Metabolomics for Evaluating Flavor-Associated Metabolites in Plant-Based Products

Metabolomic and Transcriptomic Analyses Reveal the Effects of Grafting on Nutritional Properties in Eggplant

Grafting has a significant impact on the botany properties, commercial character, disease resistance, and productivity of eggplants. However, the mechanism of phenotypic modulation on grafted eggplants is rarely reported. In this study, a widely cultivated eggplant (Solanum. melongena cv. 'Zheqie No.10') was selected as the scion and grafted, respectively, onto four rootstocks of TOR (S. torvum), Sa (S. aculeatissimum), SS (S. sisymbriifolium), and Sm64R (S. melongena cv. 'Qiezhen No. 64R') for phenotypic screening. Physiological and biochemical analysis showed the rootstock Sm64R could improve the fruit quality with the increasing of fruit size, yield, and the contents of total soluble solid, phenolic acid, total amino acid, total sugar, and vitamin C. To further investigate the improvement of fruit quality on Sm64R, a transcriptome and a metabolome between the Sm64R-grafted eggplant and self-grafted eggplant were performed. Significant differences in metabolites, such as phenolic acids, lipids, nucleotides and derivatives, alkaloids, terpenoids, and amino acids, were observed. Differential metabolites and differentially expressed genes were found to be abundant in three core pathways of nutritional qualities, including biosynthesis of phenylpropanoids, phospholipids, and nucleotide metabolism. Thus, this study may provide a novel insight into the effects of grafting on the fruit quality in eggplant.

Faba Bean Flavor Effects from Processing to Consumer Acceptability

Faba beans as an alternative source of protein have received significant attention from consumers and the food industry. Flavor represents a major driving force that hinders the utilization faba beans in various products due to off-flavor. Off-flavors are produced from degradation of amino acids and unsaturated fatty acids during seed development and post-harvest processing stages (storage, dehulling, thermal treatment, and protein extraction). In this review, we discuss the current state of knowledge on the aroma of faba bean ingredients and various aspects, such as cultivar, processing, and product formulation that influence flavour. Germination, fermentation, and pH modulation were identified as promising methods to improve overall flavor and bitter compounds. The probable pathway in controlling off-flavor evolution during processing has also been discussed to provide efficient strategies to limit their impact and to encourage the use of faba bean ingredients in healthy food design.

HS-SPME/GC×GC-TOFMS-Based Flavoromics and Antimicrobial Properties of the Aroma Components of Zanthoxylum motuoense

Zanthoxylum motuoense Huang, native to Tibet, China, is a newly discovered Chinese prickly ash, which, recently, has increasingly attracted the attention of researchers. In order to understand its volatile oil compositions and flavor characteristics, and to explore the flavor difference between Z. motuoense and the common Chinese prickly ash sold in the market, we analyzed the essential oils of Z. motuoense pericarp (MEO) using HS-SPME/GC×GC-TOFMS coupled with multivariate data and flavoromics analyses. The common commercial Chinese prickly ash in Asia, Zanthoxylum bungeanum (BEO), was used as a reference. A total of 212 aroma compounds from the 2 species were identified, among which alcohols, terpenoids, esters, aldehydes, and ketones were the major compounds. The predominant components detected from MEO were citronellal, (+)-citronellal, and β-phellandrene. Six components-citronellal, (E,Z)-3,6-nonadien-1-ol, allyl methallyl ether, isopulegol, 3,7-dimethyl-6-octen-1-ol acetate, and 3,7-dimethyl-(R)-6-octen-1-ol-could be used as the potential biomarkers of MEO. The flavoromics analysis showed that MEO and BEO were significantly different in aroma note types. Furthermore, the content differences of several numb taste components in two kinds of prickly ash were quantitatively analyzed using RP-HPLC. The antimicrobial activities of MEO and BEO against four bacterial strains and nine plant pathogenic fungi were determined in vitro. The results indicated that MEO had significantly higher inhibitory activities against most microbial strains than BEO. This study has revealed the fundamental data in respect of the volatile compound properties and antimicrobial activity of Z. motuoense, offering basic information on valuable natural sources that can be utilized in the condiment, perfume, and antimicrobial sectors.

Identification and Analysis of Metabolites That Contribute to the Formation of Distinctive Flavour Components of Laoxianghuang

In addition to volatile compounds, metabolites also have a great effect on the flavour of food. Fresh finger citron cannot be eaten directly because of its spicy and bitter taste, so it is made into a preserved fruit product known as Laoxianghuang (LXH). To investigate the metabolites that have an effect on the flavour of LXH, untargeted metabolomics was performed using an ultrahigh-performance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS), and the metabolites of the Laoxianghuang samples from different locations in the Chaoshan area were compared and analysed. A total of 756 metabolites were identified and distinct differences were revealed among the different Laoxianghuang samples. A total of 33 differential metabolites with the most significant changes were screened through further multivariate analytical steps, and each group of samples had unique metabolites. For instance, pomolic acid had the highest content in the JG sample, while L-glycyl-L-isoleucine was rich in the QS sample. Moreover, flavonoid metabolites made the greatest contribution to the unique flavour of Laoxianghuang. The metabolic pathways involved are the biosynthetic pathways of flavonoids, isoflavonoids, flavones, and flavonols. This study can provide some creative information for distinguishing the quality differences of Laoxianghuang from the perspective of metabolites and offer preliminary theoretical support to characterise the formation of flavour substances in Laoxianghuang.

