Apple Pomace as a Potential Source of Oxidative Stress-Protecting Dihydrochalcones

Abstract

Among fruits, the apple is unique for producing large amounts of the dihydrochalcone phloridzin, which, together with phloretin, its aglycone, is valuable to the pharmaceutical and food industries for its antidiabetic, antioxidant, and anticarcinogenic properties, as well as its use as a sweetener. We analysed the phloridzin concentration, total phenolic content, and antioxidant activity in the peel, flesh, seeds, juice, and pomace of 13 international and local apple varieties. In the unprocessed fruit, the seeds had the highest phloridzin content, while the highest total phenolic contents were mostly found in the peel. In processed samples, phloridzin and the total phenolic compounds especially were higher mostly in juice than in pomace. Moreover, the total phenolic content was much higher than the phloridzin content. Juice showed the highest antioxidant activity, followed by the peel and flesh. Across all samples, antioxidant activity did not directly correlate with phloridzin concentrations, suggesting that the antioxidant activity ascribed to phloridzin may need re-evaluation. In the Ferric Reducing Antioxidant Power (FRAP) assay, phloridzin only showed antioxidant activity at high concentrations when compared to its aglycone, phloretin. Considering the large amounts of apple juice produced by the juice industry, residual pomace is a promising source of phloridzin. For technical use, processing this phloridzin to phloretin would be advantageous.

Keywords: Ferric Reducing Antioxidant Power (FRAP); Malus × domestica; antioxidant activity; apple; dihydrochalcones; juice processing; phloridzin; polyphenols; pomace.

Figure 1

Chemical structure of phloridzin (A) and neohesperidin (B).

Figure 2

Phloridzin content of apple. Because of the large concentration differences, results are presented separately for peel and flesh (A), and for seeds (B). Results are shown as g phloridzin per kg dry weight (DW). The HPLC measurements were performed in triplicates. Different letters (a, b, c, etc.) between cultivars indicate statistically significant differences (p < 0.05).

Figure 3

Total phenolic content (TPC) of apple fruit parts peel, flesh, and seeds. The measurements were performed in triplicates. Results are shown as g gallic acid equivalent per kg of dry weight (DW). Different letters (a, b, c) between cultivars indicate statistically significant differences (p < 0.05).

Figure 4

Illustration showing the percentage of weight and phloridzin for each part of a dried apple (seeds, flesh, peel), as well as the phloridzin range for each. (Illustration created with BioRender.com).

Figure 5

(A) Phloridzin content of the processed juice samples (treated and untreated) and pomace. Results are shown as g phloridzin per kg dry weight (DW); (B) total phenolic content for processed juice (treated and untreated) and pomace. Results are shown as g gallic acid equivalents per kg dry weight (DW). The HPLC measurements were performed in triplicates. Different letters (a, b, c, etc.) between cultivars indicate statistically significant differences (p < 0.05).

Figure 6

Ferric Reducing Antioxidant Power (FRAP): (A) processed juice samples (treated and untreated) and pomace; (B) apple fruit parts peel, flesh and seeds. Results are shown as g ascorbic acid equivalents per kg dry weight (DW) of extract or juice. FRAP measurements were performed in triplicates. Different letters (a, b, c, etc.) between cultivars indicate statistically significant differences (p < 0.05).

Figure 7

Juxtaposition of antioxidant activity (FRAP) and the content of (A) phloridzin and (B) total phenolic compounds. Individual graphs for seeds, peels, flesh, pomace, and juices are available in the Supplementary Figures S1 and S2.

Figure 8

FRAP assay of phloridzin and phloretin standard compounds. FRAP measurements were performed in triplicates. Results are shown as µmol ascorbic acid equivalents per litre.