Causes of Spoilage
On the skins of apples there are various strains of yeast and some bacteria. These get into the juice when it is in the process of being pressed from the apples. These yeasts and bacteria can start souring ferments and turn the alcohol into acetic acid vinegar. Boiling the juice will produce a cider that will never clear - though it will destroy the troublesome yeast and bacteria. Therefore, we must destroy the yeast and bacteria without boiling. Here again, Campden fruit-preserving tablets do the job for us.
Having expressed the juice, one Campden tablet per gallon is crushed and dissolved in a little warm water and stirred into the juice. Two may be used to make sure there will be no souring ferments, but if two are used, the juice should be stirred vigorously after one hour and then again after ten minutes. This will liberate much of the gas so that the yeast added to ferment the juice is not also killed as it is put in.
The gas produced by the Campden tablets is known as sulphur-dioxide or 5.02, This method of destroying unwanted yeast and bacteria is used extensively by both commercial and amateur wine makers.
All bottles and stoppers must be washed in a solution of sulphur dioxide made up by dissolving 2 oz. sodium metabisulphite in half a gallon of warm water. Half a pint of this is poured into the first bottle, then into the next and next and so on. When a dozen have been done this half pint is discarded. The bottles are then rinsed with boiled water that has cooled; they are then ready to receive the cider. Jars for draught cider are treated in the same fashion. The bulk of the sterilizing solution may be kept for further use. Best to use a glass-stoppered or rubberstoppered bottle for this. Plenty of chemists will let you have one for about a shilling.