Chemistry Everyday for Everyone
Showing Emulsion Properties with Common Dairy Foods Carlos Bravo-Díaz1 and Elisa González-Romero Facultad de Ciencias, Campus Ourense, Universidad de Vigo, As Lagoas 32004 Ourense, Ourense–Spain Most writers on food either ignore scientific principles that underlie cooking or disparage the value of such information on the grounds that cooking cannot be reduced to the test tube. Cookbooks are sets of explicit directions, developed by an expert, intended to help us to prepare different kinds of food. However, people who have not yet logged many years preparing food might require some explanation about what is going on. They might appreciate general background information to compensate for the lack of familiarity with ingredients and techniques or simply be curious about what foods are and how cooking works (some people call it “popular science”). In fact, foods are nothing but mixtures of chemical compounds, and the qualities we sense (taste, texture, color, etc.) are all manifestations of its chemical properties. Grosser (1) pointed out that the analysis of culinary recipes is close to ideal subject matter for the introduction of chemical principles. Following his idea, our aim with this work is to show how it is possible to visualize food properties in a very simple, inexpensive way. We describe some experiments that allow both teachers and students to discuss the scientific principles that can explain, at least qualitatively, the nature and composition of foods and the effects of additives and external parameters (e.g. temperature), and we do this by using common dairy products and kitchen utensils such as graduated cylinders, funnels, and spoons. Teachers may conveniently adapt the experiments for their students in order to focus attention on chemical principles. A minimum knowledge of general chemistry, which should include some knowledge about atoms and molecules, chemical bonds, energies (thermodynamics), and the states of matter may be very helpful. The proposed experiments are related to emulsion properties. Emulsification is one of the most versatile properties of surface-active agents for practical applications and, in consequence, has been extensively studied. If we look around the different sections of a supermarket, we can find paints, polishes, cosmetics, pesticides, cleaners, etc., which are all emulsions or are used in an emulsion form. Food emulsions also cover an extremely wide area of “daily life” applications. If we look at the different products sold in the food section of a supermarket, we can find “semisolid” varieties such as margarine and butter as well as the liquid ones such as milk, sauces, dressings, and a wide array of beverages. In addition, food emulsions include materials that contain both solids and/or gases in addition to two liquid phases (e.g., ice cream). This rich variety of food emulsions means that the study of emulsion stability offers a large spectrum of scientifically interesting phenomena, although some of them are still not completely understood. We can define the term emulsion as a “stable” suspension of particles of a certain size within a second, immiscible liquid. The stability of an emulsion can be referred to as the resistance to the coalescence of the emulsion’s dispersed droplets (Fig. 1); it ranges from a 1
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few minutes to years. The rate of coalescence of droplets in an emulsion is taken as a quantitative measure of emulsion stability (2). It can be measured by counting the number of droplets per unit volume of the emulsion as a function of time, and it depends of a number of factors (2–4). The stability of a colloidal suspension can be explained
Figure 1. Schematic representation of the flocculation and coalescence processes.
Figure 2. Schematic representation of colloidal stability as a function of distance. The charged surfaces of the particles and the ions between them form an electrical double layer that gives rise to a repulsion potential (curve A) against approach of the particles. Since droplets are different from the continuous medium, a van der Waals attraction potential is formed (curve B). The resulting potential (curve C) is always negative for small distances but may be positive at intermediate distances (it goes through a minimum that is not shown). If the potential maximum is considerably larger than thermal energy (kT), the suspension will be stable.
Author to whom correspondence should be addressed.
