Generally, whole egg begins to become opaque at around 60C and increases in viscosity to 72C. At 75C it is a soft curd and increases in firmness up to 87C. Certainly, heat is the critical factor in bringing about the denaturation of the egg protein and forming structure. However, heat has many other causative factors. It should be remembered that heating the egg product itself, if (liquid or dried form) has been done during the pasteurization of eggs. This heat will decrease the functional properties of these eggs.

Again, one needs to be reminded that the rate of heating, the added ingredients, the concentration of the egg and other subtle factors will impact the range of temperature and the optimum endpoint. The work on model systems of eggs by Pawayal et al. [1946] essentially appears to indicate that there is a relationship between time and rate of heating. The faster the mixture heats the lower the point of increased viscosity.

Practically speaking, what are the implications of the differing coagulation points of the various egg parts?

When most consumers talk about egg products they likely are thinking about the whole shell eggs and the many different foods. Neff [Neff, J. 1998. Foods of Tomorrow. The Great Egg Breakthrough. Food Processing 59(1): 25.] indicates that in 1997 there were 6.443 million dozen eggs in the shell and 1.621 million dozen breaker eggs consumed. The per capita egg consumption was 171.5 shell eggs and 66.5 breaker eggs. However, there are other egg products used by the consumer, foodservice industry, and value-added portion of the food industry. Most consumers are well aware of the market for egg substitutes and egg replacers.

Although one should review egg selling sites to get the latest on available products, some of the different egg products are as follows:

Dried Eggs: (blends of whole egg and/or yolk with sugar or corn syrup) added, flake albumen, free-flowing whole egg or yolk solids, instant egg white solids, spray-dried egg white solids, stabilized whole egg yolk solids (glucose free), whole egg or yolk solids

Frozen Eggs: egg white, egg yolks, salted whole egg, salted yolks, sugared yolks, whole eggs, whole eggs with com syrup, whole eggs with added yolk (fortified), whole eggs with yolks and corn syrup, yolk and white blends w/wo sweeteners or salts

Refrigerated Liquid Egg Products: egg whites, egg yolks and various mixtures of these.

Specialty Products come in a wide variety of forms. Some of my favorites are the precooked egg yolks/whites and cooked egg white with a center tube of cooked yolk. There are boiled eggs available for purchase by institutions. One can order the "real egg" in about any form desired.

There are many products that serve as egg substitutes or replacers. Eggs have also been used as substitutes for other ingredients. Neff reported the work of Dan Neumeister in developing a heat-denatured egg yolk to substitute for potassium bromate to improve the texture and outward appearance of bread.

What is the effect of the egg product in a custard? Funk et al. 1969 has plotted the heat penetration of custards as essentially rise steadily to 74C and than slowing down in rate until approximately 82C and rising again to above 87C. They observed that the type of egg processing [foam-spray, frozen, freeze, spray-dried] did make a slight difference in the heating rate. Workers have hypothesized that the actual coagulation occurs as a two step process. The first step is the actual denaturation of the protein and the second step is the protein-protein interaction and the formation of a gel or increased viscosity. Interesting, Lowe [1942] observed that forewarming of the milk may decrease curdling and shortening cooking time. However, it is important to remember that 0.75% of the milk protein is heat coagulable.

Which of the following are egg custards?

Custards may be altered by manipulation or ingredient variation. Because egg is the primary structural ingredient of a custard, some differences are seen when fresh, frozen, old, dried or egg substitutes are used. The egg source, for example duck or turkey, also influences custard quality. Milk, although not as structurally important as egg, contributes to the viscosity or gel strength of the finished product. Calcium ions present in the milk are needed in the formation of a thicker custard, as custards made with water will not gel or thicken. Differences in processing will also influence the custard quality. Nonhomogenized milk produces a baked custard with a thinner crust, more delicate browning and better sheen than those custards made with homogenized milk. For example, stirred custard usually is considered to have "more body". Sugar is also important to the viscosity and gel strength of custards. Sugar tends to increase the denaturation temperature of the egg proteins resulting in a less stiff product. Salt and flavoring have no appreciable affect on custard quality other than for taste.

