18 April 2014

Yeast Rehydration

Choosing a yeast strain is vital to the final quality of mead (and beer, wine, sake, even spirits), as is the care that is taken to nurture them. For the purposes of mead (and wine, because we are mostly using wine yeast strains), we will most often encounter dried yeast as they are more numerous in selection, cheaper, and there is no damage caused by the drying process (beer yeast, especially lager yeast, are rather difficult to cultivate in a medium that permits drying, and can often have mutations result from the process; though techniques have improved drastically over the last few decades). The funny thing about the drying process is that it is not all that harmful to yeast, but the 'waking' is turbulent and rather tiresome for our little helpers. We are basically starting from very weak little creature who are just waking up and we want them to run a marathon; granted it's in their genetics, but without proper conditioning they may pass out at 26 miles instead of the 26.3 they were supposed to run. 

They yeast companies have done a good job feeding the yeast many things that they will need to actually get out of dormancy (high oxygen, nutrient dense, and low osmotic pressure media give them the reserves they need for the trauma about to occur), but once awake, they have to deal with their environment which can be rather harsh (lack of nutrients, and relatively high osmotic pressure), and they may give up. The number of yeast cells that die upon 'sprinkling' (the common technique of just opening the pack and adding the yeast to the must/wort) ranges a considerable gamut with some sources saying as low as 20%, and some claiming almost 2/3! I tend to think the mode number of 50% is most accurate, but even if it were only 20%, that's a lot of helpers you are killing off. The cells that do survive and work are not in very good shape either: having depleted their energy reserves (glycogen and trehalose usually) they have to spend a considerable amount of time and further energy to rebuild these levels in order to get to the point where they can actually start working.

In an environment like mead, where there is always a shortage of yeast assimilable nitrogen, lack of micronutrients, low pH buffering capacity, high osmotic pressure (usually very high compared to other fermentations), why would we want to use yeast that were even the slightest bit unhealthy? Why risk a stuck, stinky, or non existent ferment? "That's never happened to me and I don't rehydrate.", me either, but at least I've got money on the high half, first third and a zero while your all on 31! (sorry, roulette/james bond reference)

Pitching Rates

A quick note on pitching rates is in order here. Pitching rate is measured as CFU/mL (colony forming units per milliliter), but can be approximated using g dried yeast / L of must; assuming 20,000,000,000 (2*10^10)CFU/g dried yeast, we can estimate the resulting pitching rate of X grams of dried yeast added to a liquid whose total volume is Y. Great, how much do I pitch? 0.005-0.5g/L is a good range, that's 10^5-10^7 CFU/mL. More specific? Almost all manufacturers recommend 0.25g/L (~1g/gal), winemaking textbooks say 0.1-1% by volume (which is ~10^5-10^6 CFU/mL), researches have found that 0.5g/L works best for high brix musts, and brewers pitch anywhere between 6*10^6 - 2*10^7 CFU/mL! The best 'rule' to follow is

<24*Bx (SG 1.101) gets either 0.25g/L or 1g/gal, whichever measurement you prefer; 
25-28*Bx (SG 1.106-1.120) gets 0.3g/L or 1.2g/gal
>28*Bx (SG 1.120+) gets 0.5g/L or 2g/gal. 

That is not to say that many people don't cross these boundaries and make unsatisfactory meads, many people do for many reasons, it's just a guideline. Also, there has been research that suggests lower pitching rates enhance the flavors of mead, but I would stick to these guidelines for now (sometime soon I will be starting a pitching rate experiment focusing on the lower end of the spectrum). 

Basic Procedure
1g yeast strain of choice
1.25g Go-Ferm Protect (or what ever their (lallemand's) newest one is, I think evolution)
25g water

          -multiply all numbers by your chosen amount of yeast (pitching rate * volume)

  • Boil the water for a few minutes (pasteurization), I would recommend boiling more than needed and weighing afterwards
  • Put into a thoroughly sanitized flask (Erlenmeyers work the best), if you can autoclave it (truly sterile), I would
  • Wait until the temp is down to ~110F
  • Add Go-Ferm 'whatever' and swirl (this is why erlenmeyers are best)
  • Wait for the temp to drop to 104F
  • Add yeast, swirl gently to break-up clumps
  • Wait 15-30min, don't exceed 30min or the yeast will start to starve

There are actually a range of temperatures that work best for rehydration, from 95-105F, and it depends on the specific yeast strain, but most seem to prefer the upper range with a mode of around 104-105F (40C).
You'll note that the amount of water is by weight not volume, why? 50mL of water at 110F weighs less than 50mL of water at 40F (about 454mg less); if you measured the volume after boiling (while hot) you won't use enough water, and if you measure before boiling you wont be able to account for evaporation. Therefore, weight is the way to go for precision. 
As to the type of water, clean, potable water with no chlorine or chloramines is recommended. The Go-Ferm will not replace minerals found in water, therefore, the water must have a certain amount present for healthy rehydration. The hardness is a vital factor for yeast health with best results being achieved between 250-500ppm hardness.
This entire process is basically proofing the yeast and allowing their cell membranes to form properly, it does not acclimate them to the must, nor add any special property to they yeast, however, it does not damage the yeast as would happen by the 'sprinkling' method. 

