The cream of the crop - interactions of black tea and dairy:

This entry comes as a result of a lengthy discussion that took place here - scroll down to the comments, if you are so inclined. My apologies to Alex - I promised this article several weeks ago and am finally getting around to it. I also felt that the antioxidant series was first necessary to help with the clarification of some of this material. Nonetheless, here it is!

Alex had asked if the addition of milk (or cream) to black/red tea would negate any antioxidant properties that it possesses. The short answer: No, but it does significantly decrease the amount of available antioxidants - somewhere in the neighborhood of 80% of the original black tea without dairy. If this is all the information you need, stop here, otherwise I'll go into some details that some may find interesting. I'll keep it fairly simple as always, providing information that I feel is important but not too specific... On to the science!

Every wonder why black (or more appropriately, red) tea is, well, red? The answers lies in the oxidation of catechins (remember here?). During the black tea manufacturing process catechins, ever present in green tea, become oxidized via PPO, an enzyme naturally present in the plant's cells. This is why all the rolling, withering, etc is necessary to produce black tea - when the cell walls are broken down enzymes are free to interact with polyphenols without obstruction, oxidizing them into new compounds. Two of these newly formed compounds are particularly important to black tea - theaflavin and thearubigin. It's important to note that these new compounds still retain antioxidant properties - black tea is still good for you! Theaflavins are bright golden yellow to yellow-brown in color while thearubigins are brown to red-brown. Together these two compounds account for the majority of the color in black tea to varying degrees (the difference between say, an Assam and a Darjeeling depend on the relative concentrations of each). If you look at the structure of theaflavin, you can see how similar it is to the catechins that it may have been oxidized from - it looks like one was inverted and stuck on top of another (generally speaking). All you really need to know is that these are the two key players in black tea.

Perhaps the most apparent change that happens when milk/cream (I'll just say milk from here on out for simplicity, but any dairy product would do) is added to black tea is the color change. I brewed up a nice hearty Assam in my mug and took pictures of the liquor (poured into tasting cups to emphasize the color) before and after the addition of milk. Notice the orange-red color of the liquor before milk - this is the result of both theaflavin and thearubigin floating around. However, after milk is added, the color turns brown with only a hint of yellow. I corrected the photo as best as I could, but the incandescent light still made the tea with milk look more yellow than it really was. This color change isn't only because we added white milk to the tea (although the less brilliant red color is) - it's also the result of a chemical reaction. Theaflavin has somehow lost the ability to impart its golden-yellow color onto the liquor. The result is the dull, brownish color seen on the right.

Although it may not seem like the answer to Alex's question at first glance, this is exactly what we are after. It turns out that the theaflavins in solution have become "entangled" with the milk proteins, particularily one known as casein. I put "entangled" in quotes because in all honesty, I don't know exactly how theaflavin reacts with casein. Proteins are huge, tangled, conformational messes of molecules and I don't know if anyone has researched into the intricacies involved in this interaction. My guess would be that the protein has a receptor site for a molecule that is similar enough to theaflavin that it can bind in as if it were the intended molecule. But again, that's just a guess. The real point is that research has shown that theaflavin binds with casein, effectively removing it for solution. This casein-theaflavin complex (I'll just call it the CT complex from now on) is stable - more stable than if the two molecules were floating around on their own, or else the reaction would have never have happened in the first place! Since theaflavin now exists as a CT complex, it loses the antioxidant properties it once posesses. It's essentially "part" of the protein now - if there was some way for it to "unhook" itself from the protein, chances are that its conformation (3D-orientation) and/or it's chemical structure will have changed, and both are important to how it acts as an antioxidant. Basically, once the CT complex is formed, theaflavin is no longer going to act as an antioxidant.

But what about thearubigin? We discussed how the liquor takes on the color of (primarily) thearubigin and we now know that this is because theaflavin forms a complex with casein. So why doesn't thearubigin? Rather than forming a protein complex, thearubigin molecules instead polymerize with each other. Polymerization implies the formation of a polymer, which is a long chain of repeating segments - plastics are the best example. In our case, each segment is a molecule of thearubigin - they bind to each other and form long chains which are then unable to react with the proteins of milk. These long chains alter the properties of thearubigin so that they are not as effective of antioxidants as they would be on their own. In reality, it's essentially impossible to have single thearubigin molecules floating around (and even more actual, almost no molecules float around as themselves - nearly everything forms a complex, but we won't go there) because the second they enter water, they begin to polymerize.

When it's all said and done, the antioxidant potential of the tea after milk has been added is roughly 80% of it's original. I pulled this statistic from here (I think). The studies I looked at varied from noticing no change at all to masking the AOX potential by ~50%. Here is a list of some of the better studies that I consulted:

Interactions between Flavanoids and Proteins: Effect on the total antioxidant activity (same link as above).
Content of potentially anticarcinogenic Flavanoids of Tea Infusions, Wines, and Fruit Juices (PDF).
A single dose of tea with or without milk increases plasma antioxidant activity in humans (This article actually argues that milk does not effect antioxidant activity. I included this link because it's important to realize that there will always be two sides of the story, especially in matters like these. Nonetheless, I presented material that seems to be the general consensus of a number of studies.).
Comparative study of antioxidant potential of tea with and without additives (You may need to have a subscription to view this, but it's worth a shot.

Here again (in the comments) is the discussion Alex and I had - there are some more specific questions that some may find helpful (and also some speculative information from before I looked into this - consider this article to be more correct than the comments).

I hope I hit on everything without making it too overwhelming. Also, if I left anything out, please call me out on it. I don't like to be too verbose but I don't want to be neglecting necessary information at the same time.

EDIT: a rather 'ahem' large misreading took place on my part - total AOX activity is decreased by 20%, not decreased to 20%. My apologies for the bad info.

tb.