Quality Deterioration and Volatile Metabolite Profile of ‘Orah’ Mandarin During Low-Temperature Storage and Room-Temperature Shelf Life

Low-temperature stored mandarin was prone to rapid quality deterioration when moved to a room-temperature shelf (RTS). However, systematic research on the safe threshold of time for low-temperature storage (LTS) and the key metabolic mechanisms underlying the quality deterioration was lacking. The dynamic changes in color, intrinsic quality, off-flavor and volatility of ‘Orah’ mandarin during LTS (6–8 °C, 0 d to 180 d) and thereafter RTS for 7 days were determined, and the key characteristic metabolites related to deterioration were analyzed. RTS after an LTS for 0 d, 20 d, 40 d, 60 d, 80 d, 100 d or 120 d exacerbated the decline in pericarp chromatic a* value, content of soluble solid, titratable acid, vitamin C, sucrose and malic acid in the pulp, as well as the accumulation of ethanol, methanol and acetaldehyde. After the LTS for 120 days and thereafter RTS for 7 days, the accumulation of ethanol exceeded 2000 mg/L, resulting in a distinct unpleasant odor. Seven categories (alcohols, aldehydes, esters, lipids/fatty acyls, terpenoids, organic acids, tetrahydropyrans) and 43 volatile substances were detected in the pulp. A significant decrease in terpenes within the first 60 days of LTS resulted in fast loss of aroma, while the substantial increase in ethanol, methanol and acetaldehyde (ethyl octoate, caproic acid) in the middle and later stages of LTS contributed to the prominent off-flavor. In conclusion, the LTS should not exceed 100 days due to the intensified accumulation of ethanol. The above results indicated that intervention techniques should be implemented to inhibit the synthesis of ethanol in the middle of storage, and methods such as reducing temperature fluctuations should be used to block the rapid accumulation of ester compounds during the RTS.