Instantaneous gelation of acid milk gels via customized temperature-time profiles: Screening of concentration and pH suitable for temperature triggered gelation towards 3D-printing

Abstract

The current interest in manufacturing of individualized food via 3D-printing has identified a need for more information on the understanding and formulation of potential ingredients. This paper proposes a novel approach to use dairy protein-based materials matching the required fast, local, and irreversible sol−gel transition. The pH−temperature-route of acid gelation was shown to be a suitable mechanism for extrusion-based 3D-printing applications of skim milk retentates (8–12% protein, w/w), obtained by microfiltration. pH was adjusted to 4.8–5.4 by adding citric acid at 2 °C. Sol−gel transition was triggered either by linear temperature ramps (1 K/min) or with temperature–time profiles (ΔT = 15 K, 30 K/min) comparable to 3D-printing but without superimposed flow. Firm and stable gels were produced and characterized by elastic modulus in the rheometer used for gelation, and resistance to penetration and serum binding ability were analyzed with individual gel samples. Formulations were classified regarding their sol/gel-status and progression before and during gelation, in particular, a specific softening and occasional pre-gelation. Several gels which showed preferable characteristics, including a low storage modulus G’ (~0.1 Pa) during conveying under cold conditions, followed by a steep increase of G’ during sol–gel transition, were considered to be a suitable feedstock for application in extrusion-based 3D-printing of skim milk retentates, namely 10% (w/w) protein and pH 4.8.