Towards identification of postharvest fruit quality transcriptomic markers in Malus domestica

Gene expression is highly impacted by the environment and can be reflective of past events that affected developmental processes. It is therefore expected that gene expression can serve as a signal of a current or future phenotypic traits. In this paper we identify sets of genes, which we call Prognostic Transcriptomic Biomarkers (PTBs), that can predict firmness in Malus domestica (apple) fruits. In apples, all individuals of a cultivar are clones, and differences in fruit quality are due to the environment. The apples transcriptome responds to these differences in environment, which makes PTBs an attractive predictor of future fruit quality. PTBs have the potential to enhance supply chain efficiency, reduce crop loss, and provide higher and more consistent quality for consumers. However, several questions must be addressed. In this paper we answer the question of which of two common modeling approaches, Random Forest or ElasticNet, outperforms the other. We answer if PTBs with few genes are efficient at predicting traits. This is important because we need few genes to perform qPCR, and we answer the question if qPCR is a cost-effective assay as input for PTBs modeled using high-throughput RNA-seq. To do this, we conducted a pilot study using fruit texture in the 'Gala' variety of apples across several postharvest storage regiments. Fruit texture in 'Gala' apples is highly controllable by post-harvest treatments and is therefore a good candidate to explore the use of PTBs. We find that the RandomForest model is more consistent than an ElasticNet model and is predictive of firmness (r2 = 0.78) with as few as 15 genes. We also show that qPCR is reasonably consistent with RNA-seq in a follow up experiment. Results are promising for PTBs, yet more work is needed to ensure that PTBs are robust across various environmental conditions and storage treatments.

Possible sources of Listeria monocytogenes contamination of fresh-cut apples and antimicrobial interventions during antibrowning treatments: a review

Fresh-cut apples, being rich in antioxidants and other nutrients, have emerged as popular snacks in restaurants, at home and in school lunch programs, partially due to freshness, convenience, and portion size availability. Two major challenges in processing fresh-cut apples are browning of cut surfaces and contamination with human pathogens. Regarding human pathogens, contamination by Listeria monocytogenes is a major concern, as evidenced by two recent outbreaks of whole apples and numerous recalls of fresh-cut apples. Antibrowning agents currently used by the industry have little to no antimicrobial properties. The present review discusses possible origins of L. monocytogenes in fresh-cut apples, including contaminated whole apples, and contamination via the processing environment and the equipment in fresh-cut facilities. Treatment with antibrowning solutions could be an opportunity for Listeria contamination and represents the last chance to inactivate pathogens. The discussion is focused on the antibrowning treatments where formulations and coatings with antibrowning and antimicrobial properties have been developed and evaluated against Listeria and other microorganisms. In addition, several research needs and considerations are discussed to further reduce the chance of pathogen contamination on fresh-cut apples.

Canopy attachment position influences metabolism and peel constituency of European pear fruit

BACKGROUND

Inconsistent pear fruit ripening resulting from variable harvest maturity within tree canopies can contribute to postharvest losses through senescence and spoilage that would otherwise be effectively managed using crop protectant and storage regimes. Because those inconsistencies are likely based on metabolic differences, non-targeted metabolic profiling peel of 'd'Anjou' pears harvested from the external or internal canopy was used to determine the breadth of difference and link metabolites with canopy position during long-term controlled atmosphere storage.

RESULTS

Differences were widespread, encompassing everything from expected distinctions in flavonol glycoside levels between peel of fruit from external and internal canopy positions to increased aroma volatile production and sucrose hydrolysis with ripening. Some of the most substantial differences were in levels of triterpene and phenolic peel cuticle components among which acyl esters of ursolic acid and fatty acyl esters of p-coumaryl alcohol were higher in the cuticle of fruit from external tree positions, and acyl esters of α-amyrin were elevated in peel of fruit from internal positions. Possibly the most substantial dissimilarities were those that were directly related to fruit quality. Phytosterol conjugates and sesquiterpenes related to elevated superficial scald risk were higher in pears from external positions which were to be potentially rendered unmarketable by superficial scald. Other metabolites associated with fruit aroma and flavor became more prevalent in external fruit peel as ripening progressed and, likewise, with differential soluble solids and ethylene levels, suggesting the final product not only ripens differentially but the final fruit quality following ripening is actually different based on the tree position.