The proportion of ingredients is important to custard quality. The concentration of egg protein is proportional to the viscosity or gel strength of the custard. With increasing concentration, a custard sauce becomes more viscous and the gel strength of a baked custard firmer. Also, with increased egg protein, the product becomes more sensitive to end point temperature. End point temperature is the point at which optimum denaturation has occurred without curdling or syneresis. Milk serves to dilute the egg protein so less viscosity and gel strength are observed with increasing proportions of milk. Because sugar increases the denaturation temperature of egg proteins, increased sugar concentrations results in softer custards. At a 30% sucrose concentration, a custard will not gel at all.

Producing a good quality custard depends on a number of factors. In addition to the proportion and ingredient variation, temperature and rate of cooking are important. A custard heated slowly begins to thicken at a lower temperature, thickens gradually over a wider temperature range and reaches doneness at a lower temperature. On the other hand, a custard heated rapidly must be heated to a higher temperature before thickening begins and overcooking, resulting in curd formation or porosity, occurs easily. Syneresis or weeping results as the curds separate from the serum. Slow cooking can be achieved by placing the baked custard mixture in a waterbath in the oven or by using a double boiler with a stirred custard. Endpoint is indicated when the stirred custard "coats the spoon", or when a knife inserted into the baked custard "comes out clean".

Which of the following custards was cooked the longest? Which of the following custards could represent the custard having the greatest amount of sugar? Which of the following custard could likely represent the custard having egg white substituted for the whole egg?

When one turns to a recipe or formula and it lists eggs as an ingredient, it is usually understood to be whole chicken eggs. The egg itself and the method of processing of the food product it is in will make a difference in the reaction of cooking. The effect of ingredients in products will make a difference. Certainly it will vary depending upon the product and the function of the egg. Following are some affects.

	Milk's contribution in egg products such as custards is underestimated. As early as 1942, Carr and Trout reported research that indicated the cooking time for custards made with homogenized milk took 15 to 20 minutes longer than custards made with nonhomogenized milk. Heat penetration was slower. Additionally, viscosity and gel strength was greater with homogenized milk. However, this webber has observed that milk's effect on egg gelation is not well documented. Milk contains electrolytes which will enhance viscosity and gel strength.
	Sugar will inhibit and delay denaturation of the egg proteins, depending upon the product. In a custard, it delays coagulation so that equivalent thickening (from the standard) will occur only at a higher temperature, if at all. In the production of foams, it has been shown that adding sugar will delay the formation of an egg white foam. However, once a foam is formed, it is more stable.
	A variety of factors may influence the pH of eggs. Shimada and Matsushita (1980) observed that pH and protein concentration affected coagulation of egg albumin. The net charge of egg albumin increased up to pH 8, plateauing up to pH 11 at which time no coagulation occurred. The charge would increase as concentration increased. This is due to the change of the proteins themselves. Certainly if one adds acid to a egg containing product it will coagulate faster. This effect is often utilized. For example, angelfood cake foam has lemon juice or cream of tartar added as a stabilizer.
	The amount, how it is processed and type of eggs makes a difference. The part of the egg will impact the functional properties within an egg product. Researchers have determined that egg yolk will also coagulate, beginning at ~65C, losing fluidity at 70C and coagulating at 85C. Woodward and Cotterill (1987) felt that the major denaturation occurred in egg yolk at 68C to 70C. Egg white will coagulate approximately 5C higher. As with egg white, there was significant pH-temperature interdependence. Other workers [Shimada and Matsushita, 1980] have observed that coagulation increased with increased protein level and concentration. This is of particular importance when using selected fluid or dried egg products. The method of processing the egg makes a difference. Research done by Funk et al. in1969 plotted the heat penetration of custards, rising steadily to 74C and then slowing down in rate until approximately 82C and rising again to about 87C. They observed that the type of egg [foam-spray, frozen, freeze, spray-dried] did make a slight difference in the heating rate. Another study conducted by Downs et al [1970] observed that spray-dried eggs tended to coagulate faster than frozen, freeze-dried and foam-spray-dried eggs.

