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Alanine (Ala), as the most important free amino acid, plays a significant role in food taste characteristics and human health regulation. The feasibility of using near–infrared hyperspectral imaging (NIR–HSI) combined with two–dimensional correlation spectroscopy (2D–COS) analysis to predict beef Ala content quickly and nondestructively is first proposed in this study. With Ala content as the external disturbance condition, the sequence of chemical bond changes caused by synchronous and asynchronous correlation spectrum changes in 2D–COS was analyzed, and local sensitive variables closely related to Ala content were obtained. On this basis, the simplified linear, nonlinear, and artificial neural network models developed by the weighted coefficient based on the feature wavelength extraction method were compared. The results show that with the change in Ala content in beef, the double-frequency absorption of the C-H bond of CH2 in the chemical bond sequence occurred prior to the third vibration of the C=O bond and the first stretching of O-H in COOH. Furthermore, the wavelength within the 1136–1478 nm spectrum range was obtained as the local study area of Ala content. The linear partial least squares regression (PLSR) model based on effective wavelengths was selected by competitive adaptive reweighted sampling (CARS) from 2D–COS analysis, and provided excellent results (R2C of 0.8141, R2P of 0.8458, and RPDp of 2.54). Finally, the visual distribution of Ala content in beef was produced by the optimal simplified combination model. The results show that 2D–COS combined with NIR–HSI could be used as an effective method to monitor Ala content in beef.

Taking the eutectic point as the final freezing temperature, the differences of flavor substances of in hand grab mutton (HGM) frozen at three rates of 0. 26 cm/h (−18°C), 0.56 cm/h (−40°C) and 2.00 cm/h (−80°C) were determined and analyzed. The results showed that the flavor of HGM decreased significantly after freezing. With the increase of freezing rate, the contents of aldehydes, alcohols, ketones, acids, esters, others, free amino acids and 5′-nucleotides were higher, and the content of specific substances was also generally increased. All samples from unfrozen and frozen HGM could be divided into four groups using an electronic nose based on different flavor characteristics. Seven common key aroma components were determined by relative odor activity value (ROAV), including hexanal, heptanal, octanal, nonanal, (E)-oct-2-enal, (2E,4E)-deca-2,4-dienal and oct-1-en-3-ol. The higher the freezing rate, the greater the ROAVs. Taste activity values calculated by all taste substances were far <1, and the direct contribution of the substances to the taste of HGM was not significant. The equivalent umami concentration of HGM frozen at −80°C was the highest. These findings indicated that higher freezing rate was more conducive to the retention of flavor substances in HGM, and the flavor fidelity effect of freezing at −80°C was particularly remarkable.

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The dynamic changes of key volatile compounds and water distribution in the pig skin jelly with different NaCl addition (0.0%, 0.5%, 1.0%, 2.0%, and 4.0%) were identified and analyzed. A total of 28 volatile compounds were identified in all samples of pig skin jelly. Compared with the sample without NaCl addition, aldehydes, alcohols and ketones were significantly increased in the pig skin jelly with NaCl addition. In addition, 12 key volatile flavor compounds, including hexanal, nonanal, octanal, (E,E)-2,4-decadienal, (E,Z)-2,6nonadienal, (E)-2-nonenal, (E)-2-heptenal, (E)-2-octenal, 1-hexanol, 2-ethyl-1-hexanol, 1-octen-3-ol and 2,3-octanedione were further identified. With the increase of NaCl concentration, the free water located in the intercellular space was transformed into immobilized water entrapped in the myofibrillar network, increasing the water-holding capacity of pig skin jelly. Correlation analysis showed that T 21 had strong negative correlations with (E)-2-nonenal and 1-hexanol, T 22 had strong negative correlation with (E,E)-2,4-decadienal, and positive with (E,Z)-2,6-nonadienal and (E)-2-heptenol, and T 23 had strong negative correlations with (E,Z)-2,6-nonadienal and (E)-2heptenol. In conclusion, this study provides a reference for the utilization of pig by-product and salt reduction of Chinese meat products.