Rheological Properties of Some Honeys in Liquefied and Crystallised States

The paper presents the results of rheological measurements conducted on three types of Polish honey: rape, multi-floral, and buckwheat honeys. The investigations involved identification of the properties of the honeys in both liquefied (by heating) as well as crystallised states. Both steady shear as well as dynamic rheological tests were performed. As a result, it was possible to show that the liquefied honeys behave like Newtonian fluids. Good agreement of the results between the rotary shear and oscillation rotary tests was observed, thus fulfilling the Cox-Mertz rule. The structure of the honeys was subjected to qualitative scrutiny by analysing photographs of the crystals taken in the conditions of shearing interferometry. The quantitative analysis was made by presenting a numerical distribution of crystal colonies with reference to the maximum dimensions of individual crystals. The geometric measurements of the crystals were carried out using analiSIS software. In the crystallised form, the media showed a thixotropic effect, and their apparent viscosity was many times higher than the dynamic viscosity in the liquid state. After plasticising by deformation with an increasing shear rate of up to 450s⁻¹, the equilibrium melting curves of the crystallised honeys were described by the Ostwald-de Waele model. One particular reason for the research was to show that the results obtained for the honeys crystallised by the steady shear method, were qualitatively different from the results obtained in the dynamic measurements. The Cox-Mertz rule cannot be applied for the crystallised honeys.