Location: Citrus and Other Subtropical Products Research
2016 Annual Report
Objectives
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.
Approach
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.
Progress Report
Flavor volatiles were analyzed from ARS citrus (Fort Pierce, Florida) and peach (Byron, Georgia) breeding collections. For citrus, the objective was to classify hybrids that have potential tolerance to huanglongbing (HLB) disease as sweet orange for juice processing. Out of several hybrids that were HLB tolerant and had sweet orange-like shape, color and taste, one had a volatile profile similar to that of a sweet orange and could be classified as “sweet orange” for orange juice processing. For peaches, volatile production was more influenced by maturity rather than by phenotype, except for hexanol that was detected in the peach with fuzzy skin and not its nectarine (smooth skin) mutants.
Taste panels were performed with untrained panelists for ten selected avocado hybrids tolerant to laurel wilt, cold hardy and with fruit similar to commercial ‘Hass’ avocado. Consumers liked all hybrids similarly to commercial ‘Hass’, except for one. Those hybrids had a similar lipid profile to ‘Hass’. Lipids in avocado contribute to its health benefits as well as desired sensory attributes such as creaminess. With declining citrus acreage due to HLB, cold-hardy, ‘Hass’-like avocadoes adapted to Florida climate and resistant to laurel wilt present a potential for future commercial planting.
Evaluation of University of Florida strawberry genotypes was continued as in previous years, using sensory evaluation and chemical analysis. One genotype shows potential for future varietal release. Work is being continued with University of Florida collaborators to determine the genetic and environmental control of methyl anthranilate production, a volatile compound contributing to fruity flavor in strawberries.
University tomato breeding lines were compared to Florida commercial cultivars over seven years, 2-3 seasons per year for chemical and sensory traits, the analysis for which was completed and published to provide breeders with chemical traits to target to improve flavor quality.
Commercial tomato cultivars grafted on new rootstocks were evaluated by taste panels and chemical analysis, with two winter and one spring harvest. Overall, when tests showed significant differences between samples, fruit on ungrafted control plants were preferred, had higher flavor, firmness and sweetness than fruit on grafted rootstocks, except for a few exceptions.
A combination of the controlled-release sanitizer, chlorine dioxide, and a humidity-efficient clamshell was used to extend shelf-life and reduce foodborne microorganisms of strawberries, blueberries and cherry tomatoes at room temperature or under refrigeration. Currently, most of commercial applications of chlorine dioxide is at high doses (> 20 parts per million) for a short period of time (< 20 min). This work determined the efficiency of using low levels (< 10 ppm) of chlorine dioxide for long exposure time (>24 h), and found optimum combinations for each fresh commodity.
Harvest maturity was studied for several citrus hybrids over several seasons. The data is currently being analyzed. Results will provide growers information for best harvest date of new cultivars for optimum eating quality.
Chilling injury in tomatoes occurs when tomatoes are stored at temperatures below 13°C. The inhibitive effect of chilling treatments on volatile production in tomato fruits was confirmed not only for unripe fruit (simulating transportation and marketing of mature-green harvested fruits), but also for ripe fruit (simulating kitchen practices). The reduction of volatiles was also found after hot water dips that can be used to peel fresh tomatoes at home. This research indicates some consumer responsibility for decreased tomato quality in their kitchen practices.
Grapefruit is another commodity susceptible to chilling injury when fruit is stored below 10°C, with symptoms such as surface pitting and increased storage decay. A study was performed in collaboration with University of Florida where fruit were dipped in hot water before storing at 5°C or 10°C for seven weeks. The experiment was repeated with fruit harvested early (mid-October), mid-season (mid-February) and late (late March). There were effects of heat treatments or storage temperature only on fruit harvested late, with heat treatment increasing sugar content and sweetness.
University of Florida (UF) blueberry cultivars were harvested at two weeks intervals and stored for three weeks, with weekly evaluation by a trained sensory panel and sampling for firmness, soluble solids and titratable acidity. Indigocrisp and a UF numbered selection were firmest and had highest sweetness most of the time, regardless of harvest or storage. On the contrary, there was a strong harvest effect for firmness, sweetness or sourness for other cultivars. A cultivar with good eating quality that is stable over harvests and in storage is highly desirable.
