Bioaccessibility of polyphenols and antioxidant capacity of fresh or minimally processed modern or traditional lettuce (Lactuca sativa L.) varieties

Abstract

Modern city lifestyle is characterized by an increased demand for fresh or minimally processed foods. Lettuce (Lactuca sativa L.), mainly iceberg lettuce, is the main vegetable used during the manufacture of fresh-cut salads. The current study evaluated the phenolic content and antioxidant activity of ten fresh and minimally processed lettuce varieties. The phenolic content of selected lettuce samples varied significantly among varieties. Although a higher phenolic content was observed in modern lettuce varieties, when compared to the traditional ones (except for the landrace Francès 219/855), the antioxidant capacity of modern and traditional lettuce varieties was similar. Minimal processing followed by storage for a 7-day period led to an increased phenolic content in varieties Rutilaï RZ, Abago RZ, Maravilla LS044, Francès 219/855, Negre borratger 386/935, and D'hivern LS008, supporting the hypothesis that wounding can induce the accumulation of phenolic compounds in lettuce leaves. For example, the total phenolic content of Francès 219/855 after processing and storage increased from 8.3 to 11.3 mg/100 g (p < 0.05). Accumulation of phenolic compounds after minimal processing was not observed in all the studied samples, suggesting that this effect could be matrix-dependant. The amount of bioaccessible polyphenols was higher after minimal processing and storage. Indeed, the amount of bioaccessible polyphenols after a simulated gastrointestinal digestion of fresh or minimally processed Pelikan lettuce was calculated as 32.6 or 43.3 mg/100 g respectively (p < 0.05), suggesting that the increased amount of polyphenols caused by processing and storage can also lead to a higher amount of bioaccessible phenolic compounds.

Keywords: Antioxidant activity; Bioaccessibility; Lactuca sativa; Lettuce; Minimal processing; Polyphenols.

Fig. 1

Selected modern and traditional lettuce varieties. A variety means a plant grouping within a single botanical taxon of the lowest known rank. Landraces are domesticated local plant varieties grown on farm by farmers who reproduce their seeds year after year. They can be distinguished by specific traits. These varieties have a high capacity to tolerate biotic and abiotic stress, resulting in high yield stability and an intermediate yield level under a low input agricultural system (Zeven 1998). Modern varieties are those obtained after an intensive and deliberated selection during a formal breeding programme (Bitocchi et al. 2009). Variety type refers to a number of physical attributes that are common to different varieties i.e. number of leaves, thickness, undulation of margin, or venation. In the figure, variety types were selected following the guidelines for the conduct of tests for distinctness, uniformity, and stability of the International Union for the Protection of New Varieties of Plants (UPOV) (UPOV 2017)

Fig. 2

Total phenolic content of lettuce varieties at (A) day 0 and (B) after minimal processing followed by a 7-day storage period. TPC, total phenolic content. Initial, Gastric, and Intestinal refer to results obtained for methanolic extracts (initial) and for enzymatic digestive extracts after the gastric or intestinal phase, respectively. Sample A-J refer to Rutilaï RZ, Abago RZ, Muraï RZ, Pelikan, Francesca, Maravilla LS044, Francès 219/855, Negre borratger 386/935, Carxofeta LS007, and D’hivern LS008, respectively. Values represent the mean of three independent experiments (mg/100 g) ± SD. Significant differences between digestive phases are shown in capital letters. Lower case letters indicate differences between varieties at the same digestive phase. The criterion for statistical significance was p < 0.05

Fig. 3

Antioxidant capacity assessed using the FRAP assay of lettuce varieties at (A) day 0 and (B) after minimal processing followed by a 7-day storage period. FRAP, Ferric ion reducing antioxidant power assay. Legends: Initial, Gastric, and Intestinal refer to results obtained for methanolic extracts (initial) and for enzymatic digestive extracts after the gastric or intestinal phase, respectively. Sample A-J refer to Rutilaï RZ, Abago RZ, Muraï RZ, Pelikan, Francesca, Maravilla LS044, Francès 219/855, Negre borratger 386/935, Carxofeta LS007, and D’hivern LS008, respectively. Values represent the mean of three independent experiments (mg/100 g) ± SD. Significant differences between digestive phases are shown in capital letters. Lower case letters indicate differences between varieties at the same digestive phase. The criterion for statistical significance was p < 0.05

Fig. 4

Antioxidant capacity assessed using the DPPH assay of lettuce varieties at (A) day 0 and (B) after minimal processing followed by a 7-day storage period. DPPH, 2,2-diphenyl-1-picrylhydrazyl free radical assay. Initial, Gastric, and Intestinal refer to results obtained for methanolic extracts (initial) and for enzymatic digestive extracts after the gastric or intestinal phase, respectively. Sample A-J refer to Rutilaï RZ, Abago RZ, Muraï RZ, Pelikan, Francesca, Maravilla LS044, Francès 219/855, Negre borratger 386/935, Carxofeta LS007, and D’hivern LS008, respectively. Values represent the mean of three independent experiments (mg/100 g) ± SD. Significant differences between digestive phases are shown in capital letters. Lower case letters indicate differences between varieties at the same digestive phase. The criterion for statistical significance was p < 0.05