Reducing vinegar

[Blether]

At least one source on the internet says that acetic acid forms n azeotrope with water - most others say it doesn't.

I read the boiling point of pure acetic acid to be 118.1C. There's a tradition of boiling vinegars down in the kitchen - for Hollandaise, for example, and boiling vinegar is famous for the fumes it gives off. Reducing on a normal stovetop concentrates the flavour, but to what degree does it concentrate the acidity ?

This is where I have a practical question. Can I reduce 4.5% Japanese vinegar (fruit, rice, apple, they all seem to be 4.5%) to a 5% (standard western distilled or wine vinegar) concentration ? If I could boil off water only, by boiling at 100C, I reckon 631ml of 4.5% vinegar would come down to 1 pint (568ml) at 5%.

I want the vinegar for chutney-making. I'm not too concerned as I've winged it in the past, but I'm interested in understanding better.

Edited February 25, 2010 by Blether (log)

[C. sapidus]

At least one source on the internet says that acetic acid forms n azeotrope with water - most others say it doesn't.

According to Lange's Handbook of Chemistry and Physics, 15th Ed., acetic acid is zeotropic with water, so you should be able to separate the two by fractional distillation.

I read the boiling point of pure acetic acid to be 118.1C. There's a tradition of boiling vinegars down in the kitchen - for Hollandaise, for example, and boiling vinegar is famous for the fumes it gives off. Reducing on a normal stovetop concentrates the flavour, but to what degree does it concentrate the acidity ?

That would have to be experimentally determined. If your kitchen includes a still with a condensing column, you would concentrate the acid more quickly and efficiently.

This is where I have a practical question. Can I reduce 4.5% Japanese vinegar (fruit, rice, apple, they all seem to be 4.5%) to a 5% (standard western distilled or wine vinegar) concentration ? If I could boil off water only, by boiling at 100C, I reckon 631ml of 4.5% vinegar would come down to 1 pint (568ml) at 5%.

The problem is that you will not boil off water only. Some of the acetic acid will evaporate, too (hence the famous fumes). Raoult's Law may be relevant, if you are more interested in chemistry than chutney.

Good luck! (edited to clarify)

Edited February 25, 2010 by C. sapidus (log)

[Shalmanese]

This is weird, acetic acid behaves as if it has a lower boiling point than water. The fumes start coming off at sub-boiling temperatures. I know when making balsamic reduction that a low heat reduction will make a sweeter product and a high heat reduction will make a more sour product which suggests that boiling is counterproductive.

I would try and get your hands on some food grade, high percentage acetic acid and just rectify your solution rather than try and reduce it down.

[Blether]

Thanks, Bruce

I guess I'm trying to get a better handle on how much extra 4.5% vinegar I need on the one hand, and on the other to better understand the properties of these liquid mixtures.

My 568ml / 631ml figures suggest that 11% more volume of the 4.5%, contains the same absolute amount of acetic acid as 5% vinegar. But as you say, if I simply boil it, I'll lose acetic acid as well as water. So if not 11%, how much more do I need ? 20% ? 50% ?

I revised alcohol distillation. If I've got it right, you have a weak mixture of alcohol in water (say 5-15% or so) and you boil it. Around 78C it boils, and alcohol and water both begin to evaporate together. At first it's mostly alcohol. The mixture continues to boil, and the proportion of alcohol - both in the liquid and in the vapour - falls as the proportion of water rises. Once the mixture reaches 100C, that's a sign that all the alcohol has evaporated and the remaining liquid is only water.

'Azeotropic' and 'zeotropic' are a bit harder to get to grips with. Directly translated from Greek roots, 'azeotropic' means 'doesn't change on boiling' - the idea being that the mixture of (two) liquids stays at the same concentration even as you boil it - and it boils at a constant temperature, like a single liquid, even if the components have different boiling points. What confused me is the sources saying something like 'ethanol is azeotropic in water', as if it's a once-and-for-ell thing, regardless of what proportions you mix.

