| Lipid oxidation was the main biochemical reaction which resulted in the flavor quality of dry-cured meat product,and also the main factor that affected its safety quality.Lipid oxidation mechanism and its regulation is a hot research area in international dry-cured meat product.Lipid auto oxidation was caused by the attack of all kinds of free radicals to lipid substrate,which was affected by many complex factors,such as sorts of free radicals,metal ion,environment temperature and the composition of lipid substrate.Auto-oxidation played a dominant role in total lipid oxidation of dry-cured meat product.In recent years,the lipid oxidation of dry-cured meat product was mainly concentrated in enzymatic oxidation because of the complexity of lipid auto-oxidation.In this study,we attempted to start from in vitro simulation systems.The lipid auto-oxidation reaction of dry-cured meat product was simulated by introducing free radical oxidation system.The interaction of lipid auto-oxidation mechanism,auto-oxidation and enzymatic oxidation was studied by the simulation system.We also explored and revealed the regulatory mechanism of lipid oxidation in dry-cured meat product by took dry-cured bacon as research object.1.The impact of in vitro simulation systems of lipid auto-oxidation and substrate on auto-oxidation characteristics,and the formation of simulation systemWe studied the impact of diftcrent free radicals,fatty acid substrates,reaction temperature and time on in vitro fatty acid auto-oxidation by took oleic acid and linoleic acid as substrate,O2·-and OH·as oxygen free radicals.At last,response surface methodology(RSM)was used for optimization and lipid auto-oxidation simulation system was formed.The result showed that HPOD and TBARS value which were taken as lipid oxidation index first increased observably and then decreased dramatically(P<0.05)with the increase of hydroxyl free radical concentration.The oxidation index parameters almost not changed when the system concentration reached to more than 20 mM and the attack efficiency of hydroxyl free radicals was the highest when the system concentration was 4 mM.The increase of HPOD value decreased gradually when fatty acid substrate concentration was more than 10 mg/mL.The TBARS value tended to be stable when substrate concentration was more than 30 mg/mL and showed a certain hysteresis.Both HPOD and TBARS value increased persistently(P<0.05)with temperature increase,and then changed no longer after system reacted for 3 hours.Superoxide anion free radicals had no oxidation on whether oleic acid or linoleic acid when various factors changed.The parameters of lipid oxidation in vitro system that formed by the final optimization were:hydroxyl free radical system was taken as auto-oxidation system,the concentration of hydroxyl free radical system of 4.0 mM,the concentration of linoleic acid substrates of 10.0 mg/mL and the reaction temperature of 20.0?.The above results are the base of follow research.2.Influence of lipid auto-oxidation on enzymatic oxidation in dry-cured meat product and study on their interactionBased on the auto-oxidation in vitro simulation system formed in chapter one and LOX extracted and purified from pork belly,lipid auto-oxidation,enzymatic oxidation and the two mixed oxidation simulation system were formed during dry-cured bacon processing procedure.The inner connection and interaction between lipid enzymatic oxidation and auto-oxidation in dry-cured bacon were also researched compared to actual oxidation reaction at the same time.The result showed the molecular weight of LOX obtained by separation and purification was 91.3 kDa.LOX activity in HROS system was significantly(P<0.05)higher than that in H2O2 system when HROS concentration was more than 1 mM.It illustrated that auto-oxidation could activate LOX markedly.The mechanism might be that hydroxyl radical underwent electron transfer by combining LOX and oxidated LOX-Fe2+ to LOX-Fe3+,thus LOX was activated.The Ea of auto-oxidation reaction with linoleic acid as substrate decreased and arachidonic acid(AA)first decreased and then increased with the increase of substrate concentration in HROS system,which illustrated high AA concentration would inhibit auto-oxidation reaction.However,both linoleic acid and AA had certain inhibitory effect on LOX enzymatic oxidation in EOS system. Auto-oxidation and enzymatic oxidation had collaborative effect on oxidation in mixed system,and the promotion could reach the maximum when the concentration of linoleic acid and AA was 19.56 mg/mL and 17.14 mg/mL,respectively.3.Study on the function mechanism of salt and its substitute on