PRACTICAL 7 CHEMISTRY OF COLLOIDAL PARTICLES

Transcription

1 PRACTICAL 7 CHEMISTRY OF COLLOIDAL PARTICLES Structure 7.1 Introduction 7.2 Colloidal Systems in Foods 7.3 Gels 7.4 Foams 7.5 Emulsions Activity 1: Study the Effect of Various Additives on the Stability of Egg White Foam Activity 2: Demonstration of the Effect of Foaming in Preparation of Cold and Hot Soufflé Activity 3: Determination of the Best Method of Preparing a Stable Emulsion 7.1 INTRODUCTION Unit 7 in the theory course presented a detailed review on the colloidal system their nature and properties. The focus was on foams, gel, sols and emulsions. Here in this practical, we will get hands down experience of preparing these colloidal systems and analyzing their properties and factors which influence them. There are three activities in this practical. Objectives After studying this practical and undertaking the activities given herewith, you will be able to: define and identify different colloidal systems (gels, foams, emulsions etc.) recognize the different stages of foam formation and explain the factors influencing foam formation, apply the concept of foam development in preparation of various foods, prepare hot and cold soufflés and learn about factors affecting formation of soufflés, and prepare stable emulsions and recognize the factors affecting the formation of a stable emulsion COLLOIDAL SYSTEMS IN FOODS A colloidal system is a heterogeneous system. The material that forms the base of the system is called the dispersion medium or the continuous phase. The material that exists in the colloidal condition is called the dispersed medium or the discontinuous phase. Of the three states of matter (solid, liquid and gas), eight classes of colloidal systems can be formed. Solid in a solid, a solid in a liquid, a solid in a gas, a liquid in a gas, a liquid in a liquid, a liquid in a solid, a gas in a solid and a gas in a liquid. Look up sub-section in the Unit 7 in the theory course (MFN-008) which presents this classification. Colloidal particles, we also learnt, are in motion and are electrically charged. Colloidal systems may be lyophilic or lyophobic. These properties affect viscosity, as even changing the concentration of solutes like sugars and salt and the addition of electrolytes may cause precipitation. The colloidal property of adsorption makes it useful in cookery, giving protection against precipitation and agglomeration. Another property of imbibition, is the ability to pick up water and swell. Components such as starch, proteins and fat when dispersed in water forms colloids.

2 Examples of such dispersion in our food system are the formation of gels, foams and emulsions. Some of the colloidal systems in foods are listed in the Table 7.1. Table 7.1: Colloidal Systems in Food Chemistry of Colloidal Particles S. No. Name of the colloidal system Dispersed phase Continuous phase Example in food 1. Emulsion Liquid Liquid Salad dressing 2. Sol Solid Liquid Gravy 3. Gel Liquid Solid Baked custard 4. Foam Gas Liquid Egg white foam 5. Suspensoid Gas Solid Whipped gelatin In this practical, we will be highlighting the properties of gels, foams and emulsions with various factors affecting their formation. We begin with the properties of gels. 7.3 GELS Gels may be formed by the proteins of egg or flour in products like soufflés, puddings, custards, batters and doughs. When the protein particles are dispersed in water, the solution like mixture results in the formation of a sol. When a sol assumes a rigid form, it is referred to as a gel. The change of sol to gel form may be brought about by a change of concentration of the dispersed phase, a change in temperature, or a change in the hydrogen-ion concentration or electrolyte content. Gel formation takes place when the dispersed phase develops into a network structure that holds the liquid phase in its meshes. In some gels, the framework can be broken by agitation or heat. When this happens, the gel structure reverts back to the sol form. However gels formed as in case of baked custards are of non-reversible types. When only a part of sol changes to the gel form, the process is known as flocculation. An example of this process can be seen in heated milk when a precipitate coats the bottom of the pan. Syneresis is the process when the gel on shrinkage results in the loss of liquid. This process was first observed in 1861 by T. Graham, who described the process as an exudation of small amounts of liquid on standing because of a slight contraction of the gel. Although no net volume change occurs in the gel, syneresis cannot be described as a reversible process. 7.4 FOAMS Foods like meringue, ice cream and beer contain foams. A number of foods possess good foaming properties on account of the proteins they contain that can provide the stability to products and impart characteristic qualities to foods. Surface proteins are stretchy and flexible almost like a loose rubber sheet, which provide resistance to disruptive shear forces and compression/dilatation forces. This is obtained by denaturation/unfolding of polypeptide chains and exposing substantial regions of hydrophobic residues into air or lipid phases where they are stable. The ability of the protein films to resist shearing and high level stretching capability contributes to good foaming and emulsifying properties. These properties are provided from extensive interactions between protein molecules and reach a maximum at the isoelectric point of the protein where electrostatic repulsion is at a minimum. Foaming is characterized by a foam expansion index (FEI) which indicates the foam formation property of any product and a foam liquid stability (FLS) which is the indicator of the strength of foam. The foam expansion index and the foam liquid stability of proteins is given in Table

