# Valorization of Coffee Silverskin through Subcritical Water Extraction: An Optimization Based on T-CQA Using Response Surface Methodology

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Chemicals and Reagents

_{14}H

_{18}O

_{4}), 4-(dimethylamino) benzaldehyde (4-[(CH

_{3})

_{2}N] C

_{6}H

_{4}CHO), 5-methylphenazinium methyl sulfate (C

_{13}H

_{11}N

_{2}·CH

_{3}SO

_{4}), and 7,8-dimethyl-10-[(2S,3S,4R)-2,3,4,5-tetrahydroxypentyl]benzo[g]pteridine-2,4-dione (riboflavin, C

_{17}H

_{20}N

_{4}O

_{6}), were purchased from Sigma Aldrich (Singapore). 1,1-diphenyl-2-picrylhydrazyl free radical (>97% DPPH assay, C

_{18}H

_{12}N

_{5}O

_{6}) and 5-O-(trans-3,4-dihydroxycinnamoyl)-D-quinic acid (neochlorogenic acid, C

_{16}H

_{18}O

_{9}) were purchased from Tokyo Chemical Industry (Tokyo, Japan). 3-(3,4-Dihydroxycinnamoyl quinic acid (chlorogenic acid, C

_{16}H

_{18}O

_{9}) was purchased from Alfa Aesar Thermo Fisher Scientific Chemicals (Waltham, MA, USA). The solvents used for the chromatographic measurement of the target compounds were of HPLC grade. All other chemicals were of analytical grade.

#### 2.3. Extraction Procedure

^{®}, Maidstone, UK, qualitative filter paper, Grade 1). The extract after the filtration step was used as a sample for the quantification of TPC and DPPH antioxidant activities. The residues were dried at 103 ± 2 °C for 3 h in an oven for further reaction mass efficiency analysis. During the concentrating step, the filtrate was evaporated by using a rotary evaporator (Bushi, Zug, Switzerland). Then, the extracts were frozen at −80 °C in a prefreezer (Daihan Scientific, Kangwon-do, South Korea) and freeze-dried at −80 °C with a freeze drier (Gold Sim Cellular Science, Miami, FL, USA) [35]. The dried crude extract was lyophilized and used as the sample for further analysis.

#### 2.4. Experimental Design

_{0}, β

_{i}, β

_{ii}, and β

_{ij}are the regression coefficients for the intercept, linear, quadratic, and interaction terms, respectively; and X

_{i}and X

_{j}are the independent variables.

#### 2.5. Statistical Analysis

^{2}, adjusted R

^{2}, and the lack of fit. A p-value < 0.05 was used to determine whether the observed difference was statistically significant.

#### 2.6. Validation of the Optimized Conditions

#### 2.7. Measurement of the Total Phenolic Content

#### 2.8. Measurement of Antioxidant Activity

_{0}is the absorbance of DPPH only and A

_{1}is the absorbance of DPPH with a tested sample or control (Trolox solution).

#### 2.9. Quantitative Analysis of Total Chlorogenic Acid by HPLC-UV

## 3. Results

_{1}(temperature in °C), X

_{2}(time, min), and X

_{3}(solid-to-liquid ratio, unitless), which represent total phenolic content (TPC), antioxidant activity (AA), and total chlorogenic acid (T-CQA), respectively. First, a preliminary assessment of the model adequacy was carried out as presented in supplementary data Figure S1 (for TPC), Figure S2 (for AA), and Figure S3 (for T-CQA). It is recommended that a straight line of residuals should be obtained. This study revealed that all dependent variables (TPC, AA, and T-CQA) exhibit normal distributions. A long-tail distribution was detected for all four variables. No outliers were observed. The results for the residuals against fitted values and run orders revealed that they are independent of one another, have a random distribution, and have constant variance. These results, therefore, suggest that the data are appropriate and can be used to establish a model that can efficiently explain the effects of extraction conditions for obtaining TPC, AA, and T-CQA from coffee silverskin.

