Insights into profiling of volatile ester and LOX-pathway related gene families accompanying post-harvest ripening of 'Nanguo' pears

Analysis of Aroma Characteristics of 'Binzi' and 'Xiangguo' Apple-Ancient Cultivars in China

'Binzi' (BZ) (Malus domestica subsp. chinensis var. binzi Li Y.N.) and 'Xiangguo' (XG) (Malus domestica subsp. chinensis var. xiangguo Li Y.N.) are the ancient cultivars in China. The BZ fruits have a low-fragrant flavor on harvest day but a high-fragrant flavor after storage at room temperature, while the XG fruits have a stronger flavor when mature. 'Starking' (SK) and 'Golden Delicious' (GD) fruits have a rich flavor and are recognized by all countries in the world. However, information on the differences between ancient Chinese cultivars and Western apple cultivars in aroma compounds remains unknown. The apple fruits were collected for continuous two years. Aroma compounds in the skin and pulp of the fruits were detected at room temperature (20 ± 1 °C) during storage. The dynamics of VOCs in BZ and SK fruits were more similarly reflected in esters, while those of XG and GD fruits were reflected in aldehydes and alcohols. Ethyl 2-methylbutyrate, with an extremely low odor threshold, was the main source of typical apple flavor in SK, BZ, and XG fruits, while hexyl acetate was the source of the banana flavor in GD fruits. 6-methyl-5-hepten-2-one and β-damascenone were the important ketones produced in the later stage of storage, derived from the carotenoid metabolism pathway and providing a citrus and rose flavor to the four apple cultivars. SK had the highest number of characteristic aroma components, which were mainly derived from the amino acid metabolism pathway, providing fruits with a sweet and fruity flavor. Although the characteristic aroma components of GD were derived from the fatty acid metabolic pathway, the number of volatile esters was lower. Ethyl butyrate, derived from the saturated fatty acid metabolism, had the highest content in BZ, providing a pineapple flavor; the flavor of XG was mainly derived from ethyl 2-methylbutyrate, 6-methyl-5-hepten-2-one, and β-damascenone. Therefore, we suggest BZ and XG apples as the aroma-breeding material with which to enrich new cultivars' aroma components, derived from the fatty acid metabolism and carotenoid metabolism pathways, respectively.

PuERF008-PuFAD2 module regulates aroma formation via the fatty acid pathway in response to calcium signaling in 'Nanguo' pear

Calcium acts as a secondary messenger in plants and is essential for plant growth and development. However, studies on the pathway of aroma synthesis in 'Nanguo' pear (Pyrus ussriensis Maxim.) are scarce. In this study, a bioinformatics analysis of transcriptomic data from calcium-treated 'Nanguo' pear was performed, which identified two fatty acid desaturases, PuFAD2 and PuFAD3, and eight AP2/ERF transcription factors, all exhibiting the same expression patterns. Transient expression experiments showed overexpression of PuFAD2 and PuFAD3 significantly increased the levels of aromatic substrates linoleic acid, hexanal, linolenic acid, and (E)-2-hexenal, but RNAi (RNA interference) had the opposite expression. Promoter sequences analysis revealed that PuFAD2 and PuFAD3 have ERE (estrogen response element) motifs on their promoters. The strongest activation of PuFAD2 by PuERF008 was verified using a dual-luciferase reporting system. Additionally, yeast one-hybrid and electrophoretic mobility shift assays revealed PuERF008 could active PuFAD2. Transient overexpression and RNAi analyses of PuERF008 showed a strong correlation with the expression of PuFAD2. This study provides insights into the process of aroma biosynthesis in 'Nanguo' pear and offers a theoretical basis for elucidating the role of calcium signaling in aroma synthesis.

