Flavor improvement of enzymatic oyster juice by sequential heating through the accumulation of free amino acids and volatile flavor compounds.

Protease hydrolysis for production of oyster juice can greatly increase the nutrients and umami ingredients from oyster, but lacks the unique flavor of traditional oyster sauce. This study evaluated a sequential heating treatment consisting of primary heating at 107 °C for 6 h followed by secondary heating at 120 °C for 4 h and investigated the associated flavor-improvement mechanisms by detection of free amino acids and volatile flavor compounds. Compared to the unheated enzymatic juice, the concentrations of amino acid nitrogen and total free amino acids increased progressively after heating and reached their highest levels after secondary heating reaching 0.37 g/100 g and 3.02 g/100 g, respectively. The taste activity value of free amino acids in the enzymatic oyster juice was markedly promoted after sequential heating, especially some umami and sweet amino acids such as Glu and Ala. A total of 667 volatile flavor compounds were detected by HS-SPME-GC-MS, and 133 key volatile flavor compounds were screened out by OAV ≥ 1, mainly including aldehydes, alcohols, ketones, esters, heterocyclic compounds, and terpenoids. The total concentration of key volatile flavor substances in the enzymatic oyster juice increased significantly after two heating processes, rising from 34.30 mg/kg before heating to 75.51 mg/kg and then to 118.15 mg/kg. The odor profile of enzymatic oyster juice showed that the pleasant volatile flavor such as nutty, almond, fruity, green, and sweet odor was significantly enhanced by sequential heating, while the unpleasant volatile flavor such as fishy, earthy, fatty, sulfury, and spicy odor was significantly weakened. Correlation analysis and differential formation pathways showed that these changes were mainly attributed to thermal reaction such as Maillard reaction and Strecker degradation. Sequential heating is an effective method to obviously improve the flavor of enzymatic oyster juice.