On My Youngest Daughter Ai Hsing (Front) Graduation Day

On My Youngest Daughter Ai Hsing (Front) Graduation Day

Monday, August 30, 2010

Would boiling or frying an egg damage it nutritive value?

Question:

An e-mail enquiry was received from Mr. Tan Khoon Seng as follows:

Concerning all those good things said about eggs and nutrients, may I know if they are destroyed if the egg is hard boiled or fried? Frying is intense heat. Will the heat destroy the good stuffs?

We can see that the albumin and yolk protein hardens when heated. So its structure is changed. Will these changes harm the natural good nutrients in eggs? If not, why not?

Your comment will be greatly appreciated.

Regards

Khoon

Answer:

Physical, not chemical changes to eggs:

No! Moderate heat over a short time like boiling an egg for 2 minutes, or lightly fry an egg does not alter its nutritive values. Heat merely alters the physical properties of egg protein by coagulating its albumin, but this does not change it nutritive values.

However high heat and cooking the egg, especially frying it with oil in open air over a long time does cause heat damage to the proteins, and irreversibly change its nutritive values. (Digestion of Heat-damaged Egg Albumen by the Rat by Jose Valle-Riestra and Richard H. Barnes, Graduate School of Nutrition, Cornell University, Ithaca, New York 14850).

High temperatures not only physically and chemically denature the egg protein, but they also destroy almost all the vitamins, especially the heat labile B group of vitamins.

Nutritive values improve instead:

However moderate heat over short period just to cook or fry the egg on the contrary improves its nutritive value of the whole egg by inactivating a substance called avidin which is a glycoprotein found in the white of egg. This substance binds with one of the B vitamins called biotin and renders it unavailable to the body. Cooking destroys this avidin, and releases the biotin. This makes the egg more nutritive and more wholesome. However, this is compensated by the fact that the yolk contains a lot more of biotin, and it makes only a little difference even if the biotin in the white is not available because of the avidin. However, eating too much white of egg can lead to biotin deficiency.

Whole and wholesome eggs:

Egg should be eaten whole. I know of a lot of people who because of their ill-advised knowledge of nutrition throw away the best part of egg – the yolk, and eat only the white of egg.

I saw many of them doing this in front of me while we were on the same table for breakfast. Even though I did not know them as they were strangers on the same table, I still asked them why. They told me the yolk contains cholesterol and it causes heart disease. I then asked them how they knew that egg yolk contains cholesterol. They replied they got it from the Internet.

I then asked them why they thought that eating eggs high in cholesterol causes heart disease. They told me they also read this from the Internet. That’s the problem of people looking up information from the Internet instead of seeking the advice of a qualified health-care professional. I have previously written a lot on the myth of cholesterol, CHD, and health, and cited huge studies having concluded that dietary cholesterol neither raise or decrease blood lipids and serum cholesterol, and that ischemic heart disease is faintly correlated to elevated blood lipids. This was the conclusion in the follow up study from the immensely huge and longitudinal Framingham Heart Study. I don’t intend to repeat it anymore.

Raw and boiled eggs:

There is also some difference in the digestibility between raw and cooked egg in humans as assessed by stable isotope techniques. The study was conducted by Pieter Evenepoel, Benny Geypens, Anja Luypaerts, Martin Hiele, Yvo Ghoos, and Paul Rutgeerts from the Department of Medicine, Division of Gastroenterology and Gastrointestinal Research Centre, University Hospital Leuven, B-3000 Leuven, Belgium.

They used 15N-dilution technique to demonstrate that the assimilation of cooked egg protein is efficient, albeit incomplete, and that the true ileal digestibility of egg protein is significantly enhanced by heat-pretreatment. A simple 13C-breath test technique furthermore proved to be a suitable alternative for the evaluation of the true ileal digestibility of egg protein.

However others claimed the opposite that cooked eggs are more digestible than raw eggs. However, they did not cite the literature where the studies were conducted. Every claim needs research and evidence-based studies

Avidin:

Except for this slight statistical difference on protein bio availability, eggs to the best of my professional opinion are equally nutritious whether eaten raw, or lightly cooked, except for the presence of a glycoprotein called avidin which blocks the bioavailability of a vitamin called biotin. Biotin is a member of the B group of vitamins. The cooking process deactivates the avidin in the egg, allowing biotin to be made available to the body.