Development and comprehensive SBSE-GC/Q-TOF-MS analysis optimization, comparison, and evaluation of different mulberry varieties volatile flavor

Optimization of seven parameters of stir bar sorptive extraction (SBSE) on mulberry volatile components for the first time. A total of 347 volatile components were identified and quantified in 14 mulberry varieties, predominantly encompassing esters, aldehydes, terpenoids, hydrocarbons, ketones, alcohols, heterocyclics, acids, and phenols. Hexanal and (E)-2-hexenal were the dominant volatiles. Furthermore, 79 volatile compounds characterized by odor activity values (OAVs) > 1 were identified, making a significant contribution to the distinctive mulberry flavor. "Green" notes were the most intense, followed by "fatty" and "fruity". Utilizing odor ring charts, the volatile flavor characteristics of the 14 mulberry varieties could be intuitively distinguished. This study not only established a viable methodology for differentiating mulberry varieties but also laid a theoretical foundation for the quality evaluation and variety breeding of mulberry flavor.

Decoding the Effects of High Hydrostatic Pressure and High-Temperature Short-Time Sterilization on the Volatile Aroma Profile of Red Raspberry Juice

The loss of distinctive aromas due to sterilization significantly hinders efforts to enhance the sensory quality of fruit and vegetable juices. This study aimed to elucidate the impacts of high-hydrostatic pressure (HHP) and high-temperature short-time (HTST) sterilization methods on the loss of C6 aldehyde aroma-active compounds in red raspberry juice. External standard quantification and quantitative descriptive analysis (QDA) revealed a notable decline in the levels of hexanal and (Z)-3-hexenal following the HHP and HTST treatments (p < 0.05), resulting in a marked attenuation of the grassy aroma characteristic of red raspberry juice. Furthermore, a comprehensive examination of the precursors, pivotal enzymes, intermediates, and downstream aromas within the fatty acid metabolism pathway in different raspberry juice samples indicated that the C6 aldehydes loss induced by HHP and HTST sterilizations was primarily ascribed to the competitive inhibition of β-oxidation and the hindered enzymatic oxidation of fatty acids. These insights suggest that modifying sterilization protocols and enhancing enzymatic stability may help preserve the aroma integrity of raspberry juice. Our findings offer practical guidance for optimizing juice processing techniques to maintain flavor.

Discrimination of internal browning in pineapple during storage based on changes in volatile compounds

Internal browning (IB) is a physiological disorder without external symptoms of postharvest pineapples, but whether and how IB influences pineapple volatiles remain unknown. We examined eigenvalues of volatiles in 'Comte de Paris' pineapples with or without IB using electronic nose (E-nose) and gas chromatography-mass spectrometry (GC-MS). Correlation coefficients between the responses of E-nose sensors S7 and S9 and IB were 0.836 and 0.804, respectively. The multilayer perceptron neural network and radial basis function neural network models classified IB degree with accuracy of 94.77% and 91.67%. GC-MS analysis revealed 30 volatile substances upregulated in pineapple with IB compared to those without, of which 15 were esters. IB regulated volatile compound synthesis through the lipoxygenase pathway which involved lipoxygenase, pyruvate decarboxylase 1, alcohol dehydrogenases, acyl-CoA oxidase 1, and alcohol acyltransferase genes and their related enzymes. These results suggested that volatiles regulated by IB could be used to discriminate IB severity in pineapples.

Effects of cellulose nanofibrils treatment on antioxidant properties and aroma of fresh-cut apples

Horticultural products tend to deteriorate during postharvest storage and processing. In this study, cellulose nanofibers (CNFs) were prepared from wood to investigate the effects of CNF treatment on the storage quality, aroma composition, and antioxidant system of fresh-cut apple (Malus domestica) wedges. Compared with control treatment, CNF coating treatment significantly improved the appearance of apple wedges; reduced the decay rate of apple wedges; and delayed the decline in weight loss, firmness, and titratable acid during storage. Gas chromatography-mass spectrometry showed that CNF treatment could maintain the aroma components of apple wedges (stored for 4 days). Further investigations showed that CNF treatment increased the antioxidant system level and decreased reactive oxygen species content and membrane lipid peroxidation level of apple wedges. Overall, this study showed that CNF coating could effectively maintain the quality of fresh-cut apples during cold storage.

