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Advanced Lipidomics in the Modern Meat Industry: Quality Traceability, Processing Requirement, and Health Concerns

Over the latest decade, lipidomics has been extensively developed to give robust strength to the qualitative and quantitative information of lipid molecules derived from physiological animal tissues and edible muscle foods. The main lipidomics analytical platforms include mass spectrometry (MS) and nuclear magnetic resonance (NMR), where MS-based approaches [e.g., “shotgun lipidomics,” ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS)] have been widely used due to their good sensitivity, high availability, and accuracy in identification/quantification of basal lipid profiles in complex biological point of view. However, each method has limitations for lipid-species [e.g., fatty acids, triglycerides (TGs), and phospholipids (PLs)] analysis, and necessitating the extension of effective chemometric-resolved modeling and novel bioinformatic strategies toward molecular insights into alterations in the metabolic pathway. This review summarized the latest research advances regarding the application of advanced lipidomics in muscle origin and meat processing. We concisely highlighted and presented how the biosynthesis and decomposition of muscle-derived lipid molecules can be tailored by intrinsic characteristics during meat production (i.e., muscle type, breed, feeding, and freshness). Meanwhile, the consequences of some crucial hurdle techniques from both thermal/non-thermal perspectives were also discussed, as well as the role of salting/fermentation behaviors in postmortem lipid biotransformation. Finally, we proposed the inter-relationship between potential/putative lipid biomarkers in representative physiological muscles and processed meats, their metabolism accessibility, general nutritional uptake, and potency on human health.

Over the latest decade, lipidomics has been extensively developed to give robust strength to the qualitative and quantitative information of lipid molecules derived from physiological animal tissues and edible muscle foods. The main lipidomics analytical platforms include mass spectrometry (MS) and nuclear magnetic resonance (NMR), where MS-based approaches [e.g., "shotgun lipidomics," ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS)] have been widely used due to their good sensitivity, high availability, and accuracy in identification/quantification of basal lipid profiles in complex biological point of view. However, each method has limitations for lipid-species [e.g., fatty acids, triglycerides (TGs), and phospholipids (PLs)] analysis, and necessitating the extension of effective chemometric-resolved modeling and novel bioinformatic strategies toward molecular insights into alterations in the metabolic pathway. This review summarized the latest research advances regarding the application of advanced lipidomics in muscle origin and meat processing. We concisely highlighted and presented how the biosynthesis and decomposition of muscle-derived lipid molecules can be tailored by intrinsic characteristics during meat production (i.e., muscle type, breed, feeding, and freshness). Meanwhile, the consequences of some crucial hurdle techniques from both thermal/non-thermal perspectives were also discussed, as well as the role of salting/fermentation behaviors in postmortem lipid biotransformation. Finally, we proposed the inter-relationship between potential/putative lipid biomarkers in representative physiological muscles and processed meats, their metabolism accessibility, general nutritional uptake, and potency on human health.