CONCLUSIONS

Given the impact tree position appears to have on the most intrinsic aspects of ripening and quality, every supply chain management strategy would likely lead to diverse storage outcomes among fruit from most orchards, especially those with large canopies. Metabolites consistently associated with peel of fruit from a particular canopy position may provide targets for non-destructive pre-storage sorting used to reduce losses contributed by this inconsistency.

Differential Partitioning of Triterpenes and Triterpene Esters in Apple Peel

Apple peel is a rich source of secondary metabolites, and several studies have outlined the dietary health benefits of ursane-type triterpenes in apple. Changes in triterpene metabolism have also been associated with the development of superficial scald, a postharvest apple peel browning disorder, and postharvest applications of diphenylamine and 1-methylcyclopropene. Previously, studies have generated metabolite profiles for whole apple peel or apple wax. In this study, we report separate metabolic analyses of isolated wax fractions and peel epidermis to investigate the spatial distribution of secondary metabolites in peel. In addition to examining previously reported triterpenes, we identified several unreported fatty acid esters of ursane-type triterpenes (C14-C22). All free pentacyclic triterpenes and triterpenic acids, with the exception of β-amyrin, were localized in the wax layer, along with esters of ursolic acid and uvaol. All sterols, sterol derivatives and α-amyrin esters were localized in the dewaxed peel epidermis.

Survey of conspecific herbivore-induced volatiles from apple as possible attractants for Pandemis pyrusana (Lepidoptera: Tortricidae)

BACKGROUND

Studies were conducted to identify volatiles released by apple foliage untreated or sprayed with a yeast and from untreated and sprayed foliage with actively feeding larvae of Pandemis pyrusana Kearfott. Field studies then evaluated various combinations of these volatiles when paired with acetic acid as possible adult attractants.

RESULTS

The most abundant volatiles released following herbivore feeding were four green leaf volatiles (GLVs) and acetic acid. Nineteen volatiles were found to be released in significantly higher amounts from foliage with herbivore damage than from intact leaves. The combination of yeast followed by herbivore injury increased the levels of methyl salicylate and phenylacetonitrile compared with herbivory alone. Levels of acetic acid released were not significantly different among the four treatments. Only phenylacetonitrile and 2-phenylethanol with acetic acid caught similar and significantly more total and female moths than acetic acid alone. Moth catches with 12 other volatiles plus acetic acid were not significantly higher than with acetic acid alone, and were lower than with acetic acid and 2-phenylethanol.

CONCLUSION

These data show that herbivore injury does not create a unique chemical signal for adults to locate oviposition or rendezvous sites. Instead, moths may cue to the aromatic-acetic acid combination as a nutritional cue to locate sugary resources. © 2017 Society of Chemical Industry.

Delayed response to cold stress is characterized by successive metabolic shifts culminating in apple fruit peel necrosis

BACKGROUND

Superficial scald is a physiological disorder of apple fruit characterized by sunken, necrotic lesions appearing after prolonged cold storage, although initial injury occurs much earlier in the storage period. To determine the degree to which the transition to cell death is an active process and specific metabolism involved, untargeted metabolic and transcriptomic profiling was used to follow metabolism of peel tissue over 180 d of cold storage.

RESULTS

The metabolome and transcriptome of peel destined to develop scald began to diverge from peel where scald was controlled using antioxidant (diphenylamine; DPA) or rendered insensitive to ethylene using 1-methylcyclopropene (1-MCP) beginning between 30 and 60 days of storage. Overall metabolic and transcriptomic shifts, representing multiple pathways and processes, occurred alongside α-farnesene oxidation and, later, methanol production alongside symptom development.

