Imk209 - Lecture 7 - 22nd October 2012 - — food emulsions & foams

So when we disperse one liquid into another liquid, we don't always get a uniform, homogeneous distribution. That is why we have an emulsion with a mono dispersed system. So we are able to disperse the droplet, having more or less a uniform size. If you measure the diameter and we calculate the range. The range will be very narrow. Because the droplet have more or less the same diameter. So we have a mono disperse system. And usually the emulsion is quite stable. On the other hand we can have a poly disperse system. We have a wider droplet size distribution. So that is quite simple. Towards the end of the last lecture, we want thru some of the new terms and terminologies. I hope when you have made some revision on the meaning of these terms. Because when I use this terms in the lecture, you should be able to understand and relate with whatever we cover in the lecture. For example one of the important terms we have to understand is the disperse system. So food is a complex disperse system. So we actually stop on this slide last week. Chiew you. I have problems pronouncing your name. So let us say. Ice creams, McDonald ice ream. Ice cream is described as a complex disperse system. It is an emulsion as well as a foam. If we use this picture to illustrate ice cream. We take one unit volume of ice cream and put it under the microscope. So how would you describe ice cream as a complex disperse system? How do you describe this system which can represent ice cream as a complex disperse system. What do we have here? Mariam do you like to try? Andrew? This is a disperse system. So what do we have there? Atiqah. Nur Atiqah? Do you like to try to describe what is meant by a complex dispersed system? Food as a complex disperse system. We can use ice cream as an example here. What do we have in ice cream? What are the ingredients in ice cream? We have milk fat. Right? What else? Of course we must always have water, sugar, oil. Well the fat itself. Flavor. Color. So now, foam. We have air bubbles trapped like this in the system. We have maybe hydrocolloids as a stabilizer. Locust bean gum, carrageen. So all this ingredients will be mixed in the ice cream machine. There will be an agitation, mixing. So during this agitation, air will be drawn in and be incorporated into the system. The air will be dispersed in the liquid phase of the ice cream. So we get the foam. Then what happens to the fat? The fat will be dispersed as the fat droplets in the continuous phase which is water in this case. The hydrocolloids can be gelatin, a mixture of locust bean gum, carrageen and other gums. They will provide and increase the viscosity of the continuous phase. At the same time they will be also be adsorbed on the droplet surface and provide protection or the stearic hindrance that will prevent the oil droplets to come close and form a flock and maybe merged. If you want to have a good stable emulsion of foam, we do not want the dispersed phase, the droplets to come close together. We do not want them to touch each other. Or to stick to each other and finally merge. Because if that happens, the emulsion will be stabilizing. The foam will be destabilize and collapsed. So you world not get good ice cream if you do not get a good stable foam in the system. We also have sugar and the sugar is basically in the aqueous phase. Whatever which is soluble in the water phase will be available in the aqueous. We have some flavors, water soluble nutrients, we have salts, we have stabilizers. So this are all solubilized in the aqueous phase. Then we can also add emulsifiers. Emulsifies would form layer and adsorbed layer around each droplet. And this will provide a so called stearic hindrance. So this will prevent the two droplets to join and come close together. And therefore it will stabilize the emulsion. In the oil phase we have whatever is solubilized in the oil phase will be made available in the oil phase. Some flavors are soluble in fat or in oil .and any fat soluble nutrients. And during the process of making ice cream we reduce the temperate to a freezing temperature. And during the freezing process, water will start to form ice crystals. Where else the fat will also form crystal. Fat crystal. So we also have now a solid crystal formed in the oil phase. So, this is basically a complex dispersed system. The good example here that we can use to represent emulsion and foam as well as dispersion of other material is the ice cream. Any questions here? Anyway, let me know introduce a few more terms. If you want to understand more about emulsion. Two terms here which are very important. One is called surface tension and the other one is interfacial tension. So what is the meaning of this? So this is the definition. So let us read this definition very carefully and pick a few key words here. Surface tension is the property of a liquid in contact with air. Liquid and air. Liquid is one phase and another phase is air. That makes it behave as if it was covered with a thin membrane under tension. So maybe we have heard the term surface tension. So remember when we use this term surface tension it refers to the interface between liquid and air. When we have a liquid and liquid, we cannot use the term surface tension. It is not accurate. It is not accurate to use the term surface tension when we have an interface of liquid and liquid. But when we have an interface of liquid and air, we use the term surface tension. We can see the phenomena of surface tension is manifested in the physical form. As you can see here, we have observed this phenomena, what is this? The dew in the morning. They always form a spherical or a semi-spherical shape. It is very nice to look at. The water forms a nice spherical droplets and we can also see this phenomena. I remember in my childhood, when I was a small boy, there was no pipe water at that time. So everything the water comes from the well. So we can see this every day actually. The small insects can move on the surface freely without drowning. So in this case the water form like a skin on the surface of water. So this