Chemometric study on the effect of cooking on bioactive compounds in tomato pomace enriched sauces

Food waste reduction and the development of functional foods motivate the valorization of tomato processing by-products. Tomato pomace (TP), mainly seeds and peels, is rich in phenolics, carotenoids, fiber, and fatty acids and is a promising enrichment ingredient for tomato sauces. Home cooking practices affect the levels of bioactive compounds due to oxidation, degradation, and matrix effects. Innovative cooking devices such as Thermomix® (integrated chopping/mixing/heating) and Roner® (sous vide, low-oxygen, precise temperature) may influence the retention and release of bioactives and volatile organic compounds (VOCs). The research question is how different cooking techniques (Thermomix®, Roner®, pan-frying), temperatures (70 and 90 °C), times (15 and 30 min), and TP enrichment affect the profile of bioactive compounds (phenolics, carotenoids, vitamin E) and VOCs in tomato sauce. The purpose is to identify conditions and techniques that maximize health-relevant bioactives while preserving desirable volatiles, contributing to both nutritional quality and sustainability by reusing TP.

Prior studies identify tomato by-products as rich sources of polyphenols and carotenoids and show that domestic cooking significantly alters bioactive profiles in tomato sauces. Enrichment with by-products can enhance antioxidant potential. The Roner® minimizes oxygen exposure and preserves VOCs due to vacuum and controlled, typically lower temperatures. The Thermomix® simultaneously chops, mixes, and cooks, potentially enhancing release of bound compounds. Previous reports have documented caffeoylquinic acids, dicaffeoylquinic acids, and p-coumaric acid in tomato products, with lycopene as the predominant carotenoid; processing generates lycopene Z-isomers with improved bioavailability. Literature on the effect of specific cooking techniques on phenolics is mixed and matrix-dependent; some techniques (e.g., frying, steaming) may increase retention or release compared with boiling or baking. There is limited literature specifically evaluating sous vide (Roner®) effects on tomato sauce bioactives and VOCs, highlighting a knowledge gap addressed by this study.

Design: A full factorial 3×2×2 design tested three cooking techniques (Thermomix®, Roner® sous vide, pan-frying), two temperatures (70 and 90 °C), and two times (15 and 30 min), with and without TP enrichment used as a factor for comparative models. Each sauce (200 g tomatoes, 20 mL refined olive oil) was prepared with or without 12 g crushed TP added before cooking; three independent process replicates were performed. Tomatoes (Lycopersicon esculentum Mill, cv. Pera) were washed, crushed, and cooked per treatment. Refined olive oil was verified phenolic-free by LC-ESI-LTQ-Orbitrap-MS. Cooking procedures: Thermomix® TM6-1 applied set temperatures and times with continuous stirring. Roner® 9999951 sous vide water bath cooked vacuum-bagged sauce at target temperature for 15 or 30 min. Pan-frying cooked directly in a frying pan at target temperature with continuous stirring. All samples were vacuum-packed/frozen at −20 °C until analysis. Analytical determinations: Phenolic compounds were extracted (methanol:water 8:2, sonication, centrifugation, evaporation, reconstitution in 0.1% formic acid), identified and quantified by LC–HRMS (Accela LC with PDA; BEH C18 column; LTQ-Orbitrap Velos ESI−, FTMS 30,000 at m/z 900, dd-MS/MS). Identification used standards and MS/MS; quantification used calibration curves (0.1–5.0 µg/mL; R2>0.98); LOD/LOQ reported for multiple standards. Carotenoids and vitamin E were extracted (ethanol:n-hexane, sonication, centrifugation, two extractions, nitrogen evaporation, reconstitution in TBME) and identified by APCI+ MRM on QTRAP4000; quantification by UPLC-DAD with YMC C30 column and appropriate gradient. Standards and external calibration yielded linearity R2>0.99; LOD/LOQ provided for selected analytes. VOCs were analyzed by HS-SPME GC–MS (polar TG-WAXMS column). An internal standard (4-methyl-2-pentanol in refined olive oil) enabled semi-quantitation via (Aa/Ais)*Cis; results expressed as mg/kg FW. Calibration for IS covered 0.05–10 mg/kg (R2=0.99). Identification used spectral libraries and standards where available. Statistics: SIMCA and MetaboAnalyst were used. PCA explored clustering. PLS-DA assessed TP enrichment; OPLS-DA modeled effects of technique, temperature, and time. Variables with VIP>1.5 were considered markers. Model validation included R2Y, Q2Y, CV-ANOVA (p<0.01), Hotelling’s T2, and 200-permutation testing. Univariate tests (t-test for enrichment, temperature, time; one-way ANOVA for technique) applied FDR<0.05 with log-transformed data.

