In the Laboratory
Showing Properties of Food Foams with Common Dairy Foods Carlos Bravo-Díaz* and Elisa González-Romero Universidad de Vigo, Facultad de Ciencias, Pabellón Químicas, As Lagoas-Marcosende, 36200 Vigo, Pontevedra, Spain
Following the ideas pointed out in our previous work (1) about how food properties can be visualized with the aid of simple and inexpensive experiments using dairy products that can be found in any kitchen, and using almost exclusively kitchen materials, we have developed new ones to investigate the properties of food foams. It is remarkable that a variety of solid foods, most notably bakery products, have a porous and open macrostructure resulting from a dispersion of gas bubbles into a matrix. These kinds of foods are usually referred to as foams and sponges and are low-density, porous, thin-walled structures. The main difference between foams and sponges is that sponges are capable of imbibing large quantities of solvent without dissolving, whereas foams do not have that property and would normally dissolve in the presence of water. Typical dairy food foams and sponges with widely differing textures, such as meringues, cakes, marshmallow, soufflés, mousses, bread, and ice-cream, can be easily found in the bakery or frozen foods section of any supermarket. In many cases, the gas is air (occasionally carbon dioxide) and the continuous phase is an aqueous solution or suspension containing proteins. Foams and emulsions are related by the fact that each represents a physical state in which one fluid phase is dispersed in a second phase (2, 3). In foams, a continuous phase of thin layers, called lamellae, separates the gas bubbles. As with emulsions (1, 2), mechanical energy is required to create this interface. Maintaining the interface against coalescence of gas bubbles usually necessitates the presence of surface-active agents (2, 4, 5), which lower the interfacial tension and form an elastic protective barrier between entrapped gas bubbles. Foams can be classified into two morphological groups (2, 3), spherical foams (“kugelschaum”) and polyhedral foams (“polyederschaum”) and can vary greatly in size (6) (usually from 1 µm to several centimeters). There are three major methods to prepare foams (4): (i) bubbling gas through a porous sparger into an aqueous solution of low protein concentration; (ii) beating (whipping) or shaking an aqueous protein solution in the presence of a bulk gas phase; and (iii) sudden release of pressure from a previously pressurized solution. Experimental Procedures, Results and Discussion The proposed experiments are described for egg foams. The properties of eggs not only permit preparation of excellent and delicious emulsions like mayonnaise (1) but also allow eggs to make excellent foams, increasing their volume significantly in two primary ways. The first may be attributed to the albumen (the major component of egg white) because is a thick viscous solution and drains more slowly out of bubble walls than does a thin liquid; and the second is due to egg white introducing a kind of reinforcement into the bubble walls. As the egg white is beaten and air bubbles are incorporated into it, the proteins in the bubble wall are subjected to an imbalance of forces due to the air–liquid in*Corresponding author.
terface, which makes them unfold and bond to each other, forming a delicate but definitely reinforcing network.
