Dietary Protein Denaturation and Digestion

Have you every wondered why we cook eggs and meat? Or what is the difference between denatured whey protein and undenatured (native)? I have. Especially about the whey protein – denatured vs. native.

I wanted to find some answers for myself and to also put together a nice article that (hopefully) sheds some light as far as what exactly dietary protein denaturation means and is it better for us or not (as far as nutrition and possible health benefits are concerned). Moreover, in which cases is it better to aim for denaturation and in which for the raw, native form of the dietary proteins.

Whey Protein Powder

Below is the result of what I’ve found. I have to warn you, though. This article will seem slightly more academical than engaging, so it will require (slightly) more attention and focus. Not too bad, though..

Shall we begin? Before I get to what happens when proteins get denatured, you will have to undergo a crash course on proteins (just like I had to).

Structure of proteins:

1) Amino Acids – building blocs of proteins
2) Peptides – short chains of amino acids (fewer than 50 individual amino acids)
3) Proteins – long chains of of multiple peptide formations (polypeptides)

How do you get from an amino acid to a protein? By a process called polymerization.

Polymerization of amino acids (1):

1) Primary protein structure – individual amino acids bound together by peptide bonds
2) Secondary protein structure – helix – a curled (like a coiled spring) tri-dimensional peptide structure
3) Tertiary protein structure – a tri-dimensional structure formed by the multiple folding of helices (plural of helix)
4) Quaternary protein structure (2) – multiple secondary and tertiary protein structures, some times combined with other molecules, like sugars (glycoproteins) or fats (lipoproteins) (3)

Protein denaturation is the unfolding of the quaternary, tertiary and secondary protein structures. When the unfolded proteins bond to each other that’s called coagulation. These bonds are usually permanent (like what happens when you cook an egg white). Most often the denaturation of proteins is caused by physical (heat) and chemical (acids, alcohol) reactions.

I am going to make the assumption that we are all interested mostly in the denaturation of  dietary proteins that are most common in the human diet – particularly dairy proteins, meat and egg proteins.

Generally, it is a well established fact that denaturing of dietary proteins makes them more available to digestion, and in the case of milk proteins – more bio-available (4) .

Denaturing of milk proteins

There are two major proteins in milk – casein (80 percent) and whey (20 percent).

Casein undergoes little if any denaturing by heat or acid because it doesn’t have the usual secondary and tertiary protein structures. In the stomach, under the action of the stomach acids, casein forms glue-like cloths, which are difficult to digest. For that reason it stays in the stomach longer, which gives it the slow-release properties that are popular among bodybuilders.

We now know that casein releases certain bio-active peptides under the digestive powers of the proteolitic (protein-digesting) enzymes in the stomach, like pepsin, trypsin and chymotrypsin. These enzymes break down the longer protein fractions to 2 – 20 amino acids-long peptides that exhibit multiple health benefits (5).

Whey is also very popular among those who value its properties to release amino acids in the blood stream in a very short period of time, making it an ideal protein for recovery, muscle building and re-building.

Whey protein also exhibits health benefits due to its native bioactive fractions, among them beta-lactoglobulin, alpha-lactalbumin, bovine serum albumin and lactoferrin. All these bioactive fractions are shown to have cardiovascular, digestive, endocrine, immune and nervous system- modulating effects… But, only when they are in their undenatured form.

This is where I have to ask for your more acute attention..

Whey protein is derived in one of two ways – trough the cheese-making process and by direct processing of skim milk to micellar casein and native whey concentrates and isolates (via micro-filtration/ultra-filtration). The first process denatures the native whey fractions, not because the milk is twice pasteurized during the cheese-making (these biologically-active fractions are only partially lost during pasteurization) (6), but mainly because of the subsequent lowering of the Ph level to as low as 3.0 mostly by adding citric acid (at this level the most of the native fractions are destroyed, except lactoferrin). The lactic acid is added after the whey has been separated from the curd, as a part of the whey concentrate processing (7).

The processing of native way from skim milk (not from cheese) provides mostly undenatured whey protein. It undergoes one pasteurization process, but there is no need to lower the Ph to levels where the native fractions are denatured.

