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Research article
Characterization and ACE Inhibitory Activity of Fermented Milk with Probiotic Lactobacillus plantarum K25 as Analyzed by GC-MS-Based Metabolomics Approach
1Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Engineering and Technology Research Center of Food Additives, Beijing Technology and Business University, Beijing 100048, P.R. China
2Mengniu Dairy (Beijing) Co., Ltd., Beijing, P.R. China
J. Microbiol. Biotechnol. 2020; 30(6): 903-911
Published June 28, 2020 https://doi.org/10.4014/jmb.1911.11007
Copyright © The Korean Society for Microbiology and Biotechnology.
Abstract
Keywords
Graphical Abstract
Introduction
There is an increasing interest in developing probiotic foods with different functionalities, such as regulating intestinal flora, lowering cholesterol levels, regulating blood glucose level, etc. [1]. Yogurt is a fermented dairy product consumed worldwide with high nutritional value and well-established health benefits, especially when reinforced with probiotic bacteria [2-5]. Addition of probiotic bifidobacteria and lactobacilli to yogurt has become a common practice nowadays in yogurt manufacturing, but there are concerns about maintaining probiotic viability considering unfavorable factors such as various mechanical stresses, low pH, low temperature, oxygen, etc., during yogurt processing and storage [6]. To deliver the health benefits to the consumers, it is of utmost importance to maintain viable probiotic cells during cold (about 6°C) storage with viable counts ranging from 106 to 109 CFU/ml in a probiotic product till consumption [7].
Metabolomics deals with simultaneous determination and quantitative analysis of intracellular metabolites that are produced and modified by the metabolism of living organisms (
In our previous study, we isolated and identified from Chinese Tibetan kefir grains the probiotic
In this study, fermented milk added with probiotic
Materials and Methods
Bacterial Cultures
Probiotic
Preparation of Fermented Milk
Fermented milk was prepared by heat-treating reconstituted skimmed milk (12% w/v) at 95°C for 5 min, followed by cooling to 45°C, and aseptically inoculating with 0.002 g/L of the yogurt starter. The inoculated milk was divided into 2 equal portions, one without inoculation of
Enumeration of Viable Bacteria
Viable counts expressed as CFU (colony forming unit) per gram of the fermented milk samples were determined by plate counting on MRS agar at 37°C for 24 h.
Measurement of pH Change
Production of acid during storage of the fermented milk samples was expressed by measuring changes of pH (pH-250L, ISTEK, Korea) [23].
SPME–GC/MS Profiling
Headspace volatiles of fermented milk were evaluated by SPME- GC/MS. Each fermented milk sample (30 ml) was loaded in triplicate into 20 ml autosampler vials with steel screw tops containing silicone septa faced in Teflon (Microliter Analytical, USA). An internal standard solution (2-methyl-3-heptanone in methanol, Sigma-Aldrich, USA) was added to each vial to control for analysis of variability with samples of fermented milk running only after >4 h at storage temperature [24].
Samples were injected using a CombiPal autosampler (CTC Analytics, Zwingen, Switzerland) attached to an Agilent 6890N gas chromatograph with 5973 inert MS detection (Agilent Technologies Inc., USA). Samples were maintained at 10°C before fiber exposure. Samples were equilibrated at 40°C for 25 min before 30 min fiber exposure of a 1 cm divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber at 31 mm with 4 s pulsed agitation at 250 rpm. Fibers were injected for 5 min at a depth of 50 mm. The GC method was performed at an initial temperature of 40°C for 5 min with a ramp rate of 8°C/min to 250°C held for 5 min. The SPME fibers were introduced into the split/splitless injector at 250°C. An Rtx-5ms column (30 m length × 0.25 mm inner diameter × 0.25 μm film thickness; Restek, USA) was used for all analyses at a constant helium flow rate of 1 ml/min. Purge time was set at 1 min. The MS transfer line was maintained at 250°C with the Quad at 150°C and Source at 250°C.
Derivatization
L-norleucine, N,O-bis(trimethylsilyl)trifluoroacetamide with 1% trimethylchlorosilane [BSTFA (1% TMCS)], methoxyamine hydrochloride, and anhydrous pyridine were purchased from Sigma-Aldrich. A 20 μl thawed sample and 80 μl of cold methanol including internal standard (5 μg/ml L-norleucine) were combined, vortexed for 60 s, and kept at -20°C overnight. Following centrifugation (14,000 ×
GC-MS Analysis
Metabolomics instrumental analysis was performed on an Agilent 7890A gas chromatography system coupled to an Agilent 5975C inert MSD system (Agilent Technologies Inc.). An OPTIMA 5 MS Accent fused-silica capillary column (30 m × 0.25 mm × 0.25 μm; MACHEREY-NAGEL, Germany) was utilized to separate the derivatives. Helium (>99.999%) was used as a carrier gas at a constant flow rate of 1 ml/min through the column. Injection volume was 1 μl, and the solvent delay time was 6 min. The initial oven temperature was held at 70°C for 2 min, ramped to 160°C at a rate of 6°C/min, to 240°C at a rate of 10°C/min, to 300°C at a rate of 20°C/min, and finally held at 300°C for 6 min. The temperatures of injector, transfer line, and electron impact ion source were set to 250°C, 260°C, and 230°C, respectively. The electron ionization (EI) energy was 70 eV, and data were collected in a full-scan mode (m/z 50-600).
