Rheology is the study of the flow of matter, primarily in the liquid state but also as soft solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force (Schowalter, 1978). It applies to substances which have complex molecular structure, such as mud, sludge as, suspensions, polymers and other glass formers (e.g. silictates), as well as many foods and additives, bodily fluids (eg. blood) and other biological materials.
The term rheology was coined by Eugene C. Bingham, a professor at Lafayette College in 1920, from a suggestion by a colleague, Markus Reiner (Steefe, 1996). The experimental characterization of a material’s rheological behavior is known as rheometry, although the term rheology is frequently used synonymously with rheometry, particularly by experimentalists. Theoretical aspects of rheology are the relation of the flow/deformation behaviour of material and its, internal structure (eg. the orientation of polymer molecules), and the flow deformation behaviour of materials that cannot be described by classical fluid mechanics or elasticity. It is also concerned with establishing predictions for mechanical behavior (on the continuum mechanical scale) based on the micro or nanostructure of the material example the molecular size and architecture of polymers in solution or the particle size distribution in a solid suspension. Materials with the characteristic of fluid will flow when subjected to a stress which is defined as the force per unit area. Much of theoretical rheology is concerned with associating external forces and torques with internal stresses and internal strain gradients and velocities (Schowalter, 1978; Bird et al., 1960; Bird et al., 1989., Faith, 2001). Food rheology is important in the manufacture and processing of food products, it is generally referred to as the material science of food and it is defined as the study of the rheological properties of food that is the consistency and flow of food under tightly specified conditions. Understanding rheology of food is critical in optimizing product development, process methodology, final product quality and chemical analysis as well as result interpretation.
However, the consistency, degree of fluidity and other mechanical properties are important in understanding how long food can be stored, how stable it will remain, and in determining food texture. The acceptability of food products to the consumer is often determined by food texture, such as how spreadable and creamy a food product is. Rheology attribute such as texture of food, has a substantial influence on the consumer’s perception of quality and mouth feel during chewing and mastication (Fellows, 2000). The interest in product formulation is growing and stimuli acting various research activities to identify and evaluate the chemical (nutritional) and rheological properties of fruits extracts and their potential application in fruit drink production. Thus, the rheological properties of fruit products are important factors that determine the sensory properties such as mouth feel, texture and consistency. Also stressed materials deform and the rate and type of deformation characterize its rheological properties (Fellows, 2000). To build up an image of the texture properties of the food, below is an example of food eating process which may be seen as taking place in a number of stages (Szczeniak, 1963):
- An initial assessment of hardness, ability to fracture, and consistency during the first bite.
- A perception of chewiness, adhesiveness and gumminess during chewing, the moistness and greasiness of the food together with an assessment of the size and geometry of individual pieces of food.
- A perception of the rate of which the food breaks down while chewing, the types of pieces formed, the release or absorption of moisture and any coating of the mouth or tongue with food. The focus of this work is fruit product formulation where understanding rheology is critical in optimizing product development efforts, processing methodology and final product quality. One can therefore think of food rheology as the material science of food. Thus an extensive research is required to evaluate the chemical (nutritional) and rheological potentials of our locally available plant resources. Chrysophyllum albidum (“udara”)’ peels and seeds are parts of the main fruit discarded while consuming the pulp. The “udara” plants are grown abundantly in Nigeria.
1.1 Problem Statement
The African star apple fruit is perishable with a shelf life of 3-5 days after picking or harvesting. The pulp is mostly eaten while the peels and seeds are discarded. Thus, there is need to identify and evaluate the potentials of the peel and seeds of the fruit (African star apple – (Chrysophyllum albidium), and formulate acceptable shelf stable products based on their rheological properties.
1.2 Objective of the Study
The main aim is to identify the chemical components in the peel and seeds of African star apple (Chrysophyllum albidium) and evaluate the effects of processing on the rheological and chemical properties of their products.
The specific objectives are:
- To produce extracts from the peel and seeds of African star apple.
- To determine the chemical components of the extracts.
- To evaluate the effects of processing methods on the rheological and chemical properties of the products.