Abstract
This study employed UPLC-MS/MS and GC-IMS techniques to compare and
analyze the lipid metabolites and volatile flavor compounds in raw
abdominal muscle (CK), sour video abdominal muscle (SV), steamed
abdominal muscle (ST), and oven-cooked abdominal muscle (OC). A total
of 42 subclasses and 1230 lipids were identified. Among these,
lysophosphatidylethanolamine (LPE) 18:2/0:0, lysophosphatidylcholine
(LPC) 18:2/0:0, and triacylglycerol (TG) 16:0_18:1_18:1 enhanced the
aroma retention of steamed abdominal muscle, whereas
phosphatidylcholine (PC) 16:0_18:2 and phosphatidylethanolamine (PE)
P-18:18:18:2 influenced the aroma retention of roasted abdominal
muscle. Additionally, 250 differentially abundant metabolites were
identified as potential markers for differentiating various cooking
methods. Seven compounds were recognized as potential indicators for
distinguishing cooking methods: propanal-D, n-pentanal-M, n-pentanal D,
butanal-D, 3-methylbutanal, 1-hexanal-M, and 1-hexanal D. Correlation
analysis results indicated a significant positive correlation between
aldehydes and phospholipid molecules, including PC, PE, LPC, and LPE.
Keywords: Abdominal muscle, Cooking methods, UPLC-MS/MS, GC-IMS
Graphical abstract
[37]Unlabelled Image
[38]Open in a new tab
Highlights
* •
A total of 42 subclasses and 1230 lipids were identified from
abdominal muscle.
* •
A total of 49 volatile flavor compounds were identified.
* •
Cooking exerted a significant impact on both lipid composition and
flavor of pork.
* •
A significant positive correlation between aldehydes and
phospholipid molecules.
* •
LPC, LPE, PC and PE were critical in flavor development in pork
dishes.
1. Introduction
Meat and its products are popular among consumers because of their
unique flavor and significant nutritional benefits. The taste of meat
strongly affects acceptance among buyers ([39]Han, Zhang, Fauconnier, &
Mi, 2020). Uncooked meat primarily contributes a flavor reminiscent of
blood and metal ([40]Fu, Cao, Yang, & Li, 2022). Approximately 90 % of
the aroma compounds found in cooked meat originate from the breakdown
of lipids, whereas the other 10 % are produced through various other
chemical reactions ([41]Amjad et al., 2022). Lipids are vital for the
development of flavor in meat products, and this process is influenced
by numerous factors, such as the type of meat, genetic background,
feeding practices, and methods of food processing.
A variety of cooking methods exist for abdominal muscle, a widely
favored type of meat, which can influence its nutritional value by
causing the loss of lipid components while enhancing flavor and color
([42]Ángel-Rendón et al., 2020). The cooking process enhances the
creation of new characteristic flavors. Compared with other cooking
methods, baking is characterized by longer durations and higher
temperatures, leading to increased oxidation and volatile compounds.
Consequently, the Maillard reaction and lipid oxidation contribute to
flavor enhancement. Vacuum steaming, in particular, minimizes the
production of harmful byproducts, thereby promoting a healthier cooking
outcome ([43]Jiang et al., 2022). Lipids, which constitute 38 % of the
dry weight of meat, serve as the primary substrates for flavor
production in roasted meat products ([44]Liu et al., 2020).
Lipids have the capacity to generate a wide variety of aromatic
compounds via autooxidation or thermal oxidation processes. The
generation of these aromatic compounds during thermal treatment is
intricately linked to the particular lipid types and fatty acids
present (Li [45]Zhou, Zhao, Bindler, & Marchioni, 2014). [46]Liu et al.
(2024) reported that phosphatidylcholine (PC), phosphatidylserine (PS)
and phosphatidylethanolamine (PE) play crucial roles in the production
of key aroma substances in roasted pork. [47]Liu et al. (2021) reported
that PC and PE may play major roles in the formation of aromatic
compounds. Lipid molecules, such as LPE (22:6), LPE (20:4), LPE (20:5),
LPE (18:3), LPE (16:1), and LPE (18:2), are likely key lipids
responsible for generating distinctive flavor compounds during the hot
processing of crayfish ([48]Zhou et al., 2023). Unsaturated fatty acids
(UFAs) are recognized as the primary flavor components in meat (Longzhu
[49]Zhou et al., 2024).
