Location: Citrus and Other Subtropical Products Research2017 Annual Report
Objective 1: Establish bioactive and sensory characteristics of new marketable genotypes (citrus, tomato, strawberry) and new Florida crops (avocado, blueberry, peach). Objective 2: Enable real-time, commercial pre- and postharvest treatments to optimize shelf life of new genotypes and new Florida crops using packaging, coatings, and maturity markers. Objective 3: Identify new sensory targets, enable new sensors, processing methods and management strategies to predict and mitigate HLB disease effects on citrus juice nutritional and flavor quality. Sub-Objective 3a: Identify chemical and biological markers that characterize the effect of HLB on fruit/juice quality. Sub-Objective 3b: Develop methods to mitigate the effect of HLB on citrus juice quality. Sub-Objective 3c: Develop methods to mitigate the effect of HLB on citrus fruit quality.
Phenotypes for fruit quality in citrus, tomatoes, strawberry, peaches and avocados will be screened for flavor markers: volatiles, sugars and acids, and sensory characteristics by gathering chemical and sensory data on a wide range of genetically variable breeding lines (or hybrids). In the long term, plant breeders will identify genes associated with fruit quality traits and map them on the genome to aid in marker-assisted selection. For advanced selections or commercial cultivars of peach and avocado, fruit will be harvested multiple times during maturation and ethylene and respiration rate will be measured at harvest and in stored fruit. For the effect of citrus greening or Huanglongbing (HLB) disease on orange fruit/juice flavor, fruit or juice will be obtained from collaborators, or from groves undergoing various field treatments (pesticides, growth regulators, antibiotics or thermotherapy), or from trees grown on different rootstocks to study a wide range of HLB flavor symptoms. Juices will be tested for CLas infection by qPCR and for levels of sugars, acids, volatiles, flavonoids, limonoids and for flavor perception using sensory evaluation. Taste panels will serve as the biosensors for compounds isolated from HLB-affected orange juice to determine compounds responsible for these putative off-flavor taste attributes. The electronic nose and electronic tongue will be used to screen for HLB-induced off-odor or flavor. The effect of HLB on the flavor quality of grapefruit and tangerines will be investigated. HLB-induced off-flavor can be managed by blending, by modifying juice processes or by adding citrus-derived natural compounds (volatiles or non-volatiles) to mask or bind off-flavor compounds. Studies will be conducted on several citrus types using fungicide sprays (strobulorins, Topsin) targeting D. natalensis to determine if the HLB-induced fruit drop and postharvest stem end rot can be reduced.
Flavor volatiles, sugars and acids were analyzed from ARS citrus (Fort Pierce, Florida) and peach (Byron, Georgia) breeding collections for a second season. For citrus, the objective is to identify hybrids that show tolerance to Huanglongbing (HLB) that show potential for fresh or processed market. For peaches, the objective is to compare aroma volatile profiles in peach and nectarine genotypes with different melting/non-melting and yellow/white flesh. The aroma volatile profiles and interrelationships among volatile compounds will provide fundamental information to link with genetic markers and develop marker-assisted selection (MAS) in future peach breeding. Peaches of the melting flesh/free stone and non-melting flesh/cling stone were harvested from a commercial farm in Fort Pierce, Florida, and evaluated for shelf life. Clamshell packaging and edible coatings were applied to the melting flesh peaches, which presented a shorter shelf-life than the non-melting flesh peaches. An edible coating considerably delayed fruit ripening without preventing water loss, while clamshell packaging with reduced ventilation prevented water loss but sometimes increased decay. For future work, a compromise might be possible by applying edible coating on riper fruit and packaging in a clamshell to prevent water loss. Avocados of the West Indian type from a collection at ARS Fort Pierce, Florida, were harvested weekly or biweekly to establish a database for harvest maturity, phenotypes and chemicals (fatty acids and volatiles) for these hybrids under South Florida conditions. All hybrids had similar shelf life of about 2 weeks at room temperature, although the last harvests were strongly impacted by postharvest decay due to Colletotrichum