Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
As this basically got answered through questions in the comments, I'll collect up all of the various problems that might help prevent the sogginess:
- After frying, allow the croquettes to cool on a wire rack, so they don't get moist as they give off steam.
- Reheat first at a lower temperature to heat through, then turn it up the temperature to regain the crunch & get any extra browning.
- Make sure when reheating that they aren't packed too tightly, and if possible, heat on a wire rack rather than a pan.
- If reheating in a small but tightly-sealing oven, try cracking the door slightly so as to prevent humidity buildup.
(anyone who has any other suggestions, feel free to add 'em)
Best Answer
Try wrapping it in foil and cooking on a very low temperature in the oven or toaster oven. (Our oven has a "warm" setting that's ~170 F.) You can also include some water or broth in your foil packet but it won't penetrate much beyond the surface if the meat has been cooked before. It'll make that 1st bite taste more moist but in the long run it won't do much.