Effect of Freeze-Thaw Treatment on the Quality and Flavor of Toona sinensis Shoots

The effects of different freezing temperatures (−20, −40 and −80 ℃) and freeze-thaw cycles (FTC; 1, 2 and 3) on the nutritional components, flavor compounds and enzyme activities of Toona sinensis shoots were examined, aiming to identify methods for improving its quality and flavor. The results showed that after different FTCs, the T₂ relaxation time was shortened, the area of peak M₂₁ decreased, and the bound water was transformed into weakly bound water and free water. The contents of soluble protein and vitamin C both significantly decreased (P < 0.05), while the content of reducing sugars significantly increased, reaching the highest value after two FTCs with a freezing temperature of −40 ℃. The activities of polyphenol oxidase (PPO), peroxidase (POD) and lipoxygenase (LOX) all significantly increased. The activity of γ-glutamyl-transpeptidase (GGT) initially increased and then decreased with increasing number of FTCs at different freezing temperatures. The activity of alcohol dehydrogenase (ADH) was the highest after two FTCs at −80 ℃, the content of umami amino acids increased by 42.5%, and the total amino acid content increased by 38.4%. In addition, the sensory scores for overall odor intensity, cooked onion/T. sinensis-like flavor, and cooked meat-like aroma were all higher than those of the untreated control group. A total of 54 volatile compounds were identified using gas chromatography-ion mobility spectrometry (GC-IMS), with aldehydes being the most abundant, followed by terpenes. Following FTC treatment, several new compounds emerged, accompanied by a 22.2% increase in the total content of volatile compounds. By partial least squares-discriminant analysis (PLS-DA), 11 volatile compounds were selected as potential biomarkers using the cutoff of variable importance in the projection (VIP) score > 1. Correlation analysis between enzyme activities and volatile compounds revealed that GGT, LOX, and ADH exhibited a strongly negative correlation with alcohols and esters, while a positive correlation was observed with aldehydes, acids, terpenes, ketones, and other compounds. Collectively, two FTCs at −40 ℃ can effectively preserve the nutritional quality and enhance the flavor profile of T. sinensis shoots.