#### 3.1. Effect of Extraction Conditions on the TPC

_{1}), S/L ratio (X

_{3}), the square terms of time (X

_{2}

^{2}) and S/L ratio (X

_{3}

^{2}), and the interaction of temperature and time (X

_{1}X

_{2}) and time and S/L ratio (X

_{2}X

_{3}). The TPC is significantly enhanced when the extraction temperature is raised from 120 to 240 °C at the same extraction time and S/L ratio (runs 1 and 11, 3 and 9, 6 and 13, and 7 and 15). Considering the extraction time, the results reveal an insignificant effect on the TPC (p = 0.412) when using the same temperature and S/L ratio (runs 1 and 6, 4 and 14, 10 and 12, and 11 and 13). Regardless of the extraction temperature and time, TPC is enhanced when the S/L ratio is increased from 1:10 to 1:40 (runs 3 and 15, 4 and 10, 7 and 9, and 12 and 14). This reveals that the S/L ratio significantly affects the TPC in the CS extracts (p = 0.001). The magnitude of the influence of each factor was also studied. Figure 2 shows significant terms and their interactions that significantly affect the responses (i.e., TPC, AA, and T-CQA). The vertical line defines the t-value at a 95% confidence interval. The terms that pass through the line indicate that they significantly affect the response in the CS extracts, while the terms below the line are considered insignificant factors. The results confirm that the extraction temperature (A) has the highest standardized effect on the TPC. In contrast, the individual effect of extraction time is insignificant on the TPC. The descending order for the influencing factors on the TPC is given as follows: extraction temperature (A) > square term of the S/L ratio (CC) > S/L ratio (C) > interaction between extraction time and S/L ratio (BC) > interaction between extraction temperature and time (AB) > square term of the extraction time (BB).

#### 3.2. Effect of Extraction Conditions on the AA

_{3}

^{2}), and the interaction of temperature and S/L ratio (X

_{1}X

_{3}). The magnitude of the influence of each factor is shown in Figure 2. The factors that significantly influence the amount of antioxidant content (AA) follow the order of extraction temperature (A) > S/L ratio (C) > square term of the S/L ratio (CC) > the interaction between extraction temperatures and S/L ratios (AC). In addition, it was noted that the extraction time (10 to 60 min) does not affect the AA content in the CS extract.

#### 3.3. Effect of Extraction Conditions on the T-CQA

_{1}), time (X

_{2}), the square term of the temperature (X

_{1}

^{2}), and the interaction of temperature and S/L ratio (X

_{1}X

_{3}). When operating under the same temperature and extraction time (runs 3 and 15, 4 and 10, 7 and 9, and 12 and 14), T-CQA does not vary greatly when using the S/L ratio of 1:10 to 1:40. This confirms that the S/L ratio alone insignificantly affects (p = 0.6724) the T-CQA in the CS extract. The results also reveal that the use of a prolonged extraction time at each temperature (120, 180, and 240 °C) results in a decrease in the T-CQA content. The T-CQA is significantly different (runs 1 and 6, 4 and 14, 10 and 12, and 11 and 13) with distinct values for the extraction times at the same temperature and S/L ratio. Apart from temperature, T-CQA is not significantly different when the S/L ratio is increased from 1:10 to 1:40 when using the same extraction temperature and time (runs 3 and 15, 4 and 10, 7 and 9, and 12 and 14). This confirms that the S/L ratio alone insignificantly affects the T-CQA (p = 0.67). However, the interactions between the extraction temperature and the S/L ratio (X