CONCLUSIONS

Results indicate this form of peel necrosis is a product of an active metabolic transition involving multiple pathways triggered by chilling temperatures at cold storage inception rather than physical injury. Among multiple other pathways, enhanced methanol and methyl ester levels alongside upregulated pectin methylesterases are unique to peel that is developing scald symptoms similar to injury resulting from mechanical stress and herbivory in other plants.

Gene expression and metabolism preceding soft scald, a chilling injury of 'Honeycrisp' apple fruit

BACKGROUND

'Honeycrisp' is an apple cultivar that is susceptible to soft scald, a chilling injury expressed as necrotic patches on the peel. Improved understanding of metabolism associated with the disorder would improve our understanding of soft scald and contribute to developing more effective management strategies for apple storage. It was expected that specific gene expression and specific metabolite levels in the peel would be linked with soft scald risk at harvest and/or specific time points during cold storage.

RESULTS

Fruit from nine 'Honeycrisp' apple orchards that would eventually develop different incidences of soft scald between 4 and 8 weeks of cold air storage were used to contrast and determine differential transcriptomic and metabolomic changes during storage. Untargeted metabolic profiling revealed changes in a number of distinct pathways preceding and concurrent with soft scald symptom development, including elevated γ-aminobutryic acid (GABA), 1-hexanol, acylated steryl glycosides, and free p-coumaryl acyl esters. At harvest, levels of sesquiterpenoid and triterpenoid acyl esters were relatively higher in peel of fruit that did not later develop the disorder. RNA-seq driven gene expression profiling highlighted possible involvement of genes and associated metabolic processes with soft scald development. These included elevated expression of genes involved in lipid peroxidation and phenolic metabolism in fruit with soft scald, and isoprenoid/brassinosteroid metabolism in fruit that did not develop soft scald. Expression of other stress-related genes in fruit that developed soft scald included chlorophyll catabolism, cell wall loosening, and lipid transport while superoxide dismutases were up-regulated in fruit that did not develop the disorder.

CONCLUSIONS

This study delineates the sequential transcriptomic and metabolomic changes preceding soft scald symptom development. Changes were differential depending on susceptibility of fruit to the disorder and could be attributed to key stress related and mediating pathways.

Chilling-related cell damage of apple (Malus × domestica Borkh.) fruit cortical tissue impacts antioxidant, lipid and phenolic metabolism

'Soggy breakdown' (SB) is an internal flesh disorder of 'Honeycrisp' apple (Malus × domestica Borkh.) fruit that occurs during low temperature storage. The disorder is a chilling injury (CI) in which visible symptoms typically appear after several weeks of storage, but information about the underlying metabolism associated with its induction and development is lacking. The metabolic profile of flesh tissue from wholly healthy fruit and brown and healthy tissues from fruit with SB was characterized using gas chromatography-mass spectrometry (GC-MS) and liquid chromatograph-mass spectrometry (LC-MS). Partial least squares discriminant analysis (PLS-DA) and correlation networks revealed correlation among ester volatile compounds by composition and differences in phytosterol, phenolic and putative triacylglycerides (TAGs) metabolism among the tissues. anova-simultaneous component analysis (ASCA) was used to test the significance of metabolic changes linked with tissue health status. ASCA-significant components included antioxidant compounds, TAGs, and phytosterol conjugates. Relative to entirely healthy tissues, elevated metabolite levels in symptomatic tissue included γ-amino butyric acid, glycerol, sitosteryl (6'-O-palmitoyl) β-d-glucoside and sitosteryl (6'-O-stearate) β-d-glucoside, and TAGs containing combinations of 16:0, 18:3, 18:2 and 18:1 fatty acids. Reduced metabolite levels in SB tissue included 5-caffeoyl quinate, β-carotene, catechin, epicatechin, α-tocopherol, violaxanthin and sitosteryl β-d glucoside. Pathway analysis indicated aspects of primary metabolism differed according to tissue condition, although differences in metabolites involved were more subtle than those of some secondary metabolites. The results implicate oxidative stress and membrane disruption processes in SB development and constitute a diagnostic metabolic profile for the disorder.

