A methodological approach to screen diverse cheese-related bacteria for their ability to produce aroma compounds

Microorganisms play an important role in the development of cheese flavour, producing a variety ofaroma compounds for which lactose/citric acid, proteins and fat are precursors. There is a demandfrom cheese producers to diversify the flavour of semi-hard cheeses. A large screening is required toidentify strains of interest among the diversity of cheese-related bacterial species and strains. This stepcannot be bypassed and is time-consuming. Our aim was to develop a methodological approach tofacilitate and expedite the screening of various cheese-related bacteria for their ability to producearoma compounds. Our strategy was to combine: (i) the use of a curd-based broth medium incubatedunder conditions mimicking cheese manufacturing and ripening, ii) a powerful method of extraction ofvolatiles, headspace trap, coupled to gas chromatography-mass spectrometry, and iii) a metabolomicsbasedmethod of data processing using the XCMS package of R software and multivariate analysis.This approach was applied to eleven species: lactic acid bacteria (Leuconostoc lactis, Lactobacillussakei, Lb. paracasei, Lb. fermentum, and Lb. helveticus), surface bacteria (Brachybacterium articum,Brachybacterium tyrofermentans, Brevibacterium aurantiacum, Microbacterium gubbeenense),Propionibacterium freudenreichii and Hafnia alvei. All the strains grew, with maximal populationsranging from 7.4 to 9.2 log CFU/g. In total, 53 volatile compounds were identified, of which 50 variedsignificantly in abundance between bacteria. Principal component analysis of volatile profilesdifferentiated species on their ability to produce ethyl esters (associated with Brachybacterium), sulfurcompounds and branched-chain alcohols (H. alvei), branched-chain acids (H. alvei, P. freudenreichiiand Lb. paracasei), diacetyl and related carbonyl compounds (M. gubbeenense and Lb. paracasei),among others. The present approach was then successfully applied to screen collections of lactic andpropionic acid bacteria. The metabolomics-based method of data processing can be widely applied inthe field of food flavour chemistry.