Metabolome and transcriptome profiling revealed the enhanced synthesis of volatile esters in Korla pear

BACKGROUND

Flavor contributes to the sensory quality of fruits, including taste and aroma aspects. The quality of foods is related to their flavor-associated compounds. Pear fruits have a fruity sense of smell, and esters are the main contributor of the aroma. Korla pear are well known due to its unique aroma, but the mechanism and genes related to volatile synthesis have not been fully investigated.

RESULTS

Flavor-associated compounds, including 18 primary metabolites and 144 volatiles, were characterized in maturity fruits of ten pear cultivars from five species, respectively. Based on the varied metabolites profiles, the cultivars could be grouped into species, respectively, by using orthogonal partial least squares discrimination analysis (OPLS-DA). Simultaneously, 14 volatiles were selected as biomarkers to discriminate Korla pear (Pyrus sinkiangensis) from others. Correlation network analysis further revealed the biosynthetic pathways of the compounds in pear cultivars. Furthermore, the volatile profile in Korla pear throughout fruit development was investigated. Aldehydes were the most abundant volatiles, while numerous esters consistently accumulated especially at the maturity stages. Combined with transcriptomic and metabolic analysis, Ps5LOXL, PsADHL, and PsAATL were screened out as the key genes in ester synthesis.

CONCLUSION

Pear species can be distinguished by their metabolic profiles. The most diversified volatiles as well as esters was found in Korla pear, in which the enhancement of lipoxygenase pathway may lead to the high level of volatile esters at maturity stages. The study will benefit the fully usage of pear germplasm resources to serve fruit flavor breeding goals.

Characterization of key aroma compounds and regulation mechanism of aroma formation in local Binzi (Malus pumila × Malus asiatica) fruit

BACKGROUND

Volatile components are important secondary metabolites essential to fruit aroma quality, thus, in the past decades many studies have been extensively performed in clarifying fruit aroma formation. However, aroma components and biosynthesis in the fruit of Binzi (Malus pumila × Malus asiatica), an old local species with attractive aroma remain unknown.

RESULTS

We investigated two Binzi cultivars, 'Xiangbinzi' (here named high-fragrant Binzi, 'HFBZ') and 'Hulabin' (here named low-fragrant Binzi, 'LFBZ') by monitoring the variation of volatiles and their precursors by Gas Chromatography-Mass Spectrometer (GC-MS), as well as their related genes by RNA-seq during post-harvest ripening. We firstly confirmed that 'HFBZ' and 'LFBZ' fruit showed respiratory climacteric by detecting respiratory rate and ethylene emission during post-harvest; found that esters were the major aroma components in 'HFBZ' fruit, and hexyl 2-methylbutyrate was responsible for the 'fruity' note and most potent aroma component, followed by ethyl acetate, ethyl butanoate, (E)-2-hexenal, and 1-hexanol. Regarding aroma synthesis, fatty acid metabolism seemed to be more important than amino acid metabolism for aroma synthesis in 'HFBZ' fruit. Based on RNA-seq and quantitative reverse transcription PCR (RT-qPCR), LOX2a, LOX5a, ADH1, and AAT1 genes are pointed to the LOX pathway, which may play a vital role in the aroma formation of 'HFBZ' fruit.

CONCLUSION

Our study firstly investigated the aroma components and related genes of Binzi fruit, and provided an insight into the fragrant nature of Malus species.