_{1}X

_{3}) significantly affect the T-CQA content (p = 0.001). These variables can positively affect the modifying solubility and equilibrium constants, which subsequently enhance caffeoylquinic acid content [33]. A sufficiently high temperature and S/L ratio can enhance the desorption of solutes (i.e., T-CQA) from the CS matrix, diffusion of subcritical water into the CS matrix, dissolution of the T-CQA into the subcritical water, and elution of the extract from the CS matrix. In addition, a prolonged extraction time at each temperature (120, 180, and 240 °C) leads to a decrease in the T-CQA content. This is probably due to the oxidation and degradation of T-CQA. The T-CQA content is significantly different (runs 1 and 6, 4 and 14, 10 and 12, and 11 and 13) for different extraction times when operating at the same extraction temperature and S/L ratio. As shown in Figure 2, the Pareto chart reveals that the factors influencing the amount of T-CQA follow the order of AA (Temperature*Temperature) > AC (Temperature*S/L ratio) > A (Temperature) > B (Time).

#### 3.4. Regression Modeling of the SWE Conditions on TPC, AA, and T-CQA

^{2}), adjusted R

^{2}(R

^{2}(adj)), and predicted R

^{2}(R

^{2}(pred)). As presented in Table 3, the values of R

^{2}(pred) for the TPC, AA, and T-CQA are in agreement with R

^{2}(adj). For a value higher than 75%, the regression model can predict the designated response very well [36,38,39,40,42]. A multiple regression analysis was implemented to determine the coefficients and equations, which can be used to estimate the responses (i.e., TPC, AA, and T-CQA). As shown in Table 4, the R

^{2}for TPC, AA, and T-CQA was 89.64%, 96.41%, and 93.70%, respectively. Both the R

^{2}and adjusted R

^{2}values were greater than 80%. This confirms that the model exhibits precise prediction for the designated responses [40,41,42].

#### 3.5. Optimal Extraction Conditions and Model Validation

## 4. Discussion

## 5. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Pareto chart for the standardized effects on TPC, AA, and T-CQA obtained from 2

^{k}factorial designs.

**Figure 3.**Response surface plots for the effects of the mutual interactions of independent variables on the TPC, AA, and T-CQA.

Symbol | Factor | Coded and Uncoded Level | ||
---|---|---|---|---|

−1 | 0 | +1 | ||

X_{1} | Temperature (°C) | 120 | 180 | 240 |

X_{2} | Time (minutes) | 10 | 35 | 60 |

X_{3} | Solid-to-liquid ratio (g/mL) | 10 | 25 | 40 |

**Table 2.**BBD matrix with independent variables and observed values for the dependent variables (n = 3).

Run Order | Block | Independent Variables | Dependent Variables | |||||||
---|---|---|---|---|---|---|---|---|---|---|

X_{1} | X_{2} | X_{3} | Y_{1} | Y_{2} | Y_{3} | |||||

Coded | Uncoded | Coded | Uncoded | Coded | Uncoded | |||||

1 | 1 | 1 | 240 | −1 | 10 | 0 | 1:25 | 93.83 ± 6.11 | 26.12 ± 3.27 | 0.99 ± 0.26 |

2 | 1 | 0 | 180 | 0 | 35 | 0 | 1:25 | 70.05 ± 5.06 | 17.54 ± 2.28 | 1.66 ± 0.19 |

3 | 1 | −1 | 120 | 0 | 35 | −1 | 1:10 | 23.70 ± 2.10 | 5.71 ± 1.90 | 2.01 ± 0.23 |

4 | 1 | 0 | 180 | −1 | 10 | 1 | 1:40 | 65.06± 4.58 | 14.89 ± 0.85 | 2.33 ± 0.32 |

5 | 1 | 0 | 180 | 0 | 35 | 0 | 1:25 | 69.96 ± 3.25 | 17.44 ± 1.66 | 1.81 ± 0.21 |

6 | 1 | 1 | 240 | 1 | 60 | 0 | 1:25 | 75.03 ± 3.82 | 21.52 ± 2.15 | 0.50 ± 0.13 |

7 | 1 | 1 | 240 | 0 | 35 | 1 | 1:40 | 91.97 ± 4.15 | 24.94 ± 3.87 | 0.99 ± 0.27 |