Flavor Profiling Using Comprehensive Mass Spectrometry Analysis of Metabolites in Tomato Soups

Trained sensory panels are regularly used to rate food products but do not allow for data-driven approaches to steer food product development. This study evaluated the potential of a molecular-based strategy by analyzing 27 tomato soups that were enhanced with yeast-derived flavor products using a sensory panel as well as LC-MS and GC-MS profiling. These data sets were used to build prediction models for 26 different sensory attributes using partial least squares analysis. We found driving separation factors between the tomato soups and metabolites predicting different flavors. Many metabolites were putatively identified as dipeptides and sulfur-containing modified amino acids, which are scientifically described as related to umami or having "garlic-like" and "onion-like" attributes. Proposed identities of high-impact sensory markers (methionyl-proline and asparagine-leucine) were verified using MS/MS. The overall results highlighted the strength of combining sensory data and metabolomics platforms to find new information related to flavor perception in a complex food matrix.

Influence of tannic acid concentration on the physicochemical characteristics of saliva of spider monkeys (Ateles geoffroyi)

Tannins are a chemical defense mechanism of plants consumed by herbivores. Variations in salivary physicochemical characteristics such as pH, total protein concentration (TP), and presence of proline-rich proteins (PRPs) in animals have been reported as a mechanism to protect the oral cavity when consuming food with variations in pH and tannins. Variations in salivary physiochemistry as adaptations for consuming tannin-rich foods have been found in omnivorous and folivorous primates, but have not yet been reported in frugivorous species such as spider monkeys. We therefore assessed changes in pH using test strips, TP concentration by measuring absorbance at 595 nm in a spectrophotometer and salivary PRPs using the SDS-PAGE electrophoresis technique in the saliva of nine captive spider monkeys in response to the consumption of solutions with different concentrations of tannic acid. The results showed variations in pH, TP concentration and the presence and variation of possible salivary PRPs associated with tannic acid concentration. These findings suggest that spider monkeys may tailor their salivary physicochemical characteristics in response to the ingestion of potentially toxic compounds.

Predicting Mandarin Fruit Acceptability: From High-Field to Benchtop NMR Spectroscopy

Recent advances in nuclear magnetic resonance (NMR) have led to the development of low-field benchtop NMR systems with improved sensitivity and resolution suitable for use in research and quality-control laboratories. Compared to their high-resolution counterparts, their lower purchase and running costs make them a good alternative for routine use. In this article, we show the adaptation of a method for predicting the consumer acceptability of mandarins, originally reported using a high-field 400 MHz NMR spectrometer, to benchtop 60 MHz NMR systems. Our findings reveal that both instruments yield comparable results regarding sugar and citric acid levels, leading to the development of virtually identical predictive linear models. However, the lower cost of benchtop NMR systems would allow cultivators to implement this chemometric-based method as an additional tool for the selection of new cultivars.

Computational prediction of plant metabolic pathways

Uncovering genes encoding enzymes responsible for the biosynthesis of diverse plant metabolites is essential for metabolic engineering and production of plant metabolite-derived medicine. With the availability of multi-omics data for an ever-increasing number of plant species and the development of computational approaches, the metabolic pathways of many important plant compounds can be predicted, complementing a more traditional genetic and/or biochemical approach. Here, we summarize recent progress in predicting plant metabolic pathways using genome, transcriptome, proteome, interactome, and/or metabolome data, and the utility of integrating these data with machine learning to further improve metabolic pathway predictions.

Branched-Chain Volatiles in Fruit: A Molecular Perspective

Branched-chain volatiles (BCVs) constitute an important family of fruit volatile metabolites essential to the characteristic flavor and aroma profiles of many edible fruits. Yet in contrast to other groups of volatile organic compounds important to fruit flavor such as terpenoids, phenylpropanoids, and oxylipins, the molecular biology underlying BCV biosynthesis remains poorly understood. This lack of knowledge is a barrier to efforts aimed at obtaining a more comprehensive understanding of fruit flavor and aroma and the biology underlying these complex phenomena. In this review, we discuss the current state of knowledge regarding fruit BCV biosynthesis from the perspective of molecular biology. We survey the diversity of BCV compounds identified in edible fruits as well as explore various hypotheses concerning their biosynthesis. Insights from branched-chain precursor compound metabolism obtained from non-plant organisms and how they may apply to fruit BCV production are also considered, along with potential avenues for future research that might clarify unresolved questions regarding BCV metabolism in fruits.