So, for example, ethanol (alcohol) is azeotropic in solution with water at 96.4% concentration, with a boiling point of 78.1C. However much you boil it off, what is left will still be at 96.4%, and all the vapour will also be at 96.4%.

Raoult's Law... I might try to wrestle with that. It'd be better if I did have a still in my kitchen - regardless of whether I wrastle with Raoult or not.

Shalmanese, I'm starting to like your 'add some concentrated acid' idea

Edited February 26, 2010 by Blether (log)

[dougal]

At least one source on the internet says that acetic acid forms n azeotrope with water - most others say it doesn't.

I read the boiling point of pure acetic acid to be 118.1C. There's a tradition of boiling vinegars down in the kitchen - for Hollandaise, for example, and boiling vinegar is famous for the fumes it gives off. Reducing on a normal stovetop concentrates the flavour, but to what degree does it concentrate the acidity ?

This is where I have a practical question. Can I reduce 4.5% Japanese vinegar (fruit, rice, apple, they all seem to be 4.5%) to a 5% (standard western distilled or wine vinegar) concentration ? If I could boil off water only, by boiling at 100C, I reckon 631ml of 4.5% vinegar would come down to 1 pint (568ml) at 5%.

I want the vinegar for chutney-making. I'm not too concerned as I've winged it in the past, but I'm interested in understanding better.

I believe its quite hard to separate the two by distillation alone.

Even though they don't form a 'constant-boiling-point-mixture', they almost might as well.

Regarding alcohol -- it does form such a mixture (at 98%+ alcohol or thereabouts) which is why you cannot make absolute alcohol by distillation alone.

Practically, reducing the vinegar will result in less water and an only slightly smaller proportional loss of acid. It should get stronger, but not much, and you'll lose a lot of what you paid for - and that's apart from the energy cost, smell, acidic condensation, etc. (Unless you happen to have a reflux condenser column handy!)

For your pickling, the simplest thing to do might well be to just add a bit of citric acid.

Your vinegar contains about 45g/litre of acetic acid. You wish it had about 50.

Since we are dealing in kitchens and not laboratories, you might add about 5g citric acid to a litre of your vinegar to put you in the right ballpark for acidity.

No they aren't exactly equivalents, but rather than boil off maybe half your vinegar ... you should be somewhere close. And quickly and easily.

BTW, "Sarson's Pickling Vinegar" is 6% -- so don't get hung up on precision!

[Blether]

Your vinegar contains about 45g/litre of acetic acid. You wish it had about 50.

Since we are dealing in kitchens and not laboratories, you might add about 5g citric acid to a litre of your vinegar to put you in the right ballpark for acidity.

No they aren't exactly equivalents, but rather than boil off maybe half your vinegar ... you should be somewhere close. And quickly and easily.

BTW, "Sarson's Pickling Vinegar" is 6% -- so don't get hung up on precision!

Another good suggestion. And the chutney's fine, thanks for asking:

I read up some more, and I'll write up a couple of things I found, while I'm here.

As long as the link persists, this picture from Edinburgh University shows the 'Boiling Point Diagram' for acetic acid dissolved in water. The top line (curved) is the 'dew point' and the bottom line (straight) is the 'bubble point'. The Bubble Point is the temperature at which the first bubble appears as the solution is heated; the Dew Point, the temperature at which the vapour condenses as it's cooled.

Along the bottom is the 'mole fraction' - roughly speaking, the concentration (this picture shows the concentration of water rather than acid). The catch is that converting concentration by weight (e.g. 5% vinegar) to mole fraction, involves a formula with the form x /(x + y) and so it's not a linear relationship. By my calculation, some sample conversions run like this:

4.0% vinegar - 0.012 molefrac acid - 0.988 molefrac water

4.5% vinegar - 0.014 molefrac acid - 0.986 molefrac water

5.0% vinegar - 0.016 molefrac acid - 0.984 molefrac water

50% vinegar - 0.231 molefrac acid - 0.769 molefrac water

90% vinegar - 0.730 molefrac acid - 0.270 molefrac water

99% vinegar - 0.967 molefrac acid - 0.033 molefrac water

(I'm resisting the urge to abbreviate mole fraction as 'mf', but feel free to snigger along).