Electronic tongue (e-tongue), can differentiate juices based on their soluble components, and quantitative polymerase chain reaction (qPCR) can estimate the amount of deoxyribonucleic acid (DNA), for example of Candidatus Liberibacter asiaticus (CLas), the presumably causal agent of HLB disease. These techniques were used to quantify HLB disease in orange juice. The e-tongue responded to changing levels of sugar, citric acid and salt in orange juice (as determined by spiking a model orange juice), but did not respond to changes in the bitter compound limonin at levels normally found in orange juice. The e-tongue successfully separated orange juices of different HLB severities and harvest maturities. qPCR of CLas DNA in orange juice positively correlated with HLB off-flavor as defined by a highly trained taste panel, with high bitterness, tingling, metallic and stale off flavor. qPCR of CLas could be used efficiently as a marker of HLB infection in a juice and indicator of quality.
Orange juice was fractionated multiple times to isolate compounds that were tasted by trained panelists. Compounds that have high bitterness and harshness typically found in HLB juice are in the process of being further isolated and identified.
Orange juice was spiked with orange aroma and other compounds that were identified in a previous study to contribute to increased sweetness or decreased bitterness in juice. Aroma compounds could not decrease bitterness perception of juice that was severely affected by HLB with high bitterness and harshness; however, with juice that was mildly affected by HLB, adding selected aroma and other non-volatile compounds significantly improved the positive attributes of orange juice flavor.
HLB-affected trees were subjected to solar heat treatments to mitigate HLB progression, and the fruit and juice quality from these heat treatments were evaluated over several harvests. Overall, solar heat treatments not only mitigated the HLB–affected trees in groves but also restored juice quality by increasing sugars and sweetness.
Grapefruits harvested from trees that showed tolerance to HLB amidst an infected and declining grove were tasted in a preliminary taste panel, and were all of acceptable eating quality. Studies that evaluate citrus trees that show any tolerance to HLB-induced decline will be given high priority in evaluating their fruit/juice quality.
The postharvest citrus disease, stem end rot, caused by Lasiodiplodia theobromae (formerly known as Diplodia natalensis, hereafter termed Diplodia) was observed to be prevalent on fruit harvested from trees symptomatic for HLB compared to non-symptomatic trees. These fruit also produced ethylene, an abscission hormone that causes fruit drop. This led to a study where fruit that dropped off healthy or HLB-affected orange trees when shaken, were compared to fruit that remained on the tree. Dropped fruit from HLB-affected trees showed high levels of Diplodia and produced ethylene compared to fruit that remained on the same tree.
Transcriptomic studies of the abscission zone of these fruit showed upregulated genes for the ethylene, jasmonic, cell wall synthesis and phenolic pathways compared to fruit that remained on the tree or fruit from healthy trees, indicating promotion of ethylene, the abscission zone and fruit drop. Furthermore, in a field study where trees were sprayed with a fungicide, fruit drop was reduced for three out of five citrus varieties treated, and in these three varieties Diplodia levels were lower and fruit removal force (force necessary to remove the fruit from the tree) was higher. Thus incidence of Diplodia correlated to HLB, ethylene, fruit removal force and fruit drop and likely represents a secondary infection on HLB-weakened trees. Thus, what normally is a postharvest disease, is infecting fruit pre-harvest and may be contributing to HLB-associated pre-harvest fruit drop, which may be mitigated through use of fungicides.
In the Diplodia study where trees were shaken, quality of dropped fruit that were loosely held on the tree was compared to quality of fruit that remained on the tree for two citrus varieties over several harvests. There were no differences between dropped or tree-harvested fruit from healthy trees. However, fruit from HLB affected trees produced juice that was closer to juice from healthy trees if the fruit remained on the tree after shaking, while fruit that dropped were of inferior quality. Therefore, methods to prevent fruit drop, by preventing formation of abscission zone, could also minimize quality loss due to HLB.
Accomplishments
1. Pre-harvest treatments to mitigate the negative impact of Huanglongbing (HLB) on postharvest fruit and juice quality. The devastating citrus disease HLB imparts a typical off-flavor to orange juice, with high bitterness and harshness. Tree decline is the ultimate fate; however, growers and researchers are trying management practices to delay that decline. ARS researchers at Fort Pierce, Florida, applied solar heat in the field by covering trees with plastic with precise timing, with results reducing the pathogen concentration and improving tree health and fruit quality. ARS researchers also demonstrated that fruit likely to drop prior to harvest due to HLB and possibly co-infection with Diplodia natalensis have increased typical HLB off-flavor. Using solar heat in the field and preventing secondary infection by Diplodia as a field management tools can be used to reduce fruit drop maintain yield and quality of orange juice.
