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I'm completely lost as to the meaning of "grain" in 45 grain brown rice vinegar. What does the grain number mean?

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Grain is a measurement of the acidity of the vinegar. It is the acidity percentage multiplied by 10, so 45 grain vinegar would be 4.5% acidity.

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  • So what does the word "grain" have to do with "acidity"?
    – bobobobo
    Aug 19, 2013 at 20:12
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    It's just a unit of measurement. Every 10 "grain" is 1% acidity, similar to "proof" measurements of alcohol, where each "proof" is 2% alcohol.
    – SourDoh
    Aug 19, 2013 at 20:13
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    @sourd'oh I'm still curious about the connection - was there a process for making vinegar that had something to do with grain that gave rise to this name?
    – Cascabel ♦
    Aug 19, 2013 at 20:57
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    @sourd'oh acetic acid is produced by fermentation of ethanol which is produced by fermentation of starch/sugars which is your starting point of the process. The maximum acidity is predetermined by the starting amount of sugar/starch in the grain. That part is known ;) I'll turn this into a answer for longevity.
    – MandoMando
    Aug 19, 2013 at 21:32
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    @MandoMando I would think the maximum acidity would be determined more by the tolerances of the organisms producing the ethanol and then the acetic acid. No yeast is going to be able to live in a wort where all of the sugar has been converted to ethanol, and I'd assume that the acid producing bacteria would have similar limits.
    – SourDoh
    Aug 19, 2013 at 21:35
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Just an addition with some boring math ...

Grain is also a weight unit (64.8mg) based on the average weight of a barley grain. If you add 10 grains (.648g) of barley to a fluid ounce (28.4g) of water and assume that the barley contains 2/3 of fermentable starch (at least close enough for a rough estimate), you will end up with 0.286g of acetic acid dissolved in 28.4g of water if the ethanol and vinegar fermentation completes. This is very close to a 1% solution.

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-- converted from the comments regarding the origin of grain: --

It's possible the term 'grain' comes from Malt Vinegar which was traditionally made in England from barley (the grain). The 'grain' number likely referred to amount of grain in the vat of work (water plus grain) which resulted in higher acidity.

Acetic acid (main acid in vinegar) is produced by fermentation of ethanol which is produced by fermentation of starch/sugars which is the starting point of the process.

Therefore the maximum acidity of a given batch is predetermined by the starting amount of sugar/starch in the grain. And if the grain is uniform, it can be used as a unit.

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  • Yeast, which does the conversion of sugar to ethanol, dies at around 15% alcohol concentration, regardless of how much grain you start with. Starting out with more grain is just going to leave more unfermented sugar in your final product without raising the alcohol concentration beyond that limit.
    – SourDoh
    Aug 19, 2013 at 21:48
  • @sourd'oh yes. and up until that point, you can get a somewhat linear mapping of grain->acidity which likely is what they used back then. Particularly with Barley, it is difficult to get more than 5% alcohol (hence the beer), to hit 15% you really do something special. The last batch of beer we made at a local brewery had the vat nearly full of malt and still only made 4.5% alcohol. To be clear, by maximum acidity, I mean achievable within the bioprocess limits.
    – MandoMando
    Aug 19, 2013 at 22:01

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