This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
What is Ropiness? - The peculiar cloudy, stringy, oily appearance of carbonated beverages, called by the bottlers "ropiness," is caused by a peculiar viscous fermentation, by which the sugar in solution (syrup) is converted into a gummy matter and other products, instead of into alcohol and carbonic acid as by regular fermentation processes. By this imperfect (as well as by a perfect) fermentation, all the ingredients of the beverages become affected, and changed. This ropiness is attributed by the bottlers to most every possible ingredient, from the water, carbonic acid gas (marble and acid) and the apparatus, down to the extracts, essences, colors, gum foams, and whatever enters into the beverage, and has been supplied to them. On his own faults and frequent carelessness he never or but quite infrequently attributes the cause. Let us search for the causes of this much-disputed bottlers' fiend.
At a temperature of 90 to 100° F., if submitted for a considerable time, syrup suffers this fermentation, ordinarily known as viscous. Gases are evolved which are rich in hydrogen instead of being exclusively carbonic acid, and when the sugar has, for the most part, disappeared, mere traces of alcohol are found in the liquid; but in place of that substance, a quantity of lactic acid, and mucilaginous substances resembling gum arabic, and said to be identical with gum in composition. By boiling yeast or the gluten of wheat in. water, dissolving sugar in the filtered solution, and exposing it to a tolerably high temperature, the viscous fermentation is set up and a large quantity of the gummy principle generated along with a ferment of a globular texture, like that of yeast, but which is capable of producing only the viscous fermentation in saccharine solutions. Attributing the viscous fermentation to mineral acids, such as may enter the carbonated water by careless generating of gas, is quite frequent. Mineral acids, such as sulphuric and sulphurous acids, and astringent substances, such as tannin, entering with ingredients, even in small proportions, prevent or retard the viscous fermentations or precipitate the ferment, and are rather preservatives in this case, but contaminations in other respects. The fermentation is started, there can be no doubt, by some fungus introduced into the beverage. It cannot be introduced by mere essences, which are solutions of oil, and act rather as a preservative, on account of their alcoholic strength. It can only be introduced by impure and foul water, germ-loaded atmospheric air, and ingredients that permit fungoid growth, such as citric acid, lemon or lime juice, when unfiltered and unpreserved or deceptive, and solutions of impure sugar, which contain already the fungoid growth, and glucose, but also by uncleanliness. The various apparatus, syrup tanks, gauges, etc., in which or through which a saccharinated beverage passes so frequently, even if the latter is carefully prepared, are so much exposed to influences of ferments, and the traces of liquids remaining therein are so infected with it, that all liquids passing through such parts are more or less affected. In exceptional cases, enough care is taken to disconnect and scald and clean all the apparatus and its connections, and every conjointly engaged adjunct, to warrant the necessary cleanliness and freedom from any infection. Uncleaned, dirty bottles are an abominable source of ropiness, a store of ferments, and a source of fungus of all kinds, and they require the most careful attention in being cleansed. The ferment must have something to live on. Impure water, sugar-solution, and most extractive matters contain the basis, the necessary phosphatic salts for its existence and development. The action of the ferment on sugar is prevented by too great concentration of the solution, but in carbonated beverages it is highly diluted, and offers all advantages desired for its activity, first inverting cane-sugar into glucose, and finding in the other admixtures food for its subsistence.
The bottler is sometimes puzzled that some dozen or more bottles out of a lot would become ropy. Various explanations could be given therefor. That lot of bottles which turned ropy may have been imperfectly cleaned, and contained the ferment ready to develop itself, while the others were perfectly cleansed. In mouldy work-shops, or adjoining rooms, the walls and ceilings are often covered with fungus, and a draught of air detaches them, and deposits some in exposed bottles, or the air keeps the germs in suspension, and occasionally when becoming moist drops them, and thus a certain lot of bottles might become infected. These explanations account also for the occurrence of ropiness at intervals.