2. A sensory and chemical flavor analysis of tomato genotypes grown in Florida during multiple seasons. Many years of breeding tomatoes for disease resistance, yield and size has resulted in fruit that lack flavor. A collaborative study with the University of Florida tomato breeding program was conducted and evaluated 38 tomato genotypes over 7 years with 2-3 seasons per year, to understand tomato flavor. Genotypes and harvest seasons significantly influenced sensory perception and chemical profiles and one cultivar, ‘Tasti-Lee’, now readily available in supermarkets, was released from that study. When comparing Florida 47, the Florida industry standard, with ‘Tasti-Lee’, 8 out of 29 aroma compounds were higher in ‘Tasti-Lee’ (including those contributing to fruity and floral aromas) and 4 were higher in Florida 47 (those compounds giving green aromas). This provides a useful chemical model for two genotypes that differ in flavor quality that can be exploited by breeders seeking to improve flavor. Consumers will benefit from having tastier tomatoes on the market.
Work to evaluate new crops such as avocado with potential to replace citrus acreage lost to Huanglongbing will benefit rural Florida horticulture.
Collaborations with strawberry and blueberry breeders benefit small fruit growers in rural Florida by providing fruit that can have longer shelf-life and better eating quality, therefore increasing market potential.
Work that helps juice processors manage the effect of Huanglongbing on juice flavor will help rural citrus growers, processors and packers to stay in business.
Review Publications
Wang, L., Baldwin, E.A., Bai, J. 2016. Recent advance in aromatic volatile research in tomato fruit: The metabolisms and regulations. Food and Bioprocess Technology. 9(2):203-216.
Imahori, Y., Bai, J., Baldwin, E.A. 2015. Antioxidative responses of ripe tomato fruit to postharvest chilling and heating treatments. Scientia Horticulturae. 198:398-406.
Chen, H., Mccollum, T.G., Baldwin, E.A., Bai, J. 2016. Impacts of Huanglongbing symptom severity on fruit detachment force and mechanical properties of sweet oranges (Citrus sinensis). HortScience. 51(4):356-361.
Yu, Q., Plotto, A., Baldwin, E.A., Bai, J., Huang, M., Yu, Y., Dhaliwal, H.S., Gmitter, F.G. 2015. Proteomic and metabolomic analyses provide insight into production of volatile and non-volatile flavor components in mandarin hybrid fruit. Biomed Central (BMC) Plant Biology. 15:76. doi:10.1186/s12870-015-0466-9.
Sun, X., Baldwin, E.A., Ritenour, M., Plotto, A., Bai, J. 2015. Evaluation of natural colorants and their application on citrus fruit as alternatives to Citrus Red No. 2. HortScience. 50(9):1353-1357.
Raithore, S., Bai, J., Plotto, A., Manthey, J.A., Irey, M., Baldwin, E.A. 2015. Electronic tongue response to chemicals in orange juice that change concentration in relation to harvest maturity and citrus greening or Huanglongbing (HLB) disease. Sensors. 15(12):30062-30075.
Baldwin, E.A., Scott, J.W., Bai, J. 2015. Sensory and chemical flavor analyses of tomato genotypes grown in Florida during three different growing seasons in multiple years. Journal of the American Society for Horticultural Science. 140(5):490-503.
Raithore, S., Dea, S., Mccollum, T.G., Manthey, J.A., Bai, J., Leclair, C., Hijaz, F., Narciso, J., Baldwin, E.A., Plotto, A. 2016. Development of delayed bitterness and effect of harvest date in stored juice from two complex citrus hybrids. Journal of the Science of Food and Agriculture. 96:422-429.
Whitaker, V., Chandler, C., Peres, N., Do Nascimento Nunes, C., Plotto, A., Sims, C. 2015. Sensation™ ‘Florida 127’ Strawberry. HortScience. 50(7):1088-1091.
Wang, L., Baldwin, E.A., Zhao, W., Plotto, A., Sun, X., Wang, Z., Brecht, J., Bai, J., Yu, Z. 2015. Suppression of volatile production in tomato fruit exposed to chilling temperature and alleviation of chilling injury by a pre-chilling heat treatment. LWT - Food Science and Technology. 62:115-121.
Wang, L., Baldwin, E.A., Yu, Z., Bai, J. 2015. The impact of kitchen and food service preparation practices on the volatile aroma profile in ripe tomatoes: Effects of refrigeration and blanching. HortScience. 50(9):1358-1364.
Zhao, W., Bai, J., Mccollum, T.G., Baldwin, E.A. 2015. High incidence of preharvest colonization of huanglongbing-symptomatic Citrus sinensis fruit by Lasiodiplodia theobromae (Diplodia natalensis) and exacerbation of postharvest fruit decay by that fungus. Applied and Environmental Microbiology. 81(1):364-372. https://doi.org/10.1128/AEM.02972-14.