In recent years, advancements in metabolomics technology have led to
high reliability and accuracy in measuring volatile flavor compounds
and lipid components. Gas chromatography-ion mobility spectrometry
(GC-IMS) is a technique that offers high sensitivity, resolution, and
rapid analysis, making it advantageous for various applications. This
approach is especially useful for detecting functional groups,
including amino, thiol, aldehyde, ketone, and ether groups, present in
food flavorings ([50]Wang, Chen, & Sun, 2020). Currently, some scholars
have investigated the effects of various cooking methods on the flavor
components of beef ([51]Watanabe et al., 2015), mutton ([52]Roldán,
Ruiz, Del Pulgar, Pérez-Palacios, & Antequera, 2015), and fish
([53]Nieva-Echevarría, Manzanos, Goicoechea, & Guillén, 2017). However,
there is a scarcity of research reports focusing on pork, and a
comprehensive comparison of the influence of common Chinese cooking
methods on pork remains lacking. Lipidomics, which uses mass
spectrometry, has become a crucial method for studying the lipid makeup
of food products ([54]Song et al., 2022). This approach offers detailed
insights into the complete lipid profiles of samples without the need
for prior screening. Currently, lipidomics is extensively applied to
detect lipid components and potential biomarkers in both raw and
processed meats ([55]Li, Al-Dalali, Wang, Xu, & Zhou, 2022). Research
has indicated that 97 distinct lipids exist among donkey, beef, and
lamb, with 3 phospholipids proposed as potential markers for
differentiating between these meats and 13 characteristic VOCs
exhibiting negative correlations with 21 phospholipid markers ([56]Man
et al., 2023). [57]Jia, Shi, and Shi (2021) employed fragmentation
mechanisms along with UHPLC-Q-Orbitrap MS/MS to investigate the
transformations of lipids and their molecular mechanisms, revealing
alterations in the lipid profile and quality of Tannan sheep after
refrigeration periods of 12 and 24 days.
To date, few studies have combined lipidomics with volatile flavor
compounds to examine differences in the cooking of abdominal muscle.
The Nanyang black pig, a highly esteemed breed from China, is
celebrated by consumers for its flavorful meat, high fatty acid
content, and diverse nutrient profile. Unsaturated fatty acids are
prevalent in black pork and are prone to oxidation and decomposition,
resulting in the formation of aldehydes, ketones, alcohols, and other
small volatile flavor compounds during processing. Consequently, this
study aims to employ UPLC-MS/MS and GC-IMS techniques to (i) delineate
the lipidomic and volatile profiles of raw abdominal muscle, sous vide
abdominal muscle, steamed abdominal muscle, and oven-cooked abdominal
muscle; (ii) identify key lipids and volatile compounds responsible for
these differences; and (iii) explore the relationships between
significant lipid molecules and volatile compounds. The findings of
this study will enhance our understanding of lipid profile alterations
during the processing of abdominal muscle products and serve as a
reference for advancing related production technologies.
2. Materials and methods
2.1. Chemicals and reagent
HPLC-grade acetonitrile (ACN), methanol (MeOH), isopropanol (IPA),
dichloromethane (CH2Cl2), and tert-butyl methyl ether (MTBE) were
procured from Merck (Darmstadt, Germany). HPLC-grade formic acid (FA)
and ammonium formate (AmFA) were obtained from Sigma-Aldrich (St.
Louis, MO, USA). Ultrapure water was obtained via a Milli-Q system
(Millipore, Billerica, MA). Lipid standards were acquired from either
Sigma-Aldrich or Avanti Polar Lipids (Alabaster, AL).
2.2. Cooking processing of abdominal muscle
The abdominal muscle of black pigs (n = 3, 100–110 kg live weight,
about 6 months old) was sourced from a farm in Nanzhao County, Nanyang
City, Henan Province, China. The fresh abdominal muscle (1 kg) was
refrigerated and transported to the laboratory within one hour of the
pigs were slaughtered and dissected. Divide one kilogram of abdominal
muscle into four equal portions, each weighing 250 g. Prior to cooking,
the skin of the abdominal muscle was removed, and each sample was cut
to dimensions of 0.5 × 1 × 2 cm.
Four cooking methods were employed in this study: raw abdominal muscle,
steam cooking, sous vide, and oven cooking. These widely used
techniques were selected because of their significant variations in
terms of food heating mechanisms.
