K-meta powder potency?

[Nathan Shackelford]

I have obtained a pound of Potassium Metabisulfite from an online homebrew vendor.
On the package is states that 1/4 teaspoon is the amount required to kill wild yeast in 6 gallons of must. So, I'm assuming that this would be 50-150ppm in 6 gallons? Not very helpful, and potentially much more concentrated than campden tablets. Since there is no mention of ppm, it's accuracy is very limited.

I looked online and found another source that stated that 1/8th of a teaspoon of potassium metabisulfite would be equivalent to 1 campden tablet, which delivers 50ppm/gallon. There is a large discrepancy between the two statements.

So, my question is: what is the standard concentration for K-meta powder sold by homebrew (LD Carlson) shops? How does it compare to campden tablets? I'd prefer metric weights to teaspoons if anyone can help. Up until this point I've been using campden tablets, but got tired of crushing them and calculating number of tabletsx50/gal. =ppm. It seems I've made things more complicated by buying a product with no real info.
Thanks for your expertise!

[Matthew Moser Miller]

I don't have an answer for what the concentration for  K-meta is (and you didn't mention which homebrew store you bought it from), but I've had very good luck calling and asking Adventures in Homebrewing, and I know they sell LD Carlson materials. This number should work: 313-277-2739. You might have to follow a few commands to get a person on the phone, but the people at the Ann Arbor branch are very helpful.

Good luck,

Matt Moser Miller

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[Claude Jolicoeur]

1 teaspoon of Potassium Metabisulfite in 100 L gives approximately 25 ppm SO2
I say approximately because 1 tsp will not always contain the same weight depending on compaction of the powder.
The suggested dosage of 1/4 tsp per 6 gal is then quite mild at 26 ppm SO2.
For more precision, you need to use weight measure with 1 gram of Potassium Metabisulfite giving 0.5 gram of SO2.
For example, 10 grams of Potassium Metabisulfite in 100 L gives 50 ppm SO2.
You'd have complete coverage of the subject in any good book, such as Andrew's or mine...
Claude

[William Grote]

Hi Nathan

I would get a 100g x .01g scale from Amazon , they are about $10, and just weigh the right amount - its generally 1-2 grams per 5 gallon batch, depending on 50-100 target PPM and PH- again, I keep Claude's  book in my cellar as reference ( Claude, I wish you had a spill resistant edition) since it has cider specific information, the formulas that are published for wine and beer can be different.  The slight adjustment for measuring malic acid vs tartaric TA is a good example.

I do often us this on-line wine calculator for K-meta additions which you might find useful - This one in particular lets you choose either K-meta powder - tablets - or a stock solution- enter the target PMM and volume of the must, and tells you how much to add.  It also has a calculator for K-sorbate.  In any case, make sure you know that your K-meta is the same strength ( 57.6 % sulfur dioxide ) as the standard used in the linked formula.

Cheers

[Scrumpy-]

As Claude mentions below, preparation of a stock sulfite solution (10g Na or K metabisulfite/liter) is probably the easiest way to deal with sulfite. I think it is a little easier to measure out volumes of liquid in a graduated cylinder rather than weigh small masses of powder when I dose my must and much easier than grinding up campden tablets. The sulfite solution mixes with the must a little faster as well.

I found it useful to consolidate all of my record keeping and calculations in an Excel workbook (see attached).  The sheet titled Apples does some useful calculations including a sulfite calculation based on pH and volume of must. The sulfite dose is based on the curve I digitized from Andrew Lea's website and confirmed against Claude's fig 14.2. Note that the sheet calls for the volume of 5% stock sulfite solution. Enter values in the yellow cells.  All other cells and graphs are calculated from your entries.  

Sorry for the mix of SI and Imperial dimensional units but you could fix this if you have some aptitude with Excel. If you use the fermentation tracker, note that the FSU calculation has to be tweaked every time you add a new SG.

[Nathan Shackelford]

Thanks for the help, everyone! I'm equipped to dose K-meta with confidence. I appreciate the links to those web resources/calculators. I've used similar calculators for brewing processes, but never thought of finding comparable calculators for cider/wine needs. Cheers!