## 3 Results and Discussion
### Huanglongbing Effects on the Volatile Profile of \"Ray Ruby\" Grapefruit Peel Oil
Chemical groups increased and/or decreased by Huanglongbing (HLB) determined by the average concentration of volatiles in peel oil extracted from ‘Ray Ruby’ grapefruit in two harvest seasons, 2020 and 2021. * Significant differences at \(p\leq 0.05\) according to [PERSON]’s HSD test in both years. Sesquiterpene hydrocarbons were the second largest class in the peel oil, and the major VOCs in the class were (E)- \(\beta\)-caryophyllene, \(\delta\)-cadinene, \(\alpha\)-copaene, \(\beta\)-cubebene, and \(\gamma\)-muurolene, which were significantly increased by HLB affection in both years (Table 2). Similar observations were reported by [PERSON] et al. (2018) and [PERSON] et al. (2019) in orange peel  
Figure 4: Principal component analysis (PCA) score plot (**A**) and loading plot (**B**) of VOCs in peel oil of ‘Ray Ruby’ grapefruit of healthy (HLB\(-\)) and HLB-affected (HLB+) fruits from 2020 (HLB\(-\) 20 and HLB+ 20 \(\blacksquare\)) and 2021 (HLB\(-\) 21 \(\blacklozenge\) and HLB+ 21 \(\square\)) harvests. VOCs corresponding to the numbers are listed in Table 2.  
oil; stress from the bacterium preventing nutrient flow to the fruit may cause shifts in the biochemical pathways, creating a higher accumulation of those volatiles in the peel oil [18]. However, [PERSON] et al. [21] reported that HLB decreased sesquiterpenes in orange peel oil. In the 2020 harvest, HLB significantly increased the total aldehydes content in peel oil. However, there was no difference in the 2021 season (Table 2). The individual analyses of the aldehydes revealed that citronellal, perilla aldehyde, \(\beta\)-sinensal, octanal, gerani, and neral had higher content in HLB+ samples in 2020 or both seasons, while undecanal had lower content (Table 2). [PERSON] et al. [18] also found high citronellal and perilla aldehyde in oils of HLB symptomatic oranges. However, previous reports showed that HLB significantly reduced most long-chain aliphatic aldehydes, including octanal, nonanal, and decanal, of orange peel oil [4; 20; 21]. C8-C10 aldehydes are associated with peel oil quality by the pharmaceutical standard [19]. Noottadone content was consistently higher in HLB\(-\) peel oil samples, 7.9- and 2.7-fold higher in 2020 and 2021, respectively (Table 2). Noottadone is the most important and expensive aromatic compound in grapefruit [15]. In combination with other carbonyl-containing constituents, noottadone is responsible for the flavor of good-quality grapefruit oil [36]. The concentration of noottadone is used as the criterion for oil quality [15]. The accumulation of noottadone in the peel is associated with the processes of maturation and senescence of the fruits [37]. It has been suggested that HLB+ fruits have a slow maturation due to a compromised vascular system, similar to immature fruits [9]. This can explain why the sesquiterpene ketone in HLB+ peel oil was lower than in healthy fruit peel oil. Noottadone has been isolated from grapefruit peel and juice and can also be obtained from the oxidation of valencene [34; 35]. Naturally produced noottadone has a strong grapefruit odor, while the synthetic form shows a weak woody and spicy flavor, not typical of grapefruit flavor [34]. Grapefruit peel is the main source of noottadone for the industry, and due to its limited production, it has a very high price [36]. Thus, a lower concentration of this sesquiterpene in the peel oil of HLB+ fruit has a major economic impact. Previous studies reported that lower concentrations of linalool were found in orange peel oils derived from severely HLB+ fruit [18]. However, in the present study, HLB infection increased the accumulation of this compound in grapefruit peel oil in both seasons (Table 2). This terpene alcohol is an oxygenated product of D-limonene, but unlike limonene, linalool accounts for characteristic citrus and floral odors, an important aroma for citrus [38]. Because of its flavorful and fragment properties, linalool is used in the food, perfume, and cosmetics industries [39].