Experiment 1. Estimation of Required Beat Time To Form a Stable Egg Foam Whipping (4) is the preferred means of introducing gas into most aerated food products. It produces more severe mechanical stress and shear than the spargin method. Because foams have very large interfacial areas, they are often unstable. There are three main destabilizing mechanisms (3, 4): (i) drainage (or leaking) of lamellar liquid due to gravity, pressure differences, or evaporation; (ii) gas diffusion from small to large bubbles; and (iii) rupture of the liquid lamellae separating gas bubbles. Drainage and rupture are related, since rupture increases drainage and drainage reduces the thickness and strength of the lamellar film. Practical mechanisms for extending the persistence of foams include one or more of the following conditions: high viscosity in the liquid phase, high surface viscosity, surface effects, or electrostatic or steric repulsions. Foam stability can be assessed (4) by means of one or more of the following measurements: (i) the degree of liquid drainage or foam collapse reached after a given time; (ii) the elapsed time for total or half drainage; or (iii) the elapsed time before drainage starts. We have studied the stability of egg foam measuring the collected volume of lamellae 20 min. after beat stop. Materials Needed •
Food materials: fresh eggs
•
Kitchen materials: graduated cylinders, glass bowls, funnels, egg beaters (or blenders), a balance
•
Other nonfood materials: glass wool
CAUTION: Wear safety goggles. Food materials must not be eaten after experimentation and should be placed in a proper disposable container. Do not taste them. Wash your hands carefully before and after any experimentation. Procedure 1. Put some glass wool into the funnel. Put the funnel into the graduated cylinder (Fig. 1). 2. Weigh an appropriate glass bowl. 3. Carefully separate egg white from egg yolk. 4. Place the egg white in the glass bowl and weigh it. 5. Calculate the egg white weight and write it down. 6. Start to beat the egg white with the egg beater, trying to keep a beat speed as constant as possible during two minutes. 7. Transfer the foam into the funnel as quickly as you can. 8. Wait 20 min. and measure the volume of the collected lamellae. Write it down. 9. Wash gently all materials employed (C AREFUL! Do not taste any food material!) and repeat the experiment increasing the beating time each time (e.g., 3, 4, 5, 7, 10 min.). Write down the volumes collected.
Vol. 74 No. 9 September 1997 • Journal of Chemical Education
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In the Laboratory 20
Collected volume/cc
Whipped Egg Foam
Glass Wool
10
0 2
4
6
8
10
Beat time/min
Figure 1. Suggested apparatus setup to obtain experimental values. The foam is introduced in the funnel, which contains the glass wool used to retain the foam, allowing liquid drainage. The graduated cylinder is used to measure the collected lamellar volume.
Figure 2. Plot of collected volume vs. beat time.
Notes 1. The presence of any spot of egg yolk in the egg white may result in a significant reduction of foam volume, so it is necessary to be careful with step 3.
Experiment 2. Effect of Added Ingredients on Foam Stability The three most important attributes that serve to stabilize foams (4) are low interfacial tension, high viscosity of the bulk liquid phase, and strong elastic films of adsorbed proteins. Tartaric acid is often added to stabilize the foam, and sugar and salt to improve its flavor, so we will use these common ingredients to study their effect on foam stability.
2. It is very important to beat at the same constant speed in all experiments to get reproducible and comparable results. It may help to use a blender instead of a egg beater, although extra precautions must be taken because of its high power.
Materials Needed •
Food materials: fresh eggs, tartaric acid (or cream of tartar), common salt (NaCl), and cane sugar.
•
Kitchen materials: graduated cylinders, glass bowls, funnels, egg beaters (or blender), a balance. (See recommendation in experiment 1 for using blenders instead of egg beaters.)
•
Other nonfood materials: glass wool
Results and Discussion Table 1 shows the average values obtained by Table 1. Determination of us. Plotting the collected Minimum Time To Form a volume against beating Stable Foama time (Fig. 2) allowed us to Egg White Beat decide which is the reTime Weight Volume quired beating time to ob(min) (g) (cm3) tain the most stable egg 2 28.15 19.5 foam. In this experiment it 3 28.02 15.0 is obtained after beating for t = 7 min. 4 28.84 13.0 Beating is very tricky 5 29.00 7.5 in order to get a stable 7 29.70 5.0 foam. For one thing, it is 1 0 2 8 . 7 1 1 1.0 possible to beat egg white aValues for egg white weight too much, causing curdling, and collected volume are an averor too little, resulting in age of 4 runs. Beat speed was premature collapse of the maintained as constant as possible foam. When whipping bein all runs. gins, large amounts of air are trapped in the albumen, starting the process of protein denaturation. If beating stops early, the foam will be coarser and lower in volume than it should be and the lamellae will slowly drain away (4, 7) collapsing the foam (run 1). On the other hand, overbeating causes too extensive coagulation, decreasing the water-holding capacity of the protein (7) and causing partial aggregation–coagulation of the proteins at the air– water interface (4); therefore the foam will leak liquid that contains much less uncoagulated protein, clump up, and lose volume (run 6). Consequently, the ideal beating time lies between these two extremes.
