Obviously, it can be done, as dry yeast works for fermenting beer. I go sometimes a month or two between batches, and carrying over one yeast culture across beers seems a bit of a pain. How is dried yeast made/ packaged? Is it something a homebrewer can reasonably do?
I don't think the process of commercially drying yeast is straightforward - it involves a partial vacuum or a stream of filtered air to make the liquid suspending the yeast evaporate faster - and it's probably difficult to reach the levels of hygiene required in a homebrew setting.
If you want to preserve yeast for a long period, 1 year or more, you can store the yeast with glycerine and keep it in the freezer. There's a good writeup on HBT with the process and equipment needed. It's fairly simple and achieves good results. I've successfully brewed with yeast that I froze 3 years ago.
Even if you don't freeze the yeast, but keep the yeast in the fridge, the yeast remain in better condition in glycerine than when kept in wort.
If you are already making a starter then freezing is a simple process that allows you to keep a variety of strains in your yeast bank over a long period.
How did Monks and farmers do it? Farmers in Norway apparently do it with even less fanfare. http://www.garshol.priv.no/blog/342.html
Terje took some wort in a plastic bucket. Then he brought out a plastic box of pale gray flakes. This was the kveik, which Terje got from a friend in Hornindal twenty years ago, and has been using since. Terje collects the yeast after fermentation each time, and hands it to his father-in-law, who then cleans and dries it, before storing it in the freezer.
Taken from the comments on the article:
As far as I know, nobody's ever written down or in any way documented the cleaning process. From what I gathered, the cleaning is the usual process of putting the yeast in water, then draining off the non-yeast part. This is repeated a few times. The yeast is then smeared on baking paper in a room in the cellar. Once it's dried into a thin (3-4mm) crust you can just crush it by folding the paper and drop it into a small plastic box and stuff it in the freezer.
This all looks a bit like gobbledygook if you ask me. The intention is not to dry yeast commercially, simply for the home brewer. Norweigien Kveik has been dried at home for centuries, how do you think they dealt with it in the past before UV lights stainless steel chambers, pressurised canisters etc. I would suggest you look at David Heath's Youtube channel for details on how to dry your own yeast.
I know you can make a dry sourdough starter by just spreading it thinly on a sheet pan and leaving it in the sun to dry. That doesn't seem nearly sterile enough for our beery exploits, however.
There was also an interesting thread recently on HBT about farming and freezing yeast using glycerine to protect the cell walls from rupturing.
Although I do not know the exact process, if sterility is the aim, it seems that all that is required is to create a positive air pressure sterile box(PAPSB). This can be done with lexan, a strong fan, and a few high grade HEPA filters. You can create a near sterile environment in this manner with the aid of a strong sanitizer and UV lights. Using this PAPSB you should be able to perform all of the necessary procedures outlined. Of course the box will need to be outfitted with gloves to allow manipulation of objects in the box. The system I am suggesting is similar to those used by CDC members when handling various bacteria and virii. A guide to creating such a box is here.
You can make stocks of your yeast in glycerol and put them in the freezer for relatively long term storage and work with Petri dishes in the fridge for shorter storage time. You can make a glovebox for your self using house hold items. But if your sterile technique is not good, you will encounter contaminations.
The glovebox is basically a sealed plastic box that you spray with bleach or ethanol and access through gloves. If you are careful enough this should suffice.
I found excellent guidance provided at the following link on how to dry yeast in your oven. Basically, since your oven is one of the most sterile locations in your house (and even if it is not, it can easily be sterilized), just spread some yeast out on a sheet of wax paper in the oven, not with the oven turned on, but only the oven light on (assuming an incandescent lamp bulb), leave for a few days until dry, then package. I have not tried this technique yet, but I do intend to try it very soon... and not only for kveik yeasts but why not also try it for other yeasts. Hopefully it works well. See for yourself:
Just saw this on HBT: https://www.homebrewtalk.com/forum/threads/guide-to-making-a-frozen-yeast-bank.35891/
It's not drying yeasts but I think it accomplishes the same thing.
Actually the oven drying method looks quite accurate. If done correctly with wax paper, parchment paper, or foil. You can pre-heat the sheets to 212-270F in the oven. Let oven with baking sheet material cool to 100F and smear on the yeast for drying in the oven. Surely, this is what our ancestors did before high tech.
See this link for how dry yeast is made (high level overview). https://www.bobsredmill.com/blog/healthy-living/how-is-dry-yeast-made/
This question is pretty old now, but I just found this thread, while searching for the same answer. In my opinion some of the answers given are a bit ambiguous, and are lacking references.
I think that some home experimentation would be well worth it, and with a little forethought and hygienic practice, should be well within the grasp of a homebrewer to run some experiments.
It would be very easy to produce a good healthy strain of yeast in a reasonably controlled environment in a gallon jug producing a large amount of yeast available for harvest. Even a wild yeast. I primarily only ferment Cider now. I have Harvested and propagated Wild Apple Yeast to produce a working Gallon Jug that will washed out to provide me with enough tubes of yeast to make a years worth of starters. I consistently get good results and flavor from this method.
It would not be a huge effort to wash out several batches to be dried. I would experiment by making some foil pouches, placing them onto a foil covered cookie sheet and heating them an oven to 300F. Shut down the oven and monitor for the temp to drop to 100F. At 100F remove a tube of Yeast Culture from the refrigerator, open the oven door, and pour the tube into a pouch, maintain 100F until it dries to a cake. Close the pouches (before removal from the oven) and store. Once every few months pull a pouch, use it as if it were a package of dry, and add it to a cup of warm water with a 1/2 teaspoon of sugar and maybe a pinch of Nutrient and see if it blooms. Not scientifically sterile, but as good as any home practice is. Even if you pitch only a prepackaged yeast after heating your ingredients, you invite some level of unknown contamination while pitching.
I'll let you know in a year how I make out... :-)