Relationship of Thermal Treatment and Antioxidant Capacity in Cooked Foods

Citation: Navajas-Porras, B.; Pérez-Burillo, S.; Hinojosa-Nogueira, D.; Pastoriza, S.; Rufián-Henares, J.Á. Relationship of Thermal Treatment and Antioxidant Capacity in Cooked Foods. Antioxidants 2022,11, 2324. https://doi.org/10.3390/ antiox11122324 Academic Editor: Volker Böhm Received: 2 October 2022 Accepted: 21 November 2022 Published: 24 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). antioxidants Article Relationship of Thermal Treatment and Antioxidant Capacity in Cooked Foods Beatriz Navajas-Porras 1, Sergio Pérez-Burillo 1, Daniel Hinojosa-Nogueira 1, Silvia Pastoriza 1,† and José Ángel Rufián-Henares 1,2,*,† 1Departamento de Nutrición y Bromatología, Instituto de Nutrición y Tecnología de Alimentos, Centro de Investigación Biomédica, Universidad de Granada, 18071 Granada, Spain 2Instituto de Investigación Biosanitaria ibs.GRANADA, Universidad de Granada, 18071 Granada, Spain *Correspondence: [email protected]; Tel.: +34-958-24-28-41 † These authors contributed equally to this work. Abstract: Most of the foods we eat undergo a cooking process before they are eaten. During such a process, the non-enzymatic browning occurs, which generates compounds such as furosine, 5-hydroxymethylfurfural (HMF) and furfural. These are considered markers of cookedness and can therefore be used as quality indicators. In this work, we study the production of these compounds in different foods (both of plant and animal origin) that are cooked with different techniques. Additionally, we investigate correlations between the production of these markers of cookedness and the antioxidant capacity produced after in vitro digestion and fermentation. We observe that, in general, cereals and vegetables are more thermally damaged. Toasting and frying produce the highest concentrations of Maillard compounds whereas boiling the lowest. Furosine content shows a significant positive correlation with in vitro digestion data in fried foods, and with fermentation in roasted foods. Furfural content shows a significant positive correlation with in vitro digestion results in roasted foods, specifically in the Folin–Ciocalteu method. Keywords: maillard reaction; furosine; HMF; furfural; cooking; antioxidant activity 1. Introduction Cooking a food is an operation that can modify its characteristics to improve the organoleptic properties, digestibility and hygienic conditions. In addition, cooking most frequently involves a heat source in order to raise its temperature. As consequence, food undergoes physical, chemical, or biological changes. Although cooking usually improves taste, flavor or make food safe to be consumed, it can also have a negative impact on the food chemical composition and by extension on human health [ 1 ]. There are several types of cooking that can be classified according to how heat is transmitted onto the food. For example, frying or grilling uses fats as the medium to transfer heat to the food [ 2 ] whereas boiling uses water. On the other hand, others (like roasting) use the air to transfer heat to the food surface. Heating favors a plethora of chemical changes within the food, and some of those as consequence of the non-enzymatic browning, including the Maillard reaction [ 3 ]. This reaction involves a set of chemical chain reactions that is favored when food is subjected to moderate heat and gives rise to a plethora of molecules responsible for new colors, smells, tastes and textures that are usually pleasing to the consumer [ 4 , 5 ], although undesirable aromatic substances and brown compounds may also be produced [ 6 ]. In order to allow the reaction takes place, a free carbonyl group is needed (such as those from reducing sugars, oxidized lipids or B group vitamins) as well as free amino groups from an amino acid, peptide or protein [ 7 ]. The Maillard reaction is divided into three stages; during the early stage, while it is still reversible and browning has not yet occurred, sugars and amino acids begin to degrade [ 8 , 9 ]. Furosine appears during this stage [ 10 ]. It was one of the first Antioxidants 2022,11, 2324. https://doi.org/10.3390/antiox11122324 https://www.mdpi.com/journal/antioxidants Antioxidants 2022,11, 2324 2 of 14 products to be identified for the Maillard reaction. The concentration of this compound has been shown to increase as a function of the heat treatment applied and is another marker of heat damage [ 11 , 12 ]. Secondly, the