Egg Yolk

[Koo Schadler]

My question concerns egg yolk as a binder. My understanding is that yolk contains a fatty substance called egg oil. From what I've read, egg oil is a non-drying oil. Yolk also contains albumen, a powerfully drying protein. On its own albumen can be too drying and would embrittle a paint film - but in combination with egg oil it actually helps the egg oil to "cure" (and in turn, the egg oil contributes flexibility to albumen). The albumen (water soluble) and the egg oil (non-water soluble) are emulsified in water via lecithin, also found in the yolk. There are other ingredients in egg yolk but they are inconsequential to paint.

At least, that is my understanding of an egg yolk! I would appreciate hearing from anyone if they have a different or clearer explanation. Also, I have few specific questions regarding the above theory:

1. Does egg yolk cure through oxidation, polymerization, or both?
2. What actually polymerizes the yolk - oxygen, light rays, heat, or all three? Which is best?
3. If, as described above, albumen helps the non-drying egg oil to "cure", could albumen be considered a sort of "dryer" (in somewhat the same sense that, for example, cobalt helps with the drying of linseed oil)?
4. If albumen is a sort of "dryer" (so to speak) would albumen also facilitate the drying of other oils, such as linseed?
5. Another way to ask that question: would an egg oil emulsion (tempera grassa paint) cure more readily if, in addition to the yolk, the white of the egg (mostly albumen) were included in the emulsion?
5. If albumen isn't actually a sort of "dryer", than why is it that tempera grassa paints contain a large percentage of linseed (or other drying) oil - oils that usually take a long time to dry to the touch - and nonetheless the surface of a tempera grassa painting is dry to the touch within seconds? What is it in an egg that helps the slow drying linseed oil in tempera grassa to feel dry so quickly?

Apologies - I know those are a lot of questions, but all are interrelated and I think are best asked together. Any insights would be appreciated.

Many thanks,

Koo Schadler

[George O'Hanlon]

Koo, your understanding of the basic drying mechanisms of egg yolk tempera is correct. Here are some thoughts in answer to your questions. An extensive review of literature on the subject, as you probably already have discovered, will not provide much in they way of direct answers to your questions, but I will try to provide some helpful responses.

1. Does egg yolk cure through oxidation, polymerization or both?
If we examine the fatty acid content of egg yolk oil, it appears to be similar to that of non-drying oils, such as safflower (high oleic acid type) palm and soybean oils. The composition (by weight) of the most prevalent fatty acids in chicken egg yolk is as follows:[1]

Unsaturated fatty acids:
Oleic acid, 47%
Linoleic acid, 16%
Palmitoleic acid, 5%
Linolenic acid, 2%
Saturated fatty acids:
Palmitic acid, 23%
Stearic acid, 4%
Myristic acid, 1%

As a comparison, this is the fatty acid composition of palm oil, a non-drying vegetable oil:

Unsaturated fatty acids:
Oleic acid, 40%
Linoleic acid, 10%
Saturated fatty acids:
Palmitic acid, 45%
Stearic acid, 4%
Myristic acid, 1%

The low percentage of polyunsaturated fatty acids indicate that it is a non-drying oil. (Oleic acid is a monounsaturated fatty acid.) However, it is not an absolute indication that it is a non-drying oil.

Non-drying oils do not harden through autoxidation (autoxidation causes polymerization in drying oils) or thermal polymerization. So, does egg yolk oil polymerize and eventually harden, as do drying vegetable oils, such as linseed oil? There is no definitive answer to this question, because all the literature that I have surveyed does not investigate this matter directly. If egg yolk oil is a non-drying oil, than we may assume that it does not polymerize and harden. It may act as a plasticizer in egg tempera paint film, giving what would be an extremely brittle film some flexibility.

Non-drying oils, such as castor oil, have been used in paint formulations as plasticizers to increase the flexibility of the dried paint film. Perhaps this is the function of egg yolk oil in egg tempera painting.

2. What actually polymerizes the yolk—oxygen, light, heat or all three? Which is best?
As you described, egg yolk is composed of a non-drying oil that appears to “dry” along with the other ingredients in egg yolk. Egg yolk is composed of 50% water, 30% lipids (described above), 15% proteins and 5% other components, including lecithin (an emulsifying agent).[2] The proteins in egg yolk are composed of tightly folded molecular strands that are held together by “weak” hydrogen bonds. When heated these bonds break and the strands unravel, but reconnect to one another forming the hard substance we know as hard-boiled eggs. It appears that the weak bonds can also break down when the egg yolk is exposed to oxygen and light, which is what we find when we use it in egg tempera painting. But actually what is happening is the water contained in the egg yolk evaporates, allowing the protein molecules to unravel and cross-link with each other, forming a cohesive film. This is the same principal how latex paints dry and form cohesive films.