Plant-Based Proteins: The Good, Bad, and Ugly

Our global population is growing at a pace to exceed 10 billion people by the year 2050. This growth will place pressure on the agricultural production of food to feed the hungry masses. One category that will be strained is protein. Per capita protein consumption is rising in virtually every country for both nutritional reasons and consumption enjoyment. The United Nations estimates protein demand will double by 2050, and this will result in a critical overall protein shortage if drastic changes are not made in the years preceding these changes. Therefore, the world is in the midst of identifying technological breakthroughs to make protein more readily available and sustainable for future generations. One protein sourcing category that has grown in the past decade is plant-based proteins, which seem to fit criteria established by discerning consumers, including healthy, sustainable, ethical, and relatively inexpensive. Although demand for plant-based protein continues to increase, these proteins are challenging to utilize in novel food formulations.

Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Emerging nanosensor platforms and machine learning strategies toward rapid, point-of-need small-molecule metabolite detection and monitoring

Speedy, point-of-need detection and monitoring of small-molecule metabolites are vital across diverse applications ranging from biomedicine to agri-food and environmental surveillance. Nanomaterial-based sensor (nanosensor) platforms are rapidly emerging as excellent candidates for versatile and ultrasensitive detection owing to their highly configurable optical, electrical and electrochemical properties, fast readout, as well as portability and ease of use. To translate nanosensor technologies for real-world applications, key challenges to overcome include ultralow analyte concentration down to ppb or nM levels, complex sample matrices with numerous interfering species, difficulty in differentiating isomers and structural analogues, as well as complex, multidimensional datasets of high sample variability. In this Perspective, we focus on contemporary and emerging strategies to address the aforementioned challenges and enhance nanosensor detection performance in terms of sensitivity, selectivity and multiplexing capability. We outline 3 main concepts: (1) customization of designer nanosensor platform configurations via chemical- and physical-based modification strategies, (2) development of hybrid techniques including multimodal and hyphenated techniques, and (3) synergistic use of machine learning such as clustering, classification and regression algorithms for data exploration and predictions. These concepts can be further integrated as multifaceted strategies to further boost nanosensor performances. Finally, we present a critical outlook that explores future opportunities toward the design of next-generation nanosensor platforms for rapid, point-of-need detection of various small-molecule metabolites.

Overview of the current status on emerging, multi-faceted nanosensor platform designs and data analysis strategies for rapid, point-of-need detection and monitoring of small-molecule metabolites.

Aroma of peas, its constituents and reduction strategies - Effects from breeding to processing

Peas as an alternative protein source have attracted a great deal of interest from the food industry and consumers in recent years. However, pea proteins usually do not taste neutral and exhibit a distinct flavor, often characterized as "beany". This is usually contrasted by the food industry's desire for sensory neutral protein sources. In this review, we highlight the current state of knowledge about the aroma of peas and its changes along the pea value chain. Possible causes and origins, and approaches to reduce or eliminate the aroma constituents are presented. Fermentative methods were identified as interesting to mitigate undesirable off-flavors. Major potential has also been discussed for breeding, as there appears to be a considerable leverage at this point in the value chain: a reduction of plant-derived flavors, precursors, or substrates involved in off-flavor evolution could prevent the need for expensive removal later.

Bioactive Compounds, Sugars, and Sensory Attributes of Organic and Conventionally Produced Courgette (Cucurbita pepo)

Organic agriculture is considered one of the elements of sustainable food production and consumption, mainly due to its limited impact on the natural environment. At the same time, the quality features of organically produced foods, especially sensory attributes and health promoting values, are important factors determining consumers’ interest, and therefore play a key role in the organic sector’s development. The aim of this study was to investigate the sensory characteristics and concentrations of sugars and selected health-promoting bioactive compounds of organic courgette compared to conventionally grown courgette. In addition, untargeted metabolomic analysis of the courgette fruits was performed. The results of this study did not show a significant effect of the horticultural system (organic vs. conventional) on the concentrations of vitamin C, carotenoids, and chlorophylls in the courgette fruits. However, the fruits from the organic systems were significantly richer in sugars when compared to the conventionally cultivated ones (p = 0.038). Moreover, the organic fruits fertilized with manure contained significantly higher amounts of polyphenols, including gallic acid (p = 0.016), chlorogenic acid (p = 0.012), ferulic acid (p = 0.019), and quercetin-3-O-rutinoside (p = 0.020) compared to the conventional fruits. The untargeted analysis detected features significantly differentiating courgette fruits depending on the cultivar and horticultural system. Some significant differences in sensory values were also identified between fruits representing the two cultivars and coming from the horticultural systems compared in the study. Conventional courgettes were characterized by the most intensive peel color and aquosity, but at the same time were the least hard and firm compared to the fruits from the two organic systems. There was also a trend towards higher overall quality of the organically grown fruits. The presented study shows that the organic and conventional courgette fruits differ in a number of quality features which can influence consumers’ health and purchasing choices.

