[Modernist Cuisine] Sous vide custards not setting/firming?

[Anonymous Modernist 19539]

Hey all,

A few days ago I received my PolyScience Creative series immersion circulator and an Ary VP112 chamber vacuum. It's been awesome getting creative and freeing up time in the kitchen by cooking sous vide. However, I've had some significant difficulties when attempting to cook custards sous vide, particularly those that are meant to set and become firm enough to hold a shape, but light enough to be easily eaten with say, a spoon.

Some examples would be creme brulee and lemon curd to be used as a filling for a lemon tart. When I attempted creme brulee the other day (using a modified recipe from the Modernist Cuisine book for coffee creme brulee), I sealed it in a bag and cooked it sous vide at 176 F for 30 minutes. Afterwards, I poured the custard into ramekins and put them in the fridge to set. They never did; my custards tasted wonderful and were certainly cooked, but they remained liquidy and never solidified in the way one would expect creme brulee to set. I put the ramekins into a water bath and baked them in my oven until the temperatures of the custards once again reached 176 F. It eventually set perfectly.

Thinking my problem was temperature and time, I had made a lemon curd (http://www.channel4.com/4food/recipes/chefs/heston-blumenthal/lemon-tart-recipe, which by the way is absolutely fantastic tasting). I sealed the custard and cooked it sous vide at 179.6 F for 40 minutes, making sure to agitate the contents of the bag several times throughout the cooking process. Afterwards, I poured it into my blind-baked crust and let it cool to room temperature. Again, like the creme brulee, the custard was delicious and was at least this time, thicker and more viscous, but it was in no way a solid curd fit for a lemon tart.

And again, like the creme brulee, I then proceeded to put it into an oven until the custard reached temperature. It set beautifully.

What exactly is going on? Is there some sort of physics involved in a hot oven that produces results that can't be replicated sous vide? Should I reserve sous vide for custards meant to remain liquid (e.g. creme anglaise and ice cream bases) while traditionally baking more firm custards (e.g. creme brulee and fruit tarts)? Or am I simply not cooking the custard sous vide long enough?