Thin-Layer, Intermittent, Near-Infrared Drying of Two-Phase Olive Pomace: Mathematical Modeling and Effect on Recovery of Bioactive Compounds and Antioxidant Activity

Abstract

The present study examined the drying kinetics of two-phase olive pomace (OP) using near-infrared (NIR) thin layer intermittent drying at 70-140 °C. For the first time, this approach was combined with color, bioactive compound retention and antioxidant activity assessment. Among tested models, the Midilli's semi-empirical model best described the drying behavior (r² ≥ 0.99839, RMSE ≤ 0.01349). Effective diffusivity ranged from 1.417 × 10^-9 to 5.807 × 10^-9 m²/s, and activation energy was calculated at 23.732 kJ/mol. Drying at 140 °C reduced time by 68% compared to 70 °C. The corresponding sample had the highest total phenolics content, antioxidant activity (DPPH^●, CUPRAC assays) and triterpenic acid (maslinic, oleanolic) content, and a significant amount of hydroxytyrosol, despite the increased sample browning. Compared to oven-drying (140 °C), NIR was equal or better and 3.2-fold faster. The same was evidenced compared to freeze-drying, except for tyrosol recovery (1.2-fold lower in NIR). These findings were obtained using two different OP industrial samples. Given that NIR is already used industrially for food drying, the present study offers proof-of-concept for its application as a rapid and eco-friendly pretreatment of OP for food and feed uses. However, scalability challenges and the limitations of semi-empirical modeling must be addressed in the future to support industrial-scale implementation.

Keywords: antioxidants; bioactives; drying kinetics; hydroxytyrosol; mathematical modeling; near-infrared drying; olive pomace.

Figure 1

Experimental drying curves of olive pomace at five studied temperatures. Plots represent average of three measurements.

Figure 2

Experimental drying speed curves of olive pomace at five studied temperatures. Plots represent average of three measurements.

Figure 3

Plot of ln(MR) against drying time at studied temperatures (solid lines). Linear trendlines (dotted lines), their equation and R² values are displayed for each curve. Plots represent average of three measurements.

Figure 4

Arrhenius-type relationship between effective diffusivity (D_eff) and temperature (T, K).

Figure 5

(A) Total phenolics content (TPC) and (B) antioxidant activity (AA), using DPPH^● and CUPRAC assays, of olive pomace (OP1) dried at five different temperatures (70, 90, 100, 120, 140 °C). Results are expressed as mean ± standard deviation values (n = 3). Same letters above bars indicate that no significant difference was found at (p < 0.05) applying Games–Howell post hoc test.

Figure 6

Main phenolic compounds (hydroxytyrosol and tyrosol) and triterpenic acids (maslinic and oleanolic acids) of olive pomace (OP1) dried at five different temperatures (70, 90, 100, 120, 140 °C). Results are expressed as mean ± standard deviation values (n = 3). Different letters above bars indicate a significant difference (p < 0.05) applying Games–Howell post hoc test.

Figure 7

Total phenolics content (TPC) of olive pomace samples (OP1, OP2) dried with infrared radiation at 140 °C (IR), oven heating at 140 °C (OD) and freeze-drier (FD). Results are expressed as mean ± standard deviation values (n = 3). Different lowercase and capital letters above bars indicate a significant difference (p < 0.05) for OP1 and OP2, respectively, applying Games–Howell post hoc test.

Figure 8

Antioxidant activity (AA) of olive pomace samples (OP1, OP2), measured by DPPH● (A) and CUPRAC (B) assays and dried with infrared radiation at 140 °C (IR), oven heating at 140 °C (OD) and freeze-drier (FD). Results are expressed as mean ± standard deviation values (n = 3). Different lowercase and capital letters above bars indicate a significant difference (p < 0.05 for OP1 and OP2, respectively, applying Games–Howell post hoc test.

Figure 9

Main phenolic compounds (hydroxytyrosol, A and tyrosol, B) and triterpenic acids (maslinic, C and oleanolic, D acids) of olive pomace samples (OP1, OP2) dried with infrared radiation at 140 °C (IR), oven heating at 140 °C (OD) and freeze-drier (FD). Results are expressed as mean ± standard deviation values (n = 3). Different lowercase and capital letters above bars indicate a significant difference (p < 0.05) for OP1 and OP2, respectively, applying Games–Howell post hoc test.