One way to use the diagram goes: say you have some 50% vinegar, and you want to know what will happen when you boil it. You look up the mole fraction of water (0.769) on the bottom of the chart. Looking up, you find it'll bubble from about 104.4C. Looking horizontally from that point on the bubble point line, you find from the dew point line that the vapour coming off at that temperature will have a mole fraction of about 0.865 water (close to 34% vinegar by weight). You'll be able to concentrate the vinegar, but 1/3 of what you're boiling off will also be vinegar.

For 4% or 5% vinegar, all the information is in the bottom right of the bottom right box on the chart. It's hard to read at this scale, but it looks like boiling 4.5% vinegar will give off 'steam' that's itself about 3.5% vinegar.

There's a good short summary of distillation principles here from Newcastle University (you might have known) and a more detailed scientific treatise here from the University of Rhode Island..

Beyond that, my science is too rusty for the game to be worth the candle. I know where I can get some home winemaking supplies, and in the back of my mind I suspect they'll be able to point me at some acetobacter

Edited February 27, 2010 by Blether (log)

[dougal]

... It's hard to read at this scale, but it looks like boiling 4.5% vinegar will give off 'steam' that's itself about 3.5% vinegar.

...

Hence the necessity of condensing that first 'steam', and reboiling it, and repeating many times, to get a reasonable yield.

The words to lookup are "fractionating column", and "reflux condenser". The idea is that the vapour condenses and reboils many times during its passage up the column, improving the separation of the components, as though you had done many 'simple' distillations. I vaguely recall from schooldays how one constructed a 'stepladder' through diagrams such as those you link, to estimate the purification from multiple distillations ...

But a teaspoonful of citric acid powder should do the job much more easily!

I hadn't thought you would have gone ahead so soon.

However, as a lemon pickle, you'd be incorporating extra acidity anyway - and mainly citric!

Edited February 27, 2010 by dougal (log)

[Blether]

However, as a lemon pickle, you'd be incorporating extra acidity anyway - and mainly citric!

Lemon entry, Watson, dear boy.

Virtual plates and all that - I dunno, you guys seem determined to push me into temptation with your tales of distilling, but yes, whilst you can fractionally distil using a simple pot still and condenser, you do need a fractionating column / reflux setup as you say, to efficiently separate harder-to-separate combinations, which do clearly include vinegar; or just to efficiently achieve high concentrations.

I hadn't thought you would have gone ahead so soon.

Just call me Job. The truth is I measured out the vinegar and boiled up the spices in it at the same time I started the topic

It sounds like you went to a fun school - very Speyside of you.

ETA: What I haven't said: I found a local source where I can get Sarson's white distilled neutral-character vinegar in an American pint (i.e. 3/4 of a pint) for ~370yen. I can get 900ml of basic 4% fruit or rice vinegar for ~290-350yen or so (fruit vinegar is cheaper). Wine vinegar - forget it. 350ml for like 500yen. There's an Italian product available for a bit less, but it has a floral character that I don't like. I can get a 750ml bottle of good, drinkable Spanish red or white wine with the sort of flavour I like, for 300yen. So I need to buy a still start making vinegar at home.

Edited February 27, 2010 by Blether (log)

[thock]

Hope this doesn't derail this topic excessively, but I'm wondering how you all store your reduced vinegars, if you don't use them immediately. I reduced some balsamic vinegar, and I don't know whether it should be stored in the fridge, or whether pantry storage at room temperature will suffice. I know room temp is ok for unreduced vinegar, but...

[andiesenji]

It should be just fine at room temp. I've reduced cider vinegars (apple, pear, pomegranate/berry) and all of them have been stored at room temp for at least a year.

They to have to be strained into a sterilized container but after all that cooking the vinegar "mother" has been completely destroyed.

I have half a pint jar of reduced pomegranate vinegar that I made four years ago, originally in a quart jar and when I used more than half, transferred the remainder to the pint jar.

[thock]

Oh, thank you, Andie!