My+Assignments+Or+Activities

__ MY ASSIGNMENTS __   1.In your own words, describe the meaning of "yield stress". Yield stress is the stress that we must applied in order to make a stuctured fluid flow.If we put it under small stresses,the material will act or behave as solid.But if we put it beyond a critical stress,it will flow like liquid.For example,we can say that mayonnaise exhibit behavior more to soft-solid rather than a thick liquid.If we push a spatula laterally and gently on the mayonnaise,it will deform .But then it will return back to its original position when we remove the force.Then,give the extra force on it until it exceed its critical stress.It will not turned back to its original position and undergo permanent deformation.In other word,the mayonnaise flowed.

2. Why only some fluids exhibit yield stress? Give appropriate examples.

HINT: Think about the structure -- recall my demo during the lecture. Some fluid exhibit yield stress because of the differences in the composition of fluids.For example,only a very minimum force needed to make the water in the bottle flow.The gravity force is enough to make it flow.This is because there are only molecules of water that bond with each other.But,for honey,it compose of many other substances in it such as sugar or glucose.It will binds with any other substances that caused the structure to changed and form a long chain.Thus it become harder to flow and needs more force or stress to make it deform .

1)In your own words, explain the meaning of rheology?

Rheology is the study of the relation between forces exerted on the material and the deformation of that material.It also can be said as applying a force to a material and measure its flow or change in shape.It is applicable to all materials from gases to solids.For example,when we scoop a peanut butter from jar,the shape of the remainder butter in the jar will remain the same till the next day.But,for sauce in a bottle,when we turn it upside down and then let it be in its initial condition again,it will flows and its surface will flattens back in the bottle. Reference: __ What is Rheology Anyway? __ Written by F.A. Morrison 2) Why food technologist needs to know about rheological properties? Hint: Think in the context of food processing and application. Food can be characterized according to its rheological state such as solid,gel,liquid,emulsion with related to rheological behaviors and its rheological properties can be measured.Food technologist need to understand the complex relationship between the overall rheological properties of food and the types and concentration of ingredients that they contain.It enables them to optimize the ingredients the processing conditions needed to produce high quality products.Different properties will affect the design of food processing plants as well as shelf life and sensory properties that appeal to consumers.From the thin liquid such as water to the hard,solid product such as candies,food technologist need to produce the hard candies that will melt in the mouth during the consumption.As customer,of course we want to enjoy ice cream that has a good texture and give us mouthfeel right?In processing,different types of properties need different ways of process.For example,if we want to make powdery product such as instant coffee,we will use freeze drying machine.But,if we want to make liquid product such as fruit juice,we need to use other method. Shelflife also differ according to the products properties.Shelflife is the period of time a product can be stored before its quality starts to worsen. It depends a lot on handling and storage conditions.For examples,estimated shelflife for flour is about 6-8 months,cracker is 8 months.mayonnaise is 2-3 months and rice is 2 years. 3) How do we modify or control the rheological properties of food? For quality control,a simple,cost-efficient method may be appropriate.Different technique need to be used for liquids than for solids.Liquid product need precise information on its flow properties under certain conditions before designing an energy-efficient heat exchanger,which actually make it more expensive test need to be done.Besides that,the properties of food can be modify by adding or altering the composition of the product during processing.For example,addition of shortening in the baked goods give tender quality to it. References: <span style="font-family: 'Times New Roman',serif;">i) Andrew J. Rosenthal,Food Texture:Measurement and Perception,Aspen Publishers Inc,Gaithersburg,Maryland,1999.

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