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. 2024 Apr 17;13(8):1230.
doi: 10.3390/foods13081230.

Sustainable Valorization of Coffee Silverskin: Extraction of Phenolic Compounds and Proteins for Enzymatic Production of Bioactive Peptides

Wilasinee Jirarat  1 , Tanyawat Kaewsalud  1 , Kamon Yakul  2 , Pornchai Rachtanapun  3 , Thanongsak Chaiyaso  2
Affiliations

Sustainable Valorization of Coffee Silverskin: Extraction of Phenolic Compounds and Proteins for Enzymatic Production of Bioactive Peptides

Wilasinee Jirarat et al. Foods. .

Abstract

Coffee silverskin (CS), a by-product of the coffee roasting process, has high protein content (16.2-19.0%, w/w), making it a potential source for plant protein and bioactive peptide production. This study aims to develop innovative extraction methods for phenolic compounds and proteins from CS. The conditions for hydrothermal (HT) extraction of phenolic compounds from CS were optimized by varying CS loading (2.5-10%, w/v), temperature (110-130 °C), and time (5-30 min) using a one-factor-at-a-time (OFAT) approach. The highest TPC of 55.59 ± 0.12 µmole GAE/g CS was achieved at 5.0% (w/v) CS loading and autoclaving at 125 °C for 25 min. Following hydrothermal extraction, CS protein was extracted from HT-extracted solid fraction by microwave-assisted alkaline extraction (MAE) using 0.2 M NaOH at 90 W for 2 min, resulting in a protein recovery of 12.19 ± 0.39 mg/g CS. The CS protein was then subjected to enzymatic hydrolysis using protease from Bacillus halodurans SE5 (protease_SE5). Protease_SE5-derived CS protein hydrolysate had a peptide concentration of 0.73 ± 0.09 mg/mL, with ABTS, DPPH, and FRAP values of 15.71 ± 0.10, 16.63 ± 0.061, and 6.48 ± 0.01 µmole TE/mL, respectively. Peptide identification by LC-MS/MS revealed several promising biological activities without toxicity or allergenicity concerns. This study's integrated approach offers a sustainable and efficient method for extracting valuable compounds from CS, with potential applications in the food and pharmaceutical industries.

Keywords: bioactive compounds; coffee silverskin valorization; hydrothermal extraction; microwave-assisted alkaline extraction; phenolic compounds; protein hydrolysis; ultrasound-assisted alkaline extraction.

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Conflict of interest statement

The author declares no conflicts of interest.

Figures

Figure 1
Figure 1
The antioxidant activity and TPC of the HT-extracted CS liquid fraction. The effects of (a) CS loading (% w/v), (b) temperature (°C), and (c) time (min) on antioxidant activity and TPC were investigated. The experiments are performed in triplicate (n = 3). The results are reported as mean  ±  SD. Different letters (a–e) indicate significant differences at p  <  0.05 according to analysis by Duncan’s multiple range test.
Figure 2
Figure 2
Enzymatic production of bioactive peptides from CS. Protein recovery, decrease in protein concentration, and increase in peptide concentration by CAE at 50 °C for 240 min (a) and 90 °C for 30 min (b). The increase in antioxidant activity of CS protein hydrolysate derived from CAE at 50 °C for 240 min (c) and 90 °C for 30 min (d). The protein from CS was extracted by CAE, followed by enzymatic hydrolysis using protease_SE5 (200,000 U/g protein) at pH 9.5 and 55 °C for 12 h. Different letters (a–e) indicate significant differences at p  <  0.05 according to analysis by Duncan’s multiple range test.
Figure 3
Figure 3
The molecular weight distribution of CS protein hydrolysate by tris-tricine SDS-PAGE analysis. Electrophoresis was carried out under denaturing conditions in 16.5% polyacrylamide gel. The CS protein was extracted by 0.2 M NaOH at 50 °C for 240 min (CAE), ultrasound-assisted alkaline extraction (UAE) for 10 min, and microwave-assisted alkaline extraction (MAE) at 90 W for 2 min. The enzymatic hydrolysis was performed at pH 9.5 and 55 °C for 12 h using protease_SE5 (200,000 U/g protein).
Figure 4
Figure 4
Peptide concentration and antioxidant activity of CS protein hydrolysate. The antioxidant activity of CS protein hydrolysate by ABTS, DPPH, and FRAP assay. The CS protein was extracted by MAE at 90 W for 2 min using 0.2 M NaOH and subjected to hydrolysis by either protease_SE5 or Alcalase (200,000 U/g protein) at pH 9.5 and 55 °C for 12 h.
Figure 5
Figure 5
Size exclusion chromatography of <3 kDa from protease_SE5 (a) and Alcalase (b). Each fraction (2.5 mL) was eluted by deionized water at a flow rate of 23 mL/h. The active fractions were lyophilized before analysis by LC-MS/MS and de novo peptide sequencing. Antioxidant activity of active fractions from protease_SE5 (c) and Alcalase (d) obtained from the Sephadex G-25 column. The experiments are performed in triplicate (n = 3). Different letters (a–d) indicate significant differences at p  <  0.05 according to analysis by Duncan’s multiple range test.
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