3. If, as described above, albumen helps the non-drying egg oil to “cure,” could albumen be considered a sort of “dryer” (in somewhat the same sense that, for example, cobalt helps with the drying of linseed oil)?
A small matter, but albumen refers to the egg white, albumen deriving from the Latin word “alba” for white and typically not to the proteins in egg yolk.

The proteins in egg yolk do not function as “dryers” in egg tempera but rather in the manner described in the answer to your question # 2.

4. If albumen is a sort of “dryer” (so to speak) would albumen also facilitate the drying of other oils, such as linseed?
In this case, drying oils, such as linseed oil, may dry faster due to autoxidation because more surface area of the oil is exposed to oxygen when it is part of an emulsion. Tempera paints made with drying oils, such as the commercial egg tempera paints, tend to dry harder, but also behave quite differently than do pure egg yolk tempera paint. That linseed oils dry quickly when part of an emulsion can be easily demonstrated by mixing a small amount of egg yolk or other emulsifying agent, such as casein or gum arabic solution, into an oil color. The paint “sets” or “dries to the touch” rapidly, but still requires some time to completely harden into a solid, flexible film.

5. Another way to ask that question: would an egg oil emulsion (tempera grassa paint) cure more readily if, in addition to the yolk, the white of the egg (mostly albumen) were included in the emulsion?
The same principle that applies to egg tempera, which is an emulsion, is also applicable to tempera grassa. In other words, egg tempera is an oil-in-water emulsion and tempera grassa is either an oil-in-water or water-in-oil emulsion, depending upon whether the amount of oil exceeds the amount of water in the emulsion. The drying is caused by the evaporation of the water and the polymers in the paint binder coalesce into a solid film. Again, whether egg yolk oil hardens (polymerizes) in egg tempera binder or not is not conclusively known.

6. If albumen isn’t actually a sort of “dryer”, than why is it that tempera grassa paints contain a large percentage of linseed (or other drying) oil—oils that usually take a long time to dry to the touch—and nonetheless the surface of a tempera grassa painting is dry to the touch within seconds? What is it in an egg that helps the slow drying linseed oil in tempera grassa to feel dry so quickly?
Often tempera grassa recipes contain some amount of a natural resin, such as damar or mastic, which sets or hardens quickly upon the evaporation of the volatile organic solvent. This may appear to dry quickly and may even assist in the rapid drying of the oil. Even if the recipe does not contain any resin, recipes always contain some type of vegetable drying oil, which as discussed above does polymerize through autoxidation and hardens to a sold film giving the appearance of faster drying.

References
1. National Research Council, 1976, Fat Content and Composition of Animal Products, Printing and Publishing Office, National Academy of Science, Washington, D.C., ISBN 0-309-02440-4; p. 203, online: http://books.nap.edu/openbook.php?ch...ified+Page.y=0

2. Cain, et al., “Heat Changes Protein Structure: Frying an Egg,” in Discover Biology, Third Edition, W. W. Norton & Co., online: http://www.sumanasinc.com/webcontent...structure.html

[Koo Schadler]

Hello George,

You are correct - I've been searching but haven't found much information to address my questions. Thanks so much for your reply (very exciting reading for an egg tempera nerd such as myself). It makes sense and is very helpful to me.

One more question on this topic: I’ve heard that light is the best way to polymerize egg yolk tempera - that heat can cause the protein strands to become “cloudy”, but when cured by light (specifically actinic light rays) they remain transparent.

This is apparent when egg white (albumen) is cooked, certainly; but does it hold true for egg yolk as well? Have you heard of the benefits of curing with light versus heat?

(I had read somewhere – I think R. Mayer but don’t have the book with me to check - that the proteins in the yolk were albumen. I appreciate your clarification that they are not).

I presume that in the absence of heat or light an egg tempera painting would still polymerize through autoxidation, correct?

Koo

[George O'Hanlon]

Egg yolk tempera hardens due to the evaporation of the water and the coalescing of the egg yolk proteins into a solid film. This is not related to autoxidation, but may be assisted by light, air and heat. How the egg yolk oil hardens or polymerizes, if it does (see the explanation in my previous posting), may be through autoxidation, which as the term implies is by absorbing oxygen from the atmosphere. Light and heat play role in the polymerization of drying oils, but whether this is true for egg yolk oil is subject of an experiment.

In regards to the color of hardened egg yolk in egg tempera and whether light or heat may affect the color as it does egg white, is not clearly understood. It would be interesting to determine the answers to these questions.

[Koo Schadler]

Thanks for the further clarification George. I'll pay greater attention in my own workings with egg tempera to the unanswered portions of my questions, to see what I can figure out through direct experience. Your help with these issues is, as always, invaluable.

Koo