« on: October 15, 2014, 11:38:50 PM »
Was it a single-step or a two-step starter? What was your starter gravity?I'm not disagreeing, as my own experience with pitching slurries from 1.060ish beers has been quite successful, but I am curious what the biology is behind this. Is there a mechanism that the yeast has to acclimate to higher osmotic pressures?
A lot of brewers make the mistake of brewing a big beer with yeast grown in 1.040 gravity wort. Pitching a culture grown in 1.040 wort into 1.087 wort seriously stresses the yeast cells. A better approach is to pitch the vial into 1L of 1.030 gravity wort, wait twelve to eighteen hours, chill and decant the supernatant (a.k.a the clear liquid that lies above the slurry), and then pitch the slurry into 3 liters of 1.060 wort. Using this process, we are also increasing osmotic pressure with each step.
There are no hard and fast rules, but I would say that in most cases the mistake is making a high gravity starter.
The starter was well within the ratio of growth (4X at most). The gravity was low, the way every yeast lab propagates yeast. The idea is that you can pitch the correct number of cells, grown in ideal conditions (LOW GRAVITY WORT).
Whether or not this is always true is another deal. 540 is one of the few where I've had any problem at all with starter grown yeast. However, I would never encourage someone to make high gravity starters. But obviously certain yeasts benefit more than others from growing in brewing conditions.
I think the key here is the recommendation to pitch at 12-18 hours, and not after the starter has fermented to completion. I think the alcohol content in the starter is probably the biggest detriment to yeast health. By pitching earlier in the process you will avoid some of that alcohol.
Kai ran an experiment a few years ago comparing yeast growth (per gram of extract) vs starter gravity. He saw virtually no difference between a 5,7 or 10 Plato starter wort on yeast growth, and they were all at nearly 100% viability. A 20 Plato wort did see a 33% lower growth rate and was about 90% viable. He attributed the lower viability to the alcohol content. It's unclear where the break point is between 10 and 20 Plato where yeast growth and viability start to tail off in a starter, but even at 20 Plato the results weren't disastrous. It stands to reason that a 1.060 starter isn't going to be horrendously detrimental to yeast health.