intermediate stage involves dehydration of sugars by enolic isomerization, giving rise to furfural and 5-hydroxymethylfurfural (HMF) among other compounds. Furfural content of foods correlates with undesirable flavors and is therefore a good quality indicator [ 13 ]. HMF also allows one to monitor intermediate stages of the Maillard reaction and it is an indicator commonly used by the food industry to assess heat damage in plant food products [ 14 , 15 ]. The final stage involves polymerization and formation of high molecular weight-colored substances called melanoidins [16]. It is remarkable that water-soluble compounds generated during MR have shown the ability to neutralize free radicals [ 17 ]. Such antioxidant capacity is proportional to the degree of browning [ 18 ] and has a close correlation with the compounds generated from the intermediate and late stages, as well as with the type of sugar involved in the reaction [ 19 ]. Despite the partial loss of natural compounds with antioxidant activity that may occur during food processing, antioxidant properties could be maintained and even increased due to the formation of new compounds through the development of the Maillard reaction [ 20 ] or release by cell breakage [ 21 ]. In previous studies, we found that cooking techniques strongly modify the antioxidant capacity of plant [ 22 ] and animal foods [ 23 ]. Therefore, the aim of this study was to unravel the potential contribution of the development of non-enzymatic browning to the antioxidant capacity of foods. To do that, 23 of the most commonly consumed foods in Spain were submitted to common cooking techniques (including frying, roasting, toasting, boiling and grilling). Furosine, HMF and furfural concentrations were analyzed as indicators of non-enzymatic browning, related with the cookedness of foods. In addition, correlation studies were carried out between these indicators and antioxidant capacity in the same foods with the same cooking, both after in vitro digestion and colonic fermentation stages. 2. Materials and Methods 2.1. Chemicals Furosine was purchased from NeoMPS (Strasbourg, France). Furfural, 5- (hydroxymethyl)furfural, hydrochloric acid, methanol, and acetonitrile (HPLC grade) were obtained from Sigma-Aldrich (Taufkirchen, Germany). Alpha Aesar provided the pancreatin in the porcine pancreas (Heysham, UK). The remaining chemicals, which included analytical-grade salts and enzymes for in vitro digestion and fermentation as well as chemicals and solvents for the determination of antioxidant capacity, were bought from Sigma-Aldrich (Taufkirchen, Germany). 2.2. Foods and Cooking Conditions A total of 20 foods were studied, and included in these groups were: cereals (bread, bread whole grain, penne, penne whole grain, rice, rice whole grain), egg, fish (cod fish and salmon), fruits (apple and banana), legumes (beans and lentils), meat (pork, beef, chicken and lamb), tubers (potato) and vegetables (capsicum, carrot, cauliflower, onion and tomato). Different thermal processes were applied to the samples: boiling, frying, grilling, roasting and toasting) (Table S1). Fruits, tubers and vegetables were cut into different sizes so that the same texture was achieved after the different cooking processes (See Table S1). For grilling and frying, extra virgin olive oil (EVOO) was used as a cooking medium. Boiling was performed at a water/food rate of 5:1, for 20 min at 100 ◦ C. Grilling was carried out at an oil/food rate of 0.5:1, for 3 min on each side, at 220–250 ◦ C. Fried foods were obtained at an oil/food rate of 5:1, at 180 ◦ C for 8 min. Roasting was performed for 10 min at 180 ◦ C. Toasting was carried out for 3 min at 900 W, in a Grunkel TS140H toaster at the fourth level following the manufacturer’s instructions. Cooking times and food/average rates were acquired from previous work [2]. The utensils used for foods preparation were forks and knives, stainless steel spoons; frying pan, saucepan, fryer, a portable oven (1500 W), and toaster. These utensils were Antioxidants 2022,11, 2324 3 of 14 acquired at Centro Hogar Sánchez (Granada, Spain). Cooked foods were homogenized and stored at −80 ◦C under a nitrogen atmosphere. All analyses were performed in duplicate. 