Seasonal Variation of Biochemical Composition and Non-Volatile Taste Active Compounds in Pearl Oyster Pinctada fucata martensii from Two Selective Strains

Abstract

Recently, a new pearl oyster Pinctada fucata martensii strain has begun to be cultured as seafood. In the present study, the seasonal variation (February and June) in biochemical composition and flavor compounds in two P. f. martensii strains (strain for pearl production was abbreviated to PP, and seafood was abbreviated to PE) were detected to compare the nutritional and flavor differences between them, and to provide a reference for the seasonal preference of consumers for eating P. f. martensii. The ratio of soft tissues in PE-Feb was significantly higher than that in PP-Feb (p < 0.05). The contents of ash, crude protein, and crude lipid were higher in PP than those in PE in the same season, while significantly higher contents of glycogen in the PE strain were observed compared to the PP strain in the same season (p < 0.05). The major amino acids (such as Glu and Asp) and PUFA (such as DHA and EPA) were almost the same in two P. f. martensii strains in the same season, while the contents of these nutrients were significantly higher in February compared to June (p < 0.05). Taurine content in PE-Feb was the highest (19.58 mg/g wet weight), followed by PP-Jun, PP-Feb, and PE-Jun. The umami and sweet FAA contents of the same P. f. martensii strain in February were significantly higher than those in June (p < 0.05). The AMP content in PP-Jun was the highest (64.17 mg/100 g wet weight), followed by PP-Feb, PE-Jun, and PE-Feb. Succinic acid was the major organic acid, and its content in February was significantly higher than in June (p < 0.05). The betaine content in PP-Feb was the highest (23.02 mg/g of wet weight), followed by PE-Feb (20.43 mg/g of wet weight), PP-Jun (16.28 mg/g of wet weight), and PE-Jun (12.33 mg/g of wet weight), and significant differences were observed among these four groups (p < 0.05). In conclusion, the edible P. f. martensii strain harvest in February is rich in protein, glycogen, PUFA (DHA and EPA), taurine, succinic acid, and betaine, which could provide healthy nutrition and a good flavor for humans.

1. Introduction

The pearl oyster, Pinctada fucata martensii, is an economically important aquaculture bivalve that is widely distributed along the coasts of Japan, Southern China, Southeast Asia, and Australia, and is used for the production of marine pearls [1,2,3]. In China (mostly contributed to by the Guangdong, Guangxi, and Hainan provinces), the highest annual output of pearl from this species reached 34 tons in 2006 [4]. However, in recent years, the P. f. martensii industry is rapidly decreasing because of a high rate of mortality and nucleus rejection after transplantation in the process of pearl production [3,5,6].

Traditionally, after harvesting pearls, the remaining soft tissues of P. f. martensii were considered as a delicacy and consumed by pearl farmers [7,8,9]. Studies have shown that pearl oyster meat has high contents of nutrients, including proteins, glycogen, polyunsaturated fatty acids (PUFAs), trace elements, and vitamins, which may represent a valuable protein source for human nutrition [8,10]. Furthermore, many bioactive components, such as water, salt, insoluble protein fractions, and functional protein isolates, separated from P. f. martensii meat, have also been reported [11,12]. Thus, P. f. martensii is a potential edible mollusk species in aquaculture.

In order to revive the P. f. martensii industry, besides selecting high quality strains for pearl production, new applications, such as utilization of its delicious meat as seafood, have also been explored. Therefore, there are two kinds of breeding objectives in this pearl oyster species in the Beibu Gulf, China. For pearl production strains, shell size of host oyster and quality traits of donor oyster pearl (i.e., size, color, and luster) have been identified as important phenotypes; while, for edible stains, the major target traits include a high edible meat yield ratio, growth rate, nutritional composition, and good taste [13,14]. The technology for cultured pearl production in P. f. martensii has been widely studied [6,15,16]; however, only a few studies focus on its biochemical compositions and non-volatile taste active compounds, and their changes in different seasons [8,17].