• A total of 97 minor compounds were identified/quantified: 54 phenolics (≈70% phenolic acids, 30% flavonoids), 15 carotenoids, 3 vitamin E isoforms, and 25 VOCs.
• TP enrichment markedly increased phenolic content: total phenolics rose from 170.70 ± 19.82 to 439.99 ± 39.19 µg/g; phenolic acids from 115.03 ± 12.96 to 311.09 ± 28.94 µg/g. Several phenolics absent in controls appeared at high levels in enriched sauces (e.g., caffeoyl-hexose I–IV, 4-hydroxybenzoic acid, 2,6- and 2,5-dihydroxybenzoic acids, p-coumaric acid). Rutin approximately doubled (30.33 ± 4.18 to 63.29 ± 6.12 µg/g); naringenin chalcone increased (1.96 ± 1.89 to 13.56 ± 1.92 µg/g).
• PCA separated TP-enriched vs non-enriched samples on PC1 (44.5% variance), indicating strong compositional impact of enrichment. PLS-DA confirmed clear separation by enrichment.
• Carotenoids: Lycopene was the most abundant carotenoid. TP enrichment did not significantly increase carotenoid levels overall.
• Vitamin E: α-tocopherol predominated; vitamin E levels appeared overall unaffected by TP enrichment (α-tocotrienol slightly higher with TP in Results).
• VOCs: Most abundant VOCs were 6-methyl-5-hepten-2-one, 4-methyl-2-pentanol, and 2,2,4,6,6-pentamethylheptane; only 6-methyl-5-hepten-2-one increased with TP enrichment.
• Technique effects (OPLS-DA): Cooking technique had stronger influence on bioactives than temperature or time. Roner® preserved VOCs better, increasing hexanal, 2-hexenal, 3-hexen-1-ol, and 1-hexanol. Roner® produced the lowest phenolic and carotenoid levels; lack of stirring and lower heat transfer may limit release. Pan-frying yielded highest lycopene (175.12 ± 38.63 µg/g FW) and 13-Z-lycopene (2.09 ± 0.47 µg/g FW). Thermomix® yielded the highest vitamin E: α-tocopherol 16.38 ± 12.21 µg/g FW; α-tocotrienol 6.13 ± 2.66 µg/g FW. Certain phenolics varied by technique (e.g., gentisic acid higher with pan-frying; naringenin dihexose II higher with Thermomix®).
• Temperature/time: Slight increases in selected markers at 90 °C; 15 min tended to retain slightly higher bioactive levels than 30 min, though differences were modest.

Enriching tomato sauce with TP substantially boosts phenolic content, particularly phenolic acids and select flavonoids (e.g., rutin, naringenin derivatives), likely via direct transfer and release of bound forms from the pomace matrix during cooking. Despite the carotenoid richness of TP, enrichment did not translate into higher carotenoid levels in sauces, possibly reflecting matrix constraints and processing stability of these compounds under the tested conditions. Multivariate models highlighted the dominant role of cooking technique over temperature and time in shaping bioactive profiles. Thermomix® matched pan-frying in releasing bioactives, indicating that controlled stirring and chopping can effectively disrupt the matrix and promote extractability without excessive degradation. Roner® (sous vide) was superior in preserving VOCs—consistent with minimized oxygen exposure and limited evaporation—resulting in enhanced aroma-related aldehydes and alcohols, but it was less effective at releasing phenolics and carotenoids, likely due to the absence of agitation and different heat transfer. Temperature and time exerted secondary effects: slightly higher levels at 90 °C and with shorter processing suggest a balance between enhanced release and minimized oxidative/thermal degradation. Overall, integrating TP into sauces can elevate antioxidant potential while the choice of cooking technique allows tailoring toward either nutritional enhancement (Thermomix®/pan-frying) or aroma preservation (Roner®). These findings support both functional food development and sustainable by-product valorization.

• TP enrichment of tomato sauces more than doubles phenolic content and increases specific flavonoids, enhancing nutritional value without significantly altering carotenoid or vitamin E levels.
• Cooking technique is the main driver of compositional differences: Thermomix® effectively releases bioactives comparable to pan-frying; Roner® preserves VOCs but yields lower phenolic and carotenoid concentrations.
• Higher temperature (90 °C) and shorter time (15 min) modestly favor bioactive retention/release relative to lower temperature or longer time.
• Practical implication: Food producers and consumers can enrich tomato sauces with TP and select Thermomix® or pan-frying to maximize bioactives, or Roner® to optimize aroma. This approach promotes circular economy by valorizing tomato by-products. Future research should examine bioaccessibility/bioavailability of enriched compounds, sensory outcomes across techniques, broader tomato varieties and matrices, and optimization of sous vide parameters (agitation, temperature profiles) to improve bioactive release while preserving volatiles.

• Single tomato variety (cv. Pera) and specific recipe matrix (refined olive oil, defined TP source) may limit generalizability to other cultivars, oils, or processing conditions.
• Only three cooking techniques and two temperatures/times were tested; findings may not extrapolate to other methods or parameter ranges.
• Roner® conditions lacked stirring; comparisons to sous vide with agitation or different thermal profiles were not assessed.
• Study focused on composition; bioaccessibility, bioavailability, and in vivo effects were not evaluated.
• Limited literature exists for direct comparison of Roner® effects on tomato sauce bioactives and VOCs.