Evaluation of the volatile profiles in pulp of 85 apple cultivars (Malus domestica) by HS–SPME combined with GC–MS

Characterization of volatile aroma compounds in pak choi

Aroma is a key sensory factor in the flavor evaluation of pak choi (Brassica rapa L. ssp. chinensis var. Makino). The pak choi varieties Xiangqingcai (XQC) and Xiuhuajin (XHJ) have unique aroma characteristics, but the chemical profiles of these aromas are unknown. Here, the aroma profiles of three varieties of pak choi including XQC, XHJ, and Suzhouqing (CK, non-aromatic) were determined using gas chromatography-mass spectrometry (GC-MS) and relative odor activity values (rOAV). A total of 15 categories of 716 volatile metabolites were detected in the three pak choi varieties, with terpenoid metabolites identified as the major components, although in each sample the identity of the major terpenoid metabolite varied. There were 53 aroma components in XQC and 54 aroma components in XHJ with rOAV >1, which contribute to rice aroma and fishy odor of these varieties, respectively.

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.

Changes of Volatile Organic Compounds of Different Flesh Texture Pears during Shelf Life Based on Headspace Solid-Phase Microextraction with Gas Chromatography-Mass Spectrometry

Aroma is an important sensory factor in evaluating the quality of pear fruits. This study used headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) to analyze the volatile organic compounds (VOCs) of three crispy pears and five soft pears during shelf life, and the changes in soluble solids content (SSC) were analyzed. The results showed that the SSC of the soft pears such as Nanguoli, Jingbaili and Louis was always higher than that of the crispy pears throughout shelf life. A total of 160 VOCs were detected in the eight pear varieties. Orthogonal partial least squares discriminant analysis (OPLS-DA) and hierarchical cluster analysis (HCA) combined with predictor variable importance projection (VIP) showed that the eight pear varieties could be obviously classified into six groups according to the differences in their VOCs, and 31 differential VOCs were screened out, which could be used to differentiate between pears with different flesh textures. The results of clustering heat map analysis showed that, with the extension of shelf life, the content of each different VOC did not change much in crispy pears, whereas the difference in soft pears was larger. This study confirmed the potential of determining the optimal shelf life of different pear varieties about aroma evaluation and studying the mechanism of differences in VOCs in the future.

Effects of Different Irrigation Regimes, Nitrogen Levels and Storage Conditions on Volatiles of 'Gala' Apple

With a characteristic flavour and aroma, "Maçã de Alcobaça" are apples produced in the western region of the mainland of Portugal. Given the known influence of pre-harvest cultural techniques and post-harvest conservation methods on fruit quality, this work evaluated the effect of cultural factors and conservation methods on the volatile profile of 'Gala' apples. Tests were carried out during four seasons (2018 to 2021) in two 'Gala' apple orchards (F and S) maintained with different irrigation rates and nitrogen fertilisation [normal irrigation and normal nitrogen (Control, NINN), normal irrigation and excess nitrogen (NIEN), excess irrigation and normal nitrogen (EINN), excess irrigation and excess nitrogen (EIEN)], and under three storage conditions [Controlled Atmosphere + 1-methylcyclopropene (CA+1-MCP), Dynamic Controlled Atmosphere (DCA) and DCA+1-MCP]. The intact fruit volatiles were isolated by headspace solid-phase microextraction (HS-SPME) and analysed by Gas Chromatography with Flame Ionisation Detection and Gas Chromatography-Mass Spectrometry at harvest (T0) and after 8 months of storage (T8). HS-SPME volatiles from 'Gala' apples, obtained at T0 in control conditions, were characterised by trans,trans-α-farnesene dominance (36-69%), followed by hexyl acetate (5-23%) and hexyl hexanoate (3-9%). The four irrigation and nitrogen treatments did not evidence main changes in the apple volatile profile. Instead, storage conditions changed the ratio between compounds; previously undetected compounds attained high percentages and decreased the intensity of the dominant compounds in the control conditions. Although all storage conditions tested changed the volatile profile and emanation intensity, the effect was more accentuated in storage for 8 months with DCA+1-MCP.

Advances in detecting fruit aroma compounds by combining chromatography and spectrometry

Fruit aroma is produced by volatile compounds, which can significantly enhance fruit flavor. These compounds are highly complex and have remarkable pharmacological effects. The synthesis, concentration, type, and quantity of fruit aroma substances are affected by various factors, both abiotic and biotic. To fully understand the aroma substances of various fruits and their influencing factors, detection technology can be used. Many methods exist for detecting aroma compounds, and approaches combining multiple instruments are widely used. This review describes and compares each detection technology and discusses the potential use of combined technologies to provide a comprehensive understanding of fruit aroma compounds and the factors influencing their synthesis. These results can inform the development and utilization of fruit aroma substances. © 2023 Society of Chemical Industry.

