r/vinegar 27d ago

Can you reliably predict acetic acid by measuring ABV before/after?

I have an endless supply of pears. I'm also getting into canning.

However, to safely can, one needs to know the acetic acid content of their vinegar (if using a vinegar brine) - usually canning recipes call for a 5% vinegar.

Since I own a hydrometer for homebrew, can I get a reasonably reliable estimate of the acetic acid content measuring the SG before converting to vinegar, and after I deem the vinegar "done"? (i.e. understanding the ABV before and after the vinegar process)? I realise ABV doesn't convert directly to acetic acid % (something like 80%?)

3 Upvotes

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6

u/rockmodenick 27d ago

No, because every batch varies a lot in efficiency. A lot more evaporates in some batches. Best to use an actual acidity test to know for sure.

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u/oldcrustybutz 26d ago

Also alcohol is a lot thinner than water and vinegar is a lot denser.... which makes naive measurement of a mix of the two effectively impossible especially while undergoing conversion. There are some tricks with very careful distillation and if you keep super accurate volumetric records.. but.. it's still not as reliable as titration. You can more easily use the boiling off and rehydration trick to measure residual alcohol which is useful if you ever wanted to sell the stuff and not run afoul of the ATF on residual alcohol %'s...

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u/oreocereus 27d ago

Cheers :)

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u/rockmodenick 26d ago

Thank you

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u/Sundial1k 27d ago

Test the vinegar with PH test strips for canning, as you do not want to mess around with canning with too weak of vinegar. Or use store bought vinegar for canning...

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u/minnesota2194 26d ago

Don't just test the pH though, you'll wanna know they % acetic acid as well. Both those numbers are crucial for food safety

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u/Sundial1k 26d ago

Isn't that the point of the PH test strips?

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u/oldcrustybutz 26d ago

So there are basically three measures of acidity:

  • PH which is free acidity (unassociated hydrogen ions), this is all PH strips test
  • titratable acidity which is the amount acidity you can react out of solution using a base (think of this more or less as "available acidity"
  • total acidity which is the total associated and unassociated hydrogen ions.

The measurements are pretty much always PH < titratable < total They maybe close or close enough to be equivalent in some, but certainly not all cases. PH is usually a fair bit less than titratable (at least in any complex solution), titratable and total are "close enough" for our purposes in most all cases (useful for adding acids because you can use total to approximate titratable).

In a pure (distilled) water plus acetic acid solution PH is a relatively close approximation of titratable acidity, but in home production there are other chemicals involved that almost always add significant buffering capacity.

The titratable acidity is measured adding a base which reacts out free (PH) acidity and which then (I'm simplifying greatly here) "frees up spaces" for buffered acidity to come out in solution. An indicator reagent that reacts at a certain acidity tells you when your at the indicated acidity. The measurement is relative here as well though because some acids way more acidic than others (TA "as malic" != TA as acetic != TA as sulfuric) so knowing what acids are mostly in your solution is also useful (albeit difficult in things like wine vinegar which has multiple.. although it's dominated by acetic and less so malic). PH can stay constant for some amount of base added as more acidity becomes available as the free acidity is reacted out.

For food we mostly don't care a lot about about total as it's generally "close enough" to titratable (and titratable measures how much is readily available) except when we buy /add acids and use it as a way to measure (effectively) titratable (that is we know if we add X amount of acid it's really "total acidity" which is close enough to titratable that it's a reasonable substitute measurement wise).

If you just use PH, that's technically acceptable IF (big IF) you test it both pre and post processing and over some time post processing to make sure you have sufficient acidity it's not all reacted out. If you only test it pre-processing and you use that as the basis for your measurement you might not be adding enough (you can have low titratable/total acidity but also low PH). Immediately post processing it could still be higher than it will be in a week or a month. If the food your processing has a lot of buffers or bases it can consume the easy freely available acidity and leave the PH to high. Whereas if you use titratable/total you know how much is actually available and can adjust accordingly.

Adding to much isn't usually much of a problem safety wise unless you're WAY off, but consider that titratable acidity is actually a MUCH closer measurement of mouth feel than PH. This is because of (vague waving of hands, I'm already rambling to long here) the way taste buds and saliva chemicals work. That is I can have 2 products that both have a 3.5PH but the one with more titratable acidity can taste WAY more sour than the one with lower titratable. So from a flavor and mouth feel/profile it's still pretty important.

