THE CASE OF THE MISSING YOLK

PROBLEM:

Grandma calls one night. She was making your favorite lemon meringue pie when she realized the meringue was not as fluffy as usual. Knowing your background as a food chemist, grandma wants to know if you can figure out why her meringue is flat.

BACKGROUND INFORMATION:

Any foam (shaving cream, Dream Whip®, or Silly String®) can be identified as a two phase-colloidal dispersion in which the gas phase (nitrogen or air) is dispersed and enrobed by a thin film of foaming (surface active) agent, in a liquid phase. For a protein to be a successful foaming agent, it must be able to stabilize the new surface area that is continuously created during the making of foam. Therefore, the physical properties of proteins will play a major role in the production and performance of foam.

Egg white has been successful as the major foaming protein used in the food industry. The industry uses egg white as a functional ingredient in food products that require foams in several products including cakes, soufflés, meringues, candy and other suitable products that require air bubble incorporation. The success is due to the capacity of egg whites to form uniform foams of good volume while ensuring foam stability in the presence of other compounds and also upon heating. This reflects the unique combination of proteins with different physical properties that occur in egg whites.

The egg white foams have large interfacial foam areas, and this makes them unstable. The foam can break apart by 1) leakage of liquid phase due to pressure differences, 2) air coalescence from small to large bubbles and 3) rupture of liquid phase separating air bubbles. Low interfacial tension, high viscosity of liquid phase and strong elastic film of absorbed protein will stabilize the foams.

Several factors, in addition to the protein, affect the foaming properties of proteins. These include protein concentration, pH, temperature, salt, sugars and lipids.

During drying of egg white, the minute quantities of lipids (approximately 0.1%) from contaminating egg yolk, if present, can interfere with the conformation of adsorbed protein films by placing themselves at the air/water interface thus reducing the foaming ability of egg white.

OBJECTIVE:

  • To become familiar with physical changes that occur in the production of foam and the mechanism of egg white foam formation.
  • To become familiar with methods of foam measurements and investigate the effect of sugar, pH, and salt on foam production and stability.
  • To demonstrate foam production in meringue.

    MATERIALS:

  • Eggs (1 dozen per group)
  • Potassium tartrate (Cream of Tartar) saturated solution
  • Mixer
  • 250 ml beakers
  • 100 ml beakers
  • 250 ml Erlenmeyer flasks
  • 100 ml Graduated cylinders

    SAFETY AND DISPOSAL:

    There are no real safety issues or concerns regarding this lab, except do not put raw egg in your mouth. There is a remote possibility of Salmonella contamination in raw eggs. Wash hands thoroughly after finishing the lab.

    TIME:

    Approximately one hour.

    PROCEDURES:

    1. Separation of egg white from whole egg
      1. The whole egg consists of shell, yolk, and egg white liquid. Separate the egg white carefully since small amounts of yolk will contaminate the egg white. This can adversely affect the whipping properties of egg white.
      2. Collect 80 ml of egg white in a weighed 250 ml Erlenmeyer flask and note the weight of the egg white.

    2. Pretreatment of egg white
      1. pH adjustment - The pH of the native egg white is around 9.0 due to the presence of basic amino acids. Adjust the pH of egg white to about 7.0 with saturated potassium acid tartrate solution to minimize the extent of denaturation of proteins under alkaline conditions during pasteurization. Stir the egg white gently during the pH adjustment.
      2. Pasteurization - Heat egg white at 60C for 3 ½ minutes (Do not cook too long. Extensive denaturation can result in a decrease in foam production). Cool to room temperature. The heated egg white forms better foams with greater numbers of air bubbles and increased rigidity than does unheated natural protein.

    3. Foam Production
      1. Whipping - When using an electric mixer, be sure to lock beaters in place before turning it on. Turn mixer to first setting (low) and slowly work up to a setting of 10. Whip the egg white liquid for 3 minutes in a kitchen mixer on a setting of 10.

    4. Assessment of foaming properties:
      1. Foam Density - Fill a 100-ml pre-weighed weighing boat with the foam produced and weigh to get the foam weight. Calculate the foam density:

        Foam Density = Weight of foam ÷ Volume of foam

      2. Foam Capacity - The foaming capacity can be determined by "overrun" and is defined as the change in density of a given volume of a foam and is expressed as percent change in volume.

        % Change in Volume = ( Wt. of 100 ml protein - Wt. of 100 ml of foam) x 100
        % Change in Volume = ( Wt. of 100 ml of foam

    EXTENSION:

    Foam Stability Determination: Determine the weight of the remaining foam and place it on cheesecloth in a large funnel at room temperature. Collect the liquid (drip) into a 100-ml pre-weighed beaker and note the weight of the egg white drip plus beaker at 10-minute intervals for 60 minutes. Plot the weight of egg white drip against time. Estimate the time taken for 50% of the foam to drip from this plot.

    Stability Index = Time to attain 50% (w/w) drainage
    Time 0 min. 10 min. 20 min. 30 min. 40 min. 50 min. 60 min.
    Weight of drip              

    Plot the weight of egg white drip (Y axis) against time (X axis)

    C. Sucrose Addition and Meringue Preparation
    1) The egg white (80 g) is whipped at setting 10 for 2min, the speed is decreased to setting 6 and sucrose (45 grams) added in three increments at four second intervals. The whipping is continued at setting 10 for 6min. The stability index of meringue is determined as described under foam stability determination. Sucrose is a soluble polar compound, which increases bulk viscosity of egg white liquid, thereby improving foam stability.
    Time 0 min. 10 min. 20 min. 30 min. 40 min. 50 min. 60 min.
    Weight of drip              

    Plot the weight of egg white drip (Y axis) against time (X axis)

    QUESTIONS:

    1. How do proteins lower the interfacial tension during whipping of egg white?



    2. Why is mechanical energy required in foam production?



    3. What is the function of surface-active agents in foam production?



    4. How does viscosity stabilize foams?



    5. Egg white foams for product application are produced outside the natural pH and pI at which maximum foaming and stability occurs. Why?



    FOR ADDITIONAL INFORMATION CONTACT:

    Dr. Navam Hettiarachchy

    REFERENCES:

    Altschull, A.M. and H.L. Wilcke. 1985. New Food Proteins. Vol. 5. Academic press Inc., New

    York, NY. p. 155-161.

    Fennema, O.R. 1985, 2 nd ed. Food Chemistry. Marcel Dekker Inc., New York, NY.

    Chapters 5 and 14. pps 246-369 and 829-855.

    Halling, J.H. 1981. Protein-stabilized foams and emulsions. CRC Critical Review in Food Science

    and Nutrition. pps 155-203.

    Hettiarachchy, N.S. and G.R. Ziegler (Eds.) 1994. Protein Functionality in Food Systems. IFT

    Basic Symposium Series, Marcel Dekker Inc., New York. Chapter 6, p. 181-208.

    Johnson, J.M. and M.E. Zabik. 1981. Egg Albumin Protein Interactions in an Angel Food Cake

    System. J. Food Sci. 46:1231-1222, 1236.

    Poole, S., S. West and C.L. Walters. 1984. Protein-protein Interactions: Their Importance in the

    Foaming of Heterogeneous Protein Systems. J. Sci. Food Agric. 35: 701-711.

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