Studies On The Regulation Mechanism Of Lipid Oxidation During Dry-Cured Bacon Processing

Dry-cured meat products are the representative of traditional meat products, which win warm praise from the masses of consumers. Lipid oxidation is a necessary pathway of the flavour development in dry-cured meat products during processing, furthermore, it is also an important factor influencing the safety and quality of dry-cured meat products. So, studies on lipid oxidation mechanism in dry-cured meat products have always been an important research direction in meat science reaearch area. However, studies on the regulation mechanism of lipid oxidation in dry-cured meat products during processing have just started. This topic is a further extension on the basis of special studies on traditional dry-cured meat products of national scientific supporting program 2006BAD05A15 "Product development and industrialization of low-temperature and traditional meat products". The main aim of this topic is to study the lipolysis and lipid oxidation laws during dry-cured bacon salting and drying-ripening, make clear the lipid oxidation mechanism and to explore the regulation mechanism of lipid oxidation; and through all these studies, to provide a theoretical basis for regulating lipid oxidation and improving flavour quality during dry-cured meat product processing. The detailed contents and results are as follows.1. Studies on lipolysis and lipid oxidation in bacon during curing and drying-ripeningDry-cured bacons were processed using bacon belly as material by dry-salting and drying-ripening. The change rules of lipid composition, peroxide value (POV) and thiobarbituric acid reactive substance value (TBARS) in bacon sample muscles were tracked and measured during processing. Furthermore, activity changes of lipolytic enzymes and lipoxygenase (LOX) as well as their correlations with lipolytic and oxidative changes of muscle lipid were investigated. The resulrs showed that free fatty acid content significantly inceased during processing, which was negatively correlated with the significant decrease (P<0.05) in phospholipids content (r=-0.85, P<0.01). All the activities of acid lipase, neutral lipase and phospholipase decreased significantly during processing (P< 0.05), and acid lipase showed higher activity than did neutral lipase (P< 0.05) throughout the whole processing. Among all the three lipolytic enzymes, the activity decrease amplitude of phospholipase was the largest, which reached 90.9% until the end of drying-ripening. LOX activity increased significantly and reached a peak (29.4 U/minxg protein) until the end of salting (P<0.01), thereafter gradually decreased during drying-ripening period, which was positively correlated with TBARS (r=0.94). Both POV and TBARS values increased initially and reached maximal values (0.148 meq/kg lipid and 0.28 mg MDA/kgmuscle) at drying ripening 6 days and end of salting, respectively, and then decreased. All these results indicated that high salt content inhibited acid lipase and phospholipase activities but promoted LOX activity during salting period. And properly controlling processing time and other regulation factors, such as temperature and salt content, could decrease the oxidative indices of final products, and thus inproving the safety and flavour quality of the products.2. Studies on enzymatic and non-enzymatic lipid oxidation in muscle lipid oxidationThe LOX was removed from fresh bacon belly muscles by the method of LOX extraction. Then the minced muscle removed of LOX was used as material, the enzymatic and non-enzymatic lipid oxidation in the muscles were evaluated by measuring POV, TBARS and LOX activity of different experimental treatments, which set the muscles as added LOX extracts and control (not added LOX extracts) and both the samples were oxidized at two different conditions (in dark and under illumination), respectively. The results showed that, during the muscle lipid oxidation period, non-enzymatic oxidation played a dominant role in the overall lipid oxidation phenomenon of muscle lipid. Non-enzymatic and enzymatic oxidation totally explained about 89% of the total lipid oxidation variation in belly muscle and there remain have 11% of lipid oxidation variation could not be explained, suggesting that when enzymatic and non-enzymatic oxidation jointly functioned in the belly, there may be a synergistic effect on muscle lipid oxidation existed between them. In addition, during the oxidation period, the LOX activity decreased more quickly in the muscle samples of full oxidation group, in which enzymatic and non-enzymatic oxidation were functioned together, than did the LOX in the muscles of enzymatic group, indicating that the high content oxidative products could inhibit the activity of LOX.3. Studies on main enzymatic properties of muscle LOX and the regulation mechanism of process factors on its activityCrude lipoxygenase (LOX) was extracted from fresh bacon belly and studied of some main characteristics using linoleic acid as substrate. Based on the results of single-factor experiments, the interactions of temperature, sodium chloride (NaCl) and pH on LOX activity were investigated by response surface methodology (RSM), and the regulation method of LOX activity was explored by analyzing the interactive effects of process factors on LOX activity. The results showed that the optimum substrate