Rest assured, I'm a trained professional*.
In comments on my recent post on lemon pudding, I claimed that any citrus would work in the pudding. Well, that wasn't entirely correct. What I should have said that any citrus will work provided you make some modifications to the recipe.
This evening I tried making the recipe with blood oranges in the hope that the resulting pudding would be a vivid shade of red, or at least pink. I didn't get red, or anything approaching vivid--the curd (if you could call it that--more on that in a minute) was, if anything, the palest of pale pinks. It smelled and tasted deliciously orangerific, but you probably wouldn't notice the color if you didn't know it had been made with blood oranges.
But commentary on colors isn't why I sat down to write this post--chemistry is. I was wondering as I was assembling the batter whether the differences in acidity and sugar content between orange juice and lemon juice (even a rather sweet variety like Meyer lemons) would make a difference in how the pudding set up. As it turns out, it does.
Let's say you do what I did and make the recipe as written, with no modifications. Your pudding will still separate; you'll still get the light, cakey layer on top and the curd-like stuff underneath, but there will be one key difference: your curd won't be curd. It will have curdled slightly, but it will be far too liquidy to merit the name "curd." (Fortunately what it lacks in beauty and consistency it more than makes up in taste.)
I had a hunch that the acidity of the orange juice (or lack thereof) was the culprit, but I wanted verification from one Harold McGee. Out came my trusty copy of On Food and Cooking. Sure enough, in the section entitled "Egg Cookery," I found the following explanation (emphasis added):
Diluting eggs with other liquids raises the temperature at which coagulation begins, not for any subtle reason of chemistry, but simply because the protein molecuules are isolated from each other by many more water molecules. Sugar has the same effect, and for the same reason. One tablespoon of sugar surrounds each protein molecule in a large egg with a screen of several thousand sucrose molecules. Salt and acids, however, have the reverse effect and actually promote coagulation. Acid--cream of tartar, lemon juice, or the juice of any fruit or vegetable--lowers the pH of the egg, and thereby lowers the mutually repelling negative charges on individual protein molecules. The lower this charge, the smaller the repulsion, and the less force required to overcome it; the protein molecules can then bond together more easily. Salt, by dissociating into positive sodium and negative chloride ions, disturbs the electrical environment in its own way, again encouraging coagulation. So, diluting or sweetening eggs delays coagulation; salting or acidifying them accelerates it.Thus, by substituting orange juice for lemon juice, I was likely sabotaging my pudding in two ways: first, with the higher sugar content of the juice; second, with the lowered acidity of the juice. (Orange juice is still acidic, of course, but evidently not acidic enough to promote coagulation to the extent necessary in this recipe.)
I see three possible ways to address this problem:
- Reduce the amount of sugar in the batter. I think that in future orange puddings I'll do this anyway, as the pudding tonight was a bit too sweet for my taste. I'd recommend starting with 1/2 cup sugar rather than 1 cup.
- Increase the amount of acid in the batter. This could be done with cream of tartar, but I might recommend simply substituting the juice of one lemon for the equivalent amount of orange juice. The zest would still be entirely orange, so I doubt that you'd compromise the orange flavor by using a combination of juices.
- Increase the oven temperature. This theory is more tentative than the other two, namely because I'm not sure how the increased temperature would affect the other processes at work in the pudding. But baking the pudding at a higher temperature would give the protein molecules more energy, which would allow them to overcome the potential barrier that's lowered less by orange juice than it is by lemon juice. How much of an increase in temperature would you need in order to make up the change in potential? I'm not sure. So, proceed with caution--and perhaps try ideas (1) and (2) first.
In the interest of science, satisfying my own curiosity, and having an excuse to keep making pudding, I'll tweak this recipe until I get it right. And when I do, I'll share the recipe. Until then, be advised that a simple substitution of oranges for lemons won't give you the luxurious, velvety curd that I led you to expect in my other post.
* But I do encourage you to try this at home. And that image is from the fine folks at xkcd.