Vinegar is among the oldest of home-brew mould/mildew/antibacterial treatments, and for good reason. Store-bought vinegar is about five per cent acetic acid by volume. Most moulds can only survive within a certain pH range, and even low concentrations of this powerful organic acid can alter the fungi's pH balance to the point that it can no longer survive.
Mould is a fungus in the same family as mushrooms, mildew and lichens. Fungi are hearty survivors, but different species maintain different survivability ranges in terms of temperature, humidity, nutrient requirements and pH balance. While some moulds can survive in extreme environments, most don't do well outside of our own range of survivability.
Chemists refer to acetic acid as a weak acid because it doesn't release all of its hydrogen atoms when dissolved in a solution. The degree of hydrogen dissociation is what determines an acid's strength or effect on the pH level of a solution; the more hydrogen atoms it contributes, the more it can affect the solution. However, "weak" is a relative term. Citric acid is another "weak" acid, but in high enough concentrations it's more than capable of dissolving flesh from bone.
Effects on mould
Aspergillus, Geotrichum, Mucor, Fusarium, Penicillium and Scopulariopsis are common moulds that tend to grow in everyday environments, and so are collectively representative of most species you're likely to encounter. Of these, only Mucor, Fusarium and Penicillium can survive acetic acid concentrations of more than 0.35 per cent; Mucor and Fusarium die off at about 0.5 per cent, but the acetic acid's pH of 2.4 isn't low enough to completely kill Penicillium. This trend would seem to support Heinz company spokesman Michael Mullen's assertion that store-bought vinegar (which is about five per cent acetic acid) will kill 99 per cent of bacteria, 82 per cent of mould strains and about 80 per cent of viruses.
If you're looking to kill moulds and other fungi using household acid/base solutions, consider something a little more aggressive than vinegar. Lime juice has a pH of between 1.8 and 2.0, making it the strongest of the common citric acids. A pH any lower and the result is sulphuric, hydrochloric or boric acid. The alternative is to go the other direction by using a very strong base to kill the mould. Ammonia water (pH 11.6), Borax (pH 9.2), Milk of Magnesia (pH 10.5) and soda lye (pH 14.0) should take care of most strains of fungi.