Résumé:
The characterization of three essential oils (EOs) of aromatic and medicinal plants : thyme (T. willdenowii), verbena (A. citriodora) and lemongrass (C. citratus) and their incorporation into yogurt were the subject of this study. A survey of Internet users via Google Forms revealed that a high percentage did not use EOs in cooking or drinking. Analysis of the chemical composition by GC/MS revealed citral with its two isomers E-citral (38.6%) and Z-citral (29.89%) for citronella, and E-citral (34.43 %) and Z-citral (27.85%) for verbena and linalool (89.55%) for thyme. The essential oil extraction yields are 1.23±0.23%, 0.83±0.35% and 0.83±0.27% respectively for lemongrass, thyme and verbena. Thyme has a higher antiradical activity than verbena and lemongrass as well as the highest content of phenolic compounds (405.67±13.08 µg GAE/mL of yogurt). A strong inhibition of pathogenic bacteria such as B. cereus, S. enteritidis, P. aeruginosa and E. coli is obtained with thyme EO. Lemongrass EO has proven to be the most inhibitory of lactic acid bacteria in yoghurt: L. bulgaricus and S. thermophilus. The antioxidant and antibacterial activities of verbena EO were weak. However, it showed an interesting inhibitory activity of the butyrylcholinesterase enzyme, just after lemongrass EO.The inhibitory activity of the α-amylase, urease and tyrosinase enzymes was weak for the EOs studied. The evaluation of the cytotoxic effect of EOs on Artemia salina larvae did not show any cytotoxicity. The incorporation of essential oils into yogurt at a dose of 25 µL/L maintained satisfactory microbiological quality of the products. The physico-chemical analysis of the different yogurts revealed a decrease in pH throughout the cold storage period. However, the pH values, significantly are elevated in yogurts incorporated essential oils of lemongrass and verbena. The incorporation of essential oils of lemongrass and thyme significantly increased the antioxidant potential of these yogurts on the seventh day of storage evaluated by the CUPRAC test, as well as on the 21st day of storage for the yogurt incorporating the essential
oils of thyme evaluated by the test ORAC. The antioxidant potential of lactic acid bacteria increased after their fermentation with the different EOs studied. HPLC analysis of the organic acids and aromas produced revealed an increase in the production of lactic and citric acid at the end of cold storage for yogurts containing verbena and lemongrass essential oils. The three HEs also stimulated the production of the acetaldehyde aroma from the first day of storage. The evaluation of the phenolic compounds content of the formulated yogurts revealed an increase of these compounds in the lemongrass EO yogurt on the first day and in the thyme yogurt on the seventh day of storage. The yogurt incorporated with verbena EO was the most appreciated by the panel of tasters, followed by the yogurt incorporating lemongrass and thyme EO.
