If I double a mead recipe, do I need to double the yeast I put in to have the same fermentation time?
I'm doubling a small batch from a half gallon to a gallon, would I need to add a second packet?
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As a general rule, when you are re-sizing recipes you want to keep all ingredients in basically the same proportions. This includes the number of yeast cells.
If you do not double the yeast, you will be asking each yeast cell to be doing twice the work, which would tend to affect the performance of the yeast, and therefore the chemical composition of the result.
For what it's worth, almost all of my batches have been around 5 gallons, and I use two packets of dry yeast, usually a Lalvin variety. I've made a couple 3-gallon batches, where I've used one packet of yeast and half the nutrients I use in my 5-gallon batches. If I were to make a one-gallon batch, I might use a half-packet of Lalvin yeast, just scaling things down. As others have noted, it's often best to read the labels too. :)
Yeast double in population about once per 100 minutes. You can pitch 1/4 tsp of yeast into a sterile 100 gallon batch and it will eventually grow in population to start doing something useful, however you are running a huge risk that something else may make it in and take root in the must.
You know how sometimes it take 2 or three days before your airlock starts to become active even though you can see yeast growth occurring? That is because the batch could in theory have benefitted from a larger starting population.
You’d have to really try to over pitch a batch to the point that it would hurt your recipe. Don’t pinch pennies when it comes to to yeast. Spend the money and pitch at a minimum what the recipe has called for. Doubling or tripling the yeast only assure that the population reaches critical mass much sooner and that is usually a true and real benefit.
I just started some apple wine a few days ago using raw unfiltered unpasteurized apple cider from the local cider mill. Many of the apples are likely to be windfalls. I wanted some assurance that my chosen yeast were given the greatest advantage. I used two packets of yeast, and I was blessed with a quick start.
My point is, your batches are very small so I wouldn’t worry about it, but it won’t hurt to double the yeast along with the recipe and it will usually help. Meads usually get a slow start and an large starting population will get it going faster.