Effect of drying techniques on the stability of volatile compounds and antioxidative activities of dried Thai fermented black soybean (thua nao)

Thua nao, a traditional fermented soybean product in Thailand, is widely consumed as a plant-based food or a condiment due to its distinct flavor, taste, and nutritional benefits. In this research, thua nao samples were produced from black soybeans. After fermentation, various drying methods, including sun drying (SUN), hot air drying (HOT), microwave vacuum drying (MIC), and vacuum drying (VAC) were employed to produce dried thua nao. The dried samples were stored under ambient conditions for 8 months. Results revealed lipid oxidation and the Maillard reaction occurred during storage, leading to a decline in bioactive compounds and antioxidant activities. Volatile aroma compounds were analyzed using solid-phase microextraction coupled with gas chromatography-mass spectrometry, while principal component analysis (PCA) evaluated aroma profile changes during storage. PCA indicated that HOT, MIC, and VAC -treated samples remained stable for up to 4 months, while SUN-treated samples changed after 2 months. It was found that SUN treatment prominently promoted the formation of oxidized volatile compounds associated with fatty-waxy and green attributes. The HOT treatment contributed more to aroma volatile compounds associated with nutty-roasted, floral, woody-smoky, and sweet attributes. While, both MIC and VAC treatments showed a noticeable loss of aroma volatile compounds, they retained higher level of bioactive compounds compared to the other treatments. This information aids in selecting suitable drying techniques for thua nao products and enhances understanding of chemical transformations induced by drying.