Résumé:
The objective of this work is to study the effects of the incorporation of Myrtus
communis L. essential oil on the quality of stirred yogurt prepared and fermented by
freeze-dried lactic ferments in the laboratory. The essential oil was incorporated at
concentrations of 2.5%, 5% and 10% in the yogurt during the stirring process. These
effects were evaluated by monitoring the evolution of the physico-chemical and
microbiological parameters during 21 days of storage at + 4 ° C. The presence of the
essential oil at the concentrations of 2.5%, 5% and 10% in the stirred yogurt led to an
increase of St. thermophilus viability in the order of 33.7x106, 31x106 and 53.2x106
UFC/ml from the first week of storage and then a significant decrease with the highest
doses of 5% and 10% in the order of 11 × 106 and 19 × 106 CFU/ml, respectively, at the
end of storage. On the other hand, the addition of the essential oil at a low dose of 2.5%
showed a slight decrease of 1.3x107 CFU/ml at the end of storage comparable to that of the
control (1.2x107 CFU/ml). As for the acidity, the results obtained showed a decrease in pH
(from 4.6 to 3.9 for 10%, from 4.6 to 4.2 for 5% and from 4.6 to 4.35 for 2,5% of EO) and
an acidity increase of 85 to 135 ° D for 10% and 84 to 112 ° D for 5% with exception for
2.5% (81 to 94 ° D) compared to the control (83 to 125 ° D). Thus the addition of this dose
made it possible to stabilize the microbiological quality of the stirred yogurt while
extending its shelf life. Sensorially, natural yogurts (sugar-free and with sugar) were the
favorites among all samples and aromatization with myrtle EO at a dose of 5% seems to be
the least appreciated or even unacceptable because of its strong aroma, pronounced odor
and accentuated taste.