ACE-Inhibitory Activity
The probiotic fermented milk samples made with
where A
Data Analysis
Triplicate trials were carried out with each sample of 5 replicates. The peak picking, alignment, deconvolution, and further processing of raw GC-MS data were conducted by referring to previous published protocols [27]. The normalized data analysis was performed using SIMCA software (version 14.1, Umetrics, Sweden). For univariate statistical analysis, the normalized data were calculated by Student’s
Results and Discussion
Probiotic Survivability and pH Change in Fermented Milk
Change of bacterial viable counts in the samples of FM and PFM throughout the 21 days of storage at 4°C was shown in Fig. 1A. At the first day of storage, the viable counts of both the FM and PFM samples were above 9 log CFU/g with higher count in the latter. Gradual decrease in the viable counts was observed in all the samples, but the PFM sample maintained significantly higher counts than the FM sample during the whole period of storage. At the end of storage (day 21), the FM and PFM samples had viable counts of 6.4 ± 0.4 log CFU/g and 7.6 ± 0.2 log CFU/g, respectively, while the viable count of
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Fig. 1.
Changes in viable count (A) and pH (B) of FM (fermented milk made with yogurt starter) and PFM (fermented milk made with yogurt starter and probiotic L. plantarum K25) during 21 days of storage at 4°C.
Changes in pH of the fermented milk samples were monitored during the storage (Fig. 1B). Similar pH decrease patterns were observed in both the FM and PFM samples with the pH values of 4.3 and 4.2, respectively, at the end of storage. This suggested that addition of probiotic
Volatile Metabolite Profiles Determined by Headspace SPME-GC/MS
Change of volatile metabolite profiles in the FM and PFM samples during 21 days of refrigerated storage was shown in Table 1. Among various volatile compounds detected, acetoin and 2,3-butanediol, which are known as C4 compounds responsible for the typical aroma of fermented milk, were found at relatively high concentrations during the storage. These compounds could be generated from glycolysis or citrate metabolism in several lactic acid bacteria such as
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Table 1 . Identification of volatile metabolites by GC-MS and their changes during refrigerated storage of the FM (fermented milk made with yogurt starter) and PFM (fermented milk made with yogurt starter and probiotic
L. plantarum K25) samples.Identification Chemical formula Content (ng/g) FM PFM Day1 Day1 Day7 Day14 Day21 Butanal C4H8O 0.73 ± 0.01a − − − − 2,3-Butanedione C4H6O2 2.32 ± 0.11c 1.95 ± 0.03e 1.72 ± 0.2d 3.29 ± 0.06b 4.42 ± 0.07a 2,3-Pentanedione C5H8O2 3.50 ± 0.03a − − − − 2-Heptanone C7H14O 0.84 ± 0.04b 0.65 ± 0.01c 1.08 ± 0.09a 0.26 ± 0.02d 0.34 ± 0.11d Limonene C10H16 0.72 ± 0.01a 0.72 ± 0.02a 0.72 ± 0.02a 0.63 ± 0.1a 0.67 ± 0.08a Styrene C8H8 − − − − 0.66 ± 0.04a Heptanal C7H14O − 0.19 ± 0.01a 0.07 ± 0.02c 0.08 ± 0.01c 0.13 ± 0.04b Acetoin C4H8O2 5.12 ± 0.05c 4.16 ± 0.04d 5.87 ± 0.16b 7.71 ± 0.19a 7.53 ± 0.14a Octanal C8H16O 0.84 ± 0.03a − − 0.18 ± 0.02b 0.05 ± 0.01c 3-Hexanol C6H14O 0.432 ± 0.01c 0.65 ± 0.02a 0.48 ± 0.03b 0.46 ± 0.01bc 0.65 ± 0.04a Hexanol C6H14O − 0.20 ± 0.01ab 0.14 ± 0.04c 0.16 ± 0.02bc 0.21 ± 0.01a 2-Nonanone C9H18O 0.83 ± 0.02c 1.31 ± 0.08a 1.04 ± 0.03b 0.64 ± 0.04d 0.50 ± 0.13e Nonanal C9H18O 2.26 ± 0.18a 0.45 ± 0.01c 0.22 ± 0.01d 1.26 ± 0.19b 0.46 ± 0.08c Acetic acid C2H4O2 4.15 ± 0.05e 20.30 ± 0.06c 17.40 ± 0.09d 31.32 ± 0.07a 26.81 ± 0.16b Heptanol C7H16O 0.16 ± 0.05c 0.34 ± 0.01b 0.25 ± 0.03bc 0.22 ± 0.03c 0.68 ± 0.12a 1-Hexanol, 2-ethyl- C8H18O 1.86 ± 0.04b 1.19 ± 0.07c 0.91 ± 0.01d 1.23 ± 0.19c 1.59 ± 0.03a Decanal C10H20O 1.86 ± 0.06a 1.16 ± 0.03c 0.94 ± 0.04d 1.10 ± 0.08c 1.62 ± 0.01b Benzaldehyde C7H6O 5.58 ± 0.13a 2.53 ± 0.02b 1.48 ± 0.01d 1.08 ± 0.11e 2.30 ± 0.1c Octanol C8H18O − 0.49 ± 0.02c 0.89 ± 0.09a 0.53 ± 0.03c 0.74 ± 0.04b 2-Undecanone C11H22O 0.43 ± 0.02c 0.50 ± 0.01b 0.29 ± 0.06d 0.38 ± 0.01c 0.79 ± 0.01a Butyrolactone C4H6O2 0.93 ± 0.01a 0.96 ± 0.01a 0.58 ± 0.07c 0.72 ± 0.02b 0.38 ± 0.01d Ethanol, 2-(2-ethoxyethoxy)- C6H14O3 3.83 ± 0.05a 3.39 ± 0.01b 2.40 ± 0.2d 2.39 ± 0.03d 2.81 ± 0.09c Butanoic acid C4H8O2 1.32 ± 0.08a 1.07 ± 0.03c 0.87 ± 0.02d 0.65 ± 0.02e 1.18 ± 0.07b Acetophenone C8H8O − 0.61 ± 0.01a 0.26 ± 0.03b − − Nonanol C9H20O 0.97 ± 0.03c 0.54 ± 0.02e 0.76 ± 0.02d 1.32 ± 0.17b 1.88 ± 0.04a Hexanoic acid C6H12O2 4.88 ± 0.03b 5.39 ± 0.16a 4.22 ± 0.09c 3.75 ± 0.08d 5.25 ± 0.16a Benzyl alcohol C7H8O 0.04 ± 0.04b 0.15 ± 0.01ab 0.07 ± 0.03b 0.34 ± 0.26a 0.07 ± 0.03b Benzothiazole C7H5NS − − − 0.74 ± 0.19b 0.97 ± 0.03a Heptanoic acid C7H14O2 0.36 ± 0.01a 0.17 ± 0.01e 0.24 ± 0.01d 0.32 ± 0.03b 0.29 ± 0.02c Phenol C6H6O 0.32 ± 0.01a 0.29 ± 0.02ab 0.26 ± 0.03b 0.23 ± 0.04b 0.24 ± 0.03b Octanoic acid C8H16O2 8.39 ± 0.03c 9.57 ± 0.17b 7.13 ± 0.06d 6.83 ± 0.1e 9.99 ± 0.08a Nonanoic acid C9H18O2 1.33 ± 0.01b 0.28 ± 0.02c 0.22 ± 0.02d 1.84 ± 0.12a 1.35 ± 0.11b n-Decanoic acid C10H20O2 0.65 ± 0.02d 1.46 ± 0.04c 2.52 ± 0.06b 2.57 ± 0.04b 2.84 ± 0.11a Benzoic acid C7H6O2 6.69 ± 0.01b 6.89 ± 0.19a 4.927 ± 0.01c 4.53 ± 0.01d 4.32 ± 0.07e Values presented are means ± standard deviation. Means in the same row followed by different letters are significantly different (
p < 0.05).
Table 1 also shows that there were many other volatile compounds detected in the probiotic fermented milk including aldehydes, acids, alcohols and phenols. Among these volatiles, aldehydes (butanal, heptanal octanal, nonanal, decanal, benzaldehyde), alcohols (3-hexanol, hexanol, octanol, benzyl alcohol) and phenols that might be derived from free amino acids produced by proteolysis could contribute to yogurt flavor [30]. Previous studies showed that probiotic
Analysis of Differential Metabolites before Storage of Fermented Milk
Difference in metabolic products between the samples of FM and PFM with
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Fig. 2.
Difference in metabolites between the FM (fermented milk made with yogurt starter) and PFM (fermented milk made with yogurt starter and probiotic The positive axis indicates increased production of the metabolites in the PFM sample withL. plantarum K25) samples before storage.L. plantarum K25, and the negative axis indicates increased production of the metabolites in the FM samples.
