From work that has recently been done it is evident that starters are not pure cultures of S. lacticus as has commonly been supposed. Hammer and Bailey showed that pure cultures of this organism do not produce a volatile acidity at all comparable with that produced by a satisfactory starter, and that starters generally contain an associated organism which, when grown in combination with S. lacticus, yields a volatile acidity essentially the same .as that produced by a good starter. Storch found that organisms which he calls x bacteria are present in starters and that these play a part in the production of a good product. Boekhout and Ott de Vries reported the isolation from starters of organisms that, in combination with lactic acid bacteria, produce the characteristic aroma.
Hammer, B. W.
"The type of lactic acid produced by starters and by the organisms isolated from them,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 5
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol5/iss65/1