Studies On The Discoloration Mechanism Of Pasteurized Sliced Ham During Storage

Color stability is one of the important characteristics of meat and meat products. It is the primary quality attribute seen by the consumer, who may use it as an indication of freshness and wholesomeness. As the way of future development of meat products, pasteurized meat products were introduced China lately and a lot of quality defects still exist currently. Discoloration is one of the important problem which will shorten pasteurized meat products' shelf-life and limit its market competitiveness. Many producers blindly increase the amount of nitrite or add many artificial pigment in absence of the knowledge on the theory of discoloration of pasteurized meat products, which did not really improve the color stability and prolong the shelf-life, instead of decreasing the quality of their products, even gave potential health hazards to consumers. Therefore, the purpose of present study was to reveal the mechanism of discoloration of representative pasteurized meat products—beef sliced ham, and to provide a theory basis for seeking a scientific method effectively to inhibit discoloration. Based on this purpose, the research were designed from two aspects including the fading theory about products and oxidation characteristics of predominant pigment. The contents and results are as follows:1. Study on the main influencing factor of discoloration of pasteurized sliced beef hamSeven factors affected color stability in processing and preservation were designed and instrumental color of ham were evaluated in this chapter. Multiple stepwise regression analysis was adopted to develop mathematical models describing the relationship between color parameters (L*, a*, b*) and seven factors. The results showed that refrigeration temperature (< 10℃) and the level of nitrite (in the range of GB) had insignificant effect on color stability; OTR had the greatest effect, both the addition of 3000 mg/kg phosphate and cooking center temperature of 85℃significantly affect the color stability. Illumination (< 1200 lx) did not significantly affect the a* value but the L*,b* and Hue value. 2. Studies on the change of color parameters and other quality index and their relationshipPrincipal component analysis (PCA) and factor analysis were adopted to research the interrelationship between color and other parameters. The PCA results showed that the first principal component explained 59.82% of the total variation, and the second explained 22.28%. PC1 was a concentrated reflection of color changes of ham during storage and PC2 was a representative index of environment factor causing the change of color. These results indicated that discoloration of products was mainly due to the oxidation of predominant pigment and greatly related to the reductive capacity of matrix. Furthermore, significant correlations were found between pigment oxidation and lipid oxidation as well as that and protein oxidation. Matrix condition significantly influenced the color stability by change of the b* and L* firstly and caused the variation of a* value then. In addition, remarkable correlations were seen between color stability and some nitrogen compounds in matrix by PCA and factor analysis. For color parameters, great significant correlations were found in the present study among a*,C and the content of nitrosylhemochromogen (NH), meanwhile L*,b* and H had great significant correlations; The information of C was a combination of a* and b*, while the H, degrees against the backdrop of a* value, was adjusted by b* value.3. Studies on the extraction method and structure of NHExtractions of NH from beef sliced ham with many different solvents were carried out. A three-step gradational extraction and isolation procedure with petroleum ether/acetone/ethyl acetate step by step was established based on the characteristics of UV and electron paramagnetic resonance spectroscopy (EPR) of NH solution.The structure of NH was identified as a pentacoordinate mononitrosylheme complex by four kinds of spectra, which were EPR, HPLC-electrospray ionization high resolution mass spectrometry (HPLC-ESI-HR-MS), infrared spectrum (IR) and Raman spectrum.4. Studies on the oxidation properties of extracted NHThe main research content in this chapter include the kinetics of NH degradation exposed in air and/or light, the spectra characteristics of NH oxidized in air with natural light or by addition of 1.5 mg/kg H2O2 as well as the prooxidation and antioxidation properties of NH in aβ-carotene/linoleate model system and cooked muscle model systems. Based on the results of above research, discoloration mechanism by oxidation was concluded