2.2.1. Raw abdominal muscle (CK)
Unprocessed raw abdominal muscle.
2.2.2. Sous vide (SV)
The samples were pretreated and then vacuum-sealed using a Lavezzini
Univac vacuum sealer (Lavezzini Univac, Fiorenzuola d'Arda, PC, Italy)
in OPA/PP 15/65 bags (Orved, Musile di Piave, Italy). These sealed
samples were subsequently boiled at 75 °C in a water bath for 45 min.
2.2.3. Steaming (ST) (100 % steam)
The samples were subjected to steaming at an atmospheric pressure of
100 °C for 180 min in a Combo-Steam SL oven (V-Zug, Zurich,
Switzerland).
2.2.4. Oven cooking (OC)
An electric oven (T7-L3840, Guangdong Midea Kitchen Appliance
Manufacturing Co., Ltd.) was preheated to 180 °C. After 15 min, the
samples were removed and allowed to cool. During the baking process,
the samples were turned every 5 min.
Each of the four groups of experimental samples was repeated five times
in for a total of 20 samples. Once cooked, the samples were placed at
room temperature, minced using a meat grinder, vacuum-packed, and
stored at −80 °C until further analysis.
2.3. Lipid oxidation indices
2.3.1. Determination of the peroxide value (POV)
The POVs of the abdominal muscle samples were determined via the
titration method according to GB/T5009.227–2016.
2.3.2. Determination of thiobarbituric acid reactive substances (TBARS)
Based on the research method reported by ([58]Domínguez, Gómez, Fonseca
and Lorenzo, 2014a), the TBARS of various samples were determined. Six
grams of chopped and mixed abdominal muscle were combined with 20 mL of
a 10 % trichloroacetic acid solution containing 0.1 % ethylene diamine
tetraacetic acid (EDTA) and filtered through Whatman filter paper.
Subsequently, 5 mL of the filtrate was combined with 5 mL of a
0.02 mol/L TBA solution, swirled for 10 s, and incubated at 90 °C for
40 min. After cooling to room temperature, the absorbance was measured
at 532 nm.
2.4. Lipid extraction procedure
About 20 mg of sample was allowed to thaw and then transferred into a
correctly labeled 2 mL centrifuge tube, and the weights were recorded.
To this end, 1 mL of extraction solvent (methyl tert-butyl
ether/methanol: 3/1, V/V) was introduced. The resulting mixture was
vortexed for 15 min. Following this step, 200 μL of water was added,
and the mixture was vortexed for 15 min. Afterward, the mixture was
centrifuged at 4 °C (12,000 rpm) for 10 min via a 5424 R centrifuge
from Eppendorf GmbH. Subsequently, 200 μL of the supernatant was
subsequently transferred to a 1.5 mL centrifuge tube for concentration.
Once concentrated, the lipid extract was reconstituted in 200 μL of
isopropanol (1:1, v/v), vortexed for 3 min, and then centrifuged again
at 12,000 rpm for 3 min. The supernatant was then collected for
analysis via LC-MS/MS. To create a quality control sample, twenty
microliters of supernatant from each sample were pooled together.
2.5. HPLC conditions
The sample extracts were analyzed using an LC-ESI-MS/MS system (UPLC,
ExionLC AD, [59]https://sciex.com.cn/; MS, QTRAP® 6500+ System,
[60]https://sciex.com/). The column temperature was 45 °C. Flow rate:
0.35 mL/min. Mobile phase A was composed of acetonitrile (Merck,
Darmstadt, Germany) and water at a ratio of 60:40, whereas mobile phase
B was composed of acetonitrile/isopropanol (Merck, Darmstadt, Germany)
(95/5). The program for the chromatographic gradient elution was
designed as follows: at 0 min: A/B (80:20, V/V), at 2.0 min: 70:30
(V/V), at 4 min: 40:60 (V/V), at 9 min: 15:85 (V/V), at 14 min: 10:90
(V/V), at 15.5 min: 5:95 (V/V), at 17.3 min: 5:95 (V/V), then returning
to 80:20 (V/V) at 17.3 min, and finally at 20 min: 80:20 (V/V).