Ripening, storage temperature, ethylene action, and oxidative stress alter apple peel phytosterol metabolism

The chilling conditions of apple cold storage can provoke an economically significant necrotic peel disorder called superficial scald (scald) in susceptible cultivars. Disorder development can be reduced by inhibiting ethylene action or oxidative stress as well as intermittent warming. It was previously demonstrated that scald is preceded by a metabolomic shift that results in altered levels of various classes of triterpenoids, including metabolites with mass spectral features similar to β-sitosterol. In this study, a key class of phytosterol metabolites was identified. Changes in peel tissue levels of conjugates of β-sitosterol and campesterol, including acylated steryl glycosides (ASG), steryl glycosides (SG) and steryl esters (SE), as well as free sterols (FS), were determined during the period of scald development. Responses to pre-storage treatment with the ethylene action inhibitor, 1-methylcyclopropene, or an antioxidant (diphenylamine), rapid temperature elevation, and cold acclimation using intermittent warming treatments were evaluated. Diphenylamine, 1-MCP, and intermittent warming all reduced or prevented scald development. ASG levels increased and SE levels decreased in untreated control fruit during storage. Removing fruit from cold storage to ambient temperature induced rapid shifts in ASG and SE fatty acyl moieties from unsaturated to saturated. FS and SG levels remained relatively stable during storage but SG levels increased following a temperature increase after storage. ASG, SE, and SG levels did not increase during 6 months cold storage in fruit subjected to intermittent warming treatment. Overall, the results show that apple peel phytosteryl conjugate metabolism is influenced by storage duration, oxidative stress, ethylene action/ripening, and storage temperature.

Use of chemical sanitizers to reduce microbial populations and maintain quality of whole and fresh-cut cantaloupe

Whole cantaloupes either not inoculated or inoculated with Salmonella Poona were submerged in water, 180 ppm of chlorine, acidified calcium sulfate (ACS: 1.2% Safe(2)O-ACS50), 1,000 ppm of acidified sodium chlorite (ASC), 80 ppm of peroxyacetic acid (PAA), and a combination of ACS and PAA for 10 min. Although only ASC and the combination of ACS and PAA significantly reduced the aerobic plate count of samples taken from the surface of whole cantaloupe (compared with samples taken from cantaloupe submerged in water only), all treatments reduced yeast and mold counts on the whole cantaloupe. However, none of the treatments of whole cantaloupes consistently reduced yeast and mold counts for the samples of fresh-cut cantaloupes. The aerobic plate counts for fresh-cut cantaloupe were reduced by 1 to 2 log CFU/g by sanitization of whole fruit with ASC, ACS, and the combination of ACS and PAA. The low bacterial population on the fresh-cut fruit was maintained during 14 days of storage at 4 degrees C. All treatments had a limited effect on the population of Salmonella, achieving no more than a 1.5-log reduction of the pathogen inoculated on the surface of the whole cantaloupes. Salmonella was nondetectable via direct plating (with a detection limit of 0.4 log CFU/g) in fresh-cut cantaloupes prepared from whole cantaloupes treated with any of the sanitizers. However, after enrichment, Salmonella often was detectable. Color, texture, soluble solids, pH, ascorbic acid, and drip loss of cut cantaloupes were not consistently affected by any of the whole-fruit treatments. Overall, treatments of whole cantaloupe with ASC, ACS, and the combination of ACS and PAA at the concentrations tested permitted a significant reduction in Salmonella and native microflora of whole and cut fruit; however, Salmonella still could be found in cut cantaloupes from all treatments.

