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Vinegar, Preserving Of
The methods of preserving vinegar are various. It is a fact generally known that vinegar, of whatever kind, will not keep long, but in the course of a few weeks, especially in the warm temperature of summer, grows turbid; its surface is then covered with a thick mucilaginous substance, during which time the acid disappears by degrees, and at last is entirely lost; hence the vinegar must very often be thrown away. Now, in order to avoid this spoiling of the vinegar, five methods are known. The first is to prepare the vinegar very strong and sour at first, by using more sugar or other materials. It is well known that such vinegar will keep for several years; but as few people prepare their own vinegar, most persons contenting themselves with buying it at the shops, there are, of course, but few who can make use of this method. The second method is to concentrate the vinegar by freezing; after which a hole is made in the crust of ice which covers it, through which the part that is not congealed is let out, and afterwards put into bottles. This manipulation answers well enough; but nearly one half being lost, because that which forms the crust of ice is nothing for the most part but water, good economists dislike it. The third method is to prevent the access of air by filling the bottles, and keeping them corked. Though vinegar is kept long by this method, it is not much employed, probably because it is troublesome to fill up the bottle with clear vinegar from another bottle every time you make use of part of its contents. The fourth method is to distil the vinegar, and thus leave behind the impurities. Distilled vinegar does not suffer the least change, though exposed to a warm air for years; but the expense of distilling is often thought too much. The following method of purifying and preserving vinegar is the easiest, and answers sufficiently. Put the vinegar into a well-tinned vessel, and make it boil for a minute over a strong fire, or the vinegar may be put into bottles, and then into a kettle of water upon the fire, and boiled; this will coagulate the glutinous and mucilaginous matter which all vinegar contains, and this may be separated by straining; after which the clear liquor should be kept in well-corked bottles. Vinegar prepared in this way keeps long without spoiling, and should be so treated when intended for pickling vegetables.
This is a method rather than a dish: five ways of stopping vinegar from going cloudy and weak, ending with a simple pasteurisation that still works. Most of the problem it addresses has been solved at the factory.
- What the "mucilaginous substance" isThe thick skin or jelly forming on the surface is mother of vinegar, a cellulose mat produced by acetobacter. It is harmless, and in a sealed bottle of ordinary shop vinegar it is a cosmetic nuisance rather than spoilage. The loss of acid described here happens when vinegar stands open and the acetic acid is itself oxidised away, by the acetobacter once the alcohol is gone or by other organisms on the surface.
- Don't actually boil itThe hinge of the method is heat, not a rolling boil. Acetic acid is volatile and carries off in the steam, so a minute at a hard boil weakens the vinegar. Holding it at around 160-170F/70-77C for a few minutes kills the organisms and coagulates the haze without stripping the strength.
- The "well-tinned vessel"Domestic pans of the period were tinned copper or tinned iron, and the tinning is the point: vinegar attacks bare copper, brass and iron and picks up metallic salts. Use stainless steel, enamelled cast iron or glass; never aluminium or unlined copper.
- Bottles in a kettle of waterThe second version described is a water bath. Stand the bottles on a rack rather than the bare pan bottom, start them in warm water, and leave them loosely stoppered until they come off the heat, or they will crack or blow the cork.
- "Distilled vinegar" then and nowHere it means vinegar that has itself been distilled to leave impurities behind. On a modern label, distilled or white vinegar means vinegar fermented from distilled grain alcohol, which is a different thing arrived at from the other direction. Either way it is exceptionally stable, as the entry says.
- Freeze concentrationThe second method is fractional freezing, and it works because water crystallises out before the acid does. The objection stands: you throw away half the volume as ice, and the discarded ice is not quite pure water either.
- For picklingThe closing advice matters more than the clarity does. Pickling needs vinegar of known strength, at least 5 per cent acetic acid, and homemade or long-open vinegar of unknown acidity should not be used for preserving vegetables.
- Whether to botherCommercial vinegar today is pasteurised and filtered before bottling and keeps for years in a corked or capped bottle. This treatment is worth doing only for cloudy homemade or unpasteurised vinegar, or for a bottle that has grown a mother.
Cassell's treats this as a matter of principle because in the nineteenth century vinegar was a staple preservative in its own right, bought by the gallon for pickling, ketchups and store sauces, and a barrel that went flat in August wasted a season's work. The explanation offered here is pre-microbiological: the entry treats the surface growth as a "glutinous and mucilaginous matter" inherent in the liquid, to be coagulated and strained out, rather than as a living culture. Pasteur's work on the vinegar ferment in the 1860s identified the organism, though the practice of heating vinegar to preserve it was already known and in use in household books whether or not their compilers had the theory behind it. The frank note that few people now make their own vinegar, most contenting themselves with buying it at the shops, marks the point at which brewing and souring had moved decisively out of the domestic kitchen and into trade.