3 Principles of Food Science The foam expansion ratio is the ratio of the weight of a given volume of expanded foam to the weight of that same volume of unexpanded foam solution. Table 7.2: The foam expansion index and the foam liquid stability of proteins Protein FEI % FLS % Whey protein Egg albumin Casein Soya (hydrolyzed) Gelatin Lysozyme From Table 7.2 it can be observed that casein forms good foam which is not stable. Egg forms a stable foam. Whereas gelatin provides good amount of foam that is also stable. Thus, we can say that not all proteins foam equally well and not all of the foams are particularly stable. Foam is caused by the protein film lowering surface tension i.e. cohesive force of water molecules that tend to collapse a bubble, and give resistance to shearing/tearing with high level of stretching capacity. So when protein solution is whipped or stirred vigorously, air is pulled down into solution and when it tries to escape up, flexible protein surface forms a bubble. Next, let us review the properties of different food foams such as milk foam, cream foam and egg white foam. A) Milk Foams The proteins and water in milk are extended into thin films by agitation. These thin films enclose small air bubbles to make foam in which the protein and water provide the continuous network of the colloidal dispersion, and the air is the discontinuous or dispersed phase. This arrangement is possible because the native proteins in milk have a low surface tension and low vapour pressure. The low surface tension makes it possible to spread the liquid proteins into thin films, and the low vapour pressure reduces the likelihood that evaporation will occur. In fluid milks, the concentration of protein is too low to permit the production of foam with any stability. However, evaporated milk can be whipped into foam with a very large volume. The increased protein and fat concentrations of undiluted evaporated milk make it possible for the foam to form, and the foam will even have some limited stability. Foam formation and stability are enhanced if the undiluted evaporated milk is chilled until ice crystals start to form in it. This condition causes the fat to be rather firm, which concentrates the protein in the remaining unfrozen water and also helps to give some rigidity to the cell walls in the foam. Stability can be achieved by adding lemon juice because the acid promotes precipitation of the milk proteins to give more strength to the cell walls. B) Cream Foams Cream, with a fat content of at least 30 per cent, can be beaten to form foam. However, it is observed that a cream with 36 per cent fat can be beaten quickly to more stable foam. The fat contributes considerable rigidity to the cell walls in whipped cream foam that is kept chilled. Over beating whipped cream causes reversal to water-in-oil emulsion. Butter results. As fat is the principal component contributing to the strength of the cell walls, it is essential that whipped cream be stored under refrigeration until the time it is served. If cream is allowed to begin to warm to room temperature, the fat will start to soften, and the rigid cell walls containing the warming fat will weaken. If warm enough, the whipped cream foam may melt into a liquid system.