Food Metabolites as Tools for Authentication, Processing, and Nutritive Value Assessment

Secondary metabolites are molecules with unlimited applications that have been gaining importance in various industries and studied from many angles. They are mainly used for their bioactive capabilities, but due to the improvement of sensibility in analytical chemistry, they are also used for authentication and as a quality control parameter for foods, further allowing to help avoid food adulteration and food fraud, as well as helping understand the nutritional value of foods. This manuscript covers the examples of secondary metabolites that have been used as qualitative and authentication molecules in foods, from production, through processing and along their shelf-life. Furthermore, perspectives of analytical chemistry and their contribution to metabolite detection and general perspectives of metabolomics are also discussed.

Small in size, big on taste: Metabolomics analysis of flavor compounds from Philippine garlic

Philippine garlic (Allium sativum L.) is arguably known to pack flavor and aroma in smaller bulbs compared to imported varieties saturating the local market. In this study, ethanolic extracts of Philippine garlic cultivars were profiled using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC-QTOF). γ-Glu dipeptides, oligosaccharides and lipids were determined in Philippine garlic cultivars through bioinformatics analysis in GNPS Molecular Networking Platform and fragmentation analysis. Multivariate statistical analysis using XCMS Online showed the abundance of γ-Glu allyl cysteine in Batanes-sourced garlic while γ-Glu propenyl cysteine, γ-Glu methyl cysteine, and alliin are enriched in the Ilocos cultivar. Principal component analysis showed that the γ-Glu dipeptides found in local garlic influenced their distinct separation across PC1 from imported varieties. This presence of high levels of γ-Glu dipeptides and probiotic oligosaccharides may potentially contribute to the superior flavor and nutritional benefits of Philippine garlic.

Towards superior plant-based foods using metabolomics

Metabolomics is proving a useful approach for many of the main future goals in agronomy and food production such as sustainability/crop resilience, food quality, safety, storage, and nutrition. Targeted and/or untargeted small-molecule analysis, coupled to chemometric analysis, has already unveiled a great deal of the complexity of plant-based foods, but there is still 'dark matter' to be discovered. Moreover, state-of-the-art food metabolomics offers insights into the molecular mechanisms underlying sensorial and nutritional characteristics of foods and thus enables higher precision and speed. This review describes recent applications of food metabolomics from fork to farm and focuses on the opportunities these bring to continue food innovation and support the shift to plant-based foods.

Comparing the Flavor Characteristics of 71 Tomato (Solanum lycopersicum) Accessions in Central Shaanxi

Flavor is an important quality of mature tomato fruits. Compared with heirloom tomatoes, modern commercial tomato cultivars are considerably less flavorful. This study aimed to compare the flavor of 71 tomato accessions (8 pink cherry, PC; 11 red cherry, RC; 15 pink large-fruited, PL; and 37 red large-fruited, RL) using hedonism scores and odor activity values. Taste compounds were detected using high-performance liquid chromatography. Volatiles were detected using gas chromatography-olfactometry-mass spectrometry. The flavor of tomato accessions can be evaluated using the DTOPSIS analysis method. According to the results of DTOPSIS analysis, 71 tomato accessions can be divided into 4 classes. Tomato accessions PL11, PC4, PC2, PC8, RL35, RC6, and RC10 had better flavor; accessions PC4, PC8, RC10, RL2, and RL35 had better tomato taste; and accessions PL11, PC2, and RC6 had better tomato odor. The concentrations of total soluble solids, fructose, glucose, and citric acid were shown to positively contribute to tomato taste. Tomato odor was mainly derived from 15 volatiles, namely, 1-hexanol, (Z)-3-hexen-1-ol, hexanal, (E)-2-hexenal, (E)-2-heptenal, (E)-2-octenal, (E,E)-2,4-decadienal, (Z)-3,7-dimethyl-2,6-octadieal, 2,6,6-timethyl-1-cyclohexene-1-carboxaldehyde, (2E)-3-(3-pentyl-2-oxiranyl)acrylaldehyde, 6-methyl-5-hepten-2-one, (E)-6,10-dimetyl-5,9-undecadien-2-one, methyl salicylate, 4-allyl-2-methoxyphenol, and 2-isobutylthiazole. Significant positive correlations (P < 0.05) were detected between the compound concentrations and flavor scores. The above-mentioned compounds can be used as parameters for the evaluation of flavor characteristics and as potential targets to improve the flavor quality of tomato varieties.