Study On Preparing Pork Flavour By Lipid Controlled Oxidation And Thermal Reaction

Animal fat plays an important role in the formation of the characteristic flavour of cooked meat. The formation of meat flavour derives mainly from the lipid degradation or the interaction between the Maillard reaction intermediates and lipid-derived flavour precursors. Encompassing the effects of lipids on the formation of meat flavour, the lard was chosen in this paper. The processing condition of making precursor of meat flavour by lipid controlled oxidization was studied. The volatile oxidation products of lard were analyzed by headspace solid phase micro-extraction (HS-SPME) combined with GC-MS. The meat-like flavour was simulated by adding the moderate oxidized lard to the Maillard reaction mixtures. GC-O analysis was performed to identify the aroma active compounds of simulated meat flavour and stewed pork and to determine the character impact odour-active components that contribute to the overall meat flavour. We investigated the interactions between the Maillard reaction and lipids by establishing different reaction model systems. In addition, the impacts of some lipid-derived aldehydes on the Maillard reaction systems were also studied. The major contents and conclusions are as follows:1. Isolation of the volatile oxidation products of lardThe volatile oxidation products of lard were extracted by solid phase microextraction. The optimum extraction conditions were:5 mL lard samples were added to a 20 mL vial, placed in 80 C water bath on a magnetic stirrer, after equilibrating for 15 min extracted for 30min by 50/30μm DVB/CAR/PDMS fiber.2. Study on Technology and Volatile Oxidation Products of Lard under Controlled OxidationThe processing condition of making precursor of meat flavor by lard controlled oxidization was studied. Thermal oxidation of lard was assayed by peroxide value, acid value and thiobarbituric acid value. The results showed that the optimum conditions for oxidization of lard were:temperature of 120℃, time of 2h, flow of oxygen 0.025m3/h.The volatile oxidation products of lard were analyzed by headspace solid phase micro-extraction (HS-SPME) combined with GC-MS, and compared with the fresh lard. The results showed that the acids were detected to be the most abundant volatiles, representing 50.18% of the total volatile compounds in the fresh lard, while aldehydes detected to be the most abundant volatiles in the oxidized lard, representing 52.52%. The volatile compounds produced during lipid oxidation were correlated with the degree of lipid oxidation. The results showed that the volatiles including nonanal, octanal, (E)-2-nonenal,2-pentylfuran, hexanoic acid, octanoic acid and nonanoic acid were positively related to the degree of lipid oxidation.3. Preparation of pork flavour by lipid controlled oxidation and thermal reactionThe pork flavour was simulated by interaction of lipid oxidation and Maillard reaction. The thermal reaction products were evaluated by sensory evaluation, and volatile reaction products were analyzed by GC-MS. The results showed that the optimum conditions for preparation of simulated pork flavour were:the reaction mixtures were made up in presence of phosphate buffer (0.2 M, pH 5.0), containing xylose (1%), cysteine (1%) and moderate oxidized lard (1%), react at 120℃for 60min.4. Analysis of the characteristic aroma compounds in simulated meat-like flavour and stewed porkVolatile compounds in simulated meat-like flavour were qualitatively evaluated by GC-MS and compared to the volatile compounds found in stewed pork. The GC-O method was performed to determine the aroma active compounds. A total of 19 aroma active compounds were identified in stewed pork, while 20 aroma active compounds identified in simulated meat-like flavour. There are 9 co-existing aroma active components, including 1-octen-3-ol,1-octanol,2-pentyl furan, octanal, nonanal, (E)-2-nonenal, (E, E)-2,4-nonadienal, (E, E)-2,4-decadienal and 2-undecenal. These compounds may have important contribution to the characteristic aroma of meat.There were some characteristic aroma compounds not detected by GC-MS. An unknown compound of RI 918 with the strongest aroma intensity was detected to be the key compounds in stewed pork. According to the odor and RI value, we speculate that this compound may be 2-furfurylthiol.5. Study on the effects of lard on the Maillard reaction Interaction between Maillard reaction and lipid oxidation was investigated by preparing different thermal reaction model systems. The volatile reaction products derived from different model systems were analyzed by SPME-GC-MS. The results showed that the lard-containing reaction systems rendered a lower pH of MRPs than control. The MRPs derived from the lard-containing model systems showed the higher increase in absorbance at 294 nm and 420 nm.The addition of lipid had a suppressing effect on most of sulfur-containing compounds derived from the reaction between cysteine and xylose, especially for 2-methyl-3-furanthiol,2-furanmethanethiol,2-methylthiophene,3-methylthiophene and 2, 5-thiophenedicarboxaldehyde. In addition, cysteine and xylose modified lipid oxidation pathways, so that lipid-derived alcohols, alkylfurans and aliphatic acids were formed rather than aldehydes. Compared with the lard heated alone, most of aldehydes were formed at lower levels in the lard-containing reaction mixtures, especially for (E)-2-octenal, octanal,2-nonenal, (E, E)-2,4-decadienal, (E, E)-2,4-nonadienal and so on. And several aldehydes including hexanal, heptanal, (E)-2-heptenal and (E, E)-2, 4-heptadienal were absent.6、Study on the effects of aliphatic aldehydes on the Maillard reactionWe studied the impacts of several aliphatic aldehydes on the model reactions of cysteine and xylose. These aldehydes included heptanal, octanal, (E)-2-heptenal, (E)-2-octenal and (E, E)-2,4-decadienal. The unsaturated aldehydes showed the greater effects on the overall intermediates and final browning products derived from thermal reaction, which have higher reactivity and promote the formation of intermediates and final browning productsDifferent aldehydes with different contents had certain impacts on the composition and contents of volatiles of thermal reaction products. Compared with the control, the addition of aldehydes had suppressing effects on the formation of most nitrogen-and sulfur-containing compounds, while some thiophene, pyridine, pyrrole and sulfide compounds were formed.Thiophenes were mainly detected in unsaturated aldehydes-containing reaction systems, including 2-butyl-thiophene,2-hexyl-thiophene,2-ethyl-5-hexyl-thiophene and 2-pentyl-thiophene.2-Pentyl-pyridine was only detected in (E, E)-2, 4-decadienal-containing reaction mixtures. In addition, some alcohols, esters and ketones were found in aldehydes-containing reaction systems.