lipid auto-oxidation and enzymatic oxidationBased on the lipid auto-oxidation and enzymatic oxidation in vitro simulation system formed in the first two chapters and the analysis method of single factor experiment and activation energy(Ea)kinetics analysis,the influence mechanism of NaCl addition and its substitute KCl on fatty acid oxidation in auto-oxidation,enzymatic and the two mixed oxidation system was studied.The result showed the influence of NaCl addition on lipid oxidation in auto-oxidation and mixed oxidation system was similar.The addition of NaCl could promote lipid oxidation(HPOD value?TBARS value)in auto-oxidation and mixed oxidation system(P<0.05)and the Ea that needed for reaction decreased.However,the different of influence was not big with different NaCl concentration(P>0.05).The addition of NaCl almost had no influence(P>0.05)on enzymatic oxidation when temperature was low,and could result in the instability of lipid oxidation.The substitution of NaCl by KCl had no significant effect on HPOD and TBARS value of lipid in whatever system.It illustrated the substitution of NaCl by KCl had no influence on lipid oxidation reaction and can be applied in dry-cured meat product.4.Research on regulatory mechanism of salt and its substitute on lipid oxidation in dry-cured baconThe research in the third chapter showed the substitution of NaCl by KCl in dry-cured meat product was feasible,but the performance of salt and its substitute was not same with the conclusion inferred from in vitro experiment in actual process.One of the most significant was the addition of salt and its substitute in the processing of meat from different parts had different influence mechanism,and the substitute proportion by potassium salt increased in dry-cured meat product with high fat could improve lipid oxidation drastically.The phenomenon above was analyzed by DSC to find the basic reason why substitute by potassium salt could improve lipid oxidation drastically in dry-cured pork,and the realistic regulate and control parameters of salt and its substitute in dry-cured meat product could be obtained.The result was as follows:moisture(P<0.001),total fat(P<0.001),monounsaturated fatty acid(MUFA)(P<0.001)and polyunsaturated fatty acids(PUFA)(P<0.01)were significantly different in pork belly,hind leg meat and pork loin.The difference above was the main reason which resulted in the difference of unsaturated fatty acid phase change when dry-cured pork was in the range of curing,air-drying and ripening temperature.Whether in curing temperature range or air-drying and ripening temperature range,the addition of sodium salt contributed to the reduction of the fatty acid transform from solid to liquid.?H increased notably(P<0.05)when the proportion of substituted potassium salt was high(1.8%NaCl +1.2%KCl)in the range of air-drying and ripening temperature.High substitute of potassium salt could promote the change of fatty acid from solid to liquid and increase the probability that unsaturated fatty acid oxidated,which could indirectly enhance fat oxidation level.The result of meat from three different parts of pig in dry-cured process also verified that the TBARS value in meat with high fat and high substitute proportion of potassium salt was higher than that in meat with low fat and low slat substitute.Pork with high fat was not suitable to be processed with high substitute proportion of potassium salt.5.Regulation of synergistic effect between salt replacement and improved high temperature on lipid oxidation in dry-cured bacon and study on the influence on sensory quality.Based on the study in the first few chapters and laboratory studies of the past few years,we used RSM to optimize the process parameters of salt substitute combined with improved high temperature.Then the stability of lipid oxidation of dry-cured bacon and sensory quality could be regulated in concert.The result was as follows:the highest value of TBARS in each experiment was 0.65,which was far below the internationally accepted value(1.31±0.06 mg/kg).The sensory score in experiment group with improved high temperature of 65?was significantly higher than that of the other groups.The most optimal combination process obtained by RSM was:improved high temperature of 64.78?,NaCl addition of 3.05%,the substitute proportion with KC1 of 26.84%.The process was adjusted according to the actual situation and the result was:improved high temperature of 65?,NaCl addition of 3%,the substitute proportion with KCl of 26.84%.The result showed dry-cured bacon have low oxidation level and good sensory quality. |
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