الخلاصة:
A total of 123 yeasts strains was isolated from different samples of cow's milk products collected from Constantine region and spent olive "" Chemlal variety "" obtained from various mills located in Skikda. In fact, 116 strains were selected for their lipolytic activity. Six yeast isolates producing lipase were prescreened for their ability to develop important halos on tributyrine agar medium. Then, two isolates (L2 and G5) were screened for their ability to produce high levels of lipase reaching 13,32 U / ml et 7,15 U/ml, respectively. Preliminary and molecular identification showed that these strains are Yarrowia lipolytica (accession number: ACKF156787) and Candida boidinii (accession number: KF156789), respectively. In order to determine new potentialities of Y. lipolytica L2, a comparative study was carried out between the two strains (L2 and G5) taken onto consideration several parameters and under various conditions culture. The production of lipase by C. boidinii G5 is faster (24h), however, the quantity of lipase produced by Y. lipolytica L2 is higher (13,3U/ml). The effect of adding olive oil with/without tween 80 , in submerged medium, on biomass and lipase production allow showing that Y. lipolytica L2 produce biomass and lipase at low quantity of olive oil (0.5%) without tween 80. In contrast, the production of important biomass and lipolytic activity by C. boidinii G5 require adding olive oil and tween 80 to the medium.
Optimal conditions of lipase activity for L2 and G5 strains are found at pH 7.0 and temperature 37 ° C. the solid fermentation on GOC at different moisture content (70% and 90%) showed that the two strains adapt better when initial humidity is 70%. the maximal concentration of produced enzyme is 20U/ml for Y. lipolytica L2 and 19,2 U /ml for C. boidinii G5, allowing to investigate that the production of lipase in SSF (in presence or absence of tween80), is better than SmF (presence / absence of tween 80), for the two strains. This result open the way on the use of studied strains in solid oil waste degradation. Furthermore, good results were found when using Y. lipolytica L2, as a starter, in a minimal medium with olive oil. During fermentation, all fatty acids were decreased, excepted stearic acid which increases. To conclude, Yarrowia lipolytica present an important biotechnological potential allow it to be exploited at industrial and environmental level.