Metabolomic change precedes apple superficial scald symptoms

Untargeted metabolic profiling was employed to characterize metabolomic changes associated with 'Granny Smith' apple superficial scald development following 1-MCP or DPA treatment. Partial least-squares discriminant analyses were used to link metabolites with scald, postharvest treatments, and storage duration. Models revealed metabolomic differentiation between untreated controls and fruit treated with DPA or 1-MCP within 1 week following storage initiation. Metabolic divergence between controls and DPA-treated fruit after 4 weeks of storage preceded scald symptom development by 2 months. alpha-Farnesene oxidation products with known associations to scald, including conjugated trienols, 6-methyl-5-hepten-2-one, and 6-methyl-5-hepten-2-ol, were associated with presymptomatic as well as scalded control fruit. Likewise, a large group of putative triterpenoids with mass spectral features similar to those of ursolic acid and beta-sitosterol were associated with control fruit and scald. Results demonstrate that extensive metabolomic changes associated with scald precede actual symptom development.

Diphenylamine metabolism in 'braeburn' apples stored under conditions conducive to the development of internal browning

Diphenylamine metabolism and ethylene action were evaluated as factors influencing the development of 'Braeburn' apple internal browning and cavitation during cold storage. Apples treated with the antioxidant diphenylamine (DPA) and/or the ethylene action inhibitor 1-methylcyclopropene (1-MCP) were held at 1 degrees C for up to 6 months in air or a controlled atmosphere (CA) containing 1 kPa of O2 and 3 kPa of CO2. Cortex tissues from fruit without disorders as well as from symptomatic and asymptomatic areas of fruit with disorders were analyzed for DPA and DPA derivative content. Internal browning and cavities developed in control and 1-MCP-treated fruit stored in CA, whereas air-stored and CA fruit treated with DPA or with DPA and 1-MCP prior to storage did not develop disorders. Depending on the storage regimen and duration, less DPA was detected in 1-MCP-treated fruit. The 4-hydroxydiphenylamine (4OHDPA) content of control fruit decreased during air storage duration but increased between 2 and 4 months in CA storage. 4OHDPA content in 1-MCP-treated fruit increased with storage duration in CA but not air. N-Nitrosodiphenylamine (NODPA) was detected after 2 months in control fruit stored in air or CA and in 1-MCP-treated fruit stored in CA, and NODPA content in control fruit was higher compared to that in 1-MCP-treated fruit. Accumulation of 4-methoxydiphenylamine (4MeODPA) in control fruit stored in air increased with storage duration, but 4MeODPA content did not change in 1-MCP-treated fruit stored in air or CA. 2-Nitrodiphenylamine content was reduced by prestorage treatment with 1-MCP, but storage environment and duration had no effect on its accumulation. The results indicate that CA storage increases the risk of disorder development in 'Braeburn' apples, that DPA can prevent disorder development, and that the content of DPA and DPA derivatives is influenced by storage environment and ethylene action. A clear relationship between DPA derivative formation and storage conditions that promote internal browning was not apparent.

Prestorage ultraviolet-white light irradiation alters apple peel metabolome

Global metabolic profiling of 'Granny Smith' apple peel was employed for evaluating metabolomic alterations resulting from prestorage UV-white light irradiation. Apples were bagged midseason to restrict sunlight, harvested at the preclimacteric stage prior to bag removal, treated with fluorescent UV-white light for 0-48.5 h, and stored for 6 months at 0 degrees C. Trimethylsilyl (oxime) derivatized or underivatized aliquots of methanolic extracts from peel samples collected immediately after irradiation or following cold storage were evaluated using GC-MS and LC-UV/vis-MS, respectively. The profile, including more than 200 components, 78 of which were identified, revealed changes in the metabolome provoked by UV-white light irradiation and cold storage. Analyses of individual components selected using principal component analysis (PCA) models showed distinct temporal changes, before and after cold storage, related to prestorage irradiation in a diverse set of primary and secondary metabolic pathways. The results demonstrate metabolic pathways associated with ethylene synthesis, acid metabolism, flavonoid pigment synthesis, and fruit texture, are altered by prestorage irradiation, and many of the alterations are detectable after 6 months of cold storage in air.

