Rheological Characterisation of Honey: Application as an index of Quality
Adulteration and imitation of honey constitute the most important quality assurance concerns in the utilisation of this natural product. This study explores the application of rheology as a tool for assessing “honey” samples for authenticity and for quality. Sucrose melts as well as solutions of glucose and fructose are used in honey adulteration in Nigeria to increase bulk volume and enhance profit. Three samples of pure honey were serially diluted with sugar melts, glucose, fructose or water and analysed for their rheological profiles at 27 °C and 35 ºC. Other “honey” samples, gathered from different locations in Nigeria, were rheologically characterised and classified according to their levels of adulteration using the rheograms from the serially diluted honey as calibration data. Pure honey at ambient temperatures exhibits a peculiar non-Newtonian rheological behaviour while the behaviour of sucrose, glucose, and fructose tends towards Newtonian. The adulteration of honey with these materials or with water (a Newtonian fluid) drags its viscosity towards Newtonian flow behaviour. The results show that the Carreau- Yasuda model correlates the rheology of pure honey samples. Conversely, the sucrose melts followed the power law model at the elevated temperature of 35°C. The parameters of both empirical models were observed to reflect the quality of the samples and thus aided their classification. Modified with the insertion of the Mark-Houwink’s relation the Carreau-Yasuda model yielded average molecular weight data on the samples. The Structural Kinetic Model was ut i l ized to give rheological structural information and a second independent assessment of average molecular weight from the rheograms. The average molecular weight ranges of 245-252 g/mol were obtained using Structural Kinetic model while 250-260 g/mol were obtained using the amended Carreau-Yasuda model. The rheological curve fitting showed that the new model fitted the experimental data well. Using the Structural Kinetic Model, the best fits to data were obtained at 3rd order deformation kinetics suggesting the presence of three major components or groups of components in honey. The possible candidates are monosaccharides, oligosaccharides and water. The Structural Kinetic Model produced negative infinite time viscosities for pure honey. These shifted towards positive values with increasing degrees of adulteration. Three samples of “honey” obtained from Nsukka (Enugu State), Gauraka (Niger State) and Kabba (Kogi State) behaved rheologically as pure honey while the other samples purchased for this study, reflected varying degrees of adulteration. Proximate and Chromatographic characterizations of honey which provide acceptable measures of honey quality corroborated the conclusions derived from the rheological characterization of this study. The antibacterial activities of the samples al so correlated well with the rheological characterization. This work points to a novel application of rheology in honey quality assessment.