Valorization of Tomato Processing by-Products: Fatty Acid Extraction and Production of Bio-Based Materials

Abstract

A method consisting of the alkaline hydrolysis of tomato pomace by-products has been optimized to obtain a mixture of unsaturated and polyhydroxylated fatty acids as well as a non-hydrolysable secondary residue. Reaction rates and the activation energy of the hydrolysis were calculated to reduce costs associated with chemicals and energy consumption. Lipid and non-hydrolysable fractions were chemically (infrared (IR) spectroscopy, gas chromatography/mass spectrometry (GC-MS)) and thermally (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)) characterized. In addition, the fatty acid mixture was used to produce cutin-based polyesters. Freestanding films were prepared by non-catalyzed melt-polycondensation and characterized by Attenuated Total Reflected-Fourier Transform Infrared (ATR-FTIR) spectroscopy, solid-state nuclear magnetic resonance (NMR), DSC, TGA, Water Contact Angles (WCA), and tensile tests. These bio-based polymers were hydrophobic, insoluble, infusible, and thermally stable, their physical properties being tunable by controlling the presence of unsaturated fatty acids and oxygen in the reaction. The participation of an oxidative crosslinking side reaction is proposed to be responsible for such modifications.

Keywords: agricultural by-product; bio-based polymers; fatty acids; tomato pomace; valorization.

Figure 1

(A) Tomato pomace conditioning and processing to isolate the non-hydrolysable fraction (Rx) and the monomeric mixture (Hx). (B) Picture of a typical cutin-based film. (C) Infrared spectra of fibers, seeds, skins, tomato cuticle, and tomato pomace. (D) Recovery of the non-hydrolysable (Rx, empty symbols) and monomer mixture (Hx, filled symbols) in the alkaline hydrolysis of tomato pomace as a function of temperature and reaction time. The inset shows the ln k versus 1/T plot that allows the calculation of the activation energy.

Figure 2

(A) DSC of isolate tomato pomace constituents and their hydrolysable fractions (Hx). (B) TGA of Hx fractions.

Figure 3

(A) ATR-FTIR spectra of polymer films obtained from Hx pomace (poly Hx pomace) at 150 °C in air at the indicated reaction time. (B) Solid-state NMR spectra of polymers obtained from Hx pomace (black) and Hx skin (orange). The regions ascribed to –COOH, –CH–OH, –CH–O–, –CH₂–OH, and –CH₂–O– of Hx monomers (blue) are also included. (C–E) Evolution of the water contact angle, glass transition temperature, and tensile parameters, respectively, with the reaction time of the synthesis of poly Hx pomace in air. Inset in C shows a typical drop on a surface of poly Hx pomace after 20 h of reaction. DSC (inset of D) and stress-strain (inset of E) curves correspond to polymers obtained after: (blue) 3 h, (green) 6 h, (orange) 10 h, and (red) 16 h reaction.