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Procedure 1. Repeat steps 1–5 of experiment 1. 2. Add 25 g of cane sugar. 3. Start to beat the egg white with the egg beater, trying to beat at the same constant speed as before, for the beating time required to get the most stable foam experiment 1 . 4. Transfer the foam into the funnel as quickly as possible. 5. Wait 20 min. and measure the collected lamellar volume. Write it down. 6. Carefully wash all the kitchen materials employed. (Do not taste any food material!) 7. Repeat steps 1–6, adding each time, before beating, one of the following: 2 g of NaCl, 1 g of tartaric acid.
Note As in experiment 1, it is very important to beat at the same constant speed in all experiments to get reproducible and comparable results. The use of a blender instead of an egg beater may help to improve the results; however, be very careful not to overbeat. CAUTION: Wear safety goggles. Food materials must not be eaten after experimentation. Place them in a proper disposable container and do not taste them. Wash your hands before and after any experimentation.
Journal of Chemical Education • Vol. 74 No. 9 September 1997
In the Laboratory Table 2. Effect of Added Ingredients on Foam Stabilitya Run
Egg white, wt (g)
Added Ingredient Name
Amount (g)
Collected Vol (cm3)
1
28.70
caned sugar
25.0
11
2
28.30
NaCl
2.0
9
3
29.00
tartaric acid
0.1
3
aBeat time was 7 min. A constant beat speed was maintained as close as possible to that of experiment 1.Values for weight and collected volume of egg white are the average of four runs. Ingredients were added to the egg white before beating.
Results and Discussion Table 2 shows the results obtained by us. Adding a small amount of tartaric acid increases foam stability by making the albumen liquid a little more acidic (approximate pH change observed is from pH = 9 to pH = 8) and lowering the reactivity of the protein groups, thus limiting the extent to which they can bond. Adding NaCl or sugar decreases the foam’s stability. It is believed that NaCl decreases the viscosity of the solution, whereas sugar depresses foam expansion because sugar molecules get in the way of proteins and slow the rate at which the proteins bond with each other. We would like to stress that different values from those shown in Tables 1 and 2 can be obtained because several factors (beat speed, raw material, etc.) may affect results and they are almost impossible to standardize. However, similar trends should be observed.
Conclusions Students’ interest in food foams that are present in daily life can be increased through some safe, inexpensive, easily carried out experiments developed using familiar kitchen materials. The materials (food reactives and measurement apparatus) and procedures employed yield qualitative results, not quantitative, but we believe the most important outcome is that the experiments make possible scientific discussion, as we ask ourselves the reasons behind the daily observations based on an interdisciplinary subject like food chemistry. Acknowledgments We would like to thank Xunta de Galicia (XUGA 38305A94), the Department of Physical Chemistry for financial support and our students for performing the experiments. We also thank the Spanish Ministry of Education (DGICYT PB94-0741). Literature Cited 1. Bravo-Díaz, C.; González-Romero, E. J. Chem. Educ. 1996, 73, 844–847. 2. Adamson, A. W. In Physical Chemistry of Surfaces; 5th ed.; Wiley: New York, 1990; Chapter 14. 3. Myers, D. In Surfaces, Interfaces and Colloids: Principles and Applications; VCH: New York, 1991; Chapter 12. 4 Fennema, O. R. In Food Chemistry, 2nd ed.; Dekker: New York, 1985, Chapter 5. 5. Sato, T.; Ruch, R. In Stabilization of Colloidal Dispersions by Polymer Adsorption; Dekker: New York, 1990; Chapters 2–4. 6. Dickinson, E. An Introduction to Food Colloids; Oxford University: New York, 1992. 7. McGee, H. In On Food and Cooking, The Science and Lore of the Kitchen; McMillan: New York, 1984; Chapter 7.
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