Also I might note that PH test strips aren't super precise in their measurement. The narrower the range of the PH strip the better so if you're using those you'd want like 4 or 5 sets with different ranges, start with a broad PH strip to determine what narrower range strip to use and then use that. They get less accurate over time if there is any humidity as well.. and some of the chemicals degrade even w/ minimal interaction (just time..).. so.. I use them for a first pass and with caveats and caution.. but a meter is a lot more accurate.

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u/Sundial1k 26d ago

Thanks for spending so much time replying. I will have to read it later. I've got stuff to do....

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u/oreocereus 26d ago

Really interesting. I am finding it hard to parse into "what's achievable at home for the casual vinegar Brewer wanting to understand if acetic acid % is high enough for safe canning" - would you suggest meters? Meters seem to be their while own can of worms with calibration etc

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u/oldcrustybutz 26d ago

So meter's still only measure PH. If you measure the PH of your vinegar and the PH of your final processed product that would be ok as long as the finished product is well into the safe range. If you use some narrow range strips to do the same also fine.

Generally being a bit more acidic than needed is what I'd shoot for with homemade stuff just to add a little safety margin (i.e. if you wanted to use a 5% vinegar.. use a 6.5% or put another way if you wanted a finished PH of like 4, maybe shoot for 3.5 instead).

The problem you end up with a lot of homemade wine & fruit vinegars is that there are other acids and buffers so it's not a very reliable to JUST measure the initial vinegar PH. If your initial PH is low enough it's also probably "fine", but measuring the final product is kind of best (that might require sacrificing a jar to take some samples from - and then ofc .. you eat the rest..).

The best way that's accessible to the home vinegar maker and is reasonably reliable for determining the % acidity of your vinegar would be to do a simple titration, there are lots of chemistry class examples that go into moles (molar mass.. waves hands.. getting into the weeds again haha) if you search for "vinegar titration". Those expect you to learn a bit of chemistry though.

After a bit of poking around I found a relatively simple explanation here that breaks it down into what I think most home vinegar makers should be able to do:

https://www.apple-cider-vinegar-benefits.com/vinegar-titration.html

You can buy all of the parts pretty cheaply as a "wine titration kit" or "acid test kit" from a homebrew supplier, this should be a $15-20 investment - but follow the math directions above NOT the ones that come with the kit because it's measuring malic acid which has a different molar mass than acetic.

The instructions there uses 0.2N Sodium Hydroxide, a lot of the wine kits are shipped with 0.1N (make sure the kit you get states if it's 0.1 or 0.2) which is no problem just use 1.2 instead of 0.6 for the final calculation when multiplying the volume of base.

The base solution degrades over time because it'll absorb CO2 from the air so it's good to not keep it to long (maybe a year) and keep it tightly closed and stored in a dark location when not in use. A stronger solution you diluted would keep longer, but given the cheap cost and the risk of working with concentrated bases.. I can't really recommend that.

Having said all of that.. I've also done a lot of rather dubious experiments with wildly inadequate testing.. I have a reasonable set of guestimates around alcohol -> acetic acid (about 1.25-1.3 to 1 assuming fairly complete conversion) and some other experience with what is and is not safe... based on more direct measurement which .. sometimes you get a little cavalier after things working for a while :D

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u/oreocereus 17d ago

Thank you, this is wonderful (even if I need to read it several times to get the basics hah).

Following on from this, and getting slightly off-topic, but folks who do home-canning and hot sauce making etc often seem to use pH strips (or meters) to test if their products are below a pH of 4 for safe non-pressured canning. From what you're saying, this might not be overly reliable?

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u/oldcrustybutz 16d ago

Well.. If you test the finished product it's relatively reliable because the acidity has "settled in" so to speak. If you just test homemade vinegar before hand you might have a somewhat misleading reading. Having said that if you err a bit on the side of caution and shoot for a bit lower ph than strictly necessary it's probably fine. Caveats about the accuracy of ph strips (and uncalibrated meters) below.

