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Investigation of amino acid modifications derived from lipid oxidation in foods

Karademir, Yeşi
This study aimed to investigate lipid derived amino acid modifications in foods. When the revealed carbonyl pool resulted from the repeated use of frying oils and high absorption rates of the oil during frying are taken into consideration, deep-fat fried foods have a significant potential to investigate this type of reactions. In the first part, effect of heating time of the frying oil on amino acid modifications in potato chips and model doughs was investigated. 2,4-decadienal, total free amino acid, decadien-1-amine, thiobarbituric acid reactive substances (TBARs), 2-pentylpyridine, acrylamide analyses were carried out for this purpose. The results presented here revealed that 2,4-decadienal, one of the lipid oxidation products accumulated in repeatedly used oil, actively involved in Maillard type carbonyl-amine reactions occurring in potatoes during frying. To the best of our knowledge, 2-pentylpyridine and decadien-1-amine in potato chips were reported for the first time in this study as products of the reactions occurred between 2,4-decadienal and amino acids. Frying oil was found to have the highest concentration of 2,4-decadienal after thermal oxidation at 180 oC for 6 h. Expectedly, potato chips fried in this oil had the highest concentration of 2,4-decadienal (29 mg/kg). There was a positive correlation (r2=0.73) between the concentrations of 2,4-decadienal and decadien-1-amine (relative concentration as peak area) formed in potato chips fried in repeatedly used sunflower oil. No 2-pentylpyridine was detected in potato chips fried in unoxidized oil, whereas its concentration ranged between 91 and 154 μg/kg in potato chips fried in oxidized oil. According to the model system studies, addition of glucose and fructose to model doughs consisted of only whey protein and water resulted in lesser amounts of decadien-1-amine, most probably because of the competition of the reducing sugars with lipid carbonyls on the reaction with amino acids. Moreover, the highest yields of decadien-1-amine were observed in the doughs prepared at 4 h heated oil. Model doughs containing potato flour appeared to have similar trends of 2,4-decadienal and decadien-1-amine with potato chips while the decline of total free amino acids in the first 4 h of the heating was more apparent in model doughs (M4) compared to real food systems. Second part of the study included the investigation of the impact of different oil types on amino acid modifications in potato chips. Safflower, corn, canola, hazelnut and olive oils were used in this context and p-anisidine, 2,4-decadienal, total free amino acids, decadien-1-amine, carboxymethyllysine (CML), carboxyethyllysine (CEL) analyses were performed. This impact was observed in decadien-1-amine formation in the order of the safflower>hazelnut>canola>olive>corn oils. CML concentrations of the poato chips were detected to be increased significantly (p<0.05) within the first 4 hours of the heating. Mean CML concentrations were determined to be 1063, 958, 815, 680 and 522 ng/g in the chips prepared within the hazelnut, canola, safflower, corn and olive oil respectively. In case of using the oils with very high amounts of linoleic acid such as safflower oil, increase in CML and CEL levels of the potato chips within the first 4 hours of the heating was very obvious in spite of less likely gradual accumulation of the glyoxal (GO) and methylglyoxal (MGO) in the oil during the heating of the oil. For the first 4 hours of the heating, the decrease (p<0.05) in the levels of CML and CEL of the chips prepared in the oils (hazelnut, olive, canola) with high oleic acid content was remarkable while the reverse of this behavior was observed in the high linoleic acid oils (safflower, canola). Finally, in the last part, the effect of different treatments (antioxidant addition to the oil, coating, pan frying) on the amino acid modifications in potato chips and fried chickens was investigated. Similarly, p-anisidine, 2,4-decadienal, total free amino acids, decadien-1-amine, CML, CEL were measured in this part. 1% tocopherol mix which was added to the frying oil in order to increase oxidation stability triggered the oxidation of the oil. When decadien-1-amine, CML and CEL formation was monitored in the chips prepared within the more oxidized oil, the ocurrence of much more modification of the free amino acids was demonstrated. Although no significant difference (p>0.05) was detected between p-anisidine values of 0.1% BHT (T2) or 0.1% tocopherol mix (T3) added oils with the control (T1), the highest increment of 2,4-decadienal in chicken meatballs was observed in T1 group while the lowest rises were in T2 and T3 within the first 4 h of the heating the oil. In addition, T1 was found to have the lowest levels of free amino acids. However, while T3 led to the highest decadien-1-amine yields, T2 was determined to form the highest CML levels. Due to the reduced oil absorption, the protective effect of the coating treatment (T4) and pan frying (P) on lipid derived amino acid modifications was obviously observed by the lower levels of decadien-1-amine and CML formation.