4 C) Egg White Foams Chemistry of Colloidal Particles Egg white foams when whipped or beaten, the air gets entrapped in the liquid present, and an interfacial tension is established between the air-liquid interface. The three proteins which are responsible for foam formation in eggs are globulin, ovalbumin and ovomucin. On beating, ovomucin gets sheared to form hollow tubes about µ in length. The foam is further helped by some protein coagulation at the air-water interface. With agitation, the egg albumin can be spread over a large surface area, and air can be incorporated into bubbles created by beating the proteins. Some of the proteins are denatured by beating action and then aggregate to enhance stability of the developing foam. Figure 7.1 illustrates the egg white foam. Figure 7.1: Egg white foam Foams need to be stable if they are to have a role in food preparation. Stability is enhanced if the surface tension is low, if vapour pressure is low, and if a substance solidifies on the surface of the bubbles. Egg white meets all of these requirements, making it a particularly useful food when foam is desired. Egg foams at different stages of formation, as illustrated in Figure 7.2, have many applications in food preparation and processing such as: 1) Soft peak stage, as shown in Figure 7.2(a), is used in the preparation of baked products like cakes, meringues, soufflés. 2) The stiff peak stage of foam, as shown in Figure 7.2(b), is needed for hard meringues and chiffon cakes, as well as sponges and soufflés. 3) The initial peak stage can be used for clarifying soups and consomme. 4) Products like french toasts, fluffy omelets, angel cakes require egg foams. Figure 7.2: Foam formation in egg white 117

5 Principles of Food Science Having looked at the properties and applications of egg white foams, we move on to the stability of egg foams. Egg foam stability The two factors of utmost importance in egg white foams are stability and volume. Several factors influence one or both of these characteristics and are of significance regardless of the product into which the foam is ultimately incorporated. The extent of beating is an important factor in stability of egg white foam. As beating progresses, the foam becomes increasingly stable up to a critical point, after which continued beating decreases stability. Maximum stability is reached when the whites just bend over, but before maximum volume has been reached. If beating continues beyond the point of maximum stability, the surface begins to look slightly dry, and the foam exhibits some brittleness. Foam formation is delayed when the whites are well below room temperature. You would realize that there are various ingredients which influence the stability of egg white foams. We shall review these ingredients, next. Effect of addition of ingredients on egg white foam stability The addition of other ingredients also influences stability. Sometimes salt is added to an egg white foam for flavour, but this addition reduces stability slightly. Occasionally recipes include some added liquid in making egg white foam. This dilutes the proteins in the foam and decreases stability. If yolk happens to contaminate the white at all, as can happen during the separation of yolks and whites, stability of the foam formed from the whites is reduced. Not all ingredients reduce stability. In fact, the addition of sugar has a very laudatory effect on foam stability. A possible explanation is that the addition of sugar delays foam formation significantly, which means that considerably more beating is necessary to reach the proper stage of foam development. This increased beating results in foam with a finer texture and more surface area. This foam is stabilized with protein that has been partially denatured by beating. Acidic ingredients, commonly either cream of tartar or lemon juice, are useful stabilizing agents when making egg white foams, particularly when added early in the formation of the foam. Although stability is promoted by reducing the ph of the egg white foam, formation of the foam is delayed by this addition. Again, the delay in reaching the desired end point in whipping the foam results in increased total agitation and a finer, more stable foam. Cream of tartar is particularly effective as the acid ingredient when the ph of the white foam approaches ph 6.0, whereas, citric acid and cream of tartar are about comparable in their effect at ph 8.0. With this, we come to an end of our discussion on foams, next let us look at the emulsions. 7.5 EMULSIONS Emulsions, as you may already know by now, are colloidal dispersions of a liquid in another liquid with which it is immiscible. Emulsions can be formed by shaking the two liquids together until they appear to be well mixed. Shaking provides the energy needed to enable the liquid with the higher surface tension to form many small droplets or spheres which then are surrounded by the other liquid. Formation of these numerous small spheres creates a far larger surface area for the dispersed liquid that was the case when two liquids were in contact as two layers. The droplets of the dispersed phase tend to coalesce when they bump into each other as they move through the emulsion because the one large droplet represents a lower energy state than two. Because one large droplet has less surface area than two small droplets, thus the droplets in an emulsion continue to coalesce into larger droplets until the emulsion breaks or separates into two distinct phases. 118

