The NDD guideline has been challenged by a number

The NDD guideline has been challenged by a number of writers and only one year after its publication, McCallum pointed out that GSK2656157 the four categories of NDD guideline are vague and impractical in clinical applications [36]. However, rheological properties of food are highly depend on temperature and test parameters (like shear rate). Quinchia claimed that there was no scientific evidence or rationale given by NDD for the temperature and shear rate chosen for the scales. Besides, these scales only considered viscous properties and elasticity was not even mentioned [37]. Another shortcoming of NDD, mentioned by Garcia (2005) was lack of information on the clinical importance of viscosity ranges [18]. Also, no information on specific products was presented, nor was any evidence provided that the established ranges of the NDD applied equally well to the large variety of liquids that were commonly thickened (e.g., water, juice, milk and coffee). Some researchers have referred to the NDD definitions as “obscure definitions” and believed that the guideline can not be used easily in real applications [38]. As a conclusion, optimum rheological properties of food and drink for a specific kind of dysphagia are only unsatisfactorily known and have not been effectively validated [39].
Methods of measuring rheological properties Rheological measurements are performed in order to achieve some main goals, such as analyzing materials’ mechanical properties and identifying molecular interactions and composition of food stuffs [40]. Rheological methods are divided into two major categories of empirical and fundamental rheology. Both of these methods are based on the measurement of force and deformation as a function of time. Empirical methods are instrumental-dependent and specified to test a hypothesis. By contrast, fundamental tests are based on known concepts and equations of physics. In general, the quality of empirical methods, like subjective methods used in dysphagia literature, is poor [31]. The line spread test (LST) is a quick empirical test for comparing relative viscosities of various comparable products. It measures the consistency of a liquid using the distance a liquid with standard amount spread on a horizontal surface when released from a confined chamber [41,42]. Efficacy and practicability of LST is a controversial issue in the context of dysphagia problem. Some researchers have suggested that the line spread is consistent enough to be used as a screening test for product viscosity in clinical settings and can be used in staff training demonstrations to show differences in thickness of nectar and honey-like foodstuffs [38,43,44]. In contrast to the mentioned researches, the critics questioned the ability of LST to provide a reliable results. For example, Nicosia et al. reported limited functions of LST and declared that LST was only able to differentiate between liquids with very different consistencies when fluid density was held relatively constant (i.e., between nectar and honey categories); but there was no predictive relationship between line spread values and viscosity beyond this gross separation [41,43–45]. Some studies doubted the reproducibility of LST tests. Accordingly, Nicolson believed that the LST tests were prone to huge variations in operating conditions; so it cannot provide accurate and reproducible data. Furthermore, LST data can not be compared with other rheological instrumental determinations and ascertaining detailed information on foodstuff rheology was not achievable by this instrument [45,46]. Given the above considerations, LSD methods are not accurate and reliable and researchers should not refer to them and compare the data. Therefore, to acquire comparable parameters, recent publications have performed rheological experiments using viscometer and rheometer (fundamental methods). The NDD uses viscometer to categorize dysphagic patient\'s food. It is worth noting that only steady-state rheological tests can be performed using a viscometer. Foodstuffs are viscoelastic materials and possessing both viscous and elastic properties. Viscoelastic properties of hydrocolloid solutions or dispersions are particularly important for food processors in determining processing parameters, monitoring consistency and predicting stability of fluid food systems and final textural attributes of the formulated foods [47].Viscoelasticity balance of masticated food is crucial in safe and easy swallowing [48,49].