as following. The predominant pigment (NH) of sliced ham is a photosensitizer and an antioxidant and is vulnerable to be oxidized when oxygen and light are present. The changed EPR spectra of oxidized NH in acetone suggested a new proposal that the NO' group might not detach itself from iron porphyrin of NH during oxidation in air with normal lighting, but changed in conjugated structure, and the structure tended to axial symmetry of the molecule. This transformation of NH is reversible and the reductant in matrix may cause fading return to original state or maintain a dynamic balance. As the NH oxidized, the polyunsaturated substrates in matrix were also oxidized gradually and form a stronger oxidant such as H2O2. When NH was oxidized by addition of H2O2, the NO group might interact with free radicals leaving the iron porphyrin of NH to quench free radicals in the medium, and subsequently the Fe2+ was converted to Fe3+ since it lost the protection of NO, and then released from the hemoglobin molecule. Fe3+ iron is the promoter of lipid oxidation and which promoted the lipid peroxidation rapidly, and in turn the oxidative degradation of NH was accelerated by products of lipid peroxidation further. This kind of decomposition is irreversible and will make the ham fade permanently.5. Studies on the stability of NH under different conditionsIn order to reveal the effect of other factors on color fading, the effect of different factors on the stability of NH as well as the effects of nitrogen component on the color of cooked beef slurries were studied. The results were shown as following.pH had significant influence on the optical properties and stability of NH, and the conditions of strong acid and weak alkali would help to maintain the red color of NH. The pink-red color of NH turned brown yellow under strong alkali condition. For the molecular structure of yellow NH, the NO group was still bind with iron porphyrin and this structure may be the same as the one of oxidized NH exposed in air. The main reason that pH significantly influenced the color stability of ham was the remarkable instability of NH under weak acid conditions.By investigating Cys, Pro, Thr, Gly, Val, Met, Leu and Tyr, significant effects were observed on color stability of NH acetone solution and of cooked beef slurries for all listed amino acid. Cys could improve the red color of NH, Pro could maintain the stability of NH, His caused the NH acetone solution appear red precipitate, and the rest of amino acids made the NH acetone solution transform to yellow-green to varying degrees. In cooked beef slurries system, these amino acid could form colored complexes by reaction with myoglobin or hemoglobin. His and Val produced a pink to reddish coloration, similar to that of the nitrite containing controls, and Gly, Tyr, Pro produced slurries with a thin pink to Beige, Cys and Met produced slurries with pigments of various shades of pink and tan. The pink heme derivative produced by His were water-soluble. All of these different colored derivative were more instability than NH, which may be another important reason for discoloration of sliced cooked cured ham.Fe2+, Na+, K+, Mg2+, Zn2+, Cu2+ and Ca2+ enhanced obviously the stability of NH acetone solution at the concentration meet with cooked meat product respectively. The best effect was present by addition of Fe2+, the next was in the decreasing order:Na+> K+≥Mg2+>Ca2+>Cu2+>Zn2+.The loss percentage of NH did not have remarkable differnce treated with different temperature below 70℃. Refrigerated and ambient temperature storage had no direct impact on the degradation of NH. The storage stabilities of NH treated with 75℃and 80℃were remarkably inferior to that of NH treated with 72℃. The loss percentage and stability of NH treated with 85℃were subjected to significant influence.6. Effect of natural antioxidant on color stability of pasteurized sliced hamsEffect of two kinds of natural antioxidant on color and oxidative stability of beef and pork pasteurized sliced ham were studied in this chapter. The results of the present study indicated that different antioxidant had different effects on the same stored ham, and the same antioxidant produced different effects on different ham. Both apple polyphenol (AP) and rosemary successfully inhibited lipid oxidation of both pork and beef sliced ham. However, different effect on color stability was observed. Addition of 500 mg/kg AP to the samples significantly inhibited discoloration in pork sliced ham but not in beef. Addition of 500～1500 mg/kg rosemary to the samples more significantly enhanced b* value than a* value, and there was a concentration-dependent increase on the b* value.