6 Emulsions, as you may recall studying in Unit 7 in the theory course (MFN-008), are classified on the basis of the type of liquid constituting each of the phases. Let us quickly review the classification and types of emulsions once again. Classification of Emulsions Emulsions are classified as: Oil-in-water emulsion: This is a colloidal dispersion in which droplets of oil are dispersed in water. Example is mayonnaise. Look at Figure 7.3 which illustrates the oil-in-water emulsion. Chemistry of Colloidal Particles Figure 7.3: Oil-in-water emulsion Water-in-oil emulsion: This is a colloidal dispersion in which droplets of water are dispersed in oil as illustrated in Figure 7.4. Example of water-in-oil emulsion is butter. What are the types of emulsions? Emulsions can be of three types: Figure 7.4: Water-in-oil emulsion 1) Temporary emulsion: These require vigorous shaking just before they are used, as they separate out easily on storage, as used in French dressing and Italian dressings. Such emulsions exhibit a low viscosity. 2) Semi permanent emulsion: Such emulsions have viscosity of thick cream and thus are more stable than the temporary emulsion. Examples of such emulsions are salad dressings containing syrups, honey and condensed soups. 3) Permanent emulsions: These are characterized by a very high viscosity and therefore are very stable. They contain emulsifiers and stabilizers which aids to their stability. Example of this category is mayonnaise. With a basic understanding about the types of emulsions, let us also review the uses of emulsions in food preparation. Uses of Emulsions Emulsions provide many useful functions in food preparation and processing. They: 1) act as vehicles for flavour to foods, 2) dilute ingredients, 119

7 Principles of Food Science 3) hide objectionable odours or tastes, 4) provide variety in food preparation, 5) control agglomeration of fat globules in food products, 6) modify the rheological properties of doughs, by reacting with gluten proteins, 7) improve wettability and dispensability of dehydrated products, 8) modify crystals in candies, and 9) dissolve flavours and essential oils in micelles of aqueous systems. Having looked at the uses of emulsions next, let us learn about the stability of emulsions. Emulsion stability The stability of an emulsion is determined by the viscosity of the continuous phase, presence of an emulsifier, the concentration of the emulsifier, size of the droplets and the ratio of the dispersed phase to the continuous phase. Interfaces in emulsions and foams can be stabilized by: small molecule surfactants, adsorbed macromolecules often proteins, and fat globule networks. We have studied above that the stability of an emulsion is influenced by emulsifiers. What are emulsifiers? Let s get to know them. What is an Emulsifier? An emulsifier is a compound that contains both polar and nonpolar groups and thus is drawn to the interface between the two phases of an emulsion to coat the surface of the droplets. The nature of the emulsifier influences the type of emulsion that is formed. If it is attracted more strongly to water than to oil, the surface tension of water is reduced more than that of oil. The result is the formation of an oil-in-water emulsion. What is the role of the emulsifier in food systems? Let us find out next. The major functions of emulsifiers in food systems are as follows. The emulsifier: modifies the intermolecular forces which stabilize or destabilize the structure of food colloids and gels, lowers the interfacial tension between two liquid phases or liquid-air phases, reduces the pressure gradients required to disrupt the droplets when forming a dispersion, reduces the tendency of droplets once created to coalesce, alters the flow properties, modifies the crystallization of fats/oils, improves whipping quality of foam, and interacts with starch and protein components in foods, which modify texture and rheological properties. So we have seen that emulsifiers have an important role to play in food systems. There are different types of emulsifiers, which can be used. Let us look at these different classes of emulsifiers next. Classification of emulsifiers Emulsifiers can be classified under two categories. These are: Natural: These are naturally present in foods. Some examples are phospholipids and lecithin in egg yolk, which are responsible for the natural stability of mayonnaise and other products. 120