The annual changes in biochemical composition and flavor component by seasonal succession have been widely reported in many other aquatic bivalves [18,19,20,21]. For example, in Crassostrea gigas, the glycogen contents in mantle, muscle, and gonad-visceral mass were significantly lower in June than in December, and the fatty acid composition varied with the reproductive cycle [22]; Gao et al. [23] reported that the contents of free amino acids (FAAs), 5′-nucleotides, and lipid profiles in the digestive gland of C. gigas were divided into three different stages; Chen et al. [24] also studied the variation in FAAs, 5′-nucleotides, and lipids in blue mussels Mytilus edulis with the season changes. Non-biological and biological factors, such as seawater temperature, food availability, and reproductive cycle, caused changes in biochemical composition and flavor component in aquatic animals [8,23,24]. Therefore, the quality of P. f. martensii harvested in different seasons might be different, which leads to a change in quality, and influences consumer acceptance. Understanding the seasonal variations in the biochemical composition and flavor component of P. f. martensii may enable us to adjust the harvest cycle and improve product quality and economic benefits.

Generally, the harvest season of edible P. f. martensii is from February to June because of their fully-developed gonad and higher edible soft tissues. This study focused on the effect of seasonal variations on the biochemical composition (including proximate composition, amino acids, and fatty acids) and non-volatile taste active compounds flavor component (such as free amino acids (FAAs), 5′-nucleotide, organic acid, and betaine) of two-pearl oyster P. f. martensii strains (as seafood and for producing pearls). The results could elucidate the nutritional and flavor differences between these two P. f. martensii strains, and could also provide a reference for the seasonal preference of consumers for eating P. f. martensii and deep processing of pearl oyster meat products.

2. Materials and Methods

2.1. Sourcing of P. f. martensii

The two selective strains of P. f. martensii (pearl production and as edible oyster are abbreviated to PP and PE, respectively) were cultured with suspended ropes in two rafts at the same sea area (21°26′–21°55′ N, 108°50′–109°47′ E) in Beihai, Guangxi province, China. Samples of these two strains were collected at the same time in two harvested seasons, February and June 2021. Pearl oysters sampled in February and June were about 1 yr and 1.3 yr after hatching, respectively.

During P. f. martensii collection, the following environmental parameters were measured at a sampling station using RINKO-Profiler multi-parameter probes (JFE, Tokyo, Japan) positioned 1 m below the water surface: temperature (T, °C), salinity, dissolved oxygen (DO, mg/L), pH, Chlorophyll-a (Chl-a, μg/L), and turbidity (FTU) were measured.

2.2. Sample Preparation and Biological Characteristics Detection

In February and June, 36 pearl oysters of each stain were collected and transported live to laboratory at 20 °C for no more than 6 h. The shell length, height, width, and weights of the whole body and soft tissue of each individual were measured. Then, the soft tissues of 36 pearl oysters from the same group were randomly divided into three sub-groups. Twelve oysters in each sub-group were mixed, homogenized, and stored at −80 °C for further composition determination.

The meat yield and shell width index (SWI) were calculated according to the following equation:

Ratio of soft tissues (%) = (soft tissue weight/whole body weight) × 100

(1)

Shell width index = shell width/(shell length + shell height + shell width)

(2)

The biological characteristics of two P. f. martensii selective strains at different seasons are shown in

Table 1. Biological data of two P. f. martensii selective strains at different seasons.

Items	PP-Feb	PE-Feb	PP-Jun	PE-Jun
Shell height (mm)	51.02 ± 4.38 a	44.03 ± 4.22 b	53.28 ± 5.80 a	48.61 ± 5.59 ab
Shell width index	0.17 ± 0.01 a	0.14 ± 0.01 b	0.17 ± 0.01 a	0.15 ± 0.01 b
Wet body weight (g)	21.58 ± 6.11 a	19.21 ± 3.15 b	23.67 ± 4.95 a	21.09 ± 2.67 ab
Ratio of soft tissues (%)	25.41 ± 5.96 b	31.17 ± 5.70 a	23.61 ± 5.86 b	24.59 ± 4.74 b

Data are mean ± standard deviation (n = 3). Data in the same row with different superscripts are significantly different (p < 0.05).

.

2.3. Proximate Composition Analysis

Proximate compositions of P. f. martensii were analyzed as described by Zhou et al. [25]. The moisture was measured by drying the sample at 60 °C until constant weight was obtained. Ash content was determined gravimetrically in a muffle furnace (Lichen Sx2–2.5–10, Shanghai, China) by incineration at 550 °C for 24 h. Crude protein content was determined by the Kjeldahl method using a fully automated Kjeldahl nitrogen/protein analyzer (FOSS Kjeltec 