is also as phenomena of surface tension. If we put a drop of oil on the water. You can find the video on YouTube actually. If you put a drop of oil. Suddenly, this insect will drown into the water. Because something happens there. So that is the phenomena of surface tension where we can observe it in this form. Let us say we have a liquid here in a container. And above the liquid is air. So imagine now, we can see the molecules in the water. So the molecules inside. Let us say we pick this one. It would be attracted to other molecules around it. So this Molecules. We have all these molecules So this molecule. We have the molecule here, the molecule here and the molecule here. They will be attracted to each other. Each one of this. So you can imagine there is a cohesive force which holds them together. They are attracted or pull to each other. Imagine in this class, you are sitting there, and you are holding your hands together. So you can imagine there is a cohesive force which is actually binding the molecules cohesively. They are pulling to each other. Imagine now in this class, you are sitting there and you are like holding your hands together. SO there is a cohesive force that is kind of binding the molecules cohesively. They are pulling to each other. But imagine now that the molecules on the surface here, between the interface of the liquid and the air. So now you can imagine, each molecule on the surface here are now being pull down by other molecules down here. But there is no equal force which is pulling them from the top. Because on the top here is the air. So now, there is an unequal, unequal attractive force from below and from the top. So can you now imagine that the molecules inside here are pulling the molecules at the interface of air and water. So they are pulling it, such that they will form a droplet, spherical shape. Because the molecules now are pulling down the melees on the surface because there is no equal attraction on the top from the air? So that is how you get the spherical shape on the surface. And this is what we can measure, as an air liquid interface which is also known as the surface tension. So now we can describe the situation which I have just described just now. The molecule inside the liquid interact equally with other molecule, from all sides. Whereas the molecules on the surface is only affected by the molecule below it. The molecules below it pulling the molecules on the surface down. The molecules exposed to air behave differently and try to contract to the smallest possible area (Hence the spherical shape that we observe). Can you imagine? I think it is quite easy to visualize that. So now we define Surface tension as (Newton per meter) and not Nm2. If you have the unit of Newton per meter square, that will become what? In rheology, stress, which is the pressure. The intensity of the force acting on the m2 or inch2 per unit of area. So this one is Nm. So be careful of the unit. So surface tension is defined as the force acting over the surface of the liquid per unit length of the surface perpendicular to the force. So that is how we define surface tension. The force acting over the surface of the liquid per unit length of the surface perpendicular to the force. So when we, measure surface tension. How do we measure the surface tension? I will show the video shortly. Usually we put a wire inside here then we pull it up. So when we pull the wire up to the surface, it will sort of break the liquid air interface. Which is kind of like if you can imagine is sort of a membrane or a skin here. So the energy required to expand the surface area. Because this area between air and liquid is actually contracting to form a spherical shape. So now if you want to measure the surface tension. You have to expand the surface and break it up. So you need energy for that. That is why it measured as a form of force acting over the surface of the liquid per unit length of the surface perpendicular to the force. I will show the video shortly on how to measure the surface tension. But let us give a proper definition for surface tension. Sp that is the symbol to denote surface tension. It can also be defined as the amount of energy required to increase the surface area between a liquid and a gas ( e.g. air and water) by and amount of delta A. So that is the amount of energy to expand or to increase the surface area between a liquid- gas interface by this amount of area. Where else Interfacial tension is defined as the amount of energy required to increase the interfacial area between two immiscible liquids. For example, oil and water. So when we have liquid in air, we use the term surface tension. When we have a mixture of oil and water. Oil and water will have its own surface tension. When they are mix together. They will form an interfacial tension. Interfacial tension is defined as the amount of energy required to increase the surface area between two immiscible liquid. To measure a surface tension, basically we put the emulsion in a container and this case; we have the liquid and we have air above it. So this is actually a very thin wire usually made of platinum. Because platinum is used here. When it comes to contact with liquid, the whole part of the platinum wire will become wet and in full contact with the liquid. So we will drop down the wire, so that it can touch the liquid, like this. From here. We drop it down slowly. Then after that we pull it up slowly like this. And you can see at this point, that is when we. From this point to this point, number 6 to number 8, we have to pull it up very slowly. That is when we can get the delta to. So when we connect this to a computer, we can plot the force against time. And form this curve; we can do some calculation to calculate the surface tension of the liquid. So let us see this video. So this instrument is call tensiometer. That is the advance instrument. Computerize but in the old days, the one that we have in the lab, which I bought for my research last time, this one. It is still there. It is an old method. It is manual. But the good thing is that we learned it from the first principle. Now days