The interesting thing is that undenatured whey protein will provide health benefits despite the Ph level in the stomach, which is even lower than that in cheese making (Ph between 1 – 3). The reason is the digestive juices in the stomach (pepsin, trypsin and chymotrypsin), when presented with these native bioactive fractions will further break them down into smaller bioactive peptides that the body derives health benefits from. For example, from further digestion of beta-lactoglobulin the body will make Ala-Leu-Pro-Met-His-Ile-Arg (or ALPMHIR, yeah… I know), which is proven to have blood pressure-lowering properties (ACE inhibitor) (5).

Something has to said also about the cysteine-cystine ratio (cystine forms when two cysteine amino acids bind together via a disulfide bond) in native and denatured whey protein. In native whey the this ratio is more favorable to the production of glutathione (a major antioxidant) by the body. In denatured whey this ratio is skewed in favor of cystine, which makes it more difficult for the body to utilize this important amino acid (it has to break down cystine to cysteine first).

To sum it up, cheese making denatures whey, but it doesn’t denature casein. The manufacturing of native whey straight from skim milk produces undenatured whey protein.

It is important for whey protein to be manufactured in a way that doesn’t denature it before it enters the body. Once in the body, the undenatured whey is ‘denatured’ but in a way that provides the health benefits undenatured whey is well known for. Simply, the body ‘unlocks’ the bioactive substances via the action of its digestive enzymes. This is true for both whey and casein.

If the native protein fractions in whey are denatured before they enter the stomach, the body can still use the proteins by breaking them down into very short peptides and individual amino acids.

Denaturing of meat proteins

Usually we eat meat protein that’s already denatured by heat – like cooked meat. The two parts of protein that are denatured in this process are collagen, which is the connective tissue that separates the bundles of muscle fibers, and the proteins inside the muscle fibers themselves.

When meat is cooked the collagen turns into gelatin. Collagen is tough and gelatin is tender. So, cooking the meat naturally makes it more tender. But if meat is overcooked a lot of the water that is naturally trapped in muscle fibers escapes and the meat becomes tough again (3).

Denaturing of the meet proteins (mainly myosin and collagen) trough cooking makes them partially more bioavailable. The stomach proteolitic digestive enzyme pepsin can digest these proteins better, but there is no change of the digestibility level for the other two proteolitic enzymes – trypsin and chymotrypsin (8). In other words, cooked meat is still easier to digest than raw meat, although raw meat can contribute its own enzymes to aid digestion.

When is meat cooked? At 120°F the protein myosin begins to denature (coagulate). At 140°F another protein called myoglobin denatures and turns from red to tan-colored color (hemichrome). The connective tissue protein collagen starts denaturing between 140-150°F. It dissolves into gelatin at between 160-170°F.

Denaturing of egg proteins

In eggs both – white and yolk – contain protein. About 60 percent of the egg protein is in the white and 40 percent in the yolk.

The egg yolk contains two types of proteins – phosvitins and lipovitellins – phosphorus and iron-storing proteins. Albumen is the common name of the egg white proteins (not albumin). But there are a few different egg white proteins with their own names.

The most abundant protein in the white is ovalbumin – about 50 percent. It denatures at 176°F when the egg is fresh. This temperature slightly increases when the egg is several days old. Ovalbumin in its raw form inhibits the action of protein-digesting enzymes.

Ovotransferin is the second most abundant protein in egg whites – 12 percent. It denatures before ovalbumin at 145°F so it is the protein that determines the final shape of a cooked egg since it sets first.

Ovomucoid takes 11 percent of the total protein and ovomucin – 2 percent. Both proteins do not gel upon heating but they help with tightening and strengthening up the structure of the cooked egg white. When ovalbumin and ovotransferin denature and coagulate ovomucoid and ovomucin get incorporated in the final set structure.

There is one more protein that is present in minuscule amounts in the egg white – avidin (less than 0.1 percent). Interesting thing about this protein is that it binds to biotin – an essential B vitamin – preventing the body from absorbing it. This property of avidin is removed upon denaturing/cooking of the egg protein.

Heat-denatured egg proteins are digested at about 90 percent in the human body whereas raw (undenatured) eggs are digested at only about 50 percent (9). This is most likely due to anti-nutrients (digestive enzyme inhibitors, vitamin and iron absorption inhibitors) that the egg has developed during millions of years of evolution to protect the nourishing egg from being used for food by predators. Heat treatment/denaturing for the most part disables these anti-nutrients.

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That’s it in a nut shell. If you find anything missing that I need to add in this article, let me know and I will try to make it as complete as possible.

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