ABSTRACT
The word “food” in itself is a complex system comprising of a wide range of biological components with various rheological characteristics. The diversity in these biological components in different food systems impart various compositional and structural variability to the food, thus, exhibiting different types of rheological behaviors viz. low viscosity fluids (e.g., milk), high viscosity fluids (e.g., ketchup) and hard solids (e.g., candies, and gel). The rheological behavior of food decides the stability and appearance of foods such as in the form of emulsions, pastes, and spreads, etc. Moreover, food quality, apart from its nutritional value, is a function of its rheological properties viz. structure and texture. The rheological characterization of food and food forming components is vital for predicting the food quality. Depending upon the form of a specific product (e.g., suspension, emulsion, gel, paste, liquid, solid, etc.) to be analyzed, a range of rheological techniques, tests, and equipments are available. Processing the rheological data in the form of models is vital to infer its physical significance in relation to the flow behavior. Therefore, the present chapter gives an insight into the application of rheological techniques, tests, and theoretical models to predict the quality of foods.
Rheology is the study of flow and deformation of matter in response to the applied force wherein the relationship between applied stress or strain, deformation, and time is described. Viscosity and elasticity are two principle rheological parameters indispensable for describing the consistency of any material/product. Viscosity is “the resistance to flow,” whereas elasticity is “the ability of a material to resist distortion and come back to its original shape.” Apart from viscous and elastic materials, there is a third category viz. viscoelastic material exhibiting both viscous and elastic character. Food is a complex system, both compositionally and structurally. The difference in the proportion of major food constituents such as carbohydrates, proteins, fibers, fats, and others impart diverse rheological flow and deformation patterns. Therefore, food systems can range from low viscosity fluids (e.g., fruit juice) to hard solids (e.g., candy); the intermediate ones such as bread dough. Rheological tools may be detrimental for deciding the quality of the final food product as its quality is influenced by the majority of factors such as composition, processing parameters, engineering processes, etc. Moreover, the sensory attributes of any food product as perceived by the consumer are influenced by rheological properties such as creaminess, tenderness, hardness, juiciness, smoothness, etc. Thus, rheological characterization is exclusively important starting from the raw materials to the finished food product, i.e., prior to processing (raw materials), during processing (intermediates) and after processing (finished food product) (Tabilo-Munizaga, 2005). Apart from food quality, the rheological data are necessary for plant designs, heat, and mass transfer calculations, designing of mixers, extruders, pumps, etc. (Joshi and Ranade, 2003). Another area where rheology is of utmost concern is the new product development, in the last decade. An emphasis had been laid on the development of functional foods, edible films and coatings, low fat and diet foods, weaning foods and nutraceuticals foods, etc. To achieve this we are playing with the ingredient composition, addition or subtraction of some components, altering the concentrations such as use of fat replacer inulin to develop low fat ice cream (Akalin et al., 2008), addition of mango peel powder to develop high fiber biscuits (Ajila et al., 2008) and many more. These additions or subtractions have a direct influence on the textural properties, mouthfeel, and other characteristics detrimental for the acceptance of the product by the consumers. Hence, the rheological evaluation of food products can be applied for the selection of raw materials and process type, and most importantly, the quality control of the final product.
Putting in a nutshell, the study of rheology and its techniques are necessary for:
Establishing a relationship between rheological properties and sensory perception;
Selection of food ingredients for end uses;
Effect on food quality due to compositional changes;
Elucidating the textural quality;
New product development;
Effect on quality of final food product due to processing.
For rheological characterization, various tools and techniques are available. Also, a range of theoretical and empirical models are available in order to predict the material’s performance during the course of processing and experimental conditions. Foods can be liquids, semi-solid, solid solids, and hard solids (Van Vliet et al., 2009; Foegeding et al., 2011). So as to effectively study the role of rheological tools in food quality prediction, it is imperative to have knowledge of basics of rheology, their classification, different models, and rheological tests. The following chapter, apart from these topics, covers the application of rheological tests in the prediction of food quality and its processes.
Basic stress (τ) and strain (γ) relationships are keys to all rheological determinations and classifications. Stress is the force per unit area of a material, given by,
where ‘τ’ is the stress and ‘F’ is the force, and ‘A’ is an area on which force is applied. The unit of stress is N/m2 or Pa.
The strain is the deformation induced in the material in response to the applied stress, i.e., the relative change in dimensions of the material due to the externally applied force; and it is dimensionless. Rheological materials on the basis of stress-strain behavior can be classi...
