•Preparation of Casein based zinc oxide nanocolloids using the two-step approach.•Thermal diffusivity studies of prepared samples using dual-beam thermal lens techniques.•Tunable behavior of thermal diffusivity depending on volume fraction and morphology. Nanofluids with suitable heat transfer properties were found to be applied in thermal insulators, transport, nanodevices, medicine, etc. Their heat transfer properties are influenced by the particle size, morphology, and concentration of the nanoparticles present in the nanofluids. Nanofluids of different morphologies of zinc oxide such as nanospheres, nanoflowers, and nanorods were synthesized and their thermal properties were evaluated using a dual-beam thermal lens technique. All these nanofluids were found to exhibit tunable thermal diffusivity depending on concentration. The nanofluids with flower-like morphology show maximum deviation in diffusivity due to the layered structure and the increased surface area. As compared to pristine zinc oxide nanofluids, casein-capped zinc oxide nanofluids show excellent thermal insulation properties at 60 μg/ml. The findings suggest that biofunctionalized zinc oxide nanofluid is a promising candidate for a variety of thermal applications.