The effects of high shear rheology on flavour perception

Samples containing dextran, xanthan, sucrose and IAA were designed to have the same viscosity at a shear rate of 50s but significantly different viscosities at high shear rates (> 1000s). The relationship between high shear rate rheology and flavour perception will be investigated using sensory QDA method. INTRODUCTION Flavour is considered as one of the most important quality attributes that contribute to the acceptance of food. As different people have different definitions about flavour, the one proposed by the British Standards Institute “The combination of taste and odour, influenced by sensations of pain, heat and cold and by tactile sensation.” is widely agreed (1). Flavour perception is one of the most complex of human behaviours because it involves almost all the senses. The perceived flavour characteristics are a combination of taste, aroma, mouthfeel, texture and irritation/pain (2, 3). The flavour perception we experience from eating a food product is not only dependent on the nature and quality of the flavour components, but also the availability of these components to the sensory systems and the physiological mechanism of perception. Stimulation occurs when the compounds from the food come into contact with the receptors, which are taste buds for the taste and mucous membranes of the nose for the aroma. Besides taste and smell, physical food properties can affect mastication and therefore texture perception or there may be interaction with saliva which can change mouthfeel (4). These stimulations produce a complex set of signals, which are processed locally and centrally to produce the sensation of flavour (5). The flavour perception is based upon the integration of multiple, concurrent sensations and the judgment in one sensory stimulus is strongly and frequently affected by the other stimuli, even though they are not physically or chemically related (3). Understanding the relationship between physical properties and flavour perception is essential to manufacture healthy food products with maximum acceptability and the food industry would benefit from an improved understanding of the mechanisms behind such changes. In food products, hydrocolloid thickeners are common ingredients which have a profound effect on viscosity and flavour perception. It is generally accepted that increasing viscosity could lead to a decrease in the perceived intensity of volatile (aroma) and non-volatile (taste) components (6, 7). However, the mechanism behind the phenomena by which viscosity affects the flavour perception is not fully understood. One hypothesis is that the viscous or texture properties of a food system can affect the rate of tastant release and therefore modify flavour perception The effects of high shear rheology on flavour perception Chi Zhang1, Jason Stokes2, and Andy Taylor1 1 Food science, The University of Nottingham 2 Unilever Colworth Corporate Research through a change in aroma-taste interaction (6,8,9). Another hypothesis is that viscosity or texture are distinct sensory attributes and can therefore cause cross modal effects at the cognitive level (8). Most hydrocolloid solutions exhibit non-Newtonian behaviour, whereby the viscosity is dependent on the applied shear stress or shear rate. However, the relationship between perceived mouthfeel and the rheological properties of fluid and semi-solid food products are not fully established. If texture attributes could be related to a single physical attribute or a combination, then these parameters could be used to design healthy food products or monitor quality during processing. Several measures derived from instrumental measurements have been descried to correlate closely with perceived thickness. The viscosity at a shear rate of 50 s (10), oral shear rate from the Shama and Sherman curve and Kokini oral shear stress (the apparent shear stress when the tongue compresses food samples against the palate (11, 12)) have been investigated and some correlations have been established. Among these, steady-state viscosity at 50s is generally used to estimate the perceived thickness and creaminess. However, Dickinson’s’ work on oil-in-water emulsions proved that the apparent viscosity at 50s is insufficient to describe fully the perceived thickness or creaminess of some samples (13) and he suggested that the hydrodynamic conditions in the mouth correspond to substantially higher shear rates than 50 s . Furthermore, the instrumental measurement in previous studies was limited from shear rate 1 s to 1000 s and some important information, which are provided by the viscosity at shear rates over 1000 s were ignored. Malone et al (13) proposed that the shear rate on the oral surface could be estimated as being 1000 s and could be correlated to taste intensity. The novel technique of using narrow-gap parallel-plate rheometry to explore the high shear properties of multiphase fluids (14) offers opportunities for studying the effects of both bulk and high shear rheology on flavour perception. The aim of the present work was to evaluate the effects of high shear rheology of the samples on flavour perception and to determine the key rheological parameters for determining mouthfeel perception in vivo. To achieve that aim, samples were designed to have the same viscosity at a shear rate of 50s but have different viscosities at high shear rate. This particular shear rate of 50 s was chosen as it has been considered by several previous studies as the sensing shear rate in the mouth for semi-solid and fluid samples (13) The measurable viscosity has been increased from shear rate 1 s to 100,000 s and hence the effect of the high shear viscosity on flavour perception can be estimated. MATERIALS AND METHODS Materials: Dextran and xanthan were selected as the thickening polymers in this test. . Dextran is a novel food ingredient produced from sucrose by a strain of the lactic acid bacterium Leuconostoc mesenteroides. The reason why we use dextran in our study is that it has a unique rheological behaviour that is Newtonian flow even at high shear rates of several thousand reciprocal seconds. Dextran is tasteless and does not interfere with flavour perception. Dextran was supplied by Meito Sangyo Co., Ltd (Japan) and xanthan was supplied by CP Kelco, (USA). Sucrose and IAA (isoamyl acetate) were chosen as the taste and aroma components. Sucrose was purchased in a local supermarket and IAA was purchased from Aldrich (Germany). All ingredients are food grade and the water was from Evian (France). During the process of choosing the material, pullulan and maltodextrin were selected as other candidate biopolymers and their flow curves are shown in the result section.