How to make your own apple cider vinegar:
1. Apple cider vinegar and other vinegar making depends on two fermentations. The first is fermentation of the sugar to alcohol and carbonic acid gas by yeast. The second is the transformation of the alcohol into acetic acid by vinegar bacteria. The fermentations must be kept separate and distinct to get good results.
2. Pure yeast and a small amount of sulfurous acid should be used to insure good alcoholic fermentation and to eliminate lactic bacteria.
3. Most fruits can be pressed most satisfactorily if crushed and fermented before pressing.
4. The fermented juice should be stored several weeks to rid it of yeast and other solid particles and should then be ‘racked’ or drawn off the sediment before apple cider vinegar and other vinegar fermentation starts.
5. To the racked juice, should be added one-fourth its volume of new apple cider vinegar and other vinegar to start acetic fermentation and prevent vinegar diseases.
6. The acidified fermented juice may then be made into apple cider vinegar and other vinegar by the slow process in barrels or by the quick process in generators. Vinegar fermentation requires an abundance of air.
7. The progress of the acetification should be watched by the Leo tester or other means.
8. The apple cider vinegar and other vinegar should be aged for a year before sale if a high quality is desired.
9. Apple cider vinegar and other vinegar may be clarified by isinglass or Spanish clay, or by filtration.
10. Lactic-acid bacteria injure the quality of vinegar, but are avoided if pure yeast is used; wine flowers destroy alcohol, but are prevented if the fermented juice is acidified with one-fourth its volume of new vinegar; vinegar eels are destroyed by heating the fermented juice or apple cider vinegar and other vinegar to 130 degrees F.
– from W. V. Cruess, ‘Vinegar from Waste Fruits’