8 Synthetic: These are chemicals prepared in the laboratories, which are used to stabilize food products. These are further classified as emulsifiers affirmed as GRAS (Generally Regarded As Safe) and emulsifiers as direct food additives Table 7.3 gives the list of these commonly used food emulsifiers. Table 7.3: Common food emulsifiers Chemistry of Colloidal Particles S.No. Emulsifier Typical Applications Emulsifier affirmed as GRAS 1. Lecithin Margarines, chocolate products 2. Monoglycerides Margarines, whipped cream, ice-cream 3. Diacetyl tartaric acid Baked goods, confectionary, dairy products 4. Monosodium salt of phosphated monoglycerides Emulsifiers as direct food additives Dairy products, soft candy 1. Lactylated monoglycerides Baked goods, whipped toppings 2. Acetylated monoglycerides Pizza 3. Succinylated monoglycerides Shortenings, bread 4. Ethoxylated monoglycerides/ esters (TWEENs) 5. Sorbitan monostearate/esters (SPANs) Cakes, whipped toppings, frozen desserts Confectionary coatings, yeast cakes, icings 6. Polyglycerol esters Icings, salad oils, peanut butter, fillings 7. Sucrose esters of fatty acids Baked foods, fruit coatings, confectionary 8. Polysorbates Salad dressings, coffee whiteners, ice creams 9. Propylene glycol esters Cake mixes, whipped toppings GRAS: Generally Regarded As Safe With this basic knowledge, let us now carry out the activities given in this practical. There are three activities included in this practical. So get started. 121

9 Principles of Food Science ACTIVITY 1 STUDY THE EFFECT OF VARIOUS ADDITIVES ON THE STABILITY OF EGG WHITE FOAM Date:. Aim: To study the effect of various additives on the stability of egg white foam. Objectives After undertaking this activity, you will be able to: recognize the different stages in the foam formation, explain the effect of various factors in foam formation, and apply the concept of foam development in preparation of various food. Principle The two factors of utmost importance in egg white foams are stability and volume. Several factors influence one or both of these characteristics The extent of beating is an important factor in stability of egg white foam. As beating progresses, the foam becomes increasingly stable up to a critical point, after which continued beating decreases stability. Maximum stability is reached when the whites just bend over, but before maximum volume has been reached. If beating continues beyond the point of maximum stability, the surface begins to look slightly dry, and the foam exhibits some brittleness. Foam formation is delayed when the whites are well below room temperature. Addition of ingredients The addition of other ingredients also influences stability. Comment on the stability of foams as influenced by the addition of the following ingredients in the space provided herewith. We have already discussed about these effects earlier in section 7.4 under the heading egg foam stability: Addition of Salt Addition of Egg Yolk Addition of Sugar Acidic Ingredients 122

10 Equipments Measuring cylinder, funnel, stopwatch, filter paper, beater (rotatory, whisk beater). Materials Eggs, salt, castor sugar, cream of tartar, fat, distilled water. Procedure Now carry out the experiment step-by-step as enumerated herewith: A) Preparation of white egg foam. i) Separate the white from the yolk egg. ii) Beat egg white to stiff peak using a rotary beater or a whisk and note the time taken for foam formation. iii) Transfer the prepared foam to a funnel lined with filter paper and placed over a measuring cylinder. Keep aside for 45 minutes and record the volume of the liquid drained. Use one egg white for each of the variations given below and carry out the variations B to I repeating the procedure of preparation of egg white foam as given above in A (i-iii). B) Add 1/8th teaspoon salt to egg white for preparing the foam. C) Add 1/8th cream of tartar and then beat. D) Add 1/4th of egg yolk and then beat to attain the foam E) Add 10 ml of water and then beat F) Add 2 teaspoon of castor sugar and prepare the foam G) Add 10 ml distilled water and proceed H) Add 10ml oil and beat I) Use a different type of beater After keeping the foams for 45 minutes, compare the results of the samples A to I (total of 9 samples) with respect to time taken for foam formation, colour of the foam formed, stiffness, dullness or shiny texture and other sensory attributes to determine the most stable foam. Record your observations in the format given on page 124 under results and observation section. Precautions 1) Egg white should be separated carefully from the yolk so as not to mix even at trace of the yolk. 2) Beating should be done till the stiff peak stage only. 3) Added substances should be measured accurately. 4) Time should be noted when the stiff peak is reached. 5) All samples should be neatly labeled on completion and drained for exactly 45 minutes before observations are recorded. Chemistry of Colloidal Particles 123