Journal of Chemical Education • Vol. 73 No. 9 September 1996
Chemistry Everyday for Everyone
in terms of the DVLO theory (5, 6), which states that the stability is dependent on two independent interactions between colloidal particles: the van der Waals attraction and the electrostatic repulsion between electrical double layers of identical sign. The theory predicts that if the potential repulsion (PR) exceeds the absolute value of the attraction potential (AV) by a certain value (PR – AV = W >> kT) at any distance between the particles, the suspension will be stable (Fig. 2). For low values of the repulsion potential (W - kT), the suspension will coagulate as soon as the particles approach each other by the diffusion process. To stabilize the system for sufficient time it is necessary to add an emulsifying agent (4), which reduces the interfacial tension between the two liquids (and consequently the thermodynamic instability of the system) and decreases the rate of coalescence of the dispersed liquid particles by forming mechanical, steric and/or electrical barriers around them (5). Description of Experiments, Results, and Discussion
Experiment 1. Study of the Nature of the Emulsion Emulsions are usually classified as oil-in-water (o/w) and water-in-oil (w/o). The o/w type is a dispersion of a water-immiscible liquid (always called “oil”) in an aqueous phase. The w/o type is a dispersion of water or an aqueous solution in a water-immiscible (“oil”) liquid. The nature of the emulsion formed by the water and the oil depends primarily on the nature of the emulsifying agent and, to a lesser extent, on the process used to prepare the emulsions and on the relative proportions of oil and water present. These types of emulsions can be easily distinguished by means of a number of methods (3). We propose to investigate the nature of the emulsion by means of the following tests: (i) w/o emulsions will be colored by oil-soluble dyes and o/w emulsions will be colored by water-soluble dyes; (ii) filter paper test, in which a drop of an o/w emulsion produces an immediate wide, moist area, whereas a drop of w/o emulsion does not. Materials • Food reactives: milk, butter, margarine, and various sauces (e.g., barbecue, ali-oli, vinegar). • Disposable kitchen materials: spoons and dishes. • Filter paper. • Dyes: Sudan III (lipophilic) and methylene blue (hydrophilic); both are inexpensive and can be purchased from a number of chemical companies. CAUTION: Foods must never be eaten after experimentation. Do not taste them! Place them in a proper disposable container. Wearing of lab coats and gloves is highly recommended to avoid stain problems with dyes. If any solution spills on skin or clothing, wash the stained part immediately and gently with liquid soap and water. Experimental Procedure 1.
Place about 10 mL or 10 g of one of the food reactives in a dish. 2. Pour a tiny amount of one of the dyes, for example Sudan III, over the food reactive. 3. Wait one minute and check for changes in color on the food reactive. 4. Write down your observations. 5. Repeat the experiment with methylene blue. Following this experimental procedure we obtained, for different commercial emulsions, the results shown in Table 1.
Experiment 2. Effect of Heat on Emulsion Stability
Changes in temperature can break emulsions, limiting their uses in the kitchen. At high temperatures, molecules are moving very energetically, and the harder and more frequent the collisions between droplets, the more likely they are to coalesce (Fig. 1). Change in the stability of an emulsion may be irreversible and usually is accompanied by changes in physical properties as well. Materials • Food reactives: butter, margarine, and pork fat. • Kitchen material: glasses (test-tubelike), pots, and disposable spoons and knives. • Thermostatic bath at T = 60 °C. CAUTION: Wear safety goggles and face shield. Hot water may cause burns. Test tubes may break and cause injuries. Glass materials may get hot, so use of appropriate tongs, test-tube holders, or potholders to handle them is highly recommended (hot glass looks like cold glass). When heating a test tube, hold it at an angle and move it through the water bath, pointing the mouth of the test tube away from others and from yourself. After experimentation food materials must not be eaten; place them in a proper disposable container. Do not taste them! Glass materials may be used again after washing gently (at room temperature) with liquid soap and water. Experimental Procedure 1. 2.
Place about 20 g of food reactive into a glass. Check for phase separation and write down the number of phases observed at room temperature. 3. Place the glass into the thermostatic bath so that the food material is immersed completely. 4. Wait until all food material is melted. 5. Remove the glass from the bath (be careful! it is very hot), and check visually for phase separation. 6. Carefully pour the different phases on a dish; check their nature using the procedure described in experiment 1. 7. Write down your observations. 8. Repeat steps 2–4, remove the glasses from the bath, and allow them to cool slowly to room temperature. Go to step 7. Table 2 shows the number of phases we observed under the different experimental conditions. Although, according to Table 1, both butter and margarine are w/o emulsions, they have different compositions and that is the reason for the different number of phases observed. Notice that after they were melted and cooled to room temperature, the nature of butter and margarine emulsions has changed.
Experiment 3. Study of Emulsion Stability and the Effect of Different Emulsifiers As stated earlier, the nature of the emulsion formed by the water and oil depends primarily on the nature of the emulsifying agent. To a lesser extent it depends on the process used to prepare the emulsions and the relative proportions of oil and water present. In general, according to Bancroft’s rule (7), o/w emulsions are produced by emulsifying agents that are more soluble in water than in the oil phase, whereas w/o emulsions are produced by emulsifying agents that are more soluble in oil than in the water. Materials • Food materials: soybean oil, fresh eggs, table salt (NaCl), mustard.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
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Chemistry Everyday for Everyone
• Kitchen materials: glasses (beakerlike), graduated cylinders, disposable forks and spoons. • Other nonfood materials: soap powder, chronometer. CAUTION: Wear safety goggles and face shield. Oil is flammable. Extinguish all flames and conduct experiments far away from any hot device. Food materials must not be eaten after experimentation; they should be placed in a proper disposable container. Do not taste them! Wash hands gently after experimentation. Do not breathe powdered soap; it may cause irritation. Experimental Procedure 1.