everything is computerized, you treat everything as the black box. You use it and get the results, but you do not know what happens. The fundamentals. If you want to learn more, you can just go to my wiki. I have put a link down there. It is optional. I am not forcing you to use this wiki. To understand emulsion and foam, we must first understand the basic concept of surface tension and interfacial tension. So one very important quality of the colloid or the emulsion Is the large interfacial area between the dispersed and the continuous phase. That is the reason why when we prepare an emulsion or foam. What we want to try to achieve is to disperse the liquid or the air into the continuous phase to get a very,very fine small disperse phase. Small droplets with a very small diameter. The smaller the droplet, the better. Because it will increase the interfacial area between the liquid- liquid interface or the liquid air interface. To illustrate this let us compare two situations here. The importance of a large interfacial area. Let us say we prepare an emulsion. The volume of the emulsion is 20cm3 of oil in 1 cm radius droplets. So we measure the droplets about 1cm. Then we can calculate the volume, using the normal way of calculating the volume. Which has a volume of 5.5 cm3 and a surface area of 12.5 cm2? So, now we have this much oil. Each one has a volume of 5.5. Then we can calculate now. We can have about 3.6 droplets. And a total area of 45.5cm2. Starting from 20cm3 we disperse into the continuous phase, we get about 5.5cm3 and we can have 3.6 droplets or about 4 droplets. Let us say. And this is the total area. Now the same oil, but we make it into a smaller droplet Instead of 1cm, each will be about 0.1cm. Ten times smaller. Each has a volume of now instead of 5.5cm3, it will have 0.004cm3. Look at the surface area, from 12.5cm2 to 0.125cm2. Hundred times smaller. Right? Now we can have, instead of 3.6 droplets, we can now have about 5000 droplets. And a total area of 625cm2. Meaning now we are able. By having a smaller droplet it can increase the interfacial area between the liquids-liquid or liquid and air tremendously. From this much to that much. SO this is one thing we need to bear in mind, in the preparation of the emulsion. The factor of the interfacial area. Why is interfacial area important? Let us look further. Let US now talk about food emulsion properly previously we talk about the food dispersion. But now we want to zoom into two types of dispersed system. Emulsion and foam. We will start of with emulsion. Which is a dispersion of one liquid into another liquid. Oil into water or water into oil. With one liquid disperse as small spherical droplets in the other. In food, the diameter of theses droplets usually falls somewhere in the range of 0.1-100um. Two types of simple emulsion. Water-in-oil (w/o). The examples are margarine, butter and spread. And the other one is oil-in-water (o/w). The examples are mayonnaise, sold dressings, milk, beverages, cream, soups, and sauces. Bear In mind, like just earlier, when I use ice cream as an example when you have fat in the system or an emulsion. Whether it is in the disperse phase or the continuous phase, depending on the temperature the fat itself can crystallize. And form more solids crystals. When it starts to crystlize, it will change the properties of the emulsion. Imagine in the process of making margarine. In the process of making margarine we start it out with liquid oil Then we cool it down and the fat starts to crystallize. In the ice cream. In the process of making chocolate. The dispersed and/or continuous phase of much food emulsion may be partly crystalline rather than being completely liquid. In margarine. There is always at any temperature, there is always some part of the fat in the form of crystals. Some of the lipid will form it. Formation of an emulsion required the dispersion of one phase into small droplets. This results in a massive increase in interface area between the dispersed and the continuous phase. As we saw in the earlier slide. And in order to disperse the oil into the water or vice versa. So we can use the process of homogenization. Or we can use a high speed hear mixing process. By using a high speed mixer. Or there are a few other ways. Homogenization is the process by which the dispersed phase is broken down in to small droplets. So in this case actually we operate at high pressure between (10-100MPa) are now very common. So when we use a very high pressure homogenizer. We can now be able to disperse the oil into very, very fine droplets. So we get a very, very stable emulsion. So basically, we have to apply a shear force. So when we use the high shear mixer for high pressure homogenizer or high speed homogenizers, basically we are shearing the liquid and forming it into very small droplets like this. So we are using a high pressure homogenizers. In the lab we can use the hand held type like this. There are a few ways. Other ways of doing this. This is a high shear mixer. This one is using ultrasound. So you can see that is very, very efficient using ultra sound. So we have an ultra sound probe here. So it can disperse very, very efficiently Disperse the oil. Try again. This one is using a normal agitator. This one is using a high shear speed mixer. You can compare the stability. So I have shared a few more different types of method which can be used to prepare a stable emulsion. It is there in the enmodule. Feel free to go thru it when you have time. Any questions so far? Ok I guess we can stop here. Tomorrow there is no lecture for IMK221. But I have given you something to occupy the time. There are two online presentations. Please us the time tomorrow, to watch this two presentation. Make a note. Use the one hour slot tomorrow. Please watch the two online presentation tomorrow. It is not from Youtube. It is directly linked from the prepared food website. Make a note. And then I will give further instruction on what to do. Just check on enmodule what are the things you need to do f=after you have watched the two online presentations. So I will see you on Wednesday. Thank you.