**1. Introduction**

The rheological properties describe the behaviours of matter under tensions resulting from external forces. Each real matter, whether a solid, liquid or gas, strains when exposed to external forces. We distinguish elastic, plastic and viscous strains. The behaviour of elastic bodies is described by Hook's law and is characterized by its disappearance once external forces are taken away. Plastic strain is permanent and remains even after the external force is gone. A perfectly plastic body is called a Saint-Venant's body. Viscous strain, also known as flow, is characterized by a constant increase in strain under constant stress. Perfectly viscous

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. the terms of the Creative unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

fluids are described by Newton's law, which may be presented as a linear relation between shear stress and shear rate:

$$
\pi = \eta \cdot \dot{\eta}.\tag{1}
$$

The η parameter in the equation above stands for dynamic viscosity. All fluids, which do not fulfil Newton's law, are called non-Newtonian fluids. Generally, non-Newtonian fluids are divided into rheologically stable, rheologically unstable and viscoelastic. They demonstrate partially viscous, elastic and plastic properties. Rheology is tasked with the description of these properties.

The rheological properties of honey are analysed mainly within the aspect of fulfilling the basic production processes such as hydraulic transport, mixing, heating or batching [1]. Viscosity is additionally one of the parameters of quality assessment of the product [2]. In multiple published reports on the rheological properties of honey, there is a common observation that it is in fact a Newtonian fluid [3–6]. A few publications hint at the existence of a clear thixotropic effect, although it is only seen in certain types of honey such as heather honey or the Manuka honey from New Zealand [5, 7]. It needs to be stressed, however, that such reports are with regard to honey in its liquid state also known as strained honey. The parameters, which significantly influence the dynamic viscosity of the analysed product, are temperature and water content [5, 8, 9].

Bee honey is a concentrated aqueous solution of sugars. Due to this, most of the obtained types of honey undergo crystallization when in storage [10]. The crystallization process results directly from the chemical composition, as in almost all types of honey glucose are present in its supersaturated state [5, 11]. Melezitose can also undergo crystallization in honeys. The resulting solid phase is a glucose monohydrate, which has various geometrical forms in crystallization [5, 12, 13]. Honey after crystallization is called set honey and is a two-phase structure, semisolid, which substantially varies in its properties from the liquid state—strained honey [5, 14]. As a result of crystallization, the organoleptic properties undergo significant changes, mainly the honey's texture and water activity [15]. Literature regarding the rheological properties of crystallized honey is surprisingly modest. There are a few studies, which are just starting to analyse the issue [13, 14, 16, 17]. These studies only identify the specific rheological properties of crystallized honeys, as one of the characteristics which change after the crystallization process.

This analysis is an attempt at the identification of the rheological properties of honey both in its liquid state and in its crystallized form. The performance of this task has forced an analysis of additional issues, which determine the rheological characteristics. These are the measurement of the amount of solid phase formed after crystallization of the honey and its morphological characteristics. These issues are relatively seldom analysed in literature [14].
