**2. Oscillatory rheological method and hydrocolloids behavior**

Among different techniques used to distinguish between the solid and liquid-like characteristics of a food colloid, the best technique is to use an oscillatory rheological method (Dickinson, 1992). In an oscillatory rheological experiment, both stress and strain commonly present a sinusoidal variation. This is the most popular method to characterize viscoelasticity, since relative contributions of viscous and elastic response of materials can be measured. The cycle time, or frequency of oscillation, defines the timescale of these tests. By observing material response as a function of frequency, material can be tested at different timescales. The observation of material response at different frequencies is also referred to as mechanical spectroscopy (Stanley et al., 1996). Linear viscoelasticity is known as the region where stress and strain waves are set at such low values that stress is proportional to strain. This type of tests is also known as small amplitude oscillatory shear (SAOS). The relationship between stress and strain is then described and storage, loss modulus, complex shear modulus as well as dynamic viscosity can be measured. The storage dynamic modulus (G') is a measure of the energy stored in the material and recovered from it per cycle while the loss modulus (G'') is a measure of the energy dissipated or lost per cycle of sinusoidal deformation (Ferry, 1980, Stanley et al., 1996 ). The ratio of the energy lost to the energy stored for each cycle can be dened by tanδ.

The viscoelastic behavior of a simple or more complex structure can be determined in the above way. Furthermore, as structure is not disrupted, changes including sol-gel transition, gel curing, aggregation, flocs creation etc. can be monitored. Generally speaking, rheological properties could be of high interest in a) product quality characterization b) process design and flow conditions analysis (e.g. pump sizing, filtration, extrusion etc.) c) design of new foodstuffs d) basic research on ingredients' interactions.

According to Roos-Murphy (1984) solutions and gels belong to the categories of entanglement solutions, weak gels and strong gels. Hydrocolloids including starch can belong to all of the three categories revealing the wide spectrum of structures they can adopt according to their own natural state and the environmental conditions found.

Entanglement solutions (e.g. guar gum solutions) present a strong dependence of both storage and loss modulus on frequency. Weak gels behavior (e.g. xanthan gum) is characterized by gel-type mechanical spectrum, whereas strong gels (e.g. amylopectin, amylose gels) present high storage modulus values irrespective of frequency, as junction zones among macromolecules are stable on a relatively long time scale. The spectrum of hydrocolloids can vary at different concentrations.

When at low frequency the loss modulus G'' is higher than the storage modulus G', both parameters vary sharply with frequency: G''(ω) and G'(ω2). This behavior is said to be typical of a liquid-like material. As frequency increases, G'(ω) crosses G''(ω), the response of the material beyond this cross-over frequency is said to be solid-like. Entanglements of macromolecular solutions can result in such behavior. When G' is higher than G'' over most of the frequency range investigated, a weak gel behavior is observed due to the formation of a weak three-dimensional network of ordered chain segments.

Thus, viscoelastic structures of hydrocolloids may differ considerably. So, recent data about their behavior in mixtures with starch or model foods is discussed.