Combination of hot-water surface pasteurization of whole fruit and low-dose gamma irradiation of fresh-cut cantaloupe

Improvements in methods for disinfecting fresh-cut cantaloupe could reduce spoilage losses and reduce the risk of foodborne illness from human pathogen contamination. The objective of this study was to investigate the feasibility of using hot-water treatment in combination with low-dose irradiation to reduce native microbial populations while maintaining the quality of fresh-cut cantaloupe. Whole cantaloupes were washed in tap water at 20 or 76 degrees C for 3 min. Fresh-cut cantaloupe cubes, prepared from the washed fruit, were then packaged in clamshell containers, and half the samples were exposed to 0.5 kGy of gamma radiation. Native microflora populations and sensory qualities were evaluated during the subsequent 7 days of storage at 4 degrees C. The hot-water surface pasteurization reduced the microflora population by 3.3 log on the surface of whole fruits, resulting in a lower microbial load on the fresh-cut cubes compared with cubes cut from fruit treated with cold water. Irradiation of cubes prepared from untreated fruit to an absorbed dose of 0.5 kGy achieved a low microbial load similar to that of cubes prepared from hot-water-treated fruit. The combination of the two treatments was able to further reduce the microflora population. During storage, the headspace atmosphere of the packages was not significantly influenced by any of the treatments. Color, titratable acidity, pH, ascorbic acid, firmness, and drip loss were not consistently affected by treatment with irradiation, hot water, or the combination of the two. Cubes prepared from hot-water-treated whole fruit had slightly lower soluble solids content. The combination of hot-water pasteurization of whole cantaloupe and low-dose irradiation of packaged fresh-cut melon can reduce the population of native microflora while maintaining the quality of this product.

Influence of ethylene action, storage atmosphere, and storage duration on diphenylamine and diphenylamine derivative content of Granny Smith apple peel

The application of diphenylamine (DPA) to prevent the apple peel disorder superficial scald can result in accumulation of a number of DPA derivatives resulting from C-nitration, C-hydroxylation, O-methylation, and N-nitrosation during fruit storage. As the presence of these compounds may be indicative of metabolic processes leading to superficial scald development, the contents of DPA and DPA derivatives were determined in fruits treated at harvest with DPA or DPA plus the ethylene action inhibitor 1-methylcyclopropene (1-MCP), which also prevents scald development. Influences of fruit maturity, storage environment, storage duration, and a 14 day poststorage ripening period on accumulation of DPA metabolites were also assessed. Poststorage ripening, 1-MCP treatment, and controlled atmosphere storage had varied effects on DPA derivative contents suggesting that reactive oxygen and nitrogen species, such as *OH, *NO, and *NO2, or enzyme-catalyzed reactions may be present during certain ripening and senescence-related physiological processes. Definitive correlations between superficial scald incidence and contents of specific derivatives were not observed.

Relationship of superficial scald development and alpha-farnesene oxidation to reactions of diphenylamine and diphenylamine derivatives in Cv. Granny Smith apple peel

Cv. Granny Smith apple fruit, treated at harvest with aqueous emulsions containing diphenylamine (DPA) and DPA derivatives, were evaluated for the peel disorder superficial scald (scald) after 6 months of cold storage at 1 degrees C plus 0 or 7 days at 20 degrees C. Metabolism of these derivatives and alpha-farnesene oxidation were also evaluated after 6 months. Derivatives substituted at the para position prevented scald, but scald developed on fruit treated with derivatives substituted in the amino, ortho, or meta positions. The extent of scald control was also dependent on the chemical nature of the functional group used to derivatize DPA. Hydroxylation of DPA and DPA derivatives during storage was not associated with scald control. Methoxylated DPA derivatives produced during storage resulted from O-methylation of C-hydroxylated derivatives rather than C-methoxylation of DPA. N-Nitrosodiphenylamine provided partial scald control, possibly resulting from its degradation to DPA, indicating that the amino hydrogen of DPA may be crucial for scald control. Results suggest that functional group position and chemical properties both contribute to the efficacy of DPA derivatives for scald control.

