## 3 Results and Discussion
### Physico-Chemical Properties of Mayonnaises  
The pH value of mayonnaise is very important for its shelf life by reducing the risk from microorganisms, and for its structure by influencing the viscosity and elasticity of mayonnaise [4; 37]. The pH of the control sample (3.6) was slightly higher than the recommended range of 3.0 to 3.5 [38] mainly due to the lack of acetic acid in the present mayonnaise recipe. Acetic acid is currently used to acidify and to ensure the microbiological control of mayonnaise [39], but it is usually missing from artisanal recipes. The use of carotenoid-enriched oil after extraction from tomato by-products in mayonnaise manufacture determined a slight increase in pH, while no significant influence on mayonnaise pH was found by the addition of basil essential oil. The pH slightly increased during storage, remaining, however, between 3.5 and 4.0, which is still considered a safe pH range for mayonnaise [40]. Previously, [PERSON] et al. [41] reported a significant increase in the pH values after 30 days of storage both in the control and ajoene-added homemade mayonnaise, while [PERSON] et al. [42] found also that pH increased from the first day until the 60 th of storage in low-fat mayonnaise without preservatives. However, other researchers found that pH values declined continuously in mayonnaise samples during the storage period [15]. Acid value provides information on the extent of lipid hydrolysis in the oil phase resulting in the formation of free fatty acids [15]. The results indicated that acid values continuously increased during 8 weeks of storage in all samples, but the control sample showed acid values significantly higher than other samples throughout the storage period. The increase in acid value could be attributed to the action of hydrolytic and oxidative enzymes and lactic acid bacteria from the aqueous phase of mayonnaise [43; 44]. The addition of BHT or basil essential oil as well as the extraction of carotenoids from tomato by-products, reduced the growth of acid values probably as a result of the inhibition of oxidative processes. At the end of the storage period, the lowest acid values were found in MDSB5 and MDSB5 samples. The evolution of peroxide values of mayonnaise samples during 8 weeks of storage is presented in Table 2. Peroxide value is used as a measure of the primary oxidation compounds (hydroperoxides) in lipid products. The results showed that the extraction of carotenoids from tomato by-products in oil retarded significantly the oxidation process. Peroxide values gradually increased in all mayonnaise samples during storage, reaching their highest values after 6 weeks, and decreased after that. This evolution could be due to the degradation of peroxides formed in the first stages of oxidation into secondary oxidation products [45]. At the end of the storage period, the highest peroxide value was found in the control samples, followed by the MDSB2.5 sample. Significantly lower peroxide values were found in BHT-added samples during the storage, as well as in MDSB5 and MDSB5B samples, with no significant differences (\(p<0.05\)) between the peroxide values of these samples. In other words, the carotenoids extracted in oil from 5% tomato by-products lowered the peroxide value of mayonnaise compared to the control sample to an extent equivalent to the addition of 0.1% BHT. Other studies reported also an increase in the oxidation stability of mayonnaise as a result of the addition of natural plant extracts and powders, such as grape seed extract [46], purple maize extract [47], tartary buckwheat hull extract [31], or black glutinous rice extract [48]. In these studies, phenolics were the main compounds responsible for this behavior. In the present study, carotenoids are extracted in the oil from tomato by-products and reach the oily phase of the mayonnaise. Carotenoids are hydrophobic antioxidants that have the ability to quench various radical species [49] and are most likely responsible for increasing the oxidation stability of mayonnaise.