EFFECTS OF FREE FATTY-ACIDS ON PROPIONIC-ACID BACTERIA

The seasonal variations in milk fat composition, especially during the grazing period, often lead to poor eye formation in Swiss-type cheese . The influence of free fatty acids on the growth and metabolism of th e dairy propionibacteria has been studied in this work. Linoleic (C-18 :2), lauric (C-12:0), myristic (C-14:0) and oleic acids (C-18:1) inhib ited the growth and acid production of P freudenreichii subsp shermani i in the reference medium. The antibacterial activity of linoleic acid can be overcome by additions of cholesterol and soya lecithin to the medium. The four species and two subspecies of dairy Propionibacterium could be divided into two groups according to their high susceptibili ty to unsaturated fatty acids (P freudenreichii subsp shermanii and su bsp freudenreichii) or low susceptibility (P acidipropionici, P jensen ii and P thoeni,). Nevertheless, this inhibitory action of free fatty acids was not observed in milk, retentate media or lactic curd. A tota l extraction of the milk lipids led to the recovery of this inhibitory effect on Propionibacterium. The possible modes of action of these mo lecules are discussed on the basis of the observed potassium effluxes and disturbances in the cell membranes.