Evaluation of diphenylamine derivatives in apple peel using gradient reversed-phase liquid chromatography with ultraviolet–visible absorption and atmospheric pressure chemical ionization mass selective detection

A method was developed for extracting, identifying, and quantifying diphenylamine (DPA) derivatives in the peel of DPA-treated apples using gradient reversed-phase liquid chromatography with ultraviolet-visible absorption and atmospheric pressure chemical ionization detection (LC-UV-vis-APCI-MS). Compounds routinely analyzed using this method included hydroxylated, nitrosated, nitrated, and methoxylated diphenylamine derivatives. Analysis of peel treated with 0-8 g L(-1) DPA showed that peel DPA content was a limiting factor in derivative production and that recovery of most compounds over this range was linear.

Interactive responses of gala apple fruit volatile production to controlled atmosphere storage and chemical inhibition of ethylene action

Gala apples exposed to the ethylene action inhibitor 1-methylcyclopropene (1-MCP) for 12 h at 20 degrees C were stored at 1 degrees C in air or a controlled atmosphere (CA) maintained at 1 kPa O2 and 2 kPa CO2. Volatile compounds were measured after 4, 12, 20, and 28 weeks plus 1 or 7 days at 20 degrees C. Treatment with 1-MCP and then storage in air or CA or storage in CA without 1-MCP treatment reduced volatile production as compared to apples not treated with 1-MCP stored in air. The reduced production of esters, alcohols, aldehydes, acetic acid, and 1-methoxy-4-(2-propenyl)benzene was observed. Ester production by fruit stored in CA decreased throughout the storage period regardless of previous 1-MCP treatment. The production of esters, alcohols, aldehydes, acetic acid, and 1-methoxy-4-(2-propenyl)benzene by 1-MCP-treated fruit stored in air plus 7 days at 20 degrees C increased after 20 or 28 weeks of storage. Continuous exposure to 417 micromol m(-3) ethylene for 7 days at 20 degrees C after 12 or 28 weeks of storage stimulated production of many volatile compounds, primarily esters and alcohols, by fruit stored in CA or 1-MCP-treated apples stored in air. However, exposure to ethylene had no effect on the production of aldehydes or acetic acid.

Production of volatile compounds by Fuji apples following exposure to high CO2 or low O2

The emission of volatile compounds by Fuji apples following short- or long-term exposure to high CO(2) was studied. The production of ethanol, methyl and ethyl esters, octanal, nonanal, and decanal was enhanced while the production of C(3)-C(6) alcohols, propyl, butyl, pentyl, and hexyl esters and butanal decreased in fruit exposed to 10 kPa O(2) + 20 kPa CO(2) at 20 degrees C for up to 12 days. The impact of high CO(2) exposure on volatile production was dependent on fruit maturity at harvest. Apples stored for 8 months in an ultralow O(2)-controlled atmosphere (CA) (0.5 kPa O(2) + 0.05 kPa CO(2)) or high CO(2) CA (1.5 kPa O(2) + 3 kPa CO(2)) at 0.5 degrees C had reduced production of most volatiles, especially butyl and hexyl esters, as compared to fruit stored in air. Two exceptions were ethanol and ethyl acetate for which the production was enhanced by both CA regimes. Treatment with the antioxidant diphenylamine prior to storage prevented most of the high CO(2)-induced and ultralow O(2)-induced changes in volatile production. The results of this study do not indicate that changes in volatile production following the exposure of Fuji apples to high CO(2) are causally related to the development of CO(2) injury.

Influence of 1-methylcyclopropene on ripening, storage life, and volatile production by d'Anjou cv. pear fruit

d'Anjou cv. pear fruit (Pyrus communis L.) exposed at harvest to 0, 0.42, 4.2, or 42 micromol m(-)(3) 1-methylcyclopropene (1-MCP) for 12 h at 20 degrees C were stored at 1 degrees C for up to 8 months. After storage, half of the fruit was continuously exposed to ethylene (0.45 or 4-18 mmol m(-)(3)) for 7 days at 20 degrees C. All fruit treated with 1-MCP had lower respiration and ethylene production compared to untreated controls. Fruit quality changes were delayed following 1-MCP treatment, as was development of superficial scald and peel yellowing. The duration of 1-MCP-induced responses was dependent on 1-MCP treatment concentration. When 1-MCP-treated fruit began to ripen, softening and production of volatile compounds proceeded similar to that of untreated fruit. Post-storage ethylene exposure did not consistently stimulate ripening of fruit previously treated with 1-MCP. Efficacy of ethylene treatment depended on 1-MCP concentration and storage duration.

