In order to investigate the pattern of changes in taste-related metabolites during the growth process of Shitougan fruit and to reveal the chemical basis of its taste, the taste-related metabolites in two edible parts, juice sac and segment membrane, at different growth stages were determined by using untargeted metabolomics combined with quantitative analysis in comparison with the results of electronic tongue detection. The results showed that the contents of three major sweet components, sucrose, glucose and fructose, in the two edible parts generally showed an increasing trend during fruit growth and development. The contents of citric acid, tartaric acid, quinic acid and aconitic acid showed a decreasing trend, whereas the content of malic acid first increased and then decreased, and the content of fumaric acid increased only slightly. The contents of bitter components such as flavanones, limonin, and normilin showed a decreasing trend. The expression of differential metabolites in both edible parts concentrated in the biosynthesis of secondary metabolites and amino acid biosynthesis and metabolism pathways, with more substances being accumulated in the segment membrane, and the differential metabolites were dominated by the ABC transporter pathway. The contents of sucrose, fructose and citric, malic and tartaric acids in both edible parts of ripe fruit were (11.78 ± 0.79) and (10.58 ± 0.88) mg/g, (8.06 ± 0.59) and (10.32 ± 0.61) mg/g, (4.73 ± 0.04) and (4.21 ± 0.08) mg/g, (3.20 ± 0.02) and (3.24 ± 0.07) mg/g, (0.34 ± 0.12) and (0.07 ± 0.04) mg/g, respectively, which were higher than their respective lowest taste threshold concentrations. The contents of the bitter substances naringin, neohesperidin, limonin, and normilin were (0.60 ± 0.02), (3.45 ± 0.06), (0.29 ± 0.01) and (0.002 ± 0.00) mg/g in the segment membrane, respectively, which were higher than the taste thresholds, but not in the segment membrane. Sweet (serine, proline and alanine), bitter (arginine), and umami (aspartic acid and glutamic acid) amino acids all had taste intensity values greater than 1 in both juice sac and segment membrane, and these substances synergized with each other to form the distinctive taste traits of Shitougan fruit. The results of this study reveal the composition of the substances contributing to the taste of Shitougan fruits in a preliminary and comparative systematic way, and provide theoretical references for the basic research on the molecular mechanism and metabolism of the formation of important taste-contributing substances.

Bitterness affects consumer acceptance of foods as a taste sensation generally considered undesirable. The bitter taste of plant foods originates from compounds such as phenolics, terpenoids, alkaloids, and amino acids. Most bitter compounds are biologically active, contributing to disease prevention and having human health benefits. Citrus is a typical class of fruits with bitter taste. The degree of bitterness and functional activity of citrus fruits varies greatly among different varieties due to the different types and contents of bitter substances. In recent years, as the functionality of bitter substances has been widely recognized, an extensive literature has developed on citrus bitter substances. This article reviews the types, structures, biochemical properties, synthesis and metabolism pathways, and functional activities of bitter substances in citrus fruits. In addition, we summarize the main debittering techniques for citrus fruit processing, and envision future research directions toward bitter substances and their synergistic effects, rational processing pathways and breeding, aiming to provide theoretical references for the effective utilization of bitter substances in citrus fruits.