratio (25:75, 50:50, and 75:25), screw speed (120, 160, and 200 rpm), and feed moisture content (14, 18, and 22%). A total of 20 formulations were produced using a twin-screw extruder at a constant temperature of 120?°C. The results showed that both feed moisture and the apple pomace ratio significantly affected the humidity and water activity of the extruded products. The expansion ratio decreased with increases in all three independent variables, while the addition of apple pomace significantly increased bulk density. The quadratic effect of screw speed on hardness was also statistically significant. The Increase in apple pomace and feed moisture content raised the L* (lightness) value and reduced a* (redness) and b* (yellowness), whereas screw speed exerted the opposite effect on these color parameters. Both apple pomace and moisture content significantly enhanced the Water Solubility Index (WSI). The Oil Absorption Index (OAI) and Water Absorption Index (WAI) increased with higher levels of apple pomace, while, WAI decreased at higher screw speeds and moisture levels. Total phenolic content (TPC) and antioxidant activity (AA) increased up to 160 rpm and then declined. Scanning Electron Microscopy (SEM) revealed a honeycomb-like structure with smaller internal cells and thicker, rougher walls at higher apple pomace concentrations. Specific Mechanical Energy (SME) ranged from 135.07 to 235.51 Wh/kg. Process optimization using RSM and a desirability function aimed to maximize WAI and expansion ratio while minimizing bulk density and hardness. The optimized sample—comprising 53.37% apple pomace, 15.45% feed moisture, and a screw speed of 120 rpm—achieved a desirability score of 0.77 and exhibited higher levels of soluble fiber, phenolic compounds, and antioxidant activity compared to the non-extruded control. Model validation was performed using one-sample t-tests comparing predicted and experimental values. In the second phase, the optimized extrudate was milled into powder and compared with the pre-extruded powder to evaluate the effects of extrusion on the properties of the instant powder. The extrusion process improved wetting time, wettability, dispersibility, flowability, water absorption, solubility, and swelling capacity, while reducing moisture content, angle of repose, cohesiveness, hygroscopicity, caking tendency, and lightness. The resulting powder was classified as an instant product with moderate to good flowability, high solubility, non-hygroscopic behavior, and no tendency to cake. In the final phase, a D-optimal mixture design was employed to determine the final formulation of the instant porridge powder. Texturized powder (45%, 55%, and 65%), sugar (20%, 25%, and 30%), and maltodextrin (4%, 6%, and 8%) together accounted for 93% of the total composition. All formulations contained 0.5% apple flavor, 1.5% tricalcium phosphate, and 5% gum arabic. Among the developed formulations, the one consisting of 59% texturized powder, 26% sugar, 8% maltodextrin, 0.5% apple flavor, 1.5% tricalcium phosphate, and 5% gum Arabic- reconstituted with 80 g of water per 20 g of powder- achieved the highest sensory and textural scores. Overall, the final formulation of the instant porridge powder demonstrated excellent nutritional, sensory, and structural characteristics, indicating its potential as a functional semi-solid food product with high dietary fiber content and significant antioxidant capacity.

Keywords: Extruded product, Extrusion, Instant porridge powder, Oat, Broken rice, Wheat bran, Apple pomace