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In making a brine for my chicken, I came across a lot of recipes that call for sugar in the brine. I understand the general idea behind brining, but don't understand what benefit the sugar provides.

In this answer, @papin links to a PDF that states the following (emphasis mine):

The law of diffusion states that the salt and sugar will naturally flow from the area of greater concentration (the brine) to lesser concentration (the cells). There is also a greater concentration of water, so to speak, outside of the turkey than inside. Here, too, the water will naturally flow from the area of greater concentration (the brine) to lesser concentration (the cells). When water moves in this fashion, the process is called osmosis. Once inside the cells, the salt and, to a lesser extent, the sugar cause the cell proteins to unravel, or denature. As the individual proteins unravel, they become more likely to interact with one another. This interaction results in the formation of a sticky matrix that captures and holds moisture.

This makes it sound like sugar isn't really an important part of the osmosis process.

Is there a chemical/molecular reason to add the sugar, or is it just to add some flavor to the chicken breast?

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The sugar is simply used for flavoring; the fact that it helps brine to a lesser extent is just an added bonus. The sugar also aids in browning via the Maillard reaction, though this can also result in burning in a high heat application.

I suggest brining two boneless skinless chicken breasts -- one in a salt-only brine and the other with the salt & sugar brine suggested by Cook's Illustrated. You should notice a difference in both flavor and appearance.

  • Sugar has nothing to do with Maillard. Sugar browning is caramelization. Maillard reactions only occur with proteins. /nitpick. – daniel Sep 24 '10 at 21:57
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    @roux: en.wikipedia.org/wiki/Maillard_reaction You'll see that sugar has everything to do with the Maillard reaction; the reaction cannot even happen in the absence of sugar. You'll see this and other detailed information in the section titled "Foods and products with Maillard reactions". – hobodave Sep 24 '10 at 22:32
  • I am going on what I have read in McGee and in other places. Sorry, but I trust McGee over Wikipedia. The Miallard reaction--browning of meats--is a physically different set of chemical and physical processes than the phase-changes of sugar (as in table, white, sugar) when heat is applied. The end results are similar, yes, but the pathways to get there are different. – daniel Sep 24 '10 at 23:18
  • According to wikipedia, the Maillard reaction is a chemical reaction between and amino acid and a reducing sugar, using requiring heat. Note that sucrose (table sugar) is not a reducing sugar. So if that is what is in the brine, is has nothing to do with Maillard. – Mark Sep 26 '10 at 1:02
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    @Mark: The assumption you make in your last sentence is incorrect. Sucrose, along with all of the other polysaccharides still participate in the Maillard reaction, albeit with much less noticeable effect. This is noted in great detail here, here, and here. The latter of which is an actual reference for the very high level Wikipedia article. – hobodave Sep 26 '10 at 1:40
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Sugar is for seasoning. While the law of diffusion does indeed say that a solute (sugar, salt) will flow down its concentration gradient from and area of high concentration to an area of low concentration there is one fact you seem to negate which is the cell membrane within which the muscle proteins are encased. The cell membrane is impermeable to passive transport by diffusion of salt and sugar. Both these substances must be actively transported into the cell. So in a live organism water (which can easily pass in and out of cell through aquaporin proteins) would leave the cell and eventually crenate (dehydrate). However, since this animal is no longer living the cell membranes have broken down to some degree and the barrier to salt transport is relieved. The salt solution would disrupt cell membrane transport proteins and sort of punch holes in the muscle tissue allowing salt and sugar tot enter. The salt will interfere with the chemical bonding that keeps the muscle proteins shaped the way they need to be to function. This is called denaturing. The unfolded protein can now form bonds with the Cl and Na ions and water will form hydration shells around the proteins thus hydrating the meat.

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