| In this research, the moisture mobility and quality variation of chicken breast under the freezing-thawing cycles, cooking and the drying process was investigated by the Low-Field Nuclear Magnetic Resonance (LF-NMR). The dynamic mechanism of quality variation was further discussed by using the Dynamic Mechanical Analyzer (DMA) in order to provide a theoretical basis and technical support for the production, quality control and regularization of the chicken products. The main results are as follows:1. Study on the quality changes of chicken under the freezing-thawing cyclesNMR T2 relaxation results indicated changes in the migration and distribution of the bound water, the immobilized water and the bulk water of chicken in the freezing and thawing circles. The transverse relaxation time (T2b) and the signal per mass (A1) of the bound water didn’t change significantly after the freezing-thawing cycles(P>0.05). The transverse relaxation time (T21) and the signal per mass (A2) of the immobilized water reduced significantly (P<0.05) with the numbers of the freezing-thawing cycles increased. The changing regularity of the bulk water was insignificant.With the number of the freezing-thawing cycles increased, the thawing loss and centrifugal loss for 3 freezing-thawing cycles were higher than those for 1 freezing-thawing cycle (P<0.05); the cooking loss decreased (P<0.05), then increased and finally decreased; the total moisture content of the frozen-thawed chicken was significantly lower than that of the non-frozen-thawed chicken (P<0.05). The lightness and redness of the thawed chicken for 3 freezing-thawing cycles were higher than that of fresh chicken significantly (P<0.05). The pH value and TVBN value of thawed chicken increased gradually with the number of the freezing-thawing cycles increased. Freezing has a significant inhibitory effect on bacterial growth, and changes of the total viable count were variant, but it began to increase significantly (P<0.05) since 5 freezing-thawing cycles.The transverse relaxation time of the immobilized water (T21) had an extremely significant negative correlation with the number of freezing-thawing cycles and the ligntness (P<0.01), which was in significant negative correlation with the yellowness and significantly positive correlated with the total water content(P<0.05). The signal per mass of the immobilized water (A2) and that of the bound water (A1) were positively correlated (P<0.01), but the former correlated negatively with that of bulk water(A3) (P<0.05). Changes of the muscular microstructure could be reflected by the mutual migration and conversion of different water populations. The meat quality deteriorated when the transverse relaxation time and the signal per mass of the immobilized water decreased. The chicken was inedible after being frozen and thawed for 4 cycles.2. Study on the effect of cooking temperatures on the quality of the frozen-thawed chickenThe transverse relaxation time of the immobilized water (T21) increased (P<0.05) and then decreased (P<0.05) when the thawed chicken was cooked at 75℃,84℃ and 90℃ and reached the maximum for 2 freezing-thawing cycles. The T21 of the thawed chicken cooked at 87℃ first decreased, then increased and decreased finally (P<0.05) and it reached the maximum for the non-frozen-thawed chicken. When cooked at 93℃ and 100℃, the T21 of the frozen-thawed chicken increased firstly, decreased and then increased again, then reached the maximum value for 2 freezing-thawing cycles and the minimum for 3 freezing-thawing cycles. The transverse relaxation of the bound water and the bulk water of the frozen-thawed chicken cooked at all the temperaures changed to a certain degree with an insignificant regularity.With the cooking temperature increased, the total moisture content of chicken showed an overall downward trend after the first rise. The variation of the total moisture content of the frozen-thawed chicken cooked at 75℃ and 93℃ was the similar, both of which reached the maxmum for 1 freezing-thawing cycle. The springiness and cohesiveness of the frozen-thawed chicken didn’t exist significant difference (P>0.05) for all the cooking temperatures. When cooked at 75℃,84℃ and 100℃, the hardness of the chicken reached the maximum for 2 freezing-thawing cycles and then decreased significantly (P<0.05); the hardness of those cooked at 90℃ reached its maximum for 3 freezing-thawing cycles; when cooked at 87℃ and 93℃, the hadness of the cooked chicken reached the maximum for 3 freezing-thawing cycles and then decreased. The chewiness showed the same tendency with the hardness.The freezing-thawing cycles affected the quality of the cooked chicken, which could be reflected by the transverse relaxation, the total moisture content and the texture of the cooked chicken. The hardness of the cookek chicken increased due to the decreased of the transverse relaxation time (T21) and the signal per mass (A2) of the immobilized water of the chicken breast.3. Study on the moisture migration of the chicken during the drying processThe NMR T2 relaxation spectrum clearly showed the moisture transfer and distribution change of chicken during the drying process. The mobility of the bound water was almost unaffected for all the drying temperatures with the extension of the drying time. But the content of the immobilized water reduced gradually and the transverse relaxation time (T21) decreased significantly(P<0.05). The mobility of the bulk water weaken while the changing regularity was not strong. Accompanied by changes in muscle tissue structure, the higher the drying temperature, the lower the moisture content at the same time.The signal per mass of the immobilized water (A2) and the total populations (A) were extremely significant correlated (P<0.01). The total moisture content positively correlated with the signal per mass of the immobilized water and the total populations(P< 0.01), which showed that the immobilized water was the main existence form of the water in the chicken. With the drying temperatures increased, the drying time shortened. The signal per mass of the immobilized water of the chicken reduced significantly when dried for the same time at all the drying temperatures. And the changes of A2 determined the changes of the total moisture content of the dried chicken.4. Thermodynamic analysis of chicken qualityThe storage modulus and loss modulus increased with temperature increased for each freezing-thawing cycle and the former was always higher than the latter. Changes of the non-forzen-thawed chicken were more curve than those of the frozen-thawed chicken. With the programmed temperature increased, the storage modulus and loss modulus of the non-frozen-thawed chicken showed two distinct dynamic changing phases. For the first stage, the temperature was about 40℃ to 60℃ and the storage modulus and the loss modulus increased slowly. For the second stage, the storage modulus and the loss modulus increased rapidly from about 68℃. The storage modulus and the loss modulus of the frozen-thawed chicken changed similiarly without any break at about 68℃.The storage modulus and the loss modulus of chicken cooked at 75℃,87℃,93℃ and 100℃ were lower than those of the uncooked chicken. The storage modulus and the loss modulus of the frozen-thawed chicken were higher than the non-frozen-thawed chicken with the trend of gentle decrease. When cooked at 100℃, the loss modulus of the frozen-thawed chicken was lower than that of frozen-thawed chicken at the other cooking temperatures. |
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