What makes Kewpie (Japanese) mayonnaise so different from Hellmann’s

I think the problem is actually not the heating, but the refrigeration!

Consider: bacon fat solidifies easily and thickly, and does so even at room temperature. When you throw it in the fridge, the micro-droplets of bacon fat will turn to solids and clump together. These solid droplets are frozen in place when solid, but when you thaw it, they melt and reveal how wrecked your emulsion is. Now, you could add additional emulsifying agents (lecithin, extra yolks) but that's not really going to solve the problem of refrigeration.

The solution is to break up the bacon fat as it melts, and re-establish the emulsion before it can break. To do this, you have to whisk constantly as you gently heat the mayonnaise, generally in a warm water bath.

How to execute this in a restaurant setting:

Prep a big batch of bacon-mayo and throw it in the fridge. Just before service, warm some water in a pot and throw a cup of the mayo in a small bain marie or metal 6th hotel pan. Immerse the bain/pan in the pot, and whisk it as it melts. Hold the mayonnaise for service in water warm enough to melt bacon fat, and DISCARD THE WARM MAYONNAISE EVERY TWO HOURS AND THAW A FRESH BATCH. Make sure no cooks get lazy about that -- it's a food safety problem. Ideally, you should be using pasteurized eggs to reduce the risk of salmonella. Basically, you're treating your mayonnaise like a Hollandaise or Bearnaise.

I've found that the thawing trick works fairly well for home hollandaise (which most people say shouldn't refrigerate), and which does the same thing if heated fast. It should apply to easily-broken mayonnaise too.

It's important to determine whether your emulsion actually broke immediately or was just creaming.

I'm going to trot out this diagram again from yesterday:

(source: Cube Cola)

Creaming occurs when the oil drops, which are less dense than the water, float to the top. As long as the droplets don't coalesce, you can still fix this with agitation (whisk, blend, or shake vigorously).

Emulsions with coarse particles (of oil) are much more prone to creaming, because their increased buoyancy makes them more able to push up past the water molecules. You want to have a very fine "mist" of oil suspended in the water; if you were using a squeeze bottle, perhaps the individual drops were simply too large. A ketchup bottle in particular would not be appropriate for this sort of thing, you'd want to use something closer to a syringe (or just get an actual syringe).

Now if you keep adding oil to a creamed emulsion or let it sit too long then it will also start to coalesce, and coalescence and creaming together are what cause an emulsion to break completely. This, you really can't recover from, except to let it separate completely, skim off the fat, and start over.

So, to recap:

• Your oil drops may have been too large;
• You may not have been agitating enough, especially when it started to cream;
• You may have added too much oil after creaming had already started.

I also agree with commenter Henrik that the amount of oil sounds a little high; 3/4 cup would be more reasonable for 1 egg yolk. But since you say you only got 1/3 of the way through, that's clearly not your problem here.