concentration, temperature and pH value for LOX catalyzing linoleic acid oxidation were 3.47 mM,40℃and≧9.0, respectively, and the Michaelis-Menten constant (Km) and maximum velocity (Vmax) were 68μM and 0.26 U/min, respectively. The NaCl also existed a critical value for LOX activity, which was determined as 3.1%(w/w) at 20℃, and above which inhibited the LOX activity. Temperature had significant interactions (P<0.05) with NaCl and pH. The temperature critical value decreased linearly with NaCl content increasing in the enzyme solution, while increased with pH increasing. The NaCl critical value for LOX activity also decreased linearly with temperature increasing. All these results suggested that decreasing salt added amount at salting stage and increasing temperature during early drying-ripening period could promote LOX activity, which will accelerate lipid oxidation and promote the formation of flavour precursors (lipid hydroperoxides). In addition, during the last drying-ripening stage, the salt content quickly increases with continuous increase in temperature, but meanwhile the NaCl critical value decreases with temperature increasing, therefore LOX activity will be inhibited and the formation rate of flavour precursors will decrease, while their further decomposition will be accelerated, which could promote the flavour compounds development.4. Effect of antioxidant enzymes on lipid oxidation during dry-cured bacon processing and the influence of processing factors on antioxidant enzyme activities.Both the oxidative stability and antioxidant enzyme stability changes were evaluated by assessing the changes of POV, TBARS and activities of catalase, glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) during dry-cured bacon processing. The effect of antioxidant enzyme activities on muscle lipid oxidation were investigated by analyzing the correlations between the changes of antioxidant enzyme activities and lipid oxidation stability during dry-cured bacon processing. And the effects of process factors on antioxidant enzyme activities were investigated by response surface methodology. The results showed that all three antioxidant enzyme activities were negatively correlated with POV (P> 0.05) and TBARS values (P<0.05), and the correlations decreased with temperature increasing. The activities of all three antioxidant enzyme continuously decreased (P< 0.05) untill the end of process, among which GSH-Px was the most unstable one followed by catalase. Within the temperature range of lower than the temperature critical value for LOX (40℃), the higher drying-ripening temperature was in favor of all the antioxidant enzyme activities. But NaCl always showed an inhibitory effect on all the antioxidant enzyme activities, and which was concentration dependent. The antioxidant enzyme activities in dry-cured bacon during processing were influenced interactively by the temperature promoting effect and salt inhibitory effect. Increasing salt content in bacon decreased the promoting effect of temperature on antioxidant enzyme activities, but raising temperature enhanced the inhibitory effect of salt on antioxidant enzyme activities. All the above results indicated that antioxidant enzymes have important influence on controlling lipid oxidation in muscle. And, under the condition of low salt (NaCl) added level, greatly increasing processing temperature could inhibit antioxidant enzyme activities, accelerate the decomposition lipid hydroperoxides, decrease oxidation indices (POV and TBARS) and promote flavour development.5. Studies on process reulation mechanism of lipid oxidation in dry-cured baconKinetic characteristics of primary and secondary lipid oxidation in dry-cured bacon and the influence of different NaCl contents on them were investigated by measuring POV and TBARS values of different bacons treated with different temperatures or NaCl levels according to temperature and NaCl single-factor experiments. The interactive effect of process factors on lipid oxidation rate was further studied using response surface methodology, and the regulation method of primary lipid oxidation was optimized. The results indicated that, within the range of temperature lower than 40℃and NaCl added level lower than 5.0%(w/w), both temperature and NaCl showed significant (P<0.05) prooxidant effects, but temperature showed relatively greater effect than did NaCl. The activation energy (Ea,92.35 kJ/mol) for POV was higher than that (65.66 kJ/mol) for TBARS. Adding NaCl (<5%, w/w) could decrease both the Ea, and when NaCl added level were 3.1% and 3.4%, respectively, the corresponding Ea for primary and secondary lipid oxidation were the lowest. Temperature and NaCl had extremely significant (p<0.001) interactive on primary lipid oxidation rate. And elevating temperature could significantly (p<0.05) and linearly (y=-0.067x+5.113) decrease the threshold value of NaCl concentration influencing lipid oxidation during dry-cured bacon processing. When temperature was 35℃and NaCl level was 2.77%, the primary oxidation rate was the fastest. These results suggested that raising temperature and decreasing salt used amount could accelerate primary lipid oxidation rate in dry-cured bacon, which will make the change inflexions of POV and TBARS advanced, and thus provide more times for decreasing oxidation indices and improving flavour quality of products by secondary or more further oxidation.