8 | 1 | 0 | 180 | 0 | 35 | 0 | 1:25 | 72.81 ± 3.25 | 17.42 ± 2.48 | 1.62 ± 0.19 |

9 | 1 | 1 | 240 | 0 | 35 | −1 | 1:10 | 89.29 ± 4.67 | 19.742 ± 2.87 | 0.48 ± 0.15 |

10 | 1 | 0 | 180 | −1 | 10 | −1 | 1:10 | 42.65 ± 3.00 | 9.98 ± 1.39 | 2.04 ± 0.32 |

11 | 1 | −1 | 120 | −1 | 10 | 0 | 1:25 | 19.01 ± 3.54 | 4.98 ± 1.28 | 1.66 ± 0.21 |

12 | 1 | 0 | 180 | 1 | 60 | −1 | 1:10 | 77.69 ± 9.24 | 18.44 ± 2.26 | 1.69 ± 0.19 |

13 | 1 | −1 | 120 | 1 | 60 | 0 | 1:25 | 24.00 ± 2.36 | 6.65 ± 1.93 | 1.25 ± 0.33 |

14 | 1 | 0 | 180 | 1 | 60 | 1 | 1:40 | 76.24 ± 3.38 | 19.20 ± 2.09 | 1.49 ± 0.21 |

15 | 1 | −1 | 120 | 0 | 35 | 1 | 1:40 | 19.09 ± 2.29 | 4.15 ± 0.43 | 0.61 ± 0.41 |

_{1}= Extraction temperature (°C), X

_{2}= Extraction time (min), X

_{3}= S/L ratio, Y

_{1}= TPC (mg GAE/g CS), Y

_{2}= AA (mg TE/g CS), and Y

_{3}= T-CQA (mg/g CS).

**Table 3.**p-Values for the significant terms in the polynomial regression equations and the coefficients in the final model.

Parameters | p-Values for Each Parameter in the Polynomial Regression Equations | ||
---|---|---|---|

TPC | AA | T-CQA | |

Model | 0.000 | 0.000 | 0.000 |

Linear | 0.000 | 0.000 | 0.001 |

Temperature (X_{1}) | 0.000 | 0.000 | 0.001 |

Time (X_{2}) | 0.457 | - | 0.003 |

S/L Ratio (X_{3}) | 0.000 | 0.000 | 0.160 |

Square | 0.000 | 0.000 | 0.000 |

Temperature × Temperature (X_{1}^{2}) | - | - | 0.000 |

Time × Time (X_{2}^{2}) | 0.044 | - | - |

S/L Ratio × S/L Ratio (X_{3}^{2}) | 0.000 | 0.000 | - |

2-Way Interaction | 0.010 | 0.009 | 0.001 |

Temperature × Time (X_{1}X_{2}) | 0.036 | - | - |

Temperature × S/L Ratio (X_{1}X_{3}) | - | 0.009 | 0.001 |

Time × S/L Ratio (X_{2}X_{3}) | 0.010 | - | - |

Lack of Fit | 0.169 | 0.097 | 0.213 |

Model Summary | |||

R^{2} (adjusted) | 87.91% | 94.97% | 90.21% |

R^{2} (predicted) | 84.94% | 90.07% | 78.07% |

R^{2} | 89.64% | 96.41% | 93.70% |

Responses | Polynomial Regression Equations | R^{2} | R^{2} (Adjusted) | Lack of Fit |
---|---|---|---|---|

TPC | Y_{1} = −126.7 + 0.5457 X_{1} + 1.989 X_{2} + 6.063 X_{3} − 0.01056 X_{2}^{2} − 0.0884 X_{3}^{2} − 0.00448 X_{1}X_{2} − 0.02419 X_{2}X_{3} | 89.64% | 87.91% | 0.169 |

AA | Y_{2} = −10.28 + 0.0441 X_{1} + 0.805 X_{3} − 0.02022 X_{3}^{2} + 0.002586 X_{1}X_{3} | 96.41% | 94.97% | 0.097 |

T-CQA | Y_{3} = −1.019 + 0.05592 X_{1} − 0.01043 X_{2} − 0.1023 X_{3}− 0.000207 X_{1}^{2} + 0.000532 X_{1}X_{3} | 93.70% | 90.21% | 0.213 |

**Table 5.**Predicted and actual response values for T-CQA, and the experimental values of the TPC, AA, and extraction yield obtained from the optimized extraction parameters.