Characterization and Preliminary Application of a Novel Lipoxygenase from Enterovibrio norvegicus

Lipoxygenases have proven to be a potential biocatalyst for various industrial applications. However, low catalytic activity, low thermostability, and narrow range of pH stability largely limit its application. Here, a lipoxygenase (LOX) gene from Enterovibrio norvegicus DSM 15893 (EnLOX) was cloned and expressed in Escherichia coli BL21 (DE3). EnLOX showed the catalytic activity of 40.34 U mg-1 at 50 °C, pH 8.0. Notably, the enzyme showed superior thermostability, and wide pH range stability. EnLOX remained above 50% of its initial activity after heat treatment below 50 °C for 6 h, and its melting point temperature reached 78.7 °C. More than 70% of its activity was maintained after incubation at pH 5.0-9.5 and 4 °C for 10 h. In addition, EnLOX exhibited high substrate specificity towards linoleic acid, and its kinetic parameters of Vmax, Km, and Kcat values were 12.42 mmol min-1 mg-1, 3.49 μmol L-1, and 16.86 s-1, respectively. LC-MS/MS analysis indicated that EnLOX can be classified as 13-LOX, due to its ability to catalyze C18 polyunsaturated fatty acid to form 13-hydroxy fatty acid. Additionally, EnLOX could improve the farinograph characteristics and rheological properties of wheat dough. These results reveal the potential applications of EnLOX in the food industry.

Integrated metabolome and transcriptome analysis of the regulatory network of volatile ester formation during fruit ripening in pear

'Nanguo' pear (Pyrus ussuriensis Maxim.) is a typical climacteric fruit with an attractive aroma after postharvest ripening. Esters are the key volatile compounds determining the typical aroma formation. However, the mechanism of aroma-related ester formation remains largely unknown. In this study, we performed transcriptome and metabolome analyses to reveal the changes of aroma-related compounds during pear ripening in the optimal taste period (OTP). During the pear ripening process, typical fatty acid-derived volatile organic compounds (VOCs) are transformed from aldehydes, alcohols, and ketones to esters, where ethyl hexanoate, hexyl acetate, and ethyl butanoate are the dominant esters in the OTP. Rich aroma-related esters in the OTP are associated with the accumulation of important precursors of aroma volatiles, including linoleic acid, α-linolenic acid, γ-linolenic acid, and oleic acid. Genes encoding key biosynthetic enzymes are associated with the altered levels of aroma-related esters. The candidate genes associated with the high levels of aroma-related esters in 'Nanguo' pears are PuFAD2, PuLOX2, PuLOX5, and PuAAT. Additionally, transcription factor (TF) genes such as PuWRKY24, PuIAA29, and PuTINY may play crucial roles in aroma formation during fruit ripening. Hence, we summarized the TFs that regulate VOC metabolism in different fruit species. The results provided a foundation for further research on aroma-related esters in 'Nanguo' pears and could help to elucidate the mechanisms regulating fruit quality improvement.

Comparative elucidation of bioactive and volatile components in dry mature jujube fruit (Ziziphus jujuba Mill.) subjected to different drying methods

This study investigated the effects of convective drying(CD) and freeze drying(FD) on bioactive and volatile components in jujube. No significant difference in total phenolic, total flavonoids and antioxidant capacity among CD60, CD70, CD80 and FD samples (P > 0.05). LC-MS/MS analysis showed that this trend mainly originated from the dynamic equilibrium relationships between caffeic acid, chlorogenic acid, p-hydroxybenzoic acid, rutin, epicatechin, and quercetin. HS-SPME-GC-MS identified 31 volatile organic compounds (VOCs) comprising more than 80% aldehydes and acids. Principal component analysis distinguished the VOC characteristics of samples subjected to different drying methods. Six VOCs had an odor activity value (OAV) >1, most of which were fatty acid oxidation or Maillard reaction products. Combined with the precursor components, these reactions were speculated to be the major VOC-producing pathways in dried jujube. Considering the bioactive components and flavor retention, CD at 60 °C was an effective drying method with potential to replace FD.