2.6. ESI-MS/MS conditions
The experimental parameters for ESI-MS/MS were established with
ionization voltages of (+) 5.5 kV and (−) 4.5 kV. The temperatures for
the ion source (TEM), as well as the settings for nebulizer gas (GC1),
auxiliary gas (GC2), and curtain gas (CUR), were controlled at 500 °C,
45 psi, 55 psi, and 35 psi, respectively. Instrument tuning and mass
calibration were performed with 10 and 100 μmol/L polypropylene glycol
solutions in QQQ and LIT modes, respectively.
2.7. Volatile compound analysis
Volatile compound assessment was performed utilizing a GC-IMS device
(FlavourSpec®, G.A.S., Dortmund, Germany). A 2 g sample of abdominal
muscle was introduced into a 20 mL headspace vial and incubated at
60 °C for 15 min. Next, 500 μL of the sample was injected, with the
injector temperature set to 85 °C and the incubation speed adjusted to
500 rpm. The MXT5 (15 m × 0.53 mm ID) capillary column was used, and
the temperature of the column was held constant at 60 °C throughout the
entire procedure. Nitrogen gas, with a purity of ≥99.999 %, was
utilized as the carrier gas. The flow rate commenced at 2.0 mL/min for
the initial 2 min, subsequently ramped linearly to 10.0 mL/min within
an 8-min timeframe, and further increased linearly to 100.0 mL/min over
the course of 10 min, which was then maintained at 100.0 mL/min for an
additional 10 min. Ionization of analytes occurred within the IMS
chamber before being directed into the drift tube (150 mL/min).
2.8. Relative odor activity value (ROAV)
The ROAV method was employed to assess how particular aroma components
influence the overall flavor profile. The component that had the
greatest impact on the flavor of the sample was designated
ROAV[x] = 100. The ROAV values for the other components (x) were
derived via the formula outlined below:
[MATH: ROAVX≈100×C%X
C%stan×TstanTX<
/mfrac> :MATH]
In the formula, C%X signifies the percentage content relative to the
volatile component, whereas TX denotes the sensory threshold associated
with it. Additionally, C% reflects the percentage of the component that
has the most significant impact on the sample's overall flavor, and T
is the sensory threshold of that particular flavor-contributing
component.
2.9. Statistical analysis
Data on lipids were analyzed via Analyst 1.6.3 software, whereas R
software (available at [61]www.r-project.org) was used for multivariate
statistical analyses. Flavor data analysis was carried out utilizing
the instrumental data processing software VOCal. Analysis of variance
(ANOVA) and Duncan's multipole difference test (P < 0.05) were used to
evaluate the differences between samples. Both PCA (utilizing UV
scaling) and OPLS-DA (employing Par scaling) analyses were executed
with Simca 14.1 to differentiate the smell and taste characteristics of
abdominal muscle samples.
3. Results and discussion
3.1. Lipid oxidation
The effects of various cooking methods on abdominal muscle POV and
TBARS are illustrated in [62]Fig. 1A and [63]Fig. 1B, respectively.
Compared with those in the control group (CK), the POV and TBARS levels
in the SV, ST, and OC groups tended to increase. Specifically, the CK
group presented POV and TBARS values of 0.28 meq/kg and 0.35 mg MDA/kg,
respectively, whereas the ST group presented values of 1.91 meq/kg and
3.85 mg MDA/kg, indicating increases of approximately 7 and 11 times,
respectively, compared with those of the CK group. Although the POV and
TBARS values in both the SV and OC groups were lower than those in the
ST group were, they remained elevated compared with those in fresh
abdominal muscle. This finding is consistent with Broncano et
al.([64]Broncano, Petrón, Parra, & Timón, 2009), who demonstrated that
cooking significantly increases physicochemical indicators of pork,
such as the TBARS value and cholesterol oxide content, thereby
reflecting the extent of fat oxidation. These cooking methods can
considerably increase the degree of oxidation of pork fat, potentially
contributing to the development of flavor compounds.
Fig. 1.
[65]Fig. 1
[66]Open in a new tab
Changes in lipid oxidation in abdominal muscle with different cooking
methods: (A) changes in the content of peroxide value (POV); (B)
changes in the content of thiobarbituric acid reactants (TBARS). Lipid
composition of abdominal muscle treated with different cooking methods:
number and percentage of lipid subclasses (C). Changes in lipid content
of abdominal muscle under different cooking methods (D). Changes in
lipid subclass content in abdominal muscle under different cooking
methods (E-H). The asterisk above the bar chart indicates the results
of a t-test comparing the two groups of data, with the following
criteria for P-value determination: * P < 0.05; ** P < 0.01; and ***
P < 0.001. CK: raw abdominal muscle, SV: sous vide abdominal muscle,
ST: steamed abdominal muscle, OC: oven-cooked abdominal muscle.
