Effect of Huanglongbing on the Volatile Organic Compound Profile of Fruit Juice and Peel Oil in 'Ray Ruby' Grapefruit

Abstract

Along with orange and mandarin, grapefruit production in Florida has declined sharply due to Huanglongbing (HLB), or citrus greening disease, caused by Candidatus Liberibacter asiaticus (CLas). HLB affects the volatile profiles of juice and peel oil in oranges, but there is limited information on grapefruit. In this research, 'Ray Ruby' grapefruit were harvested in 2020 and 2021 from healthy (HLB-) and HLB-affected (HLB+) trees. Peel oil was extracted by hydrodistillation, and the volatiles were analyzed by direct injection of the oil samples into gas chromatography-mass spectrometry (GC-MS). Volatiles in the juice were analyzed by headspace (HS)-solid-phase microextraction (SPME) coupled with GC-MS. HLB significantly altered the volatile profiles of peel oil and juice in 'Ray Ruby' grapefruit. Juice samples of HLB+ fruits had lower decanal, nonanal, and octanal, important citrus juice flavor compounds. HLB+ samples also showed reduced content of nonterpene compounds, other aliphatic and terpene aldehydes, and terpene ketones. Ethanol, acetaldehyde, ethyl acetate, and ethyl butanoate were increased in HLB+ juice samples, indicating an HLB-induced stress response. The most abundant compounds D-limonene and β-caryophyllene, as well as other sesquiterpenes, were increased in HLB+ juice and peel oil samples. On the other hand, the oxidative/dehydrogenated terpenes were increased by HLB in peel oil but decreased in the juice sample. Nootkatone, the key grapefruit volatile was consistently reduced by HLB in both peel oil and juice samples. The impact of HLB on nootkatone deteriorated the quality of both juice and peel oil in grapefruits.

Keywords: Citrus paradisi; gas chromatography–mass spectrometry; hydrodistillation; nootkatone; principal component analysis; solid phase microextraction.

Figure 7

Summary of metabolic pathways leading to monoterpenes, sesquiterpenes, and nootkatones [49,52,53,55]. Bar graphs show changes (%) caused by HLB affection in peel oil and juice in 2020 and 2021. MEP, methylerythritol phosphate pathway; MVA, mevalonic acid pathway; IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; Cstps1, valencene synthase.

Figure 1

Precipitation, maximum, and minimum temperatures for 2019 and 2020. Fort Pierce, FL, USA. (Source: FLORIDA CLIMATE CENTER, 2021).

Figure 2

Fruit appearance of ‘Ray Ruby’ grapefruit of healthy (HLB−) and HLB-affected (HLB+) fruits (2021).

Figure 3

Combined cluster analysis and heat map of grapefruit peel oil samples from healthy (HLB−) and HLB-affected fruits (HLB+) from 2020 (HLB− 20 and HLB+ 20) and 2021 (HLB− 21 and HLB+ 21) harvests (columns) and identified VOCs (rows). 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 ≤ 0.05 according to Tukey’s HSD test in both years.

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 ) and 2021 (HLB− 21 and HLB+ 21 ) harvests. VOCs corresponding to the numbers are listed in Table 2.

Figure 5

Combined cluster analysis and heat map of investigated grapefruit juice samples of healthy (HLB−) and HLB-affected fruits (HLB+) from 2020 (HLB− 20 and HLB+ 20) and 2021 (HLB− 21 and HLB+ 21) harvests (columns) and identified VOCs (rows). Chemical groups increased and/or decreased by Huanglongbing (HLB) determined the average concentration of volatiles in juice extracted from ‘Ray Ruby’ grapefruit in two harvest seasons, 2020 and 2021. * Significant differences at p ≤ 0.05 according to Tukey’s HSD test in both years.

Figure 6

Principal component analysis (PCA) score plot (A) and loading plot (B) of VOCs in the juice of ‘Ray Ruby’ grapefruit of healthy (HLB−) and HLB-affected (HLB+) fruits from 2020 (HLB− 20 and HLB+ 20 ) and 2021 (HLB− 21 and HLB+ 21 ) harvests. VOCs corresponding to the numbers are listed in Table 3.