[HowTo] Making my own Roasted Malt Beer Vinegar (Cerveza Negra vinegar) at home

Making vinegar is easy. Arguably too easy in fact, since you do not actually need any special methods or equipment to make vinegar. Some might even say that it is the result of the failure to make alcoholic beverages (of course there's no reason why you should not make home-made vinegar on purpose — they can be a lot better tasting to store-bought ones). Leave a beverage with adequate sugar and naturally occurring yeast in the air will ferment it into alcohol and certain bacteria will, in turn, ferment the alcohol into vinegar after some time has elapsed. Yes, making vinegar is a distinct 2 step process, and while that doesn't mean the two processes cannot occur simultaneously, different conditions will favor one activity over the other. Relying on wild yeast will drastically slow down the process and typically it doesn't produce alcohol as high as when using brewer's yeast (brewer's yeast has a higher alcohol tolerance to keep on converting sugars to alcohol) which in turn will produce a weaker vinegar (in terms of acetic acid content - the chemical conversion is theoretically same part alcohol gets converted to same part acetic acid but will also depend on other factors like evaporation, the strength of the alcohol which may inhibit the acetobacter activity among others).

On a food safety perspective, generally, the acetic acid content of most commercially bought vinegar is 4% at the minimum (the most common legal minimum for most countries) but can go as high as 7% and should be clearly stated in the label.

The fermentation of alcohol into vinegar is an aerobic process (it needs the presence of oxygen to progress). So fermentation containers should be properly covered (to prevent insects from going in) but are left to breathe so that the surface of the liquid gets ample supply of oxygen. The best way is to cover the mouth with a fine cheesecloth or some paper towels secured with some rubber bands.

In this particular example, we are making things simple by starting off with an alcoholic beverage already at the ideal percentage alcohol range for vinegar fermentation 5 to 10% alcohol by volume (or optimally at 7 to 9%). Too low and you risk too slow fermentation times and weak acetic acid concentrations (which is problematic in terms of mold and other harmful bacteria control - as mentioned, a final acetic acid strength of 4% is usually recommended), and too high an alcoholic content will inhibit the activity/presence of the acetobacter which converts the alcohol into acetic acid.

I used Cervesa Negra, a dark roasted malt beer available here in the Philippines. It has a 5% ABV (alcohol by volume) and has a rich slightly sweet taste (that is the best description I can come up with, apologies as I am not an avid beer drinker). 3 bottles of 330ml plus 1/4 cup of unpasteurized apple cider vinegar with the mother (I used Bragg ACV to inoculate the beer - succeeding batches can use the mother of the previous batch). I just mixed everything into a large wide-mouthed glass container, covered the top in paper towels (folded twice so there are 4 layers) secured with a rubber band, labeled the source material, the date of mixing, and the due date of checking after 4 weeks. I stored it in a dark corner of my cupboard and tried my best to ignore it (sometimes I can't help sneaking a peek to check on the progress and now I understand that waiting for something to happen to a long fermentation project such as this may be painfully frustrating for impatient individuals).

After 4 weeks have passed, a thin layer of "mother" (a slimy substance made up of cellulose and acetobacter that floats on the surface of the liquid) has formed. The cupboard has the unmistakable smell of vinegar (before it just smelled of beer). It tasted of weak vinegar but has a pleasant beer-like flavor. I have decided that the 4 weeks of fermentation was sufficient so I carefully transferred the vinegar mother to another batch of beer vinegar I was making and transferred the resulting vinegar back to the amber beer bottles (which I have cleaned thoroughly of course) using a funnel. There were no sediments observed so I did not do any filtering of the liquid. I have also decided to do an extra pasteurization step to ensure that the fermentation stops and the shelf life to improve (over fermentation happens when the bacteria further breaks down the vinegar - also overoxidation breaks down the acetic acid so it should also be tightly covered). I did this by immersing the bottles in a constant temperature circulation bath with the temperature set at 68.3 degrees C for 30 minutes. I am not that sure how effective this is considering that the top parts of the bottles are not fully submerged (but that is also the reason why I increased the temperature a bit from the recommended temperature I read somewhere online of 140 degrees F but not to exceed 160 degrees F).

The final results? The final malt beer vinegar was dark brown in color with a strong acetic acid smell. Taste-wise, the vinegar tasted a bit weak (not unusual considering the low ABV of the alcohol used) with hints of pleasant roast malt beer-like flavor. The second time I made this, I used San Miguel's apple-flavored beer and I can say that the result was even better than the first attempt. I did however increased the ABV from a very low 3% to 7% by using vodka and skipped the pasteurization process (I found out that I can consume about a litter of this specialty fermented vinegar in a surprisingly short time at home).

I can't wait to try out variations to the starting liquid to be fermented and find out the best results for my taste.