INTRODUCTION
The global meat industry is a continuously growing sector with an ever-increasing demand. The Organization for Economic Co-operation and Development (OECD) and the Food and Agriculture Organization of the United Nations (FAO) have recently stated that worldwide meat production is projected to expand by approximately 44 million tons by 2030, despite the detrimental impacts of the coronavirus disease 2019  pandemic and other possible restrictions (1). Meat is considered a unique animal-derived food providing high biological value proteins with all essential amino acids and various micronutrients (2). Alongside proteins, lipids are abundant constituents in meat and meat products that play critical roles in providing desirable mouth-feel perception, characteristic flavor, favorable texture, juiciness, and enhanced cooking yield (3). According to the complexity of structure and biosynthesis, lipids are divided into eight categories, depending on their differences in the level of unsaturation, the type of the covalent bond, the fatty acyl chain length, double bond location, the head groups, Z/E geometric isomerism, and the branched functional groups (4,5). Specifically, these muscle-derived lipid species mainly include non-esterified/free fatty acids (FFAs), glycerolipids (GLs), glycerophospholipids (GPs), sphingolipids (SLs), sterol lipids, prenol lipids, saccharolipids, and polyketides (6). Among them, triglycerides (TGs) or triacylglycerols (TAGs) and phospholipids (PLs), two common and most abundant categories of lipids, are highly associated with health and nutritional functions in the body (7,8). TGs are mainly composed of FAs, such as capric acid (Ca) and lauric acid (La), but their contents can vary greatly among different breeds and muscle tissues (9). Membrane phospholipid composition may play a critical role in subsequent lipid oxidation development in raw and cooked meats (10,11), while SLs and glycolipids contribute more functions to human health, such as increasing anti-inflammatory and anticarcinogenic activities and alleviating the risk of cardiovascular diseases and cholesterol absorption (7,12). Traditional studies utilize gas chromatography (GC) with known standards or mass spectrometry (MS) to detect and identify total fatty acids in the samples, where saponification and derivatization protocols are usually required to generate the volatile fatty acid analytes and may result in the loss of information about the original esterified structure (neutral or polar lipids) (9,13). Thus, the modern meat industry deserves the exploration of lipid composition from different biological sources through global profiling for the qualitative and quantitative characterization of individual lipid species (11).
During the latest decade, scientific expertise and technologies are constantly being developed with commercial or industrial aspects to advance the traceability and authentication of meat products and to address the safety concerns of the public, and for economic and quality reasons as well (14). Although meat from different species can be easily detected using deoxyribonucleic acid (DNA)-based techniques, the mixing of meat from different biological sources (e.g., geographical origins) is more difficult to detect. In this regard, MS-and nuclear magnetic resonance (NMR)-based lipidomics, such as both untargeted and targeted approaches, have been suggested as a promising strategy for this detection (15)(16)(17). The high-throughput untargeted analysis has the advantage of detecting lipid metabolites as comprehensively as possible by emphasizing the changes in quantity in biological importance (14,15). Additionally, the targeted approach focuses on identifying and acquiring a number of specific fractions of known lipid species, for instance, FFAs, PLs, and cholesterol, in the presence of external chemical standards or available databases (18,19). To achieve more persuasive results, the sample/lipid-extract preparation, chromatographic separation or direct-infusion MS (DI-MS, "shotgun lipidomics"), method validation, multivariate data processing, and bioinformatics evaluation have been well considered in the entire analytical platform (4,15).
Exploratory analysis, classification/discriminant analysis, and regression analysis/prediction models have been proven to be useful in lipidomics data analysis. Apart from these chemometric tools or machine learning methods, however, bioinformatic validation should be crucial for determining the potential biomarkers of lipid species present in a biological system and providing molecular insights into alterations in energetic/metabolic pathways of lipid biosynthesis (4). Hence, in this short review, we examined the mainstream of recently published investigation available with discussions regarding the applications of MS-/NMR-based lipidomics in muscle foods, such as meat origin and adulteration identification, meat safety assessment, and dietary lipid nutrition during representative processing conditions and/or in vitro treatment.

MEAT LIPIDOMICS IN TRACEABILITY AND MICROBIAL SAFETY
Meat is originally skeletal muscles of livestock and thereby suffers from some factors in the livestock production system, mainly such as animal genetic/breed background, feeding types, geographical location, and environmental stress, particularly associated with spoilage developed by microbiological activity (20). As highlighted in literature evidence (Table 1), different efforts have been undertaken to achieve lipid-label validation depending on diagnostic and reliable features that can reflect the origins/authenticity of suspected muscles and their freshness status through the lipidomics strategy. The consequent lipid molecules that are inherently from muscle tissues or result from the metabolism of indigenous microorganisms vary among different species, such as beef (14,18,(21)(22)(23)(24), pork (24)(25)(26)(27), sheep/goat (28), poultry (16), and marine products (29)(30)(31)(32).