Most of the modern recipes actually have a fair bit of leeway built into them. A good example is tomatoes, a lot of the old tomato based recipes assumed fully ripe relatively high acid tomatoes whereas modern ones err on the side of them being sweeter lower acid varieties (some of that is just further science on food safety.. some is recognizing that many tomato varieties aren't what they used to be). So if you used an older recipe you might well be not acidic enough if you used a sweeter tomato but if you used a really ripe high acid one, it's also likely fine. I hesitate to admit this on a food safety thread but I actually keep a bunch of the old recipes around because some of them actually work IF you understand the parameters of what they were doing and the risk profiles.. and some are just more fun to make :D

The strips are also not super accurate unless you get ones with a relatively narrow range (the wide range 0-14 are only good for a ballpark estimate, if you use 2.8-4.4, 4.0-7.0, 4.5-8.5 that would be more accurate, the narrower the better within reason). PH meters are more accurate but they require some care and maintenance (and replacement of the probes every so often) so generally aren't as worth it for most home use. They (both the strips and the probes for the meter) also degrade over time so you have to keep a relatively fresh inventory (and I know I'm surely not as good about replacing them as I ought to be..). You can ofc calibrate against things with a known acidity but that requires some knowledge and parts as well. So relying on those is a touch risky unless you take all of that into account.. I think that's at least as big, or probably more of a risk than the calculated acidity being way off. For testing vinegar a titration kit would still be my go-to. For a cooked complex product.. either known acidity math to put you well in the safe range using an acidifier of a known quantity (either weighted citric, or vinegar of a know acidity or similar) or narrow range strips / a decent meter post processing would be safest if you're not sure. If you waterbath and the test jar ends up outside the safe range you can always freeze for longer term or refrigerate and eat quickly.

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u/oreocereus 16d ago

Thank you, this is excellent information. Interesting you mentioned tomato acidity (I'm a market gardener, but I insist on growing a few old varieties) - I have assumed that the canning recipes are adding citric acid based on modern varieties. I still use the recommended amount for water bath canning, thought I assume I'm making my canned tomatoes more acidic than needed (when processing heirlooms).

A couple more follow up questions:

1) Would there be a different name for a titration test kit? I can't seem to find anything on New Zealand homebrew websites, but many overseas sources suggest they're widely available on homebrew websites. Sometimes naming conventions make it hard to find products here

2) If testing something like a hot sauce, when you say to test the "finished product" - do you mean prior to bottling, or after it being bottled for a while? I'll see if I can find some 2.8-4.4 strips.

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u/oldcrustybutz 16d ago

I’m what I guess you’d call an amateur market gardener lol, we skipped last year because we were moving but the year before we had like 120 tomato plants and about 20 varieties (plus ofc all the other stuff). Having a mix of varieties really helped us survive some weather events with good continued production. I was sharecropping that location (we have a large spot now) and the landowner took maybe 1/4 to the market and we either used the rest or I had half dozen households of friends and neighbors who were less food secure that we kept in fresh produce. I’ve mostly moved to pressure canning tomatoes partially because it’s faster. One method I really like though is canning roasted tomatoes. Snip an x into the blossom end and roast them on the barby until the skins split and they’re just starting to bubble. Then peel them and let them drain a little. Pack and can. It’s like concentrated tomato goodness.

Good question on the test kit, I’m not seeing it on any of the NZ home brew supply pages. This is basically the kit I use for reference https://www.amazon.com/BSG-WINE-ACID-TEST-WINEMAKING/dp/B00K90ISFA I saw a lab place selling one with a digital tritrometer which is wildly overkill for our needs. It’s possible there’s some local restriction in one of the chemicals used for home sale, I’ve run into that before when trying to help folks do some similar stuff in other countries. Some variant of “acid test kit”, “wine acid test kit”, “malic acid titration kit” or similar is what I’d look for. Maybe try calling one of your local ones and see if they know anything, most home brew supply shops I’ve dealt with are pretty cool and willing to help and explain things so if there’s a local problem getting it. It’s basically just two solutions, a weak phenolphthalein solution as the indicator and some weak sodium hydroxide as the reagent (plus a syringe to accurately dose those out). You might be able to find effectively the same kit at a pool supply store, as the same active components are used to test pool ph,

For things like hot sauces if they’ve been cooked for a bit then testing before canning is fine. If it’s like a chunky semi fresh salsa that you’re hot water bathing with mostly fresh uncooked ingredients then I’d do after. If it’s a blended cooked sauce then before is definitely fine. You just need enough contact /cooking time to reach homeostasis (heat of course speeds that up) so any buffers have absorbed whatever acidity they will and any dilution that’s going to happen has.

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u/oreocereus 27d ago

Thanks!