Abstract

Utilization of supercritical fluid extrusion (SCFX) to alter the microstructure of biopolymers offers new opportunities to design novel products of enhanced utility. In this project, SCFX was utilized to modify the microstructure of milk protein concentrate (MPC) extrudates to develop quick, in-mouth disintegrating products. Quick in-mouth dissolution is an important characteristic of convenient baby foods which is achieved by using predominantly starch based formulations, raising nutritional concerns. Thus, there is a need to develop protein-rich baby food alternatives with a solid-like texture that break down and dissolve easily with minimal chewing. The reactive role of supercritical carbon dioxide (SC-CO2) in altering the physicochemical properties of MPC-sucrose (MPC-S) extrudates was evaluated by comparing the effects of steam extrusion (SX) at 115 oC and SCFX at 85 oC. SC-CO2 injection at 2.5 (SC-2.5) and 5 (SC-5) wt. % of feed moisture, temporarily lowered the pH of the melt to 4.90 and 4.75, respectively while pH 6.2 was maintained during SX. Compared to unextruded samples, free sulfhydryl content was reduced by 12.6% in SC-5 and 75.9% in SX extrudates indicating decreased sulfhydryl interactions in SCFX extrudates. The temporarily induced acidity and lower operating temperature of SCFX were found to prevent protein aggregation during extrusion. The equilibrium solubility of carbon dioxide in water was also simulated at supercritical conditions and its effect on the expansion characteristics of the extrudates was quantified. Reduction in interfacial tension with increasing SC-CO2 concentration favored homogeneous nucleation up to CO2 saturation levels, resulting in uniformly porous extrudates with mean cell size < 500 µm and mean cell wall thickness < 40 µm. SC-CO2 input rate above the saturation levels resulted in undissolved gas vacuoles in the melt, which let dissolved CO2 diffuse into them when the pressure was lowered at die exit, favoring coalescence, and formation of fewer large cells (mean cell size > 700 µm; mean cell wall thickness > 80 µm). Thus, SCFX with CO2 injection rate corresponding to the saturation level contributed to improving the physicochemical and structural properties of milk protein extrudates and was useful in designing the quick dissolving attribute in the extrudate. To further prevent the protein-protein interactions during SCFX and to induce ionic destabilization in the extruded milk proteins, sodium hexametaphosphate (SHMP) as a functional ingredient was utilized. Textural analysis demonstrated that the compression strength of SCFX-SHMP extrudates reduced by 93% on hydration (30 s at 37 oC) as compared to 31% for SCFX-control samples. Reduction of protein interactions contributed to the weakening of the scaffolding that makes the expanded structure of the extrudate resulting in extrudates with quick in-mouth dissolution characteristics. In-mouth dissolving properties of MPC-S puffs with an optimized level of SHMP addition (0.4%) were found to be comparable to the commercial starch-based baby puffs by the sensory panel. This research identified the role of SCFX, extruder operating conditions, and the mode of action of specific ingredients to create desirable physicochemical and microstructural changes in milk protein concentrate during extrusion. The developed and demonstrated process was successfully used to design milk protein based in-mouth dissolving puffs.

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