Oh, we're back with Heineken again, by the way. Just in case you were wondering.
Finding stuff in unexpected place is why it's always a good idea to exhaust every possible source. Despite reading through lots of very detailed Heineken brewing records, I'd been able to unearth only patchy information about their lagering regime. Now I think I've tracked it down. Not in any Heineken document, but in a newspaper article.
An article that isn't even about the Amsterdam or Rotterdam brewery. Instead it's discussing the Heineken plant in modern day Indonesia.
The “classic” system introduced in India.
These days, the management of N.V. Heineken's Ned.-Indische Bierbrouwerij Mij., more popularly known as the Java beer brewery in Soerabaja, received a number of invitees and the press to view its new yeast and lager cellar department.
Mr. De Man set out in a short speech the purpose of the meeting. The Java breweries follow a system known as Nathan system. It consists of the beer fermenting and fermenting in the same tank. That setup is economical and good. It can be used to market cheap beers that are of excellent quality. A great success has been achieved and these breweries gradually reached their maximum capacity.
However, many consumers have a preference for lager, according to the classic system. And in connection with this, the brewery will now also lager beer.
The group was then shown around and they first visited the fermentation plant. From here the split in processing of the beers continues. One saw the Nathan tanks where the Java beer ferments and matures, after which the lager system was viewed. To this end, they came to a location where the fermentation vessels are set up at a temperature of approximately 5 degrees Celsius. Glass discs allowed to follow the fermentation process. When the beer has been exposed to this operation for about nine or ten days, it goes down into the lagering tanks. It stays here for three months to get its fine and aromatic taste. It lies in large tanks at a temperature of 0.5 degrees C. Each of these tanks contains 250 H.L.
It should also be noted that a specialty in the field of lagered beers, Dr. Mendlik, who previously worked as leader of the brewing laboratory of Heineken's Brewery in Rotterdam, came especially to Soerabaja to supervise and direct the preparation of the new lagered beers."
De locomotief 17-09-1937, page 4.
I had been wondering what the difference was between Java beer and Heineken. Now I know. The former wasn't lagered.
As they'd brought out Dr. Mendlik from Rotterdam to set up their lager cellar, I think it's safe to assume that the procedure on Java was the same as back home in Holland.
Nine to ten days primary fermentation matches what went on in Rotterdam. So I'm guessing that was also followed by 3 months lagering at 0.5º C. Which is pretty much what I had expected. Nice to have confirmation, mind.