Production of Volatile Compounds in Cheese by Pseudomonas fragi Strains of Dairy Origin

Volatile compounds produced in cheese by five Pseudomonas fragi strains isolated from 1-day-old raw milk cheeses were investigated. Each strain was representative of a different biochemical group of isolates of identical phenotypic characteristics, according to identification with API 20 NE strips. The five strains were ascribed to the species P. fragi after 16S rRNA sequencing because of their high degree of coincidence with P. fragi ATCC 4973. In each of two experiments, carried out on different days, five cheeses were made at laboratory scale from pasteurized milk separately inoculated with approximately 10⁵ CFU/ml of each P. fragi strain. After 12 days at 10°C, mean counts of P. fragi strains were close to 10¹⁰ CFU/g in the outer part of cheeses and close to 10⁸ CFU/g in the inner part. A total of 131 volatile compounds, 49 of which were further characterized, were identified in cheeses by gas chromatography–mass spectrometry after extraction with a purge and trap apparatus. Abundances of compounds were generally higher in the outer part of cheeses. Production of volatile compounds was clearly strain dependent. Only two strains produced ethyl esters, and three produced nonethyl esters. Ethyl acetate, ethyl butyrate, ethyl caproate, methyl acetate, isopropyl acetate, and propyl tiglate were the major esters, and ethanol, 2-propanol, and 3-methyl butanol were the major alcohols. Undecene was the major hydrocarbon, dimethyl sulfide and methyl thiocyanate the major sulfur compounds, and 2-pentanone the major ketone. Two aromatic compounds, styrene and o-dichlorobenzene, were present in all cheeses.