Hello everyone welcome to the first lecture of the  course Fundamental of Food Process Engineering. I  

am Jayeeta Mitra assistant professor, Agricultural  and Food Engineering department IIT Kharagpur.  

My two research scholars Soumen Ghosh  and Amaresh will be associated with me  

for development of this course throughout.  

Today I will start the first chapter which  is importance of rheology in the food.  

So, let us first discuss why rheology is  so important in the context of food.  

Food having a diverse nature we know that various  kind of a solid, semi solid liquid products are  

available, purely solid we not get that much in  case of food only for very very low amount of  

strain this kind of behaviour elastic  behaviour is visible, but mostly fluid  

and semi solid nature is widely available. So,  let us first see that what are the various kind of  

food, we see normally around us and what is the  nature of those food, right. So, first let us see,  

this is a picture of a glass of water ok, water we  all know that a it is a fluid and it is considered  

as you might have heard that Newtonian fluid  ok. So, water is having a linear relation  

among the stress and shear rate ok. So, if we  try to plot the behaviour let us do that.  

Let us plot the shear stress and  shear rate behaviour for pure water.  

So, we will get a straight  line and the slope of this line  

is an important parameter that define  the rheological property of water ok.  

So, so this water is a fluid, that is a  Newtonian fluid and it has a linear relation  

between shear stress and shear rate. Now, if  we see the clarified juice ok, clarified in  

the sense there is no suspended particle in the  juice. So, a fully clarified juice showing the  

similar behaviour that is it also show a linear  relation between the shear stress and shear rate.  

Pure honey, so pure honey it is a viscous  material, pure honey also follow the  

similar trend that is a constant slope we are  getting if we apply a changing shear rate and  

plot it with the corresponding share stress. Finally, edible oil we all are very much aware of  

all kind of edible oil whether it is from ground  nut source, whether it is from mustard seed.  

So, this edible oil also follow the  similar nature. So, one thing in common  

in all such food material is that there is  only one property, one rheological property  

that can express the nature of all such  material and that is called viscosity right.  

Now, it is not so simple in many other kind  of food ok, in them if we see the shear stress  

and shear rate behaviour. So, we may not  get a straight line nature, so let us see  

those food. So, now coming to the next slide  

we see that there is tomato ketchup,  tomato soup there is mustard sauce  

and mayonnaise, all four food are very common  to all of us we normally have this in my,  

in our house we eat day to day basis. So, see what  it what the title says it shear thinning behaviour  

with yield stress right. So, all such  food having a shear thinning behaviour,  

now what is that actually? If we go back to this diagram,  

so shear thinning; that means, in x axis where  our shear rate is increasing and corresponding  

shear stress we are measuring in y axis.  So, it will give us a plot of this kind;  

that means, with increasing shear  rate it is having a decreasing  

slope ok. So, viscosity is decreasing at each  instant, if we measure the slope here, here, here  

and then here. So, we are getting that constantly  lowering in the viscosity we may see for those  

kind of material ok. So, it is see how much varied  nature in different food sample we are getting, so  

coming back to the slide. So, these are the fluid food  

showing the viscous property and we also say  that plastic behaviour. So, plastic behaviour;  

that means, it is it is somewhat related with  the this phrase that is yield stress. Now,  

again we will move on to its shear thinning  behaviour which was visible by this,  

now when yield stress is added to that, so  let us say this is sigma 0 and from here  

this kind of a plot we are getting. So, it  shear thinning behaviour with yield stress  

right. So, that means, a initial threshold  is required to cause the deformation  

or flow of such material right. So, again very  special kind of category we can observe here.  

So, from this we can see that different  kind of food which is having shear thinning  

behaviour with yield stress. Now, we will see the next category,  

here we are getting food having shear thickening  behaviour, so just the reverse of the earlier case  

that we are getting. So, these are what sample  this is partially gelatinised starch first one,  

then we are getting a cream, this is also cream  used as a toping of shake kind of material. So,  

again this kind of product is showing a different  nature. So, if just we look into the structure of  

this material we can say the structure is somewhat  more rigid. So, they have a properly framed  

structure and to break that structure; that means,  to cause the flow its needs a higher amount of  

stress or higher shear rate is required. So, this phenomena we can again observe here. So,  

this is called the shear thickening behaviour.  So, in the stress strain shear rate diagram  

we can get this kind of a plot right. So, one  first which was showing the linear relation  

then we have got which is showing non-linear  relation shear thinning behaviour then we have  

seen that the shear thinning with yield stress  and finally, we are getting a behaviour which is  

shear thickening behaviour. That means, as shear  rate is increasing in x direction we are getting a  

higher slope at each and every point ok, so that  means, viscosity is continuously increasing right  

ok. So,  

why this happened because, when we are putting  a shear rate and we are increasing a shear rate,  

so there are certain product which may  form their structure and their structure  

is so ah, you know the structure modification  is such that to break or to cause the flow  

from them it needs a higher  amount of stress.  