Impact of UHT processing on volatile components and chemical composition of sea buckthorn (Hippophae rhamnoides) pulp: A prediction of the biochemical pathway underlying aroma compound formation

The effects of ultrahigh-temperature sterilization (UHT) on the volatile components and chemical composition of sea buckthorn pulp (SBP) were evaluated firstly. UHT had significant effects on the volatiles of SBP (mainly occurring at 140 °C for 2 s and 4 s), in which 140 °C for 2 s resulted in a decrease of 3.48% and 14.60% in total volatiles and esters, and an increase of 6.73% in alcohols, while alcohols contents sharply decreased by 6.90% at 140 °C for 4 s. Moreover, 140 °C for 2 s and 4 s decreased the amino acid content by 35.39% and 29.75%, respectively, while UHT significantly promoted the increase of fatty acids, but only a small increase at 140 °C for 4 s. The speculation is that a large number of volatiles were formed during the 140 °C for 2 s and 4 s, mainly from amino acid reactions and lipid oxidation.

Fingerprints and changes analysis of volatile compounds in fresh-cut yam during yellowing process by using HS-GC-IMS

The purpose of this study was to investigate the dynamic change in volatile components during the yellowing process. The volatile components were analyzed by headspace gas chromatography-ion mobility spectrometry (HS-GC-IMS) and the critical enzyme activities were determined by commercial kits. The results revealed that 29 signal peaks for 20 compounds were identified, which were quantified in all samples: 1 furan, 1 ester, 15 aldehydes, 3 ketones, and 9 alcohols. The contents of most of these compounds increased first and then decreased at 36 h, which were basically consistent with the enzyme activities of LOX, HPL, ADH and AAT. Subsequently, principal component analysis (PCA) results clearly showed that the fresh-cut yams for different yellowing processes were well distinguished by the volatile compounds. These results showed that the potential of HS-GC-IMS-based approaches to evaluate the volatile compound profiles of fresh-cut yam at different stages in the yellowing period.

Pear genetics: Recent advances, new prospects, and a roadmap for the future

Pear, belonging to the genus Pyrus, is one of the most economically important temperate fruit crops. Pyrus is an important genus of the Rosaceae family, subfamily Maloideae, and has at least 22 different species with over 5000 accessions maintained or identified worldwide. With the release of draft whole-genome sequences for Pyrus, opportunities for pursuing studies on the evolution, domestication, and molecular breeding of pear, as well as for conducting comparative genomics analyses within the Rosaceae family, have been greatly expanded. In this review, we highlight key advances in pear genetics, genomics, and breeding driven by the availability of whole-genome sequences, including whole-genome resequencing efforts, pear domestication, and evolution. We cover updates on new resources for undertaking gene identification and molecular breeding, as well as for pursuing functional validation of genes associated with desirable economic traits. We also explore future directions for "pear-omics".

Volatile Profile and Biosynthesis of Post-harvest Apples are Affected by the Mechanical Damage

Mechanical damage to fruit causes flavor changes during post-harvest supply chains. It is important to identify the main volatiles and explore their biosynthesis mechanism. In this study, the volatile changes in apples caused by mechanical damage were analyzed by gas chromatography-ion mobility spectrometry. Hexanal and ethyl acetate were accumulated and identified as potential volatile biomarkers to detect damaged apples. The study on the lipoxygenase (LOX) pathway and transcription factors (TFs) shows that mechanical damage up-regulated the expression of MdLOX-like, MdLOX3b, MdLOX7b, MdLOX7c, MdLOX2a, and MdAAT in the LOX pathway and that of one MYB TF (MdMYB-like), five ERF TFs (MdERF073, MdERF003, MdERF114, MdERF15, and MdERF2), and five WRKY TFs (MdWRKY23, MdWRKY17, MdWRKY46, MdWRKY48, and MdWRKY71). Notably, MdAAT was significantly correlated to MdMYB-like, MdWRKY23, MdWRKY71, MdERF15, and MdERF2. Thus, TFs may attribute to the accumulation of hexanal and ethyl acetate by regulating the expression of LOX pathway-related genes.