The experiment: Collect fruit scraps (apple cores, lemon peels, the white bits of a watermelon, etc.) in the 'fridge for whatever length of time is convenient. Add to a solution of 1/4 cup honey per liter of water, in a wide-neck bottle or jar. Cover with a cloth held on by a rubber band, and leave in a cool, dry place for a week. Next week, I'll find out whether watermelon rind makes a decent vinegar.
The science: The first and obvious step of any sugary solution left at room temperature is that wild yeasts will ferment the sugar into alcohol. Once there's alcohol in solution, the naturally occurring acetic acid bacteria will ferment that into vinegar, quickly lowering the pH below that favored by the various species of microbes and fungi that cause spoilage. (If the culture contains Acetobacter xylinium or similar, they'll form a harmless mat that floats at the top of the jar which is known as "mother of vinegar".)
The science: The first and obvious step of any sugary solution left at room temperature is that wild yeasts will ferment the sugar into alcohol. Once there's alcohol in solution, the naturally occurring acetic acid bacteria will ferment that into vinegar, quickly lowering the pH below that favored by the various species of microbes and fungi that cause spoilage. (If the culture contains Acetobacter xylinium or similar, they'll form a harmless mat that floats at the top of the jar which is known as "mother of vinegar".)