As well as crying over the lack of milk while Bella is dry, I was also worried about not having whey. Whey is an essential ingredient in so many of my new-found fermented foods (from sauerkraut to ginger beer, and soaking brown rice before cooking) that I didn't know what I would do without it. As an experiment I started freezing small batches of whey each time I made cream cheese, and it seems to work perfectly. Each batch of cream cheese makes about half a litre of whey (from one litre of milk), and for each fermentation I only need about half a cup of whey, so I just pour a little whey into small snap-lock bags and freeze them. When I need the whey I just get out the bag and either defrost, or just run the bag under water to so I can get the whey-block out and put it straight into rice or whatever I'm fermenting.
frozen whey ready to use for fermenting things
I also often substitute kefir for soaked flour recipes where whey or yoghurt is in the recipe. I find the kefired milk works just the same and as there's always some ready in the fridge its the easiest one to use. You can use yoghurt, but if its been made from milk powder or pasturised milk with pure yoghurt culture, it may not create a good ferment at room temperature, as yoghurt bacteria prefers slightly higher temperatures. Cream cheese or kefir that has been fermented at room temperature is more likely to contain bacteria that do well at room temperature, so I think they are better for fermenting beverages and vegetables.
Do you save whey? What else can you freeze for later?