T-CQA (mg CQA/g CS) | TPC (mg GAE/g CS) | AA (mg TE/g CS) | Yield (%) | |
---|---|---|---|---|

Predicted value | Experimental value | Experimental value | Experimental value | Experimental value |

2.38 | 2.70 ± 0.23 | 51.86 ± 5.98 | 13.72 ± 2.04 | 27.25 ± 1.57 |

Raw Material | Extraction Method/Conditions | TPC | Antioxidant Activity | T-CQA (Based on 3-, 4-, and 5-CQA) | References |
---|---|---|---|---|---|

(Based on DPPH Assay) | |||||

Coffee silverskin | Subcritical water | 51.86 mg GAE/g CS | 13.72 | 2.70 mg/g CS | This study |

Ratio 1: 10 (w/v) | |||||

Time 10 min | |||||

Coffee silverskin | Solid–liquid extraction | 22 mg GAE/g CS | 11.5 mg TE/g CS | 1.5 mg 5-CQA/g CS | [18] |

Subcritical water | |||||

Ratio 1:50 (w/v) | |||||

Temperature 80 °C | |||||

Time 60 min | |||||

Coffee silverskin | Solid–liquid extraction (Soxhlet) | 13.2 | 5.3 | 3 mg/g CS | [46] |

60% Isopropanol | |||||

Ratio 1:10 (w/v) | |||||

Temperature 27 °C | |||||

Time 20 min | |||||

Coffee silverskin | Solid–liquid extraction | 16.1 | n.a. | 4.3 mg/g CS | [14] |

(Maceration) | |||||

1% formic acid | |||||

Ratio 1:10 (w/v) | |||||

Time 90 min | |||||

Coffee silverskin | Solid–liquid extraction | 12.81 | 4.49 | n.a. | [17] |

(Waterbath) | |||||

60% Ethanol | |||||

Ratio 1:35 (w/v) | |||||

Temperature 60–65 °C | |||||

Time 30 min | |||||

Coffee silverskin | Solid–liquid extraction | 22.2 | 13.9 | n.a. | [35] |

Mild hydrothermal pretreatment | |||||

Ratio 1:30 (w/v) | |||||

Temperature 120 °C | |||||

Time 20 min |

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## Share and Cite

**MDPI and ACS Style**

Ginting, A.R.; Kit, T.; Mingvanish, W.; Thanasupsin, S.P.
Valorization of Coffee Silverskin through Subcritical Water Extraction: An Optimization Based on T-CQA Using Response Surface Methodology. *Sustainability* **2022**, *14*, 8435.
https://doi.org/10.3390/su14148435

**AMA Style**

Ginting AR, Kit T, Mingvanish W, Thanasupsin SP.
Valorization of Coffee Silverskin through Subcritical Water Extraction: An Optimization Based on T-CQA Using Response Surface Methodology. *Sustainability*. 2022; 14(14):8435.
https://doi.org/10.3390/su14148435

**Chicago/Turabian Style**

Ginting, Agita Rachmala, Thavy Kit, Withawat Mingvanish, and Sudtida Pliankarom Thanasupsin.
2022. "Valorization of Coffee Silverskin through Subcritical Water Extraction: An Optimization Based on T-CQA Using Response Surface Methodology" *Sustainability* 14, no. 14: 8435.
https://doi.org/10.3390/su14148435