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The yeast plant is a fungus, whereas the characteristic vinegar-forming organisms are acetic acid bacteria. Calling both vegetable growths reflects an older classification of microscopic organisms; it does not make them green plants or two stages of one species.

The familiar brewing and baking yeast is Saccharomyces cerevisiae. Its oval or rounded cells possess a cell wall, cell membrane, cytoplasm, cell nucleus and other eukaryotic structures. Ordinary multiplication is by budding: a small outgrowth enlarges, nuclear division supplies it, and separation leaves a scar or a temporary attached cluster. Under suitable conditions, the life cycle also includes mating between compatible haploid cells and formation of spores by diploid cells. Those processes are distinct from simply breaking a chain of accidentally adherent cells. Neither roots nor a leaf-like apparatus is required for its growth.^[ref]

Yeast obtains organic nutrients from its surroundings. In alcoholic fermentation it converts fermentable sugar into ethanol and carbon dioxide; the idealized glucose balance isA real culture also uses material to build cells and produces additional compounds. The process does not require oxygen as a reactant in that net balance, although yeast can respire and its growth and product yields depend on conditions. In bread the gas expands the dough; in a fermenting liquid both gas evolution and chemical change can be followed. Gas bubbles alone do not identify the organism responsible.

The historical vinegar plant or mother of vinegar appears as a surface film or gelatinous mass in a suitable alcoholic liquid. Louis Pasteur used the name Mycoderma aceti for the film-forming agent in his investigations of acetification. In modern terms, members of several acetic-acid-bacterial groups can occur in vinegar cultures. Rod-shaped bacterial cells reproduce by binary fission; some form an extracellular, often cellulose-rich, matrix that helps produce a visible pellicle. The gross mother is therefore a growth structure or microbial community, not a single large cell and not necessarily a pure culture of one modern species.^[ref]

Its essential chemical role is oxidation of ethanol to acetic acid using atmospheric oxygen:Thus the historical term acetic fermentation denotes an oxidative microbial process, unlike the oxygen-independent net alcoholic-fermentation reaction above. Surface access to air favours the process. With suitable organisms and conditions, further oxidation of the acid can occur, so indefinitely leaving an exhausted culture exposed to air does not guarantee indefinite accumulation of vinegar.

Louis Pasteur's Mémoire sur la fermentation acétique by Louis Pasteur, published in 1864, supplies contemporary experimental evidence appropriate to this paper. Inoculating an appropriate alcoholic medium with material from an active film, observing film development and measuring the rise of acidity connects the growth with the chemical process. Comparisons with uninoculated or differently aerated preparations distinguish mere atmospheric exposure from the action of the growth. His experiments also examined the effects of submergence and the conditions under which the film performed acetification.^[ref]

Wine turning to vinegar therefore illustrates succession: yeast can first supply an alcoholic substrate from sugar, and acetic acid bacteria can then oxidize that substrate when oxygen is available. A white surface growth is not automatically the active vinegar agent; other yeasts and moulds can form films or spoil the liquid. Microscopy, controlled transfer and chemical measurement together provide a natural-history account stronger than identifying every surface skin by its household name.