A Rheological Analysis of Biomaterial Behaviour as a Tool to Detect the Dilution of Heather Honey

Abstract

Heather honey is a valuable and rheologically special type of honey. Its above-average selling price may motivate its intentional violation with a mixture of honey from another botanical origin, the price of which is lower on the market. This work deals with the rheological properties of such devalued heather honey in order to determine the changes in the individual rheological parameters depending on the degree of dilution of the heather honey. For this purpose, a differently diluted heather honey sample series was created and the following rheological parameters were determined: hysteresis area, n-value, yield stress (τ0), parameter B (Weltman model), parameter ϕ, or parameter C (model describing the logarithmic dependence of the complex viscosity on the angular frequency). Part of the work was research into whether the set parameters can be used as comparative parameters. It was found that the hysteresis area does not appear to be a suitable relative comparison parameter due to the high variability. The parameters that appear to be suitable are the relative parameters n-value and the parameter ϕ, which showed the greatest stability. The change in the determined rheological parameters is, depending on the degree of dilution, non-linear with a step change between the samples containing 40% (w/w) and 60% (w/w) of a heather honey.

Analytical Rheology of Honey: A State-of-the-Art Review

“…Most honeys are Newtonian fluids [18][19][20][21][22][23][24][25][26] characterized by constant viscosity (η) at a fixed temperature, which can be described by Newton's law for flow (Equation (1)) showing a linear relation between shear stress (σ) and shear rate ( (1) However, some honeys, including heather honey, New Zealand manuka honey, Indian karvi honey, Nigerian honey, buckwheat, white-clover honey and several eucalyptus honeys show non-Newtonian behaviour, with viscosity values that change with shear rate at constant temperature [2,8,11,23,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. For non-Newtonian fluids, the shear stress (SS)/shear rate (SR) ratio defines the apparent viscosity (η app ) of the fluid at constant temperature, similarly to that of Newtonian fluids; however, this coefficient changes with SR while the dynamic viscosity of Newtonian fluids is SR-independent.…”

“…Steady shear and dynamic shear assays have been performed to analyse the rheological properties of honey of different botanical sources and origins, either in its liquid state or in its crystallised form [2,8,11,15,18,23,24,27,31,32,34,35,[37][38][39]43,45,50,54,64,68,74,75]. Additionally, parameters calculated from oscillatory measurements are very susceptible to physical and chemical changes, justifying their usefulness in the rheological evaluation of honey.…”

“…Due to its high market value, honey is often adulterated with less expensive industrial sweeteners which mimic its sugar profile [3,49,51,56,60,[77][78][79]. Common adulterants in honey include starch syrups (high fructose corn, corn, rice), inverted syrups (for example, inverted syrup from sugar cane/sugar beet, and jaggery), starch or inverted syrups fed to bees (providing honey with a high level of indirect sugar), or even low quality honey added to high priced honey (for example, acacia honey adulterated with rape honey, or heather honey adulterated with common honeys) [45,49,51,56,66,77,79].…”

“…Pridal et al [45] analysed the rheological changes in heather honey, a favourite target of fraud, after its dilution with lime honey (a common honey). Results of the application of the Power-law (Equation ( 2)) and the Herschel-Bulkley (Equation ( 3)) models to SR-SS data showed that n, K and τ 0 changed accordingly with the variation in the honey dilution.…”

“…Authors did not recommend the use of hysteresis area as an evaluation parameter for thixotropy, due to its great instability resulting from the sensitivity of the hysteresis loop to both internal and external factors [45]. Still regarding the dependence of the apparent viscosity versus time, the parameter φ (the ratio between η in the first second of the assay and η in the 300th second of the assay) followed the same trend observed for parameter n. After a gradual initial variation, authors observed a 13% change between samples with 40% and 60% heather honey.…”