Genetic analysis and QTL mapping of aroma volatile compounds in the apple progeny 'Fuji' × 'Cripps Pink'

Aroma is an essential trait for apple fruit quality, but the understanding of biochemical mechanisms underlying aroma formation is still limited. To better characterize and assess the genetic potential for improving aroma quality for breeding, many efforts have been paid to map quantitative trait loci (QTLs) using a saturated molecular linkage map. In the present study, aroma profiles in ripe fruit of F₁ population between 'Fuji' and 'Cripps Pink' were evaluated by gas chromatography-mass spectrometry (GC-MS) over 2019 and 2020 years, and the genetics of volatile compounds were dissected. In total, 38 volatile compounds were identified in 'Fuji' × 'Cripps Pink' population, including 23 esters, 3 alcohols, 7 aldehydes and 5 others. With the combination of aroma phenotypic data and constructed genetic linkage map, 87 QTLs were detected for 15 volatile compounds on 14 linkage groups (LGs). Among them, a set of QTLs associated with ester production identified and confirmed on LG 6. A candidate gene MdAAT6 in the QTL mapping interval was detected. Over-expression of MdAAT6 in tomato and apple fruits showed significantly higher esters accumulation compared to the control, indicating it was critical for the ester production. Our results give light on the mode of inheritance of the apple volatilome and provide new insights for apple flavor improvement in the future.

Analyzing Volatile Compounds of Young and Mature Docynia delavayi Fruit by HS-SPME-GC-MS and rOAV

This study focused on the examination of the volatile compounds and fragrance properties of the young and mature fruit of Docynia delavayi. Headspace solid-phase microextraction combined with gas chromatography−mass spectrometry (HS-SPME-GC-MS) was applied for identifying 42 volatile compounds, with young and mature fruit containing 36 and 42 compounds, respectively. Heat map cluster analysis, principal component analysis (PCA), orthogonal partial least squares discriminant analysis (OPLS-DA), and independent sample t-testing were used to analyze sample differences. Based on a variable importance in projection (VIP) > 1 and p < 0.05, 23 key volatile compounds such as octanal, geranylacetone, butyl acetate, and dihydro-β-ionone were screened. β-Ionone and phenethyl acetate made the largest contribution to the aroma of D. delavayi after analyzing the relative odor activity value (rOAV) of the key volatile compounds and their aroma descriptors. Young D. delavayi fruit exhibited a prominent woody scent, while mature D. delavayi fruit had more intense floral and rosy aromas. The findings may lay a foundation for comprehensively developing and utilizing D. delavayi fruit.

Variation in Cell Wall Metabolism and Flesh Firmness of Four Apple Cultivars during Fruit Development

Fruit ripening and softening are highly complex processes, and there is an interplay and coordination between the metabolic pathways that are involved in the biological processes. In this study, we aimed to elucidate the variation in the characters and possible causes of cell wall materials and morphological structure during apple fruits development. We studied the cell wall material (CWM), structure, cellular morphology, hydrolase activity, and the transcriptional levels of the related genes in four apple varieties 'Ruixue' and 'Ruixianghong' and their parents ('Pink Lady' and 'Fuji') during fruit development. The decrease in the contents of CWMs, sodium carbonate soluble pectin, hemicellulose, and cellulose were positively correlated with the decline in the hardness during the fruit development. In general, the activities of polygalacturonase, β-galactosidase, and cellulase enzymes increased during the late developmental period. As the fruit grew, the fruit cells of all of the cultivars gradually became larger, and the cell arrangement became more relaxed, the fruit cell walls became thinner, and the intercellular space became larger. In conclusion, the correlation analysis indicated that the up-regulation of the relative expression levels of ethylene synthesis and cell wall hydrolase genes enhanced the activity of the cell wall hydrolase, resulting in the degradation of the CWMs and the depolymerization of the cell wall structure, which affected the final firmness of the apple cultivars in the mature period.

GC-MS Metabolite and Transcriptome Analyses Reveal the Differences of Volatile Synthesis and Gene Expression Profiling between Two Apple Varieties

Aroma is a key quality attribute of apples, making major contributions to commercial value and consumer choice. However, the mechanism underlying molecular regulation of aroma formation genes and transcription factors remains poorly understood in apples. Here, we investigated the aroma volatile profiles of two apple varieties with distinctive flavors using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS). A total of 35 volatile compounds were identified in Granny Smith and Jonagold apples. Aldehydes were the most abundant volatiles contributing to the aroma in Granny Smith apple while esters were the dominant volatile compounds in Jonagold apple. In order to know more about the expression levels of aroma-related genes involved in the metabolic pathways, transcriptome sequencing of these two different apple varieties was conducted utilizing the Illumina platform. In total, 94 differentially expressed genes (DEGs) were found in the fatty acid metabolism, amino acid metabolism, the mevalonate pathway and phenylpropanoid pathway. Furthermore, compared to the Granny Smith apple, the expression of multiple genes and transcription factors were upregulated in the Jonagold apple, which might play important roles in the synthesis of aroma volatile compounds. Our study contributes toward better understanding on the molecular mechanism of aroma synthesis in apples and provides a valuable reference for metabolic engineering and flavor improvement in the future.