11 Principles of Food Science Results and Observations Record the observations made as indicated in the format given herewith. Record the effect of added substances on foam formation: Variation Time (min) Colour Foam Liq. Texture Foam Liq. Gloss (shiny or dull) Stability (Yes/no) Drained Liquid (ml) Applications Egg white A. B. C. D. E. F. G. H. I. From the above observations, inferences may be drawn regarding the stability and suitability of various foams for food preparation and processing. Inference Egg white foam when treated with acid results in Egg white foam when treated with salt results in Egg white foam when treated with sugar results in Egg white foam when treated with oil results in Egg white foam when diluted with water results in 124

12 Conclusion (Comment regarding the stability and suitability of various foams for food preparation and processing) Chemistry of Colloidal Particles Submit the activity for evaluation... Counsellor Signature 125

13 Principles of Food Science ACTIVITY 2 DEMONSTRATION OF THE EFFECT OF FOAMING IN PREPARATION OF COLD AND HOT SOUFFLÉS Date:. 126 Aim: To demonstrate the effect of foaming in preparation of cold and hot soufflés. Objectives After undertaking this activity, you will be able to: learn how to prepare hot and cold soufflé, and explain about the various factors affecting formation of hot and cold soufflés. Principle Soufflés are egg white foams, which can be modified to prepare savory or sweet dishes. These preparations usually involve beating of egg whites till they foam to peak stage and then folding them into a flavoured base of other ingredients. If served hot, they are usually baked just before service so as to prevent the foam from collapsing on cooling. In such preparations the air entrapped in the mixture during foaming expands on heating to give volumes, which may be 2-2 ½ times greater on cooking. This results in products having a light soft texture and mouthfeel. However, on keeping the product, its volume decreases due to some of the entrapped air bubbles escaping where the protein fibers are weak, having lost their elasticity. A well-made product shows only slight variations in the volume because of denatured surface proteins on foaming, provide enough elasticity to stabilize the product. When served cold they are refrigerated immediately after the mixture is ready so that the protein films formed around the air bubbles get set preventing their escape and maintaining the volume of the desert. Equipments Baking dish, soufflé dish, frying pan, weighing scale, measuring cups, spoons, spatula, beater refrigerator and oven. Materials Eggs, cream, lemon or orange, castor sugar, gelatin, water, essence, butter paper. Procedure Now carry out the activity step-by-step as enumerated herewith. Start with the preparation of cold soufflé. A) Preparation of a cold soufflé i) Prepare a soufflé dish. ii) Wash and dry a small lemon. Grate the rind, extract the juice and keep aside. iii) Take one egg, separate the white from the yolk, putting the yolk in a bowl along with ¾ of lemon extract and 30 g of sugar. iv) Place the bowl containing the mixture in a double boiler in which the water is simmering at 97ºC and beat or whisk until creamy. v) Remove the bowl from the boiler and continue beating till the mixture is cool. vi) Sprinkle 5 g gelatin in 30 ml water and keep aside for few minutes to swell. Then stir to dissolve. vii) Add the gelatin to the egg yolk mixture gradually and mix well. Place in the refrigerator to set. viii) Whip 80 g cream, and beat egg whites till peak stage. ix) Fold in whipped cream and the stiff egg white into slightly set yolk mixture. x) Pour the mixture into the prepared soufflé dish and leave in the refrigerator till set. Gently remove the paper from the dish before serving.