2.3. Furosine, HMF and Furfural Assays Furosine assay was carried out following the method of Delgado-Andrade et al. [ 24 ]. Samples were hydrolyzed for 23 h at 120 ◦ C with 7.95 M HCL. The hydrolysate was purified with a Sep-pack C 18 cartridge (Millipore, Burlinton, MA, USA), and the resulting solution was analyzed by ion pair RP-HPLC. The analysis was performed in duplicate, and the data are mean values expressed as µ g/g food and µ g/g of protein. Protein in each food was estimated from a database [25]. HMF and furfural were determined following a previously described protocol [ 14 ]. Ground samples were suspended in deionized water, clarified with Carrez I (potassium ferrocyanide, 15% w/v) and Carrez II (zinc acetate 30% w/v) solutions. The resulting solution was analyzed by RP-HPLC. The analysis was performed in duplicate, and the data are mean values expressed in µg per g of food. 2.4. In Vitro Gastrointestinal Digestion and Fermentation and Antioxidant Capacity All samples were submitted to in vitro digestion-fermentation according to the protocol previously described [ 26 , 27 ]. Five g of each food was subjected to in vitro gastrointestinal digestion followed by in vitro fermentation, in triplicate. The in vitro fermentation was carried out using fecal material from five healthy donors (with a mean Body Mass Index = 21.3 , and who had not taken antibiotics for three months prior to the assay). All fecal samples were pooled together to restricted inter-individual variability. The fermentation was carried out for 24 h, at 37 ◦ C. After the in vitro gastrointestinal digestion and fermentation, two fractions were obtained: digestion supernatant, which is available for absorption at the small intestine, and fermentation supernatant, which is available for absorption at the large intestine. A control fermentation was carried out, using only the fecal fermentation solution (inoculum composed of peptone, cysteine, and resazurin). The antioxidant capacity was evaluated in the two supernatant fractions obtained after in vitro digestion and fermentation, which represent the total antioxidant capacity that each food could exert in the human body [ 28 ]. Three different methods were used to determine the antioxidant capacity (DPPH, FRAP and Folin–Ciocalteu). The results of such analyses were described in previous work for plant [22] and animal foods [23]. 2.5. Statistical Analysis Statistical differences were calculated using the unpaired Kruskal Wallis test with 95% confidence, comparing the amount of furosine, HMF and furfural in each of the food groups, as well as within each group, and the comparison was made by cooking. Thus, we show whether a particular food group has a higher or lower amount of these indicators of cookedness. Pearson’s parametric statistic was calculated to show the linear relationship between the heat damage markers and between these and the antioxidant capacity produced in the same foods with the same thermal processing at p-value < 0.05. Correlations were made for both antioxidant capacity after in vitro digestion of foods and after in vitro fermentation with healthy adult microbiota. The correlations were based on the cooking methods used for the different food groups. The Statgraphics Plus software (version 5.1) was used to perform all statistical analyses. 3. Results 3.1. Furosine Content of Cooked Foods For the furosine assay, the food group with the highest concentration after thermal processing was cereals, followed by vegetables, meat, legumes, fish, eggs, fruits and tubers. The values for cereals were significantly (p< 0.05) higher than the mean of the other food groups (Figure 1A). When we consider cooking techniques, toasting and frying gave the Antioxidants 2022,11, 2324 4 of 14 highest (p< 0.05) concentrations. Grilling, on the other hand, showed significantly ( p< 0.05 ) lower concentrations than the rest (Figure 1B). Antioxidants 2022, 11, x FOR PEER REVIEW 4 of 15 3. Results 3.1. Furosine Content of Cooked Foods. For the furosine assay, the food group with the highest concentration after thermal processing was cereals, followed by vegetables, meat, legumes, fish, eggs, fruits and tubers. The values for cereals were significantly (p < 0.05) higher than the mean of the other food groups (Figure 1A). When we consider cooking techniques, toasting and frying gave the highest (p < 0.05) concentrations. Grilling, on the other hand, showed significantly (p < 0.05) lower concentrations than the rest (Figure 1B). (A) (B) Figure 1. (A) Furosine levels in different food groups. Statical analysis was performed via Kruskal– Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: *: p < 0.05. **: p < 0.01, ns: not significant. (B) Furosine levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: *: p < 0.05, **: p < 0.01, ns: not significant. Figure 1. ( A ) Furosine levels in different food groups. Statical analysis was performed via Kruskal– Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: *: p< 0.05. **: p< 0.01, ns: not significant. ( B ) Furosine levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: *: p< 0.05, **: p< 0.01, ns: not significant. Furosine Content by Specific Foods and Cooking Methods Table 1shows the furosine content of each food depending on the kind of thermal treatment used for cooking. In the cereals group, the highest furosine value was reached with frying (138.9 µ g/g), while in the case of eggs it was obtained for grilled eggs (24.5 µ g/g). In Antioxidants 2022,11, 2324 5 of 14 the fish group, again fried foods had the highest furosine content (fried salmon, 34.6 µ g/g) but for fruits, was boiled banana (42.0 µ g/g). In legumes, roasted kidney beans demonstrated a high reactivity (furosine values of 62.6 µ g/g), but were meat and vegetables the groups with the highest levels of furosine: 412.5 µ g/g for fried cauliflower and 183.6 µ g/g for fried meat. Thus, in general, frying (followed by roasting) was the cooking method that produced the highest levels of furosine. Table S2 shows the correlations depending on the food group. Table 1. Furosine values depending on the cooking method applied to the food. Food Group Food Boiled Fried Grilled Roasted Toasted µg/g Food mg/100 g Protein µg/g Food mg/100 g Protein µg/g Food mg/100 g Protein µg/g Food mg/100 g Protein µg/g Food mg/100 g Protein Cereals Bread - - 138.9 163.4 - - - - 95.4 127.2 Penne 3.1 5.8 - - - - - - - - Rice 19.5 84.8 - - - - - - - - Egg Egg 14.5 7.1 n.d. n.d. 24.5 11.5 21.0 8.6 - - Fish Cod fish - - 23.5 8.9 4.7 2.6 - - - - Salmon - - 34.6 8.9 17.6 7.9 18.6 7.2 - - Fruits Apple - - 18.6 620.0 n.d. n.d. 2.2 73.3 - - Banana 42.0 323.1 - - 26.0 123.8 3.9 32.5 - - Legumes Beans (Kidney) 8.5 14.9 - - - - 62.6 88.2 - - Lentils 40.5 61.4 - - 11.2 14.7 9.5 11.7 - - Meat Beef 72.4 29.7 183.6 67.0 5.6 1.9 39.2 19.2 - - Chicken 26.8 9.6 54.2 18.4 7.7 3.0 28.9 12.6 - - Pork 59.6 22.7 30.2 9.7 3.6 2.0 8.0 3.7 - - Lamb - - 45.4 18.5 5.4 3.6 19.0 11.7 - - Tubers Potatoe 5.0 0.010 14.8 0.062 0.9 0.002 - - - - Vegetables Capsicum 3.4 37.8 53.2 110.8 0.5 10.0 1.4 15.6 - - Carrot 14.6 162.2 99.4 764.6 4.1 41.0 0.9 5.6 - - Cauliflower 6.5 31.0 412.5 808.8 1.5 7.5 13.7 52.7 - - Onion 3.9 32.5 74.0 528.6 0.8 5.7 2.8 4.4 - - n.d. = not detected. The sign—denotes that such cooking method was not used for that particular food. Furosine is a good indicator of the thermal damage suffered by proteins during heat treatment [ 3 , 5 , 10 ] since it is correlates with the loss of available lysine. Thus, furosine can be also expressed in mg/100 g of protein to show the thermal damage (or heat load) of the food. In this sense, the highest thermal damage was suffered by vegetables (fried cauliflower and carrot with values surrounding 800 mg furosine/100 g of protein) closely followed by fruits (fried banana with a value of 620 mg furosine/100 g of protein) and cereals (fried bread, 163.4 mg furosine/100 g of protein). Again, frying was the heat treatment with the highest thermal damage, while boiling and grilling were the milder cooking option. 3.2. HMF Content of Cooked Foods Regarding HMF content, the food group that presented the highest amount of this compound after the different thermal processes was cereals, followed by vegetables, fish, fruit, tubers, meat, legumes and eggs (Figure 2A). As for cooking techniques, toasting generated the highest concentrations (p< 0.05). Frying, roasting, grilling and boiling followed toasting in HMF production, though only boiling produced significantly lower levels (p< 0.05) than the rest (Figure 2B). Antioxidants 2022,11, 2324 6 of 14 Antioxidants 2022, 11, x FOR PEER REVIEW 6 of 15 (A) (B) Figure 2. (A) HMF levels in different food groups. Statical analysis was performed via Kruskal– Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: ns: not significant. (B) HMF levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: **: p < 0.01, ***: p < 0.001, ns: not significant. HMF Content by Specific Foods and Cooking Methods The levels of HMF in each food (Table 2) were also used to study the effect of the cooking method on the development of non-enzymatic browning. As expected, the heat treatment of cereals produced the highest HMF concentration, up to 10,305 μg/g for toasted bread and five times