Now, next we are having food having time  dependent shear thinning behaviour ok,  

now the new thing came that is time dependent. So,  till now we were discussing about the foods which  

is behaving differently with changing shear  rate ok. Now, we are getting certain amount,  

certain kind of food material like salad  dressing, then soft cheese, whipped cream.  

For them what we are observing is that, that  if we apply a constant shear rate, but for a  

prolong time so shear thinning behaviour; that  means, the lowering of viscosity we can observe  

ok. So, here the time is important that  for how long we are giving that shear rate.  

So, all such different kind of nature is  available in the in the diverse food system.  

So, next is food having viscoelastic behaviour  that is semisolid food. Now, this is again how  

different from the earlier discussion is  that by now we were discussing about the  

fluid food, the food which is having  a the nature is viscous right. Now,  

here we are getting a viscoelastic food  so; that means, it has the property of  

viscous material and is also having  the property of elastic material.  

So, very interesting feature we can observe in  this kind of material, mostly different kind of  

dough sample comes under this. So, this one  is chapatti dough with normally we prepare in  

household cooking every day, then there is ice  cream mix, there is chocolate brownie batter ok.  

So, all such material will have nature that they  will, if if they are being applied certain force.  

So, we can observe the nature that some  amount of deformation will be there that  

cannot be recovered. However, certain amount  of deformation can be recovered so; that means,  

they show both the elastic property  which says that upon withdrawal of force  

the product will come to its initial  shape, at the same time some deformation  

we can observe here. So, that is why these  are called the viscoelastic food and mostly  

this is widely visible in the different kind  of food sample, the semisolid pattern ok.  

So, we can see that a vary diverse nature of the  food available and for that sometime when the,  

when it is better to be explain by this diagram  that sometime one rheological property is enough  

that is this slope to express the food  characteristics or nature. Sometime we  

need to have different different slope at  different different location; that means,  

we need to cover a range of shear  rate. So, that what is the behaviour  

of the material of the food within that strain  rate that is of important, important issue.  

So, in that case not maybe 1 parameter may be  more than 1 parameter we require to characterize  

properly the food in terms of rheological  behaviour. And this is important because we  

may not need to give the shear rate or the range  of shear rate that is applicable for processing of  

all kinds of food, this rate depends on what kind  of processing do you want for your product. So,  

that is why for rheological properties are very  important for processing point of view.  

So, we will see now or try  to just you know try to just  

think over that, where the rheological  properties will help and why it is  

important? So, we have taken a case  of tomato ketchup processing unit and  

we can see that what are the various steps maybe,  first the proper quality good quality of tomato,  

possibly of similar variety a good coloured,  a shape freshness those are received. Then  

they will be grading of the material after  that there will be pulping of the juice  

and we need then deseeding mechanism then sometime  

we mix a certain ingredient to it,  certain spices ah, certain preservative.  

So, all such thing we add ok, then what we do? We  do certain processing like we do pasteurization  

or then after processing we  do filling or canning ok. So,  

the properties of this pulp with seed,  without seed and its rheological behaviour  

all this we need to understand to  design the system in a better way. So,  

design in terms of the consistency  of this pulp from the making the pulp  

to final product is very important and  while we are designing the processing  

line the pipe or the pumping requirement. So, all things depends on the rheological  

behaviour of this product, let us say if you are  heating this and then the viscosity is changing  

or if the shear rate is increasing, so how the  viscosity is behaving. So, all such thing is very  

you know is of interest and is of  preliminary understanding before  

designing such kind of a system. So, one  case I can just discuss is that.  

Let us say this is the pipe flow  

and we consider a fully developed flow in laminar  

region, so we may get this kind  of a profile velocity profile  

ok. So, one  

cross section we have taken and we are getting  this kind of velocity profile. So, this is the  

centre r and here it is  

r o the radius at the outside of the wall. So,  

so this is um, so r o this at the wall and r is  at the centre and we also know that if we try  

to draw the stress, so this stress will be minimum  at the centre and it will be higher at the wall.  

So, this is the stress at  the wall that is tau w and  

this is let us say z direction velocity v z  

ok. So, velocity will be maximum at this  centre and it will be 0 at the wall because,  

it is fixed and there will be no flow at the  wall. So, if we want to know the velocity profile  

so we can have r to r 0, dv z  

by d r, into d r and also we know that  

stress at any distance r will be equal to stress  at the wall into radial distance, where we want to  

measure the shear stress and divided by r o that  is the distance from the centre to the pipe wall.  

