The interaction between muscle proteins and aroma compounds during processing can lead to a loss of aroma in the meat products, thereby altering the aroma profile for the consumer preference. Heat treatment has been commonly used in meat processing. The addition of spices during stewing meat can be used to eliminate the off-flavor for the enhanced color and the fragrance. Among them, terpenoid compounds are the key volatile flavor compounds in spices. But it is unclear on the mechanism of their interaction with muscle proteins. Myofibrillar protein (MP) and sarcoplasmic protein (SP) also play important roles as components of muscle proteins. This study aims to clarify the patterns and mechanisms of the interaction between muscle proteins and terpenoid compounds during heat treatment. Various physicochemical properties of MP and SP were determined at different heating times, such as surface hydrophobicity, total sulfhydryl content, particle size, and secondary structure content. Three terpenoid compounds (3-carene, limonene, and linalool) which significantly contributed to the aroma of stewed pork, were selected to investigate the interaction between aroma compounds and muscle proteins. Solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) was used to study the changes in the aroma absorption ability of proteins with heating time. The correlation was examined between aroma absorption and conformational changes of MP and SP. Moreover, the molecular docking analysis was also implemented to reveal the mechanism of muscle proteins binding to the terpenoid compounds. The results indicated that the heat treatment significantly altered the conformation of MP and SP. The surface hydrophobicity and total sulfhydryl content of MP and SP initially increased and then decreased, as the heating time increased, due to their unfolding and aggregation behaviors. Both MP and SP exhibited an increase in particle sizes during heating, particularly with a higher degree of SP aggregation, due to the lower thermal stability. Furthermore, the secondary structure content analysis showed that the intramolecular hydrogen bond of proteins was rearranged into intermolecular hydrogen bond, and then the ordered structure was transformed into a random coil conformation following heating, resulting in a decrease of α-helix structure content and an increase of β-sheet and random coil structure content. MP and SP demonstrated enhanced aroma adsorption during the initial 5 min of heating, due to their unfolded secondary structures and exposed binding sites. However, the continued heating led to the aggregation of MP and SP, burying the binding sites, and reducing their adsorption ability. Although the MP and SP exhibited similar adsorption within 0-5 min of heating, the SP displayed weaker adsorption than the MP after 10-60 min of heating. This difference was attributed to the higher degree of aggregation in the SP, which resulted in greater steric hindrance and the burial of more aroma-binding sites. Molecular docking results demonstrated that the hydrophobic interactions dominated the interactions between 3-carene/limonene and muscle proteins, whereas the linalool bound to muscle proteins through hydrophobic interactions and hydrogen bonds. Overall, the heat treatment can be expected to modify the conformation of muscle proteins, and then regulate the exposure or burial of aroma binding sites. Consequently, there was some impact on the interaction between muscle proteins and terpenoid compounds. The findings can provide theoretical guidance to control the aroma retention/release behavior during the thermal processing of meat products for the higher quality of meat products.
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This study clarified the escape behavior of key volatile compounds in stewed beef with soy sauce during air-cooling process and refrigeration, aiming to provide the theoretical basis for the flavor preservation and conformity of stewed beef with soy sauce.
The volatile flavors in stewed beef with soy sauce were identified by the electronic nose (E-nose) and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME/GC-MS) combined with odor activity value (OAV) and principal component analysis (PCA), which defined the dynamic change of flavors during cold air-cooling process (the central temperature was reduced from 50 ℃ to 10 ℃, the wind speed was 2 m∙s-1, and the cooling time was 90 minutes) and refrigeration (0, 6, 18, 30, 42 and 54 h at 4 ℃). Then the escape behavior of key flavor active compounds in stewed beef with soy sauce were revealed.
According to the E-nose analysis, the flavor profiles of stewed beef with soy sauce exhibited significant differences among the three stages (“out of the pot”, before air-cooling and after air-cooling). Nitrogen oxides, alcohols, aldehydes, ketones, aromatics and organic sulfides were considered the main compounds for the discrepancy. On the other side, the stewed beef with soy sauce after 0, 42 and 54 h refrigeration showed differential flavor profiles, which mainly caused by the variety in nitrogen compounds, alcohols, aldehydes and ketones. However, there was no significant difference in the flavor profiles among 6, 18 and 30 h refrigeration. The HS-SPME-GC-MS results showed that 39, 31 and 33 volatile compounds were identified in three cold air-cooling stages, and the contents were 13 636.18, 9 799.21 and 8 213.86 μg∙kg-1, respectively, with total amounts reducing by 39.8%. Morever, 36, 36, 34, 34, 31 and 29 volatile compounds were identified on different refrigeration periods, the contents were 7 712.65, 6 196.00, 5 319.42, 4 732.69, 5 295.05 and 4 281.82 μg∙kg-1, respectively, and the total contents decreased by 44.5%. Therefore, the flavor escape of stewed beef with soy sauce mainly occurred in the refrigeration stage. Besides, OAV analysis showed stewed beef with soy sauce in the cold air-cooling stage had 18 key active compounds, while eucalyptol, lauryl alcohol, valeraldehyde, and L-carvone were all lost before cold air-cooling, and acetoin, phenethyl alcohol, geranyl acetone, as well as linalool escaped severely, with their OAV values reducing by 81.3%, 64.0%, 63.7% and 55.1%, respectively. 15 key active volatile compounds were identified during refrigeration. Among them, isobutyl acetate and ethyl heptanoate were lost after 6 h. Anethole, eugenol, 4-allylanisole, and linalool were severely escaped, and the OAV values decreased by 63.9%, 63.8%, 58.1% and 53.9%, respectively. Refrigeration was the main stage for the loss and dissipation of flavor compounds in stewed beef with soy sauce; alcohols, ketones, esters and phenolic ethers were more susceptible to lose and escape during air-cooling and refrigeration; the loss and dissipation of flavor compounds were mainly derived from spices.
This study revealed the escape behavior of flavor active compounds in stewed beef with soy sauce during air-cooling process and refrigeration, and provide the theoretical basis for stewed beef's flavor retention and regulation with soy sauce.
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