Isolation and Identification of Geotrichum candidum from Kefir Grains and Analysis of Volatile Compounds Produced by It

Strains K2, Z7-1 and Z8-4, all of which could produce compounds that contribute to pleasant floral and fruity aromas, were isolated from kefir grains collected from Kumming, Yunnan. Based on genome resequencing, the genome similarity of the strains to Geotrichum candidum LMA-244 was analyzed, showing that the DNA-DNA hybridization values were 99.50%, 99.40% and 99.50%, and the average nucleotide identity was 98.66%, 98.65% and 98.68%, respectively. Based on morphological characteristics and 26S rDNA sequence homology analysis, all the strains were identified as G. candidum. Gene function annotation of G. candidum K2 was carried out using the Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Cluster of Orthologous Group (COG) databases. The results showed that 40.74% of the annotated genes were associated with cellular components, 47.54% with biological processes and 11.73% with molecular functions. KEGG metabolic pathway analysis revealed the presence of genes and pathways related to terpenoid biosynthesis, indicating that G. candidum K2 has the genetic basis for the synthesis of volatile aroma components. By gas chromatography-mass spectrometry (GC-MS), a total of 50 volatile components were detected in the fermentation broth of G. candidum K2. Among them, 3-alkene, α-pinene and p-cymene contributed to the floral aroma, L-α-terpineol was responsible for the pine-like aroma, terpinen-4-ol and (-)-terpinen-4-ol was responsible for the clove-like aroma, and α-terpineol acetate contributed to the fresh and sweet aroma. Therefore, it was inferred that G. candidum greatly contributes to the formation of aroma substances in kefir grains. This study may provide a theoretical basis for the application of G. candidum K2 in fermented dairy products and other fermented foods.