Analysis of Metabolite Changes during Storage of Fermented Milk
To further understand the mechanism of formation of metabolites by probiotic
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Table 2 . Metabolomics analysis of the changes of metabolites and their correlation to ACE-inhibitory activity in the probiotic fermented milk (PFM) with
L. plantarum K25 during 21 days of refrigerated storage.Metabolites PFM Correlation coefficient Fold change Day 7 vs Day 1 Day 14 vs Day 1 Day 21 vs Day 1 γ-Aminobutyric acid ↑ (0.93c) ↑ (1.50b) ↑ (1.88a) 0.74 Alanine ↑ (0.66b) ↑ (0.63c) ↑ (0.77a) 0.36 Glycine ↑ (0.67c) ↑ (0.55b) ↑ (0.86a) 0.28 Glyceric acid ↑ (0.44c) ↑ (1.68b) ↑ (2.06a) 0.74 Malic acid ↑ (0.74c) ↑ (1.01b) ↑ (1.12a) 0.72 Succinic acid ↑ (0.24c) ↑ (0.49b) ↑ (0.59a) 0.72 2-Hydroxy-3-methylvaleric acid ↑ (0.16a) —— ↑ (0.12b) -0.41 2-Hydroxybutyric acid ↑ (0.12b) ↑ (0.22a) —— -0.31 2-Hydroxyisocaproic acid ↑ (0.39b) ↑ (0.34c) ↑ (0.54a) 0.36 2-Hydroxyisovaleric acid ↑ (0.34a) ↑ (0.22c) ↑ (0.29b) -0.55 Pyroglutamic acid —— ↑ (0.19b) ↑ (0.28a) 0.73 2-Ketoglutaric acid ↓ (0.37c) ↓ (0.87b) ↓ (0.91a) -0.76 Fumaric acid ↓ (1.21c) ↓ (2.46b) ↓ (2.84a) -0.76 Citric acid —— ↓ (0.11b) ↑ (0.10a) 0.10 Pyruvic acid —— ↓ (0.25a) ↓ (0.39a) -0.78 1,3-Dihydroxyacetone ↑ (0.52a) ↑ (0.42b) ↑ (0.38c) -0.84 Ribose ↑ (0.12c) ↑ (0.21b) ↑ (0.30a) 0.67 N -Acetyl-D-glucosamine↑ (0.06b) —— ↑ (0.15a) 0.26 Xylulose —— ↑ (0.10a) ↑ (0.08b) 0.57 Fructose-6-phosphate ↓ (0.21b) —— ↓ (0.34a) -0.16 Trehalose ↓ (0.31a) —— ↓ (0.31a) 0.13 Glycerol ↓ (0.19b) —— ↓ (0.22a) 0.03 Ethanolamine ↓ (0.24a) ↓ (0.15b) ↓ (0.23b) 0.10 Leucine —— ↑ (0.26b) ↑ (0.38a) 0.73 Ornithine ↓ (0.17c) ↓ (0.33b) ↓ (0.57a) -0.75 Tyrosine ↓ (0.54a) ↓ (0.50b) ↓ (0.42c) 0.59 Data in the same row followed by different letters are significantly different (
p < 0.05).
Among the metabolites detected, the major sugars were 1,3-dihydroxyacetone, ribose, and xylulose in the PFM sample. The concentrations of 1,3-dihydroxyacetone and xylulose decreased during the storage probably due to bacterial utilization of sugars for energy to survive. Ribose, mainly formed from hydrolysis of RNA, might not be used by the microbes, as reported earlier with
Organic acids, amino acids and sugars are known to be directly associated with taste and flavor quality, as well as the functional properties of fermented products. The increased formation of these metabolites as observed in this study might modify the sensory quality and functionality of the probiotic
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Fig. 3.
Schematic representation of metabolites produced in PFM (fermented milk made with yogurt starter and probiotic Metabolites identified by UPLS-Q-TOF are marked red and green. Red represents increased metabolites, and green represents decreased metabolites.L. plantarum K25) sample during 21 days of storage at 4°C.
Angiotensin-Converting Enzyme Inhibitory Activity
Probiotic
-
Fig. 4.
ACE inhibition activity of the FM (fermented milk made with yogurt starter) and PFM (fermented milk made with yogurt starter and probiotic L. plantarum K25) samples during 21 days of storage at 4°C.
Previously, ACEi activity was found to increase gradually during cold storage of probiotic fermented milk [58]. The permeate of probiotic fermented goat milk was also shown to possess high ACEi activity [55].
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 31871823), and Beijing Talent Cultivation Quality Construction - First-class Professional Construction (Municipal Level) - Food Science and Engineering (Project No. PXM2019_014213_000010).
Conflict of Interest
The authors have no financial conflicts of interest to declare.
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