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Can I use SM powder to stop a ferment?
All the recipes say crush a Campden tablet into 1 Gall of brew but Campden tablets are 10 times more expensive to buy than powder and are made of........Sodium Metabisulphate. Iwas thinking of about 1/2 teaspoon of SM powder per Gallon. Any thoughts?

The quantity of sodium metabisulphite added to a gallon should be minute. You rarely need to add more than 0.25g/gallon. That is 1/2 a campden tablet per gallon, unlike advice given in many books.

Several factors dictate how much sodium metabisulphite you should add, these include:

• Wine pH
• Red or White
• Sugar content
• Existing sulphate levels

I strongly recommend the sulphite calculator on Winemakermag, found as a link at the bottom of this page:
http://www.winemakermag.com/sulfitecalculator/index.html

For that particular calculator, you can get away without knowing the pH and if you've only just started your wine, your sulphate level is at 0.00ppm.

You need a digital scale sensitive to 0.01g and these can be obtained from Ebay at around £15 inc. p&p.

Ideally, you want a free sulphate concentration of 25ppm for red wines and 35ppm for white wines. The difference is due to tannins in red wines that add to wine preservation.

Sodium metabisulphite will not stop a wine from fermenting because it only kills bacteria in safe concentrations. You need potassium sorbate to stop fermentation because it acts upon yeast cells.

I would have a look at this page (go to articles; sulphur dioxide):

http://www.brsquared.org/wine/

Thanks for that.

Gary.