Investigations of aroma volatile biosynthesis under anoxic conditions and in different tissues of "Redchief Delicious" apple fruit (Malus domestica Borkh.)

Disks from different tissues were obtained from "Redchief Delicious" apple fruit (Malus domestica Borkh.) and analyzed for the ability to metabolize 1-pentanol as well as synthesize constitutive esters and alcohols under anoxic and aerobic conditions. The skin tissue displayed a greater capacity to synthesize pentanal, pentyl acetate, pentyl propionate, pentyl butyrate, and pentyl hexanoate than the hypanthial and carpellary tissues during incubation with 1-pentanol. With the exception of pentyl acetate and pentyl propionate biosynthesis, the hypanthial tissue synthesized these compounds at a higher rate than the carpellary tissue. Anoxia inhibited both constituent and 1-pentanol-derived ester biosynthesis. While anoxia inhibited ester biosynthesis, ethanol biosynthesis increased at a greater rate in tissue disks held under these conditions. Biosynthesis of 1-butanol, 2-methyl-1-butanol, and 1-hexanol was greater in tissue disks held in air during the first part of the measurement period and dropped off more rapidly than those transpiring in tissue disks held under anoxic conditions. The biosynthetic rates of all esters, both constituent and 1-pentanol-derived, increased as a result of air exposure. While hypoxic or anoxic conditions may promote ethanol synthesis, these conditions also appear to inhibit the formation of the ethanol-derived esters partially responsible for the off-flavor in apples attributed to ultralow O(2) controlled atmosphere storage.

Development of apple superficial scald, soft scald, core flush, and greasiness is reduced by MCP

1-Methylcyclopropene (MCP) was used to evaluate the role of ethylene in development of apple (Malus x domestica Borkh.) physiological disorders during storage. Granny Smith, Red Chief Delicious, and Fuji apple fruit were treated with MCP at a concentration of 1 microL L(-)(1) for 12 h at 20 degrees C. For all varieties stored at 0 degrees C, ethylene production and respiration rates were reduced for several months following MCP treatment, and firmness and titratable acidity of treated fruit were higher compared to controls. Apples treated with MCP did not develop superficial scald or peel greasiness through 6 months storage plus ripening at 20 degrees C for 7 days. Core flush was not observed in MCP-treated fruit until 6 months after treatment when the incidence was still lower compared to control fruit. MCP delayed the rise in production of alpha-farnesene and reduced accumulation of its oxidation products.

Impact of 1-methylcyclopropene and methyl jasmonate on apple volatile production

Climacteric Fuji apples were treated with 10 microL x L(-1) MCP (1-methylcyclopropene), 2 mmol x L(-1) MJ (methyl jasmonate), or a combination of 10 microL x L(-1) MCP and 2 mmol x L(-1) MJ. Fruit were kept at 20 degrees C for 15 days after treatment. Production of ethylene and other volatile compounds was measured prior to and 3, 7, 11, and 15 days after treatment. Ethylene production decreased 3 days following MJ treatment and then increased. MCP treatment alone or in combination with MJ inhibited ethylene production. MJ and MCP inhibited production of many volatile alcohols and esters. The production of individual alcohols and esters appears to be differentially inhibited by MJ or MCP. MJ and MCP inhibited not only production of alcohols but also formation of esters from alcohols.

Identification of geosmin as a volatile metabolite of Penicillium expansum

Cultures of Penicillium expansum produce a musty, earthy odor. Geosmin [1,10-trans-dimethyl-trans(9)-decalol] was identified by gas chromatography-mass spectrometry from headspace samples of P. expansum cultures. Olfactory comparison of P. expansum cultures with a geosmin standard indicated geosmin is the primary component of the odor associated with P. expansum.