sp. Flavor volatiles, sugars and acids were analyzed from guava and tropical strawberry collections in south Florida, in collaboration with the University of Florida tropical fruit breeder. Initial data will be used by the breeder to determine selections for potential progenitors and to understand the genes that control flavor in fruit. Lychee is another tropical fruit with potential for Florida growers and for which there is a gap of knowledge with respect to optimum maturity and storage. Hakip, Mauritious and Brewster varieties were harvested multiple times in 2016 and effects of cultivar, harvest maturity, antibrowning treatments and clamshell packaging on fruit shelf life and flavor were evaluated. Humidity-efficient packaging significantly improved appearance and flavor quality of stored fruit. Seven selections from the University of Florida (UF) strawberry breeding program and two commercial cultivars were evaluated each month by a taste panel, and fruit were analyzed for sugars, acids firmness and volatiles. The fruit of FL 12.121-5 were evaluated for the third consecutive year, and were always rated high in sweetness and strawberry flavor. As a result, FL12.121-5 was released in August 2016 by UF, and named ‘Florida Beauty’. Ten tomato selections, including both round and plum tomato types, from the UF tomato breeding program, were evaluated by a consumer panel, and the fruit prepared for analysis of sugars, acids and volatiles. In another experiment, heirloom and commercial tomato cultivars were tested for quality with maturity and post-storage cold- or heat-treatments. The heirloom ‘Cherokee Purple’ had much higher soluble solids content and titratable acidity than other cultivars. An electronic tongue was able to separate samples by cultivar and maturity, and could be used to predict soluble solids content. Commercial tomatoes with intrinsic (genetic) low flavor, good-flavored hybrid, and a favorite heirloom cultivar were harvested at different maturities, exposed to chilling or blanching, coated with paraffin oil, and treated with methyl salicylate or salicylic acid. The ripening behavior, chilling injury response, and flavor quality were determined by volatile analysis and gene expression. Refrigeration (chilling) and blanching (heating) did not change sugar or acid levels, but refrigeration induced more changes as measured by e-tongue compared with blanching. Winter melon (Benincasa hispida) is a new Florida crop that is of interest to the juice industry because it has potential for blending with other juices to reduce sugars while still preserving qualification for the “100% fruit juice” label. Two winter melon types, “long” and “short”, were processed and evaluated for volatiles, nutrient composition and sensory evaluation in collaboration with CRADA partner. Juice made with winter melon of the short type had a mild acceptable taste, whereas juice made with the long type had a vegetable flavor, unacceptable in a fruit juice. Processing unpeeled fruit enhanced unacceptable vegetable flavor. Humidity-efficient packaging research was continued and experiments were conducted with cherry tomato, sweet cherry, litchi, peach and apricot. Among various postharvest handling conditions, the common and key factor for the quality deterioration was produce water loss. A humidity-efficient clamshell improved quality of many fruit and extended storage life. Controlled release chlorine dioxide (ClO2) technology was improved by using polymer films with different permeabilities. The ClO2 concentration in perforated packages with produce was monitored more accurately. Experiments were performed in collaboration with the ARS Food Quality and Safety Laboratory in Beltsville, Maryland, to determine the effect of ClO2 on foodborne microorganisms. Results indicate that ClO2 possesses strong antibacterial and antifungal activities on inoculated cherry/grape tomatoes, improved fruit safety, reduced decay and weight loss, and maintained firmness during storage. A survey of fresh-cut mango quality on supermarket shelves was implemented in a three-way collaboration between ARS researchers in Fort Pierce, Florida, UF, and University of California in Davis. Cut mangoes in clamshell packaging were purchased bi-weekly from local stores in each of the cities, brought to the laboratories and evaluated for quality. It was found that generally, the initial whole fruit was cut at an immature stage, resulting in sour and firm slices which do not represent the eating quality of a ripe fruit. Furthermore, packaging was generally gas restrictive, leading