14 Next, follow the steps given herewith and prepare the hot soufflé. B) Preparation of hot soufflé i) Melt 12 g butter in a saucepan. Add 12 g flour and stir cook for 1-2 minutes. ii) Gradually add ¼ cup of scalded milk stirring continuously till it comes to a boil. Cook till the mixture coats the back of a spoon and remove from fire. iii) Prepare orange rind and extract juice of ½ orange. Stir the juice and 10 g of castor sugar into the sauce prepared in (ii). iv) Allow the mixture to cool and beat in ½ egg yolks. v) Whisk the egg whites to stiff peak stage and gently fold them into the prepared yolk mixture. vi) Spoon the mixture into a greased oven-proof dish and bake at 175ºC for 20 minutes. vii) Serve hot decorated with orange rind. C) Make variations in the soufflé recipes A and B with regard to: i) The kind of beater used rotatory or electrical blender. ii) Improper yolk white separation i.e. partial mixing of egg yolk and white. iii) Amount of added ingredients (vary the amounts of different ingredients required for the preparation of soufflé like using more flour than required, using less egg white also one can vary the process as egg white not beaten to a stiff peak stage or in cold souffle addition of cream without whipping). Subject the samples to sensory evaluation and carry out the percent sag as given below. Percentage sag Percentage sag is used to determine the firmness of a product. To calculate the percentage sag for cold souffle follow the steps given below: 1) Pierce a toothpick vertically in the centre of the set soufflé and pull it out. 2) Measure the moist level with the help of a scale. 3) Note down the reading (mm). This reading is R 1. 4) Loosen the sides of the set gel with a moist knife and invert the gel on the centre of a plate. 5) Again take the reading as in steps 1-3. This is R 2. 6) Calculate the percentage sag using the formula: R1 R2 R To calculate the Percentage Sag in Hot soufflé carry out the following process: 1) Pierce a toothpick vertically in the centre of the hot soufflé immediately after it is taken out of oven and pull it out. 2) Measure the moist level with the help of a scale. 3) Note down the reading (mm). This reading is R 1. 4) You will observe the centre sag of the product after some time, now pierce a toothpick vertically in the centre of the hot soufflé which has sagged. 5) Again take the reading as in steps 1-3. This is R 2. 6) Calculate the percentage sag using the formula: R1 R2 R Chemistry of Colloidal Particles 127

15 Principles of Food Science Precautions 1) Use dishes with vertical sides for making soufflés. 2) To prevent sticking, coat the baking dish with butter and sugar. For cold soufflés rinse out the dishes with water and do not wipe dry. 3) Bake or refrigerate immediately after the foam mixture is ready. Do not freeze. 4) Preheat oven and bake at 180ºC to obtain a dry and stable product. 5) Leave soufflés in the oven for 5-10 minutes after switching off. 6) To prevent their sudden collapse on serving, open the oven door slightly to enable temperature to gradually come down and stabilize the product. Results and Observations Record the observations made through evaluation of i, ii and iii in point C above, with respect to volume, % sag after 10 minutes at room temperature, texture, taste and acceptability (sensory evaluation) in the format given herewith. S. No. Variations Percent Sag Sensory evaluation 1) Variations in cold soufflé a) b) c) d) e) f) 2) Variations in hot soufflé a) b) c) d) e) f) 128

16 Inference (with regard to) i) Quality of souffle with regards to the kind of beater used Chemistry of Colloidal Particles ii) Quality of soufflé with regards to improper yolk white separation iii) Quality of soufflé with regards to amount of added ingredients Conclusion (Indicate the applications of the foams as used in food preparation). Submit the activity for evaluation.. Counsellor Signature 129