Also we know that if the nature of the  

fluid that is the rheological behaviour  of the fluid is known to us so stress is,  

I mean any any point of time  if we want to measure.  

So, we know that it is viscosity eta and  

dv z by dr to the power n will  come to detail of all the nature of  

thedifferent fluid. However, just to express this  particular case we will take that n is equal to  

1. So, this will be a case of Newtonian fluid  so dynamic viscosity into dv z by dr right.  

So, now if we want to know the velocity profile  of this fluid which we are handling in in the  

in a processing line that that we are intended to,  let us say in this case the tomato processing line  

we want to develop or design. So, velocity profile  analysis will be easy if we know the nature of the  

rheological behaviour. That means, the velocity is  can be calculated by this equation where we need  

to know the dv z by dr this is nothing, but shear  rate. So, this behaviour if we know then we can  

calculate the vz from this or else if we know  the velocity, we can calculate what will be the  

shear stress at different section  and the wall and based on that we can  

design the whole processing line ok. So, so similarly very much interesting  

features can be identified or the nature  of different food can be analyse, different  

inference of the nature of the food can be drawn  by analysing the rheological property ok.  

So, we have understood that  food is a complex material  

and the complexity depends on the composition and  nature of the food. So, mostly this foods are you  

know these are they contain high molecular weight  polymers like protein, pectin and other material  

and often they contribute significantly to the  structure formation and viscosity of the food.  

So, when studying the rheological behaviour of a  food a knowledge of the composition of the food  

specially the important structuring component,  that is the dissolved polymer or suspended solids  

this should known to us and structure  of the food itself. That means,  

it is a homogeneous or a heterogeneous composition  or the, what is the phase of that emulsion or what  

kind of materials. So, the structure should be  known to us and processing and storage condition  

because sometime with changing temperature  or with the shear rate with prolong time. So,  

all this change the rheological nature of the  food. So, this is very important to know.  

Now, structure of the food particle if we  consider we will take one example like dispersion.  

So, dispersion we call if large molecules of  or fine molecules, fine subdivided bulk matter  

they are considered to be colloidal  matter, if the particles are in the  

range of 10 to the power minus 9 to 10  to the power minus 6 meter in dimension.  

So, these kind of colloidal matter  which are dispersion, so they are exist  

like milk or cloudy fruit juice and egg white  ok, then there are may be phase transition.  

So, phase transition we know that this is  a change of one phase to the other that is  

liquid to solid liquid and gas this  interchange is called the phase transition.  

So, most of the cases and in case of food we are  getting liquid to solidified state or vice versa  

ph and pressure ph and presence of the  divalent ion as well as enzymatic action and  

these are helpful in conversion of liquid  to solid ok. For example, gels can be  

created from casein, either by enzymatic action  followed by precipitation with calcium ion  

or by acid coagulation. So, by that the  liquid to solid conversion can happen.  

There is another which is gel. So,  by definition gel can be defined as  

a two component system that is the  gelling polymer and the solvent,  

formed by a solid finely dispersed or dissolved  in a liquid phase, exhibiting solid like behaviour  

under deformation. So, both components  that is the gelling polymer and the solvent  

they extend continuously throughout the entire  system and each phase being interconnected.  

So, what will be the structure of  this? So, at the molecular level,  

gelation is the formation of a continuous  network of polymer molecules, in which  

the stress resisting bulk properties which is  the solid like behaviour these are imparted by  

a framework of a polymer chains that  extend throughout the gel phase.  

So, when we call gel setting, that signify the  formation of cross link, while softening or  

liquefaction of the gel; that means, melting  or the destruction of the structure.  

So, to sum up I can say that in the food  industry rheology has a significant role,  

process engineering calculation involves a  wide range of equipment such as pipeline, pump,  

extruder, mixer, coater, heat exchanger,  homogenizers and online viscometer.  

So, all this need a proper rheological  understanding that what is the effect  

of shearing or increasing shear or increasing time  on it, similarly final product quality control is  

very easy if we know before and the rheological  nature of the material. Then evaluating the food  

texture by correlating the sensory data because  sensory perception is very important for any food  

material based on with which we design a food  because consumer acceptance is very important.  

So, to mimic properly the sensory data  texture analysis is being done which is  

you know getting an insight important  insight of the rheological nature of the food  

and analysing rheological equation of  state or constitutive equation which given  

understanding of the flow phenomena or the  behaviour. So, this is very important.