Protein   
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Proteins mostly function as humectants, and as emollients and moisturizers. These can give a smooth feel to hair and skin, but they can't repair hair. They also are too big to be absorbed into the hair shaft, but are often processed to make them small enough to cling better to the hair.

Proteins work by coating the outside of your hair, filling in any gaps in the cuticles. This can make your hair feel softer. However, even though proteins are building blocks to our hair and bodies, it can't repair our hair. —This is the same thing as dumping yarn on a wool sweater and expecting your sweater to be repaired—T Protein in no way can permanently attach to hair, and in no way can repair hair. To say otherwise is purely magical thinking [Begoun (Hair) pg 83].

Used for conditioning, and many believe it can repair hair. Protein is known to improve hair gloss, give hair body, moisturizer, and even make hair easier to comb. It may even help reduce some irritation from harsher surfactants. Because they are film forming, they temporarily can “glue” together split ends. They also can make hair feel better after damaging treatments such as bleaching, perming, and rough combing. Proteins are often put in products because of their marketing appeal.

Protein is derived from animal sources, such as collagen, elastin, milk, and keratin or from silk, soy, vegetables, yeasts, and even marine animals. Interestingly, egg, beer or milk conditioners are usually not marketed as protein conditioners, even though they actually do contain true proteins.

Some studies have found that protein can actually penetrate hair and deposit on the cortex (though nothing can bring back severely damaged hair once it has been damaged-—T). The more damaged your hair, the more protein it absorbs. What most affects how much protein is absorbed into the hair is the damage in the hair (the more damaged, the more porous, so the more it can soak up), the size of the molecules, the pH, how long the protein has been left in the hair, and the concentration of the protein being used. The most absorption occurs within 15 minutes. Concentrations up to 5% seem to work best on damaged hair.

Sometimes proteins may contain Sodium chloride (salt). This happens during the chemical reaction of breaking them down to make them water-soluble. The high salt content may mess up the stability of the products they are used in. The good news is that salt free versions are available. (The trick is finding out which variety is in a product-—T).

When proteins are used in a product, it’s actually a broken-down form of protein (peptides). The protein is able to dissolve in water only after it’s gone through a process that breaks it down (by hydrolysis). However, so long as the form of protein used doesn’t fall below a certain size, it still qualifies as protein (if the protein keeps getting broken down, eventually it becomes the amino acids that compose it. Amino acids are the building blocks of protein—long chains of amino acids are what make proteins. However, these are no longer considered proteins, and have different properties). First the protein is broken down into somewhat smaller chains of amino acids known as polypeptides. When the chains are further broken down, then they are amino acids.

When proteins are broken down by hydrolysis, they become water-soluble. This is necessary because if they are still made up of the long chains of amino acids, they are too big to really be of any use to hair. They are too big to stick to the hair. The molecules work best between a very narrow range of sizes, smaller generally being better. There is an optimum protein size that sticks best to hair (but they can’t get too small because then they no longer qualify as protein, but are now amino acids) [Hunting (Conditioning) Pages 349-352].
See also: Amino acids Keratin Keratin amino acids Hydrolyzed elastin Collagen Hydrolyzed
Source(s): Begoun Hunting