I've been exploring ways to concentrate ordinary vinegar at home for those who can't buy it easily and need some in a pinch or have a child in need of a science project. The methods aren't very fast, and the yield might not be great. The first order of business is to establish a way to measure the product acidity at home. I found this titration explanation on the Web at a URL that is now inactive to me, and I'm repeating it substantially as I found it. The suggested steps are:
1. Shred or chop a good-sized chunk of red/purple cabbage, maybe a half of a medium sized cabbage, and steam it well in a small amount of water. The darker purple the water is, the better. Pour the colored water into a bottle or jar with a lid, put it in the refrigerator to cool, then eat the cabbage.
2. Put a couple of ounces (an equal amount) of water into two clean glasses. I use a small syringe to measure the water into each glass, but any fairly accurate measure should work. In a third glass, dissolve enough baking soda so that a little of the soda settles to the bottom.
3. Use a clean eye dropper to add an equal amount of enough of the cabbage water to impart a noticeable color to the water in each glass. Rinse the eye dropper in clean water.
4. Rinse it with store-bought 5% (5 grain) vinegar. Add 7 drops of the store-bought vinegar to the first of the colored water glasses, which you should label as "control." Rinse the eye dropper in clean water.
5. Rinse the eye dropper with the unknown acetic acid sample that you want to test. Add 7 drops of the unknown to the second colored water glass, which you can label as "unknown." Rinse the eye dropper with clean water.
6. Rinse the eye dropper with the baking soda solution, then add 20 drops of the baking soda solution to the control glass. Swirl the control or stir it with a plastic spoon between drops or after each couple of drops. Observe the color of the control solution.
7. Add the baking soda solution drop by drop to the unknown glass, stirring or swirling between each drop. Count the number of drops of the baking soda that are required to bring the unknown solution to the same color as the control. To derive the concentration of the unknown, divide the number of drops of baking soda solution added to the unknown by 4. The amount of baking soda needed can vary from user to user due to water quality. A similar approach could be used to make soap using wood ashes, if a person ever is on a survival TV show and happens to have an anthocyanin source available.
After using this several times, a person can shortcut the assay method to get a fair estimate. One of the methods for strengthening vinegar that is available to the home user is to freeze concentrate it. The basic idea is discussed here:
I'm working on some trials of that method and also a trial of a simple evaporation process, and I'll try to update this post with results when I get them. If anyone else tries it, please post your results. The theoretical yield of 10% vinegar by that method would be 50% of the original volume, but I doubt if that is likely as a practical matter. Besides those two approaches, the other basic choice for the home user is to produce additional acetic acid by fermentation. That would involve using either unpasteurized unfiltered vinegar, probably cider vinegar, or mother of vinegar (acetobacter), and adding it to ethanol or to a yeast and sugar source combination that will ferment into alcohol. I believe 10% acetic acid would be the practical limit for that process, and a person generating hard cider as an intermediate product might not want to publicize that fact in some states. Deriving high strength acetic acid rapidly in high yield really requires a distillation or catalytic process, but I don't figure they are available to the average home user.
The physical qualities of vinegars can vary among type, and I would expect a raw cider vinegar to act slightly different from a clear distilled product. It appears to me that there is some misleading/incorrect information about the physical properties of vinegar around. For example, the boiling point of typical 5% vinegar available in the grocery store often is listed as 116C (240F) at sea level. That probably is correct for glacial acetic acid, but my observed boiling point for 5% distilled vinegar is quite different. I'd be interested to hear what others observe for 5% vinegar's boiling point. The freezing point of the 5% product would not be very easy for the home user to measure.