Screening of Bioactive Compounds in Lettuce: Multivariate Optimization of an Ultrasound-Assisted Solid–Liquid Extraction Procedure

Lettuce (Lactuca sativa L.) is one of the most consumed vegetables worldwide and is considered a good source of health-promoting compounds. The objective of this work was to apply a multivariate design of experiments through a screening stage followed by optimization of factors that affect the extraction of phytochemicals present in lettuce. A response surface methodology was employed and, specifically, a central composite design model, for the optimization of the methods. The optimal conditions for the extraction of chlorophyll a, b, and total carotenoids were as follows: 4.8 mL of acetone:water (80:20 v/v) and 10 mg of sample with ultrasound-assisted extraction for 5 min. On the other hand, for anthocyanin and phenolic compounds, the optimal conditions were as follows: 5 mL of acetone:acidified water (80:20 v/v) and 20 mg of sample for 5 min with ultrasound-assisted extraction. The experimental model was followed by spectrophotometry methodology for a comprehensive characterization. Optimal conditions were applied for the simultaneous extraction and determination of the above-mentioned compounds in different lettuce cultivars. Both extraction methods successfully extract different phytochemicals by reducing the volume of solvent, sample mass, and extraction time compared to previously reported methods. The results obtained showed a significant variability in the content of phytochemicals; therefore, it could be estimated that the different cultivars of lettuce evaluated will exhibit different beneficial health effects.