Interaction mechanisms for the muscle proteins with terpenoid compounds during heat treatment

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.