To investigate the effect of genetic background, MALDI-TOF-MS and Phosphorus-31 NMR ( 31 P NMR)-based lipidomics were successfully applied to capture the differences in fatty acid biosynthesis among German Simmental bulls fed with different diets. Consequently, TGs, phosphatidylethanolamines (PEs), phosphatidylcholine (PCs), phosphatidylinositol (PIs), cardiolipins (CLs), and cholesterol were identified as potential biomarkers (22). Furthermore, untargeted and targeted MSbased lipidomics based on the use of ultra-performance liquid chromatography (UPLC) coupled with high-resolution MS (HRMS) has shown good discriminative power between different species/breeds and feeding conditions toward extended global lipid information with appropriate multivariate data analysis (Table 1). A panel of lipids, particularly GLs [diacylglycerols (DAGs) and TAGs], lysophosphatidylcholines (LPCs), lysophosphatidylethanolamines (LPEs), and n-6 polyunsaturated fatty acids (PUFAs) have been screened out as specific markers for differentiation of animal diets (18,21,25,28,29). More intermuscular differences traceable properties in lipidomics can be evaluated as a function of geographical origin and for adulteration ( Table 1). For instance, GLs [TGs and diglycerides (DGs)] and GPs were screened as main lipid biomarkers by principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) modeling through UPLC-Q-TOF-MS/MS approach for China's domestic pork (from Tibetan, Jilin, and Sanmenxia black pigs), suggesting the difference in production systems, feeds and genetic backgrounds (26).
It is also noticeable that exposure to microbes may significantly influence the lipid compositions during meat production where other indicators are usually involved, e.g., isotopic ratios and feeding (23). Particularly for marine products, such as fish, fatty acids, and GPs, metabolism is identified as the major pathway through microbial contamination during cutting, storage, and distribution processes after slaughter. A study was conducted to evaluate the spoilage of farmed Atlantic salmon (Salmo salar L.) during storage at 4 • C for up to 15 days by adopting the UPLC-Q-Exactive-MS with high sensitivity (30). According to the results, the increase of LPC (C17:0) and LPC (C18:0) could result from the hydrolysis of PC (C18:4/C16:1) as a major freshness index. Assisted by "shotgun lipidomics, " PCs, PEs, PIs, phosphatidylserines (PSs), and sphingomyelins (SMs) were profiled as the lipid biomarkers of interest for the naturally spoiled muscle from Ctenopharyngodon idellus during room-temperature storage. UPLC-HRMS-based lipidomics has also shown good strength in identifying PCs, ceramides (CERs), and SLs metabolism as differentiated by pork and beef ground meat from different grades or due to death from diseases/abnormalities (24,27). However, NMR-based lipidomics can screen more polar lipid metabolites and thus provides characteristic information on the metabolic profile of adulterated muscle. The presence of o-phosphocholine and a reduced level of Myo-inositol in turkey breast muscle injected with protein hydrolysates were observed through Proton NMR ( 1 H NMR) untargeted lipidomics, suggesting the possible role of Myo-inositol deficiency in enhanced lipolysis (16,33).

