New milk protein anisotropic structures formed by high moisture extrusion

In this work, the formation of anisotropic, layered structures from milk protein concentrates was investigated using high moisture extrusion. These structures were obtained in combination with three concentrations of starch (0 %, 14 %, and 33 %, d.b), and at three different moisture levels: 58 %, 63 %, and 68 %. Two different temperature profiles were investigated, reaching a maximum barrel temperature of 80 and 115 °C (and maximum melt temperature of 56 and 64 °C, respectively), with a constant screw speed and feed rate. In situ monitoring of torque, as well as the evaluation of water partitioning via proton relaxometry highlighted the interplay between moisture content and barrel temperature in starch transitions and water intake. For example, the presence of starch displayed water populations of distinct mobility, featured by higher relaxation times, and the apparition (or intensification) of new peaks representing free water in the system. The extruded samples showed a multi-phase system with swollen starch granules embedded in a protein network when 14 % starch was used, at moisture <63 %, as depicted by confocal microscopy. Tensile strength and cutting force tests showed a broad range of textures, from more brittle at high starch concentration, to softer and mushier when extruded at high moisture. Increasing starch and moisture content disrupted the protein network structure. This work demonstrated that milk protein concentrates show great potential to create novel anisotropic structures by extrusion, with tunable texture properties depending on starch, moisture and temperature conditions.