3.2. Lipid changes in abdominal muscle under different cooking methods
Abdominal muscle samples treated with four cooking methods were
analyzed by UPLC-MS/MS to assess changes in lipid components resulting
from these techniques. A mixed solution was utilized as the QC sample.
As depicted in Fig. S1, the curves representing the total ion flow for
lipid detection showed considerable overlap, with consistent retention
times and peak intensities. These findings indicate the robust
stability of signals when identical samples are analyzed across various
time points, which suggests high reproducibility and dependability of
the data. The lipid composition of the abdominal muscle processed by
the four cooking methods was shown in [67]Fig. 1C, and a total of 1230
lipids were identified. These included 553 glycerophospholipids (GP),
248 sphingolipids (SP), 345 glycerolipids (GL), 74 fatty acids (FAs),
and 10 other lipid compounds. Among these lipid subclasses, GP were the
most prevalent, followed by GL and SP. This observation aligns with
earlier studies on pork lipids, which also indicated that the GL, GP,
and SP subclasses constituted more than 90 % of the total lipid content
(H. [68]Liu et al., 2024). These lipid compounds can be further
categorized into 42 subclasses, predominantly consisting of
triglycerides (TG), phosphatidylethanolamines (PE),
phosphatidylcholines (PC), and ceramides (Cer). Triglycerides are the
primary form of lipids found in both animals and plants, and their
degradation into free fatty acids is crucial for flavor development.
This finding aligns with previous research on pork lipids ([69]Wu, He,
Yang, & Li, 2024).
The alterations in the lipid content of the abdominal muscle following
various heat treatments are illustrated in [70]Fig. 1D. GLs were the
most abundant lipids both before and after heat treatment, followed by
GP, FA, and SP. The contents of various lipid subclasses were further
analyzed, among which the lipid contents of DG and TG were the highest
in ST, significantly higher than those of the other three samples
([71]Fig. 1E). Under heat treatment conditions, DGs and TGs are
susceptible to thermal degradation and hydrolysis, yielding free fatty
acids (FFA). These FFAs can subsequently undergo β-homolysis,
ketone-enol tautomerization, or isomerization at carbon‑carbon
double-bond reaction sites, forming various types of hydroperoxides
([72]Yin, Xu, & Porter, 2011). These compounds are then decomposed into
smaller molecular species, including hydrocarbons, ketones, aldehydes,
and alcohols ([73]Zhang, Qin, Lin, Shen, & Saleh, 2015). These findings
indicate that the FA content in the abdominal muscle decreased
following different heat treatments, likely due to the oxidation of FAs
into smaller molecular weight compounds ([74]Fig. 1H). Compared to CK,
the levels of LPC, LPE, and LPS were significantly elevated in both ST
SV and OC, with the highest concentration observed in ST ([75]Fig. 1F).
This increase may be attributed to the hydrolysis of PC and PE into LPC
and LPE under aerobic heating conditions. Cooking temperature is a
crucial factor influencing phospholipase activity, which in turn
regulates the conversion of PC and LPC in abdominal muscles ([76]Jia,
Li, Wu, Liu, & Shi, 2021). Notably, CK and SV do not contain LPA. The
contents of PE, PG, PI, PS, PC, and PA were highest in OC, followed by
SV ([77]Fig. 1F). This finding may be associated with the alteration
and degradation of lipid molecules during oxidation ([78]Tu et al.,
2022). Additionally, the content of SM increased significantly
following heat treatment, particularly during baking ([79]Fig. 1G).
The contents of TG, LPC, and LPE in the ST group were significantly
greater than those in the other treatment groups. The highest levels of
LPE, LPC, and TG were observed for LPE (18:2/0:0), LPC (18:2/0:0), and
TG (16:0_18:1_18:1), respectively. Moreover, the concentrations of PE
and PC in the OC group significantly increased relative to those in the
other groups. The lipid molecules found in the highest abundance within
PC and PE were identified as PC (16:0_18:2) and PE (P-18:0_18:2),
respectively. It has been reported that lipid content has a greater
influence on aroma than does lipid type (M. [80]Zhou et al., 2023).