17 Principles of Food Science ACTIVITY 3 DETERMINATION OF THE BEST METHOD OF PREPARING A STABLE EMULSION Date:. 130 Aim: To determine the best method of preparing a stable emulsion like mayonnaise. Objectives After undertaking this activity, you will be able to: learn the process of preparation of a stable emulsion, and discuss the various factors affecting the formation of a stable emulsion. Principle Mayonnaise is an emulsified salad dressing, which requires ingredients in certain proportions to be mixed together to form a stable oil-in-liquid emulsion. The egg forming about 20% of the total weight provides the emulsifying agent lecithin, present in the yolk. This helps in stabilizing the other ingredients in the mixture. While whole egg may be used in the preparation of the product, it is the yolks, which have better stabilizing ability as they hold moisture in dispersion. Therefore the ratio of other ingredients such as salt, vinegar and spices should not exceed 65:10:5 percent of the weight of the egg contents, since the yolks have limited assimilating power. Continuous beating breaks up the oil into fine globules, which get simultaneously dispersed in liquid to convert it into a semisolid stable product. Equipments Rotary beater or whisk, blender, bowl, standard cups and spoons Materials Eggs, 1 cup refined oil, 1tablespoon vinegar, ½ teaspoon salt, ½ teaspoon sugar, ½ teaspoon mustard powder. Procedure The activity is divided into three sections A, B and C to study the effect of variations on the end result and determine the most suitable method of preparing mayonnaise for different applications. So carry out the activity following the steps indicated herewith: A) Method of preparing mayonnaise i) Separate yolks from the eggs and drop into small bowl. ii) iii) iv) Add all the ingredients except vinegar and oil to the yolks. Beat with a whisk or beater till ingredients are well blended. Add the oil drop by drop alternating it with the vinegar while continuously beating the mixture. Continue beating till the mixture begins to thicken. v) Continue the process, gradually increasing the quantity of oil added at one time, till all the ingredients have been used up and the resultant product is spoonable into a jar. vi) Observe samples for microscopic structure, colour, consistency, taste, flavour and applicability. B) Effect of varying the method of combining ingredients i) Add seasonings and vinegar to the yolks and then start the whisking. Add oil as in A (iv v) and when ready, keep sample aside for assessment.

18 ii) iii) Beat the egg yolks first and then add the oil drop by drop till thickened, followed by spices and vinegar. Set the sample aside. To the yolk, add seasoning, 1/3 vinegar and then oil drop by drop beating till thickened, followed by 1/3 vinegar again and then the rest of the oil and the remaining 1/3 vinegar to complete the product. Keep sample aside. For quick and even continuous beating, an electrical blender may be used and the ingredients continuously added without much pause following the method in A(i-vi). With this method, the time can be monitored for equal number of revolutions when variations are used. C) Effect of substituting emulsifying agents i) Make A starch gel with ½ T (tablespoon) of cornstarch add ½ cup of water. Cool the gel. Take 2 T of this gel and add it to the egg yolks with all the other ingredients except oil, and mix together. Add oil as in A (iv) and follow procedure till A (vi). Keep sample aside. ii) Prepare a gel using 2T gelatin in ½ cup water and cool to room temperature. Use 1T of gel to make the emulsion as in (i) above. Keep sample aside. iii) Substitute egg white for yolk and proceed as in A (i-vi). Keep sample aside. iv) Substitute whole egg for yolk and proceed as in A (i-vi). Keep sample aside. D) Effect of temperature of ingredients on formation of emulsion i) Add vinegar and seasoning to egg yolk at room temperature, then mix and refrigerate till cold. Keep the oil for some time in the fridge too. Remove from fridge and note the temperature of the mixture and oil. Place bowl in ice and add cold oil as in experiment A till ready. Keep aside. ii) Heat oil and vinegar separately to 100ºC. Combine the ingredients as in A till ready. Keep aside for assessment. Note: Temperature plays an important role in the formation of an emulsion. As temperature rises the surface tension of the continuous phase gets reduced, and the oil too becomes more mobile and less viscous. Precautions 1) The oil should be added drop wise initially and then gradually as the emulsion forms. 2) Beating should be continuous while preparing the product. 3) Proportion of ingredients should be exactly controlled for all samples even when substitutes are used. 4) Proportion of ingredients should be strictly controlled for all samples even when substitutes are used. 5) Adding oil and vinegar alternately is essential for dispersion. 6) Temperature must be kept constant throughout the experiment. Results and Observations All the samples should be labeled correctly and assessed for appearance, consistency, taste, mouthfeel and application in food preparation and service. Record the results in the format given herewith. Chemistry of Colloidal Particles 131

19 Principles of Food Science Sensory quality of mayonnaise for different variations. Variation Appearance Stability Taste Mouth feel Application A B (i) (ii) (iii) C (i) (ii) (iii) (iv) D (i) (ii) Inference Conclusion (Comment regarding the applications of emulsions as used in food preparation) Submit the activity for evaluation... Counsellor Signature 132