Ohmic-heating treatment as a new process for development of functional ingredients through valorisation of tomato and grape by-products

Student thesis: Doctoral Thesis


Tomato and wine production are the main agro-industrial sectors in Portugal. These activities produce and accumulate large amounts of waste, especially seeds, peels and pulp residues, with high economic and environmental concerns worldwide. These residues are typically rich in bioactive compounds (BC), such as proteins, sugars and lipids, and phenolic compounds (PC) and carotenoids. Therefore, they can be an economical source of high added value, with potential food, cosmetic, or pharmaceutical industry applications. However, current extraction methods (e.g. extraction with chemical solvents) and environmental hazards can also degrade BC and promote toxicity, reducing biological properties and health benefits, making it difficult to use as value-added products. The main objective of this thesis was to explore the value of grape and tomato bagasse to promote the principle of sustainable development, with the achievement of ingredients of added value and zero waste. Thus, an optimisation study of BC extraction from these by-products was carried out, using ohmic heating (OH) method and solvents used in the food industry, to ensure the maintenance of compositional integrity, functionality and safety of bioactive extracts obtained and compared with the traditional method in which it is used organic solvents (CONV). Initially, a group of grape by-products was used, namely stalk and bagasse samples from white and red wine and tomato bagasse, obtained from the processing industry. The compositional characterisation and phytochemical properties of the byproducts were carried out to verify the potential of the matrix to be used and to define the recovery strategy. Grape bagasse presented higher protein, carbohydrate, and BC content than stalks, mainly characterised by high fibre content. Concerning tomato bagasse, samples of the same Heinz tomato cultivar from two different industries were used, and the results were compared. Both samples presented protein contents between 16.3 and 19.4 g / 100 g DW; fibre content between 57.8 and 59.0 g / 100 g DW, mainly polyunsaturated, linoleic acid, oleic and palmitic acid. After complete characterisation, the grape and tomato bagasses were selected as the most promising byproducts and OH was applied to extract the BC and reduce the environmental impacts based on a circular economy strategy. As solvents, water was used alone or combined with ethanol to promote the extraction of more lipophilic compounds, such as carotenoids. In parallel, a CONV method was used to compare the results. After extraction, two different fractions were obtained: the liquid fraction (LF) rich in BC solubilised and the remnant, the solid fraction (SF) with relevant nutritional and functional potential. In the case of bagasse, LF did not present significant differences (p>0.05) between CONV and OH for total phenolic compounds (TPC), 2.84 ± 0.037 and 3.28 ± 0.46 mg / g DW equivalent of gallic acid, respectively. The same trend was found for antioxidant activity (AA), where CONV and OH presented values of 2.02 ± 0.007 g / 100 g and 2.34 ± 0.066 g / 100 g ascorbic acid equivalent, respectively. The main anthocyanins identified were malvidin-3-O-acetylglucoside, delphinidin-3-Oglucoside, petunidin-3-O-glucoside. These extracts exhibited antimicrobial potential against the microorganisms Yersinia enterocolitica, Pseudomonas aeruginosa, Salmonella Enteritidis, Methicillin-sensitive Staphylococcus aureus, methicillin-resistant S. aureus (MRSA) and Bacillus cereus. SFOH showed a composition with potential functional ingredients, showing higher proteins and BC bound to fibre than CONV. Regarding tomato bagasse, a LF rich in carotenoids, polyphenols, and sugars and a SF rich in fibres linked to polyphenols and carotenoids were obtained. Also, new molecules were identified by UPLC-qTOF-MS analysis, such as phene-di-hexane and N-acetyl-Dtryptophan. After first evaluating the feasibility of applying OH, an experimental design was made to obtain better yields. Different moderate electric fields (MEF) of different intensities (i.e., 4, 6 and 11 V.cm-1) were also used to identify non-thermal effects in the extraction process and their influence on the bioactive properties of the extracted compounds. BC extraction using OH was successfully optimised with the best extraction conditions corresponding to 70 ºC for 15 min using 70% ethanol as a solvent, which allowed a more significant recovery of rutin in 77% than the control samples. It allowed to recover up to 4.93 μg / g lycopene of tomato by-products without resorting to organic solvents, showing itself as a selective extraction method, depending on the compounds of interest. Since there are no studies on the impact of the extraction methodologies and the action of the gastrointestinal tract (GIT) on the bioaccessibility of BC obtained from tomato by-products, the GIT was simulated for each of the LF and SF obtained by the OH and CONV methods. In LF, the results showed that extraction significantly influences the bioaccessibility of the BC present, with OH demonstrating a positive impact on the preservation of BC and consequently on the associated biological properties such as antioxidant, antihypertensive, prebiotic and anti-inflammatory. The main BC identified by UPLC-qTOF-MS were p-cumáric acid (163 m/z), naringenin (271 m/z) and luteolin (285 m/z). In addition, extract obtained by OH after GIT simulation exhibited a prebiotic effect on different strains of Bifidobacterium and Lactobacillus. Regarding FS, the results showed that the OH treatment originated a flour (SFOH) with higher total fibre than the flours obtained with CONV (SFCONV), 62.47 ± 1.24 and 59.06 ± 0.67 g / 100 g DW, respectively. Both flours presented high resistant protein content, representing between 11 and 16% of the insoluble dietary fibre. The main carotenoids identified were lycopene, phytofluene and lutein, all known as health promoters. Despite the higher initial polyphenols and carotenoids in SFOH, BC were more bioaccessible and presented higher antioxidant capacity than those present in SFCONV, throughout the simulated GIT. Finally, and because there is scientific support on the positive effects of prebiotics on the intestinal microbiota, the fermentability of SFOH and SFCONV of tomato were evaluated. The results showed a higher growth of Bacteroidetes with SFOH and the highest values of Bacteroides with SFCONV. A correlation between the growth of microorganisms and short-chain fatty acids was also found. Therefore, the OH treatment allowed similar recovery yields with reduced treatment times and without the need for organic solvents (green extraction routes). In a sense, this work will contribute to the sustainability of the wine and tomato processing industries in the context of circular economy, as it presents an environmentally friendly, fast and economical process capable of recovering BC with high application potentials, such as functional ingredients for food, nutraceutical or cosmetic application.
Date of Award20 Dec 2021
Original languageEnglish
Awarding Institution
  • Universidade Católica Portuguesa
SupervisorMaria Manuela Pintado (Supervisor), José António Couto Teixeira (Supervisor) & António Sebastião Rodrigues (Co-Supervisor)


  • Wastes
  • Grape bagasse
  • Tomato bagasse
  • Ohmic heating
  • Circular economy
  • Bioaccessibility
  • Health benefits


  • Doutoramento em Ciência e Tecnologia Alimentar e Nutrição

Cite this