Additionally, the findings revealed that the aroma retention of steamed
abdominal muscle was influenced by LPE (18:2/0:0), LPC (18:2/0:0), and
TG (16:0_18:1_18:1), whereas PC (16:0_18:2) and PE (P-18:0_18:2) could
significantly contribute to the aroma retention of roasted abdominal
muscle.
With respect to double bond equivalents (DBE), triglycerides (TG),
phosphatidylethanolamine (PE), and phosphatidylcholine (PC), with DBE
values of 2, dominate the abdominal muscle subjected to various cooking
methods. The most frequently observed DBE value for free fatty acids
(FFA) is 1 ([81]Fig 2ABC). An analysis of lipid compounds with
different chain lengths revealed that TG (chain lengths: 46, 48, 50,
52, and 54) presented the highest content in the abdominal muscle.
Additionally, PE (chain lengths: 36, 38), PC (chain lengths: 34, 36,
38), and FFA (chain length: 18) were among the most abundant (Fig. S2).
The concentration of TG (chain lengths: 46, 48, 50, 52, and 54) in the
ST sample was significantly greater than that in the other three
samples, whereas the concentration of PE (chain lengths: 36 and 38) was
significantly lower than that in the other samples ([82]Fig 2EF).
Compared with that in the control (CK), the concentration of PC (chain
lengths: 34, 36, 38) increased significantly following heat treatment,
whereas the concentration of FFA (chain length: 18) decreased
significantly ([83]Fig 2GH).
Fig. 2.
[84]Fig. 2
[85]Open in a new tab
Content of different double bond equivalents (DBEs) in abdominal muscle
(A) TG, (B) PE, (C) PC, (D) FFA. Content of lipid compounds with
different chain lengths in abdominal muscle (E) TG, (F) PE, (G) PC, (H)
FFA. * P < 0.05, ** P < 0.01, *** P < 0.001. CK: raw abdominal muscle,
SV: sous vide abdominal muscle, ST: steamed abdominal muscle, OC:
oven-cooked abdominal muscle.
3.3. Differential lipidomics analysis in cooking abdominal muscle
Principal component analysis (PCA) allows for the visual interpretation
of complex datasets, revealing groupings, trends, and outliers within
the data ([86]Guo et al., 2022). Orthogonal partial least squares
discriminant analysis (OPLS-DA) is a supervised pattern recognition
multivariate statistical analysis method that effectively removes
unrelated influences to identify differentially abundant metabolites.
Utilizing a dataset comprising 1230 lipids, PCA and OPLS-DA analyses
were conducted on the metabolite data of cooked abdominal muscle to
discern differences among samples subjected to various cooking
treatments ([87]Fig 3AB). The PCA results showed that PC 1 and PC 2
account for 36.19 % and 20.1 % of the total model, respectively,
highlighting differences among the four sample groups. Notably, the
difference between SV and OC was minimal, whereas the differences
between CK and ST, and OC were substantial. The classification metrics
R2Y and Q2 were 0.995 and 0.785, respectively, demonstrating that the
OPLS-DA model is reliable and has good fitting and predictive
capabilities ([88]Bi et al., 2023). These findings indicate that
OPLS-DA is effective for differentiation applications. Furthermore, the
results demonstrate that the metabolic profile of abdominal muscle
undergoes significant changes after cooking, aligning with previous
studies.
Fig. 3.
[89]Fig. 3
[90]Open in a new tab
PCA score plot (A), orthogonal partial least squares discriminant
analysis (OPLS-DA) score plots (B). Venn diagram of the number of
differential lipid molecules in abdominal muscle between different
cooking methods (C). Volcano plot of differential lipid molecules in ST
vs CK (D), SV vs CK (E), OC vs CK (F). Each point in the volcano plot
represents a metabolite, with the x-axis reflecting the fold change in
lipid differences between groups (log[2]FoldChange) and the y-axis
indicating the level of significance (-log[10]FDR). Significantly
upregulated lipids are represented as red points, significantly
downregulated lipids as green points, and gray points indicate
metabolites detected but not significantly different. The size of the
points corresponds to the VIP value. Effect of heat treatment on the
amount of differentially changed lipids in abdominal muscle: ST vs CK
(G), SV vs CK (H), OC vs CK (I). (For interpretation of the references