Steam Explosion (STEX) of Citrus × Poncirus Hybrids with Exceptional Tolerance to Candidatus Liberibacter Asiaticus (CLas) as Useful Sources of Volatiles and Other Commercial Products.

Florida citrus production has declined 75% due to Huanglongbing (HLB), a disease caused by the pathogenic bacterium Candidatus Liberibacter asiaticus (CLas). Methods to combat CLas are costly and only partially effective. The cross-compatible species Poncirus trifoliata and some of its hybrids are known to be highly tolerant to CLas, and thus can potentially serve as an alternative feedstock for many citrus products. To further investigate the commercial potential of citrus hybrids, three citrus hybrids, US-802, US-897, and US-942, were studied for their potential as feedstocks for citrus co-products using steam explosion (STEX) followed by water extraction. Up to 93% of sugars were recovered. US-897 and US-942 have similar volatile profiles to that of the commercial citrus fruit types and as much as 85% of these volatiles could be recovered. Approximately 80% of the pectic hydrocolloids present in all three hybrids could be obtained in water washes of STEX material. Of the phenolics identified, the flavanone glycosides, i.e., naringin, neohesperidin, and poncirin were the most abundant quantitatively in these hybrids. The ability to extract a large percentage of these compounds, along with their inherent values, make US-802, US-897, and US-942 potentially viable feedstock sources for citrus co-products in the current HLB-blighted environment.

Analysis and Potential Value of Compounds Extracted From Star Ruby, Rio Red, and Ruby Red Grapefruit, and Grapefruit Juice Processing Residues via Steam Explosion.

Culled whole grapefruit (WG) and grapefruit juice processing residues (GP) are currently incorporated into low-cost animal feed. If individual chemical components found within these side streams could be recovered as high-value coproducts, this would improve the overall value of the grapefruit crop. In this study, pectic hydrocolloids, sugars, volatiles, phenolics, and flavonoids were extracted from Star Ruby, Rio Red, and Ruby Red GP and WG using a continuous pilot scale steam explosion system. Up to 97% of grapefruit juice oils and peel oils could be volatilized and contained 87-94% d-limonene. The recovery of pectin, as determined by galacturonic acid content, was between 2.06 and 2.72 g 100 g-1. Of the phenolics and flavonoids analyzed in this study, narirutin and naringin were extracted in the amounts of up to 10,000 and 67,000 µg g-1, respectively.

Pectic hydrocolloids from steam-exploded lime pectin peel: Effect of temperature and time on macromolecular and functional properties.

Previously, we showed the weight average molecular weight (M w) and intrinsic viscosity ([ƞ]) of pectic hydrocolloids recovered from steam-exploded citrus peel were low, suggesting fragmentation due to process temperature and/or time-at-temperature. We have tested this hypothesis on a commercial lime pectin peel, washed to remove soluble sugars and dried for stabilization, using a static steam explosion system. We examined temperatures of 120-150°C at 1-3 min hold times. Galacturonic acid recovery and M w ranged from 22% to 82% and 142-214 kDa, respectively. Recovery of most major pectic sugars increased concomitantly with galacturonic acid as temperature and time-at-temperature increased. [ƞ] ranged from 1.75 to 6.83 dl/g. The degree of methylesterification ranged from 66.5% to 72.1%. Tan (δ) (Loss modulus/Storage modulus; G″/G') values of sugar-acid gels for 120-140°C treatments were <1.0. Ideal optimization analysis, where time, [ƞ], and percent recovery were maximized, identified processing conditions that favor either increased [ƞ] or percent recovery. The results presented here support our hypothesis that temperature and time-at-temperature affect M w and [η] of the recovered pectic hydrocolloids. These results also demonstrate that manipulating either temperature or time-at-temperature enables the production of structurally varied populations of pectic hydrocolloids. Based on optimization analysis, commercially viable values of [ƞ] can be obtained while recovering approximately 50% of the pectic hydrocolloids.

Quercus leaf extracts display curative effects against Candidatus Liberibacter asiaticus that restore leaf physiological parameters in HLB-affected citrus trees.

Citrus greening, also called Huanglongbing (HLB), is one of the most destructive citrus diseases worldwide. It is caused by the fastidious gram-negative α-proteobacteria bacterium Candidatus Liberibacter asiaticus (CLas) and vectored by the Asian citrus psyllid (ACP), Diaphorina citri. Currently, there is no cure for HLB, no compounds have been successful in controlling HLB, and no sustainable management practices have been established for the disease. Thus, searching for alternative citrus greening disease mitigation strategies is considered an urgent priority for a sustainable citrus industry. The aim of this study was to use compounds extracted from oak, Quercus hemisphaerica, and to assess the antibacterial effects of these against CLas-infected citrus plants. The application of aqueous oak leaf extracts showed substantial inhibitory effects against CLas in citrus plants and the activity of genes related to starch. Significant differences were also observed in plant phenotypic and physiological traits after treatments. Citrus plants treated with oak extracts displayed an increase in stomatal conductance, chlorophyll content and nutrient uptake concurrently with a reduction of CLas titer, when compared to citrus plants treated with just water. The information provided from this study suggests a new management treatment program to effectively deal with the HLB disease.