These are extra steps that can/should be added to the basic procedure when the circumstances dictate.

The best 'add-on' is an acclimation phase for temperature. Simply put, take your slurry and add some must to it so that the yeast aren't shocked too much by the temperature. More technical you say? For every 18F (10C) difference between your must and starter, add an amount of must equal to the original starter size to the flask.
So, I have 6gal of mead must and want to pitch 8g D47.
I rehydrated with 10g Go-Ferm in 200g of water. I now have a starter/slurry of about 200mL (give or take), that is at 84F, and my must is at 66F. 
I need to take 200mL of must and slowly add it (over a few minutes) to my slurry and wait 15-30min.
If the slurry was at 92F and my must at 56F, I would need to make 2 x 200mL additions to my starter. 
This process works to slowly acclimate the yeast to the temperature, gravity and pH of the must. Because it works for gravity and pH also, it is usually recommended to do this even if the temperature is within the 18F tolerance.

Very High Bix Musts
For very high brix musts (>35*Bx, SG 1.154), there are more delicate steps that should be taken to avoid osmotic shock that can happen even if utilizing the routine above. The method I describe here was put forth by Kantkanen et al. fot the production of ice wines. They tested several procedures across 2 pitching rates (4*10^6 and 10^7 CFU/mL): acclimated rehydration vs simple rehydration, and utilization of Go-Ferm vs none in the rehydration step. The starting gravity of the must was 37*Bx (SG 1.164), and the results showed that only those fermentation pitched at 0.5g/L (10^7 CFU/mL) were able to reach 10%abv, and that utilizing the following process with a lower pitching rate did not achieve desirable results; starters that underwent acclimation showed an increase in consumed sugar compared to non acclimated starters. 
Interestingly, while the use of Go-Ferm decreased fermentation time and acetic acid production, it also decreased the amount of ethanol produced as a function of sugar consumed (ie. sugar was consumed quicker than the non Go-Ferm batches, but it's utilization for ethanol production was relatively lower).

Here's what you do using a 25mL slurry as made above (just multiply the volumes for larger amounts)
  • Take sample of must and dilute with sanitized water to 20*Bx (SG 1.083)
  • After initial 15-30min wait (above), add 25mL of dilute must to starter (the result should be a 50mL starter at 10*Bx)
  • Place flask in water bath at 77F (25C), for 1hr swirled every 30min
  • Take a second must sample and dilute it with sanitized water to 30*Bx (SG 1.129)
  • After 1hr wait add 50mL of the second dilute must sample; resulting in a 100mL starter at 20*Bx
  • Place flask in water bath at 68F (20C) for 2hrs, swirl every 30min

Be warned, this works really well and creates super tolerant yeast that can pass their alcohol tolerance by a few percentage points (actual abv accounting for shift in composite density). Technically this process results in a solution that is equal to 
                                                       25g dried yeast / L of water,
meaning that you would need to pitch 20mL starter / L of must to equal the 10^7 CFU/mL (0.5g/L dried yeast) recommended.
If you want to ferment something higher than 40*Bx (SG 1.179), I would high suggest step-feeding (adding honey in multiple increments during fermentation). However, if trying a polish style mead, or some other absurdly high gravity style, and you do not want to step-feed, I might suggest trying this method with an additional step of adding another equal amount (100mL in our example) of must to the starter for ~3hrs with swirling at 30min intervals (or stirplate the whole time!). Note that each addition has made a 10*Bx jump; in keeping with this the gravity of several additions can be estimated:
20*Bx addition to make 10*Bx starter
30*Bx addition to make 20*Bx starter
40*Bx addition to make 30*Bx starter
50*Bx addition to make 40*Bx starter
60*Bx addition to make 50*Bx starter
Obviously I did not have to list those as they are in 10*Bx increments with a 10*Bx offset, but I did want to show that at 50*Bx and above, it is almost useless in trying to ferment without some sort of gradual addition of fermentable material during the process. 

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