Place 5 mL of soybean oil in the beaker. Add 5 mL of water, beat it with a fork for 2 min (trying to maintain a constant beating speed), and record the elapsed time until phase separation is seen. To minimize errors, repeat the experiment at least six times and calculate the average value. Repeat step 1 but use vinegar instead of water. Repeat step 1, each time mixing soybean oil, water, and 0.01 g of one of the following: NaCl, egg yolk, soap powder, mustard. Table 3 shows the average values we obtained. As-
2. 3.
properties of egg yolk (5) and permits discussion of mayonnaise preparation as an example of preparation of emulsified sauces, showing the rationale for the procedure (8, 9) (i.e., why the egg yolks and water are always the first materials in the bowl, and the oil is added to them rather than the other way around). There are at least two main reasons for this procedure. The first is that the main job of the sauce-making is to divide the oil into microscopic droplets, and this is easier to do if one begins with one drop of oil in the water phase rather the other way around. The second is that emulsifying agents will coat the oil droplets more quickly if initially there is more emulsifier present than oil. Run f allows us to demonstrate seeds as excellent emulsifiers and to discuss, if desired, the usual procedure (8) to “rescue” separated sauces (e.g., mayonnaise). The trick is to repeat the initial procedure to prepare mayonnaise except that we start now with the water and oil phases partially mixed before the beating begins. We have to point out that depending on the food or soap-powder brands used (i.e., composition of the raw material), different times can be obtained; however the relative time scale should be similar. Conclusions
Table 1. Nature of the Tested Emulsions Emulsion
Type
Milk
O/ W
Butter
W/ O
Margarine
W/O
Barbecue Sauce
W/ O
Ali-Oli Sauce
W/ O
Vinegar Sauce
O/ W
Students’ interest in scientific principles that underlie procedures in daily life (such as cooking, even if they do not cook) can be increased through some easy, inexpensive, and safe experiments developed using familiar kitchen materials. It is necessary to recognize that the type of materials (food reactives and measurement apparatus) and the procedure employed yield qualitative, not quantitative, results. However we consider the most important factor to be that the experiments make possible scientific discussion of the reasons for the phenomena observed in daily life, based on an interdisciplinary subject like food chemistry. Indeed, concerns about food exist throughout the world although the aspects of concern differ with location.
Table 2. Effect of Heat on Emulsion Stability Temperature
Condition
Number of Observed Phases Margarine
Butter
Fat
Acknowledgment
25 °C
before melting
1
1
1
60 °C
completely melted
2
3
1
We would like to thank Universidad de Vigo and the Department of Physical Chemistry for financial support, R. A. Mosquera for helpful discussions, and our students for performing this experiment.
25 °C
after melting
2
2
1
Literature Cited
Table 3. Average Phase Separation Time for Different Emulsifiers Run
Oil
Water
Emulsifier
Time (s)
a
soybean
water
none
25
b
soybean
water
0.01 g NaCl
30
c
soybean
water
0.01 g powdered soap
d
soybean
water
0.01 g egg yolk
e
soybean
water
f
soybean
vinegar
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1. Grosser, A. E. J. Chem. Educ. 1984, 61, 362–363. 2. Boyd, J.; Parkinson, C.; Sherman, P. J. Colloid Interface Sci. 1972, 41, 359. 3. Rosen M. J. In Surfactants and Interfacial Phenomena; Wiley: New York, 1978; Chapter 9. 4. Fennema, O. In Food Chemistry; Dekker: New York, 1985; Chapter 4. 5. Frieberg, S. E.; Gourban, R.F.; Kayali, I.H. In Food Emulsions, 2nd ed.; Larsson, K.; Fribera, S. E., Eds.; Dekker: New York, 1990; Chapter 1. 6. Werwey, E. J. W.; Overbeck, Th. G. In Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. 7. Bancroft, W. D. J. Phys. Chem. 1913, 17, 514. 8. McGee, H. In On Food and Cooking, The Science and Lore of the Kitchen; McMillan: New York, 1984; Chapter 7. 9. Paul, D. C.; Palmer, H. H. In Food Theory and Applications; Wiley: New York, 1972.
211
0.01 g mustard
58
none
12
suming that the recorded times represent a relative stability scale, discussion about the role of the emulsifier can be opened. Comparison of runs a–d shows, qualitatively, the role of different ionic emulsifiers on emulsion stabilization, but bear in mind that some of the soap-powder components may be nonionic. Comparison of runs a and e shows the emulsifying
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