to extreme levels of O2 and CO2 in the package atmosphere and resulting in off flavor. This study led to funding of a second study to optimize mango cutting conditions with post-processing treatments and new packaging. Grapefruit trees from a commercial grove appearing to be free of Huanglongbing (HLB) symptoms were harvested fruit and tasted by the in-house trained panel in comparison with fruit from the same grove but from trees affected by HLB. Taste panels indicated that fruit from trees free from HLB symptoms were similar or sweeter than fruit from the trees affected by HLB. A second harvest was performed and waiting for analysis to confirm previous year data. Volatile and non-volatile compounds from orange juice extracts were added to orange juice in order to mitigate the off flavor induced by the HLB. Overall, the non-volatile orange juice extract reduced some of the bitterness and harshness found in HLB-affected juice. Volatiles added in juice at various levels and combinations generally increased sweetness perception and decreased bitterness perception in HLB juice, but results were affected by the base juice used for spiking. Orange juice from healthy or HLB-affected fruit was analyzed using chromatographic separation combined with solvent extractions and resulted in five fractions and 10 sub-fractions composed of various flavonoids. The fractions and sub-fractions were evaluated by taste panels and high performance liquid chromatography (HPLC) analysis. Compounds other than the known bitter limonoid aglycones (limonin and nomilin) and astringent flavonoids were present in the extracts with high bitterness, astringency and harshness and are suspected to be derivatives of hydroxycinnamic acids. Transcriptome analysis was done on abscission zones of orange fruit affected or not by HLB, and that were tightly or loosely held on the tree. Results showed that the expression of genes that respond to biotic stress and fungal pathogens were upregulated in HLB, especially in HLB dropped fruit, whereas those that respond to abiotic stress (such as water or carbohydrate shortage) were down regulated. Genes for plant defense response and ethylene production were upregulated in HLB dropped fruit, which produced ethylene, an abscission hormone. This supports evidence from previous studies that pre-harvest fruit drop related to HLB, is partly due to an opportunistic fungus that normally causes postharvest stem end rot, but that is invading fruit on weakened HLB-affected trees in the field, exacerbating fruit drop.
1. ‘Florida Beauty’ strawberry cultivar release. The two leading cultivars grown in Florida in 2016-2017 are ‘Florida Radiance’ and Sweet Sensation® (‘Florida127’) have high yield, uniform and attractive fruit, with Sweet Sensation® having excellent eating quality and long shelf life. However, these two cultivars do not produce well, with small unmarketable fruit in November/early December, when market prices are highest. The newly released ‘Florida Beauty’ seems to be filling that production gap. ARS researchers at Fort Pierce, Florida, evaluated ‘Florida Beauty’ for three consecutive seasons, and found it had eating quality equal or superior to Sweet Sensation® throughout the production season (December through March). Production of ‘Florida Beauty’ will allow strawberry growers in Florida to garner optimum prices in the early market window while offering consumers a strawberry fruit with superior eating quality.
2. Identification of a fungus that is exacerbating pre-harvest fruit drop and postharvest stem end rot of citrus affected by citrus greening disease. Citrus greening disease or huanglongbing (HLB) is devastating the citrus industry, especially in Florida where yields have declined 70% from optimal pre-disease years. Part of the decline in yield is due to pre-harvest fruit drop from trees affected by Huanglongbing (HLB). ARS researchers at Ft. Pierce, Florida, determined that an opportunistic fungus, that usually causes postharvest stem end rot, is infecting fruit from HLB-weakened trees in the field and contributing to pre-harvest fruit drop, in addition to increasing postharvest stem end rot. This is resulting in pre- and postharvest losses for the industry. Molecular analysis confirmed the presence of the fungus and its inducement of a plant hormone that promotes fruit drop. Use of fungicides in the field reduced the incidence of fungi and subsequent fruit drop, which growers are now using as part of their production practices to bolster yields.
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