Powrie, W.D., H. Little, and A. Lopez. 196 . Gelation of egg yolk. Journal Food Science :38.

Egg safety is critical to minimize the problems of food infection and poisoning. There are a number of truisms necessary:

albumen beading chalazae clarify coagulate cream of tartar custard denaturation disulfide linkages eggs egg grades emulsion flocculated foam gel lipoproteins protein efficiency ratio sol souffle surface activity surface tension syneresis viscosity vitelline

REVIEW

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Are brown eggs better than white eggs from a food quality viewpoint? From a nutritional viewpoint?

What are the grades and weight of eggs? Are they the same or different? Why should the food preparer care?

What is the structure of a typical eggs? Of what importance is this?

Why does a egg float?

What are the changes which occur in an egg during storage?

As eggs age they lose carbon dioxide and become more alkaline in pH. Several visual changes accompany this. Describe four of the visual changes that indicate a lower quality egg.

What is the significance of a thin runny versus a thick viscous egg white?

What is the difference between egg albumen and egg albumin?

What are the temperatures at which the different components of eggs coagulate?

How does the following affect the coagulation temperatures of egg:

Describe the differences and/or similarities between the following terms:

What is the most striking difference between a poached fresh egg and a poached deteriorated egg?

What would be the ideal conditions for storing eggs in the home and why?

Which component of the egg coagulates at the lowest temperature? What is the effect of combining the egg yolk and egg white? Why?

If you have made baked eggs and found them to be tough and undesirable, what you do differently next time?

Why does a hard cooked egg have a undesirable green film on the yolk?

How would one avoid a dark ring around the yolk of a hard-cooked egg?

Why do dried egg whites turn brown? How do they prevent it?

Does the egg substitute make a suitable replacement for fresh eggs? Why or why not?

What are egg replacers?

What are the top grades of eggs?

What are the weight classes of eggs? Why do we care what type of egg (from what species) one has?

Are eggs with blood spots safe to eat?

What is the likely reason for the thinning of egg white- indicate major protein involved and indicate possible mechanisms?

Why do we care what the physicochemical properties of eggs are? What is the advantage of knowing them?

Complete the following chart and indicate the nutritional and medicinal protein sources and their effect or advantage for those that eat cooked and/or raw eggs.

Explain the possible effect of removing ovalbumin and replacing it with ovomucoid in an angel food cake.

Which component of the egg coagulates at the lowest temperature? What is the effect of combining the egg yolk and egg white? Why?

Which component of the egg coagulates at the lowest temperature? What is the effect of combining the egg yolk and egg white? Why?

Why do scrambled eggs stick to the bottom of the pan particularly?

An egg in the shell which has been simmered in water for 20 to 25 minutes rather than boiled would likely have what quality characteristics?

Can one decrease the cholesterol content of eggs?

Why and/or how do egg substitutes and/or replacements differ from whole eggs? What is the affect of such replacement?

A recipe I have calls for 10 eggs. I don't want any left over as I am leaving on vacation. What size should I get?

Practically speaking, what are the implications of the differing coagulation points of the various egg parts?
The importance of this will be dependent upon the role of the egg protein in the food product. Particularly, if it is suppose to form structure it may cause a problem. For example, some culinary experts will think they will substitute egg yolks for egg whites and get a "richer" products. If this is done with a foam in a sponge cake, it means that the relationship between leavening and the formation of structure will not be identical. It means that the something like an angelfood cake might collapse as the structure would not be set before the leavening was done. In a custard a substitution of egg white for egg yolk or whole egg would mean that the coagulation point would be lower and it would be more likely to curdle due to the change in coagulation point.

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Updated: Wednesday, October 24, 2007.