The avidin-biotin blockage in the white of eggs is compensated by its yolk which is very rich in biotin. In fact the yolk contains one of the highest biotin content among all foods. Hence it is wise to eat the entire egg, the yolk and white together. Even the shell is a rich source of calcium. In fact it is so rich in calcium that traditional beliefs (as told to me by my mother when I was a child), that eating egg shell may cause may cause the flow of urine to be blocked.

This does make scientific sense to me as I see it now. The difficulty of micturition (urination) may be caused by renal calculi (stones in the kidneys) passing down into the urinary tract caused by excessive calcium leaching out into the urine and blocking the urinary passage-way with calcium stones. Of course I am aware of other causes impeding the flow of urine, like benign prostatic hyperplasia (BHP) or prostate enlargement. But we are not talking about this here. This is an irrelevant topic here.

Biotin deficiency:

Thus it may be true that eating too many raw egg whites by themselves may cause biotin deficiency, which is normally missed out in the diagnosis by doctors who are not competent in nutritional disorders unless they have additional postgraduate degrees in clinical nutrition to be clinically competent in nutritional medicine. In fact the assessment of nutritional status is one of the hardest and most challenging fields of medicine and even for a very well qualified and expert nutritionist.

Salmonella in eggs:

An added advantage of cooking an egg reasonably well, but not over fry it, is that it may prevent the risk of a food poisoning caused by a bacterial infection called salmonella. This is a serious health risk. Salmonella are gram stain-negative intracellular anaerobes (microbes) that cause a wide spectrum of gastrointestinal diseases. They include gastroenteritis, enteric fever (caused by typhoid and paratyphoid serotypes), bacteremia, focal infections, to a convalescent lifetime carrier state. The type of infection depends on the serotype of Salmonella and host factors. Each type is distinctly identifiable by its specific protein coating. They are known to cause disease in humans, animals, and birds (especially poultry) worldwide.

No more about eggs please!

Kindly do not ask me anymore things about eggs as I have a lot of other things I want to complete writing on other subjects and other work to do, but unable to do so because of all these distractions. My mails contain too many questions people ask off and on.

jb lim

4 comments:

Tai Onn said...

Dear Dr JB,

I was one of those "ignorant" people you mentioned about not eating the egg yolk (because of the hearsay about its causing high cholestrol)for many years in the past. Ever since I read about the fact from such a highly qualified nutritionist like yourself a few months ago, I have started eating the whole egg. Thanks for your kind advice.

Ir T O Lau

suzanne stoeckle said...

I am trying to establish what happens to the EFAs in Egg yokes when they are dried? DHA and AA ?
does drying effect the nuttritional values of the Yoke ..does the drying damage nutrients?
thank you Suzanne

lim said...

Dear Suzanne,

Thank you for asking.

This depends on the extent you dry the egg yolk, how long you keep it, the humidity and temperature of storage, and whether or not an appropriate antioxidant has been added to prevent the EFA and DHA from suffering lipid peroxidation and oxidative damage.

Generally speaking, if you accelerate freeze dry egg yolk or vacuum evaporate it under low temperature, or spray dry it, all the nutritive values in egg yolk or in whole egg would be retained.

If you then reconstitute it with correct amount water, you should get back all the nutrients intact, including the fatty acids, the B group of vitamins as in the original amounts.

Only the physical texture, and perhaps taste and colour may change slightly.

Drying foods is another alternative method to preserve them instead of freezing them.

Examples are milk powders, traditional sausages, dried fish and dried prawns as practised in Asian food preparation and cultures

Dr JB Lim

Vis8 said...

Heating a protein will NOT affect its nutritional value. If it did, all the cooked meat we eat would have no nutritional value.

People get confused with the term 'heat inactivation' (or 'denaturation') of proteins.... This means that when heated, the biological function of the protein is destroyed, but, the protein structure remains intact.

All proteins that are ingested are broken down into amino acids, the basic building blocks of proteins in the gastro-intestinal tract, and get absorbed into the body. The body then makes its own proteins by combining these amino acids together.