lower for fried bread. Eggs and legumes where not too much affected by cooking, with grilling the most damaging cooking method (169 and 179 μg/g for grilled egg and lentils). Meats showed a higher content than fish after cooking, with being grilling again being the most harmful thermal treatment (1287 and 613 μg/g for pork and salmon, respectively). In addition, cooking potatoes generated large amounts of HMF, ranging from 550 μg/g during frying till 737 μg/g after grilling. On the other hand, fruits and vegetables were highly reactive during cooking, showing high HMF levels for Figure 2. ( A ) HMF levels in different food groups. Statical analysis was performed via Kruskal–Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: ns: not significant. ( B ) HMF levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: **: p< 0.01, ***: p< 0.001, ns: not significant. HMF Content by Specific Foods and Cooking Methods The levels of HMF in each food (Table 2) were also used to study the effect of the cooking method on the development of non-enzymatic browning. As expected, the heat treatment of cereals produced the highest HMF concentration, up to 10,305 µ g/g for toasted bread and five times lower for fried bread. Eggs and legumes where not too much affected by cooking, with grilling the most damaging cooking method (169 and 179 µ g/g for grilled Antioxidants 2022,11, 2324 7 of 14 egg and lentils). Meats showed a higher content than fish after cooking, with being grilling again being the most harmful thermal treatment (1287 and 613 µ g/g for pork and salmon, respectively). In addition, cooking potatoes generated large amounts of HMF, ranging from 550 µ g/g during frying till 737 µ g/g after grilling. On the other hand, fruits and vegetables were highly reactive during cooking, showing high HMF levels for grilled banana and fried apple (around 1500 µ g/g) and close to 4000 and 2000 µ g/g for fried onion and cauliflower, respectively (Table 2). Opposite to furosine, in the case of HMF generation, there was not a single cooking method with a higher thermal damage, since frying, grilling, roasting and toasting produce large amounts of HMF, depending on the food matrix. Table S2 shows the correlations depending on the food group. Table 2. HMF values (expressed in µ g HMF/g of food) depending on the cooking method applied to the food. Food Group Food Boiled Fried Grilled Roasted Toasted Cereals Bread - 2057.0 - - 10,304.9 Penne 6.7 - - - - Rice 15.3 - - - - Egg Egg 47.4 43.8 168.9 26.5 - Fish Cod fish 30.8 429.5 336.0 - - Salmon - 275.2 613.2 115.3 - Fruits Apple - 1505.6 114.1 444.0 - Banana - n.d. 1572.8 179.4 - Legumes Beans (Kidney) 175.8 - - 154.1 - Lentils 5.0 - 178.5 134.5 - Meat Beef 13.3 1222.5 720.8 52.9 - Chicken 33.5 351.8 720.9 333.0 - Pork 4.5 63.7 1286.9 423.3 - Lamb 2.6 52.0 61.7 6.2 - Tubers Potatoe 708.0 550.4 737.4 - - Vegetables Capsicum 88.0 71.7 352.6 63.2 - Carrot 55.1 104.1 223.3 89.5 - Cauliflower 48.9 1868.4 89.8 581.1 - Onion 127.6 4065.5 237.0 1832.1 - n.d. = not detected. The sign—denotes that such cooking method was not used for that particular food. 3.3. Furfural Content of Cooked Foods Regarding furfural, tubers showed the largest levels (p< 0.05), followed by cereals, fruits, vegetables, meat, fish, legumes and eggs (Figure 3A). Toasting generated the highest concentrations (p< 0.05). After toasting, in decreasing order of furfural content, we found frying, grilling, roasting and finally boiling. Only the latter showed significantly (p< 0.05) lower concentrations than the average of the rest (Figure 3B). Furfural Content by Specific Foods and Cooking Methods Furfural was another furanic compound used as an indicator of thermal treatment (Table 3). Boiling produced low levels of furfural in cereals such as penne or rice, but roasted and fried bread generated large amounts of this furanic compound (7859 and 1192 µg/g , respectively). In the case of protein-rich foods, eggs and salmon had a relatively high furfural content (from 352 till 545 µ g/g), but fried meats (pork and chicken) were those with higher values (over 1100 µ g/g in both cases). The highest furfural levels were obtained in cooked tubers and vegetables, reaching very high furfural values: around 17,600 and 19,200 µ g/g for fried onion and potatoes, respectively (Table 3). As in the case of HMF, there was not a single cooking method producing the largest furfural contents, Antioxidants 2022,11, 2324 8 of 14 since frying, grilling, roasting and toasting