Beverage Question

Hello Fans of Brideshead Revisited,
I'm hoping not to sound to triffeling.
During the first episode, at the end of the dinner with Jawkins, Mr. Ryder (senior-John Guilgouds character) has a beverage after all have gone (as he and Charles are discussing the evening). It doesn't look like wine or appertiffe (in fact he mentions earlier in the episode that wine is "forbidden him").
The beverage looks like a watery-milkish substance. I think "Bridey" (the older brother of Sebastian) has something like it in the second episode, as well, although I might be wrong about that.
I was wondering if anyone might hazaard a guess as to what it was? Or if someone who has read the book would know what it is?
I thought it might be bicarbonate of soda (after dinner, stomach settling) but I confess, I'm curious to distraction.
Thanks for any ideas.
Gypsyohara in FL