Nice to see another great question coming in from brewers in Norway! The process outlined above may sound a bit extreme to the modern brewer, but the temperature and time progression described essentially follow traditional cold-lager fermentation and a subsequent cold maturation. In Wolfgang Kunze’s textbook Technology, Brewing & Malting, this basic method is covered in Section 188.8.131.52. So, the answer to your basic question is “yes,” this technique is a thing. The only bit of information in your question that looks unusual is the cold crash, or rapid cooling, before fermentation is complete.
Let’s roll back the discussion a bit and review some of the basics. I will use Kunze’s description here as a benchmark. Kunze references pitching yeast into wort chilled to 6–7 °C (43–45 °F), followed by a natural rise to a peak fermentation temperature in the 8–9 °C (46–48 °F) range, tank spunding, a slow cooling period to 3–4 °C (37–39 °F), and finally lagering at -1 °C (30 °F). One thing to note with this description is that the timeline is gradual. Kunze does not suggest lagering longer than about five weeks because yeast autolysis and off-flavor generation is a real risk. If very long aging is desired, a racking step is a good way to protect beer from autolyzing yeast. An alternate to this when using cylindroconical vessels is the periodic removal of yeast from the bottom of the cone. Aside from the details about yeast removal, this process is generally what you have described. I will leave the topics of crash cooling, priming, and kräusening for the moment.
The elephant in the room with this general overview is the relatively fine temperature control and the slow changes made to temperature. When these methods are read with modern eyes, a brewer may wonder how these methods came to be and why they were used. The answer is likely found in the environment surrounding the production of these traditional lagers. The temperature changes were probably not designed into a process, rather the specifics were teased out of established practices.
Wort cooling to 6–7 °C (43–45 °F) likely became the norm because that’s the temperature resulting from cooling methods used in lager brewing centers before closed wort coolers and the ability to control wort temperature to any temperature desired by the brewer. Pitch yeast at this temperature, conduct fermentation in relatively small, open, wooden fermenters, housed in 7–8 °C (45–46 °F) cellars, and the fermentation temperature freely rises to about 8–9 °C (46–48 °F). As fermentation rate slows, it makes sense to move the beer into a closed, lagering vessel where fermentation completes, carbonation increases, and beer aging occurs. This is where a bit of conjecture is required, but if we assume that lagering cellars were located “downhill” from fermentation cellars to facilitate gravity beer transfers, and the cellars were cooled using ice stored in ice rooms, the temperature of lagering cellars would have been colder than the fermentation cellars because cold air is denser than warm air and tends to move downhill. This is true of the labyrinth beneath many old breweries, such as Pilsner Urquell in Pilsen.
Assuming that lagering cellars from the past were in the 2–4 °C (36–39 °F) range, it follows that beer racked from open fermenters at ~8 °C (46 °F) slowly cooled to 2–4 °C (36–39 °F). Bear in mind that cooling rate slows as beer temperature approaches the coolant temperature, in this example air. None of these traditional vessels were equipped with cooling coils and temperature changes occurred slowly as the beer warmed with yeast metabolism and slowly cooled as fermentation subsided. A feature of these slow cooling rates was the absorption and reduction of diacetyl from the aging beer because of viable and metabolically active yeast cells. Today, brewers are careful not to cool beer too cold before diacetyl reduction is complete; “crash cooling” before fermentation and diacetyl reduction is complete is the one step in your question I would reconsider.
The modern brewer may raise a suspicious brow at these low lagering temperatures and question if yeast is actually active at such low temperatures. The world may move at a faster pace today than it did in the past, and waiting weeks for a beer to finish fermenting may seem like an eternity, yet lager yeasts are indeed capable of very slowly fermenting at cold temperatures. The practical brewer can empirically determine the lower limit for their yeast strains and cellar designs as there is no single number for the practical minimum.
Some of the challenges of these slow processes include the risks of incomplete attenuation, incomplete diacetyl reduction, contamination by opportunistic bacteria that may find the residual carbohydrates in green beer appetizing, and the questionable economics of very long aging practices. While kräusening is a very traditional lager brewing method, it was most surely developed as a practical and effective way of speeding up the aging process. Kräusen beer brings fermentable sugars and a fresh charge of viable and vital yeast to the party and helps to speed things up so that the aging beer can perform its real function to the brewer, and that is being converted into revenue by the business end of the brewing operations! Gyle or priming sugar additions can also be used at home if beer is racked to a closed secondary with insufficient fermentable sugars to carbonate the beer.
Here are a few practical considerations for both commercial and homebrewers:
- Temperature stratification (layering) easily develops in beer and water tanks because of the relationship between liquid temperature and density. This is especially a problem when tanks are cooled with cooling jackets or coils. Very slow cooling (and heating) rates are a challenge in unstirred tanks.
- Small vessels cool very quickly, whether air-cooled or equipped with cooling jackets.
- Temperature stratification also develops in closed coolers with little to no air movement.
- Perhaps the easiest way to slowly change temperatures to mimic cold fermentation and maturation is to create air-cooled cellars at home. Chest freezers are relatively inexpensive and can be reliably controlled using external thermostatic controllers to cycle power. Probe placement is important because freezers do not have fans and gradients can easily develop if probes are placed too high or too low in the freezer. Slow cooling rates are easy to achieve by changing the cellar temperature by a degree every couple of days.
- Be careful not to reduce the temperature of your lagering cellar too much lest your yeast will become sleepy and not finish what was begun before cooling things down. My inclination would be to use 4 °C (39 °F) for the final bit of fermentation, natural carbonation, and diacetyl reduction before reducing the temperature to -1 to -2 °C (30 to 28 °F) for extended lagering.
- There are many modern methods aimed at speeding things up while maintaining beer quality. If you want to use the cold fermentation and aging methods that have piqued your interest, ignore much of what you may find written about modern lager fermentations and enjoy the slow life.