produced high furfural levels depending on the food. It is noteworthy to mention that boiling was the less aggressive heat treatment, giving rise to low furfural levels or even not detected in meats and legumes. Table S2 shows the correlations depending on the food group. Antioxidants 2022, 11, x FOR PEER REVIEW 8 of 15 (A) (B) Figure 3. (A) Furfural levels in different food groups. Statical analysis was performed via Kruskal– Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: ns: not significant. (B) Furfural levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: **: p < 0.01, ***: p < 0.001, ns: not significant. Furfural Content by Specific Foods and Cooking Methods Furfural was another furanic compound used as an indicator of thermal treatment (Table 3). Boiling produced low levels of furfural in cereals such as penne or rice, but roasted and fried bread generated large amounts of this furanic compound (7859 and 1192 μg/g, respectively). In the case of protein-rich foods, eggs and salmon had a relatively high furfural content (from 352 till 545 μg/g), but fried meats (pork and chicken) were those with higher values (over 1100 μg/g in both cases). The highest furfural levels were obtained in cooked tubers and vegetables, reaching very high furfural values: around 17,600 and 19,200 μg/g for fried onion and potatoes, respectively (Table 3). As in the case of HMF, there was not a single cooking method producing the largest furfural contents, since frying, grilling, roasting and toasting produced high furfural levels depending on the food. It is noteworthy to mention that boiling was the less aggressive heat treatment, giving rise Figure 3. ( A ) Furfural levels in different food groups. Statical analysis was performed via Kruskal– Wallis test. Each of the groups were compared to the average of all of them (i.e., base-mean). Statistic labels: ns: not significant. ( B ) Furfural levels depending on the cooking applied. Statical analysis was performed via Kruskal–Wallis test. Each group was compared to the average of all of them (i.e., base-mean). Statistic labels: **: p< 0.01, ***: p< 0.001, ns: not significant. Antioxidants 2022,11, 2324 9 of 14 Table 3. Furfural values (expressed in µ g Furfural/g of food) depending on the cooking method applied to the food. Figure Food Boiled Fried Grilled Roasted Toasted Cereals Bread - 1191.6 - - 7858.6 Penne 47.3 - - - - Rice 98.2 - - - - Egg Egg 42.9 n.d. 13.1 351.8 - Fish Cod fish 268.0 55.5 n.d. - - Salmon - 494.3 544.5 191.1 - Fruits Apple - 14,028.8 1.8 810.3 - Banana - 425.4 338.3 - Legumes Beans (Kidney) n.d. - 449.0 - Lentils n.d. 61.5 27.4 - Meat Beef n.d. 613.6 131.5 90.3 - Chicken n.d. 514.7 1168.0 533.9 - Pork n.d. 54.6 1821.1 41.7 - Lamb n.d. n.d. 18.5 20.0 - Tubers Potatoe 864.2 19,164.0 1386.5 - - Vegetables Capsicum 3.8 232.7 1748.4 1844.1 - Carrot 83.7 76.2 844.3 139.7 - Cauliflower 3.3 3496.7 1199.2 851.8 - Onion 396.4 17,596.9 524.5 3672.6 - n.d. = not detected. The sign—denotes that such cooking method was not used for that particular food. 3.4. Correlation of Heat Damage Markers with Antioxidant Capacity of Cooking Foods after Digestion and Fermentation The results of antioxidant capacity are shown in Table 4and are deeply described in previous work for plant [ 22 ] and animal foods [ 23 ]. In general, it was found that intense cooking methods, such as frying, increase the antioxidant capacity of foods. In the case of animal foods, meat was the group with the highest antioxidant capacity [ 23 ], while cocoa and legumes were the most antioxidant plant foods [ 22 ]. Taking all this information into account, we generated correlations between heat damage markers (furosine, HMF and furfural) and antioxidant capacity obtained after in vitro digestion and fermentation (Figure 4). Table 4. Antioxidant capacity measured via Folin–Ciocalteu (mg gallic acid equivalents/kg of food), FRAP (mmol Trolox equivalents/kg of food) and DPPH (mmol Trolox equivalents/kg of food) depending on the type of cooking applied. AOX Method In Vitro Pre-Treatment Boiled Fried Grilled Roasted Toasted Folin-Ciocalteu Digestion 1259 ±1144 1368 ±1018 1409 ±1374 2262 ±1609 3536 ±268 Fermentation 33,396 ±12,455 38,221 ±19,990 43,498 ±21,926 43,837 ±18,024 16,573 ±5625 FRAP Digestion 2.3 ±2.0 6.8 ±5.7 4.1 ±2.4 4.6 ±3.0 4.9 ±1.0 Fermentation 179 ±66.9 202 ±115 239 ±118 243 ±98.4 97.4 ±26.0 DPPH Digestion 13.2 ±9.6 22.3 ±24.1 12.1 ±8.7 18.0 ±13.8 1.1 ±0.8 Fermentation 213 ±219 222 ±206.9 253 ±199 290 ±215 108 ±21.5