MEAT LIPIDOMICS AS AFFECTED BY PROCESSING FACTORS
Up to date, various meat processing strategies, such as castration (34), thermal/non-thermal techniques (8,17,19,(35)(36)(37)(38), freezing/thawing intervention (39), in vitro oxidation (40,41), postmortem aging/storage (11,42), modified atmosphere packaging (MAP) (42), and brining/drying-curing/preservatives treatments (43)(44)(45)(46)(47) have been implicated to improve the microbiological safety, color, flavor, and texture for the development of favorable meat products ( Table 2). Accordingly, DI-MS and/or a combination of GC and liquid chromatography (GC/LC) and MS techniques have been utilized in the application of thermal processing to identify dozens of different lipids as potential biomarkers. Consequently, the lipolysis of TGs and PLs was noted as a strong flavor-binding precursors through different thermal degradation (boiling, steaming, and roasting) and showed specific losses of representative lipids (19,35). It is worthwhile to note that fresh fish fillets, particularly Pleuronectes platessa upon high-pressure processing (HPP) were characterized by a distinctly high level of lipidderived polar metabolites, serine-phosphoethanolamine species (Ser-PETA) (36). Specifically, studies on the discrimination of untreated/irradiated raw/ground meat from commercially produced goat, chicken, turkey, and pork through global lipid profile using UPLC-Q-Exactive-Orbitrap-MS/MS, targeted GCresolved fatty acids composition, and chemometric tools (PCA and PLS-DA) have been recently reported (8,37,38). These authors observed γ-ray irradiation dose-dependent increase in docosahexaenoic acid (DHA)-enriched PC (C18:4/C22:6) + H in goat meat, while X-ray irradiation tended to result in n-3 PUFA-enriched lipids in chicken and turkey meat, accompanied with a noticeable accumulation of PLs and oxidized shortand long-chain FFAs. Comprehensive lipidomics based on GC-technique and phosphorus-31/carbon-13 NMR ( 31 P/ 13 C NMR) spectroscopy was devoted for hoki co-product (i.e., roe) treated with a pulsed electric field (PEF), where abundant PLs and a high level of lyso-diphosphatidylglycerols (LDPGs), LPEs, lysophosphatidylserines (LPSs), and LPCs were finally characterized, and PEF transformed more sn-2 phospholipid eicosapentaenoic acid (EPA) and DHA into sn-1,3 positions with potentially compromised bioavailability in terms of reesterified structures (17). Additionally, when frozen Atlantic salmon and bullet tuna were thawed, an untargeted MSbased lipidomics approach detected abundant water-soluble phospholipid metabolites (i.e., L-α-glyceryl-phosphoryl-choline and N-methyl-ethanolamine phosphate) (39). However, hydroxyl radical ( · OH) attacks can significantly alter the lipidomics profiles of shrimp and yak muscle to a large extent. PEs enriched in PUFAs were highly vulnerable to in vitro oxidation but both GPs metabolism and fatty acid biosynthesis were enriched during the subsequent deterioration and spoilage process of oxidized yak hindquarter meat (40,41). On the other hand, PLs in porcine meat can undergo enzymatic hydrolysis during postmortem aging. For example, by employing targeted UPLC-TQ-MS/MS (with PLs as internal standards) to determine the PLs lipolysis tendency, researchers found that postmortem porcine loin (M. Longissimus) up to 21 days at 4 • C was rich in PIs, PSs, and PAs, particularly C38:4 and C36:2 lipid species. Phospholipase A 2 (PLA 2 ) can be activated by postmortem calcium influx from the sarcoplasmic reticulum (SR) with a surge of LPCs in aging muscles (11). Apart from the targeted MS approach, untargeted "shotgun lipidomics" [electrospray ionization (ESI)-QTrap-MS/MS] became useful in identifying the lipid biomarkers derived from water-boiled dry-cured Pekin duck as a function of salting and ripening times (43,44). In    those studies, the findings pointed out that low-salt (< 6%) dry-cured duck significantly promoted the degradation of individual PLs (e.g., PCs, PGs, PEs, PSs, and PIs) probably resulting from a robust release of phospholipase, though some LPCs were damaged possibly due to oxidation and thermal degradation provided by boiling. In other cases, by combining the GC-system and untargeted hydrophilic interaction liquid chromatography (HILIC) coupled to QTrap-MS, saturated fatty acids (SFAs) (C16:0), monounsaturated fatty acids (MUFAs) (C18:1), and PUFAs (C18:2) were observed to be important lipid-derived flavor precursors in low-salted salmon and PCs content was kept at a high level even at 30% NaCl replacement rather than PSs (45

DISCUSSION ON POSSIBLE MECHANISMS AND FUTURE REMARKS
Lipids are inherently abundant in muscle tissues, which play critical roles in a series of cellular processes and physiological/biological activities (e.g., cell membrane architecture, cell signaling and energy-storing) (48). Breeds and feeding conditions, which usually have a large impact on animal physiology, determine the skeletal muscle growth and maturation, the final meat yield, and nutritional/flavor quality (21,28,29). NMR-/MS-based approaches have been attempted to obtain lipid-metabolite signatures and their relationship to physiological alteration in tissues as well as the potency in meat production. Differences in muscle energy metabolism, lipolysis in adipocytes, fat digestion/absorption, and cholesterol metabolism, β-oxidation of fatty acids, in particular, would offer the dynamics of redox status, oxidative stability, and consumer acceptability as influenced by the nutrients and ingredients in the animal feeds (20). For instance, concentrate-fed sheep are generally more obese compared with pasture-grazing sheep. High-fat diets may thereby promote the levels of isoleucine, lipids, glutamate, and 3-methylhistidine and lead to decreased citrate and glycerophosphorylcholines (GPCs) in animals (28). Consequently, as putatively revealed from lipidomics analysis, DAGs tend to accumulate in meat from concentrate-fed sheep/goats due to fat deposition, while some saturated TGs (e.g., C40:0, C42:0, and C44:0) in meats can be favored by pasture-grazing feeding strategy, showing a positive biological function of energy storage (28). PLs are the primary structural constituents of biological membranes and serve as critical nutrients owing to their physiological and nutritional properties. Following lipidomics, some species of fatty acid components and PLs have been useful markers in muscle tissues for differentiating breeds/dietary supplementation (21,28,29), detecting adulteration (27), and monitoring microbial accessibility (30)(31)(32). For instance, n-3 long-chain PUFA, such as EPA (C20:5n-3) and DHA (C22:6n-3) are essential fatty acids (EFA) for marine fish and crustaceans. An appropriate ratio of DHA/EPA feeding diet can improve the lipogenesis, integrity of membrane PLs, and DHA biosynthesis/deposition, meanwhile, inhibiting the mitochondrial β-oxidation of fatty acid and supporting growth performance in the hepatopancreas of S. paramamosain (29). Lipids in the meat matrix are usually involved in thousands of metabolites that may be affected by species, nutrients, microbial diversity, production, and storage. These muscle-derived lipid metabolites are not only the phenotypic consequences of physiological muscle metabolism but also the major molecular basis for characterizing organoleptic components following different processing conditions (20,(49)(50)(51). For dry-cured meats, lipolysis usually involves a set of endogenous adipose tissue TGs lipases [e.g., neutral and basic ones including hormonesensitive lipases (HSL) and lipoprotein lipases (LPL)] and some phospholipases (classified as A 1 , A 2 , C, and D) responsible for PLs degradation followed by auto-oxidation, which contributes to the formation of aromatic volatile compounds (10,52,53). However, we should note that the identification efficacy of bioactive lipid species through classical MS-based lipidomics approaches would be significantly affected by the applied lipid extraction protocols [e.g., liquid-liquid extraction (LLE), some alternative methods, such as methyl tert-butyl ether (MTBE)] mainly due to the characteristic amphipathic properties of lipids to achieve a differential partition (54,55). In particular, the good recovery, ionization efficiency, and identification of global phospholipid species by LC-MS are still difficult to achieve, arising from their complexity in molecular structures (i.e., the length of the fatty acid chains and the difference in fatty acyl substitution at the glycerol backbone), and hydrophilicity across the entire chromatographic separation (28,45,56). Regarding the MS instruments showing high sensitivity, multi-sourced trace impurities that could result from biological matrices (e.g., remaining proteins in muscle tissues), solvents used for lipid extraction, preparation devices, such as siloxenes and phthalates, and even sample containers, such as plasticizers could be detected when they are carried to the lipid extract and thus these impurities may influence the reproducibility of the lipidomics profile (54). So, the extension of GC-system and NMR-based lipidomics and their combination with LC-resolved MS would exert unique superiority to enrich the entire lipid metabolism pathway (e.g., biosynthesis, oxidative decomposition, and enzymatic lipolysis) by detecting FAs, TGs, and sterols as well as some specific short and polar secondary lipid-metabolites (8,16,17,19,22). Indeed, during non-thermal processing, such as HPP, some important water-soluble lipid-metabolites (e.g., Ser-PETA) might be active in fatty acid transformation and participate in GPs metabolism (36). Overall, the lipolysis of TGs and PLs in meats are closely related to some relevant factors, mainly including (i) the feeding processes of animals (28,29), (ii) circumstances in slaughtering (30,31), (iii) postmortem aging (11,42), (iv) thermal/non-thermal processing (8,13,17,19,35,37,57), and (v) brining/dry-ripening processes (45)(46)(47)58). As a result, the physical/redox status of muscle/adipose tissues can be changed with the difference in bioavailability and bioactivity of endogenous and microbial lipases/phospholipases   Semi-quantification in the abundance of each lipid using the integrated response of the NMR spectra, two-way and one-way ANOVA PAs, PEs, PSs, PIs, PCs, LDPGs, LPEs, LPSs, LPCs, CLs, and SMs; n-3 fatty acids, i.e., DHA/EPA esterified at sn-2/sn-1,3 High PEF input resulted in abundant phospholipids without affecting n-3 fatty acid content, and generated LDPGs, LPEs, LPSs and LPCs. PEF transformed more sn-2 phospholipid EPA and DHA into sn-1,3 positions, indicating a negative change in re-esterified structures to phospholipids and decreased bioavailability of hoki roe lipids.   (10,13,52,53,(59)(60)(61)(62)(63), consequently determining the fat deposition, lipolysis, and lipidomics profile (9,20). In most cases, PLs are the main substrates for lipolysis in dry-cured meat products (10). However, some protein chaperones (heat shock protein 90, Hsp90) are reported to stabilize cell membranes and preserve membrane integrity in muscle tissues, and particularly act as an inherent antioxidant by providing additional protection against ROS-induced PLs oxidation (64). For seafood, such as in fish fillets, the total lipid content generally decreases during cold storage as TGs and PLs are either hydrolyzed by lipolytic enzymes, such as lipase and PLA 2 and/or susceptible to oxidative damage from the myoglobin-mediated mechanism of action (39,65,66), though the activity of mitochondrial enzymes may be different during subsequent thawing. Indeed, the water-enriched external medium could induce hydrostatic pressure in cells and impair plasma membrane integrity, provoking an enrichment of intracellular enzymes in the final exudate (39). These knowledge of the adipose tissue TGs and PLs hydrolysis and oxidation during meat processing suggests a complicated overall lipid degradation mechanism and cellular protection under oxidative stress. A more comprehensive understanding based on multiomics techniques is still required for improving both the quality and nutritional value of specific end products.

CONCLUSION
During the transition from "farm-to-fork" to the modern meat industry, the lipidomics disciplines successfully encompass a comprehensive and high-throughput understanding of meat composition, nutritional value, and safety with a combination of biochemical and mechanical mechanisms. Overall, the techniques for lipidomics have been steadily progressing, particularly regarding the omics-data-mining and multivariate statistical analyses, whereby new efforts are contributed toward new algorithms of developed prediction models for identified lipid biomarkers. Untargeted MS-/NMR-based lipidomics gives molecular insight into meat origin/adulteration and microbial safety with more tentative lipid markers being screened out on a global scale, though additional targeted analytes (e.g., the lipolysis fate of PLs resulting from foodborne microbe bred in muscle) still require further validation in their adulteration detection. In addition, the exhaustive analysis of lipids and their alterations during meat production favors the selective design of processing methods for specific muscle matrices (e.g., irradiation and PEF). Many putative lipid biomarkers following computational approaches and possible metabolism pathways enriched by bioinformatics provide valuable suggestions on food safety and health concerns regarding their potential during the treatment with preservatives, fermentation, aging, and storage. However, challenges remain due to the complexity of meat lipidome, the nature of key intermediate lipid-metabolites, and their evaluation concerning the quality and nutritional value of the final product.

AUTHOR CONTRIBUTIONS
CL and BO-K performed literature review, analyzed and interpreted the data, and drafted the manuscript. CL, BO-K, and GJ reviewed the first draft and revised the manuscript accordingly. All authors contributed to the article and approved the final submitted version.

FUNDING
This research was supported by the National Natural Science Foundation of China (No. 31871824).