I am a huge fan of drinking marmite, partly because of the vitamins it offers, as I am a vegan. However, the price of it is quite high and up until recently I was able to buy a supermarket's own brand version, but they have disappeared from the shelf. I use a lot of vegetables in my everyday cooking, steaming and stewing, so I am always generating potential vegetable stock. I would like to find a way to add the Vitamin B12 to my vegetable stock before simmering it down into a paste which I know can be done by adding yeast, but I'm not sure of what would be the best process for doing this. Would I activate the yeast first with some sugar, or activate it in my stock? At what point would I heat it up to kill the fermentation? Any suggestions would be really appreciated. Also, I know there are blogs about this, but most of them suggest a very slow process for creating marmite that can take up to tn days. I simply want to create a yeast extract, add it to my vegetable stock and then simmer it down into a paste. Kind of a quick and dirty method for the nutritional properties rather than taste. 
It isn't about pasteurization, it is about texture. Egg yolks whip very differently at different temperatures. When you whip egg yolks with sugar at 60 C, you will get a very airy foam, which will also be more stable than a foam which has been whipped at room temperature. So, for optimal results, you should whip in a bain marie with a handheld mixer. It is not completely impossible to do it by hand, but you will need to whip for a very long time, until you have the perfect foam. If you want to skip the bain marie, then you can do so. You will simply have a slightly denser zabaione. I would be wary of trying to use yolks which have been pasteurized in a sous vide machine. The texture of a yolk which has been taken to 60 C without agitation is different than the texture of a yolk which has never been warmed up. If you want to pasteurize your yolks for food safety, and then go into the stand mixer, do so, but you will have to contend with a zabaione that is a bit worse in quality than if done with never-heated yolks. 