## 3 Results and Discussion
### Volatile Compounds in Mandarin Juices and Juice Powders
It has also been reported as one of the most important aroma-active compounds in citrus fruits, being responsible for a pleasant citrus-like odour [42]. In addition, D-limonene is listed as \"Generally Recognized as Safe (GRAS)\" in the Federal Regulations Codes and, therefore, has been widely used as a flavoring agent for food items of soft drinks, puddings, ice creams, fruit juices, and chewing gums, etc., as well as a fragrance additive in personal hygiene productions (soaps, perfumes, etc.) [43]. It is noted that there is no significant change in the concentration of D-limonene between the original mandarin juice and spray-dried powder (Table 4), which indicated a generally good preserving effect of the wall materials on juice flavour. In addition, \(\beta\)-myrcene, \(\gamma\)-terpinene, \(\alpha\)-pinene, and \(\beta\)-pinene are also major flavour compounds in mandarin samples, which was accordance with the free volatile profile of mandarin juice that was reported by [PERSON] et al. [41]. However, the retention of these compounds showed different trends across samples. For example, the concentration of \(\alpha\)-pinene in RS2 powder after spray-drying significantly increased from its original juice sample (juice vs. powder, 688 ug/L vs. 6613 ug/L), while, in the spray-dried RS1 sample, it dropped to the half value observed in the original juice (juice vs. powder, 218.4 ug/L vs. 112.2 ug/L). \(\beta\)-Myrcene, which was characterized by the sweet and balsamic odour, showed a decrease in concentration among all of the spray dried samples, except for RS1 and RS2. There are some reasons that are related to these phenomena. Firstly, the high temperature applied for spray drying could increase the relative volatility of volatile compounds during processing and thus, leading to the loss of volatiles in the final products. This was supported by a previous study on the odour retention of noni juice powders that were prepared by MFJ spray dry and freeze dry, respectively [15]. They discovered that the retention of octanoic acid and hexanoic acid in the freeze-dried powders were 90.2 and 90.1%, respectively, while, in the spray-dried microcapsules, their retentions were all below 50%. The difference was ascribed to the increased volatility of aromatic compounds induced by spray-drying [15]. On the other hand, the degradation of volatiles during thermal treatment might also result in decreased concentrations in powders. Secondly, glycosidically bound volatiles are widely found in most fruits with higher concentrations than their free counterparts [27], and mandarins showed a more abundant profile of glycosidic volatiles than other citrus species, i.e., grape fruit, as reported by [PERSON] et al. [41]. During spray drying, the acidic juice matrix with the aid of thermal processing condition could lead to the liberation of free volatile aglycones from their corresponding glycosidic precursors. Consequently, this will cause the increase of aroma compounds in the final powders. Finally, volatile compounds would go through transformation under heat treatment, for example, linalool and D-limonene would transform into \(\alpha\)-terpineol [42], and this could contribute to the accumulation of specific compounds. \(\alpha\)-Terpineol and terpinene-4-ol are reported as two off-flavour compounds in mandarin juices [42]. In this study, the heat that is induced by spray-drying did not cause their augmentation. Especially for \(\alpha\)-terpineol, its concentration in powder samples were much lower (\(p<0.05\)) than that in the initial mandarin juices. This indicated that microencapsulation might have a masking effect on the unpleasant odour that is present in mandarin juices. In addition, three aldehydes (hexanal, nonanal, and decanal) and two alcohols (1-heptanol and 1-octanol) were found in both juices and microcapsules samples.