Valorization of Waste Orange Peel to Produce Shear-Thinning Gels

A laboratory experiment was developed to introduce students to waste valorization. This is the process of reusing, recycling, or composting from wastes, useful products, or sources of energy. In this laboratory experiment, waste valorization is demonstrated through transforming waste orange peel (WOP) into a marmalade-type gel by extracting a pectin-based mixture (or sol) and forming a gel in combination with an acidified sugar solution. Upon isolating the pectin sol, students examined how the rheological properties varied as a function of temperature using capillary viscometry. Gelation was followed via rotational viscometry, and non-Newtonian, shear-thinning properties were demonstrated by monitoring the viscosity change as a function of spindle RPM. In addition to providing a safe and green alternative to the traditional borax-cross-linked poly(vinyl alcohol) gel for students to study, this experiment demonstrates that valuable products for everyday use can be created from household waste such as WOP. Students making the transition from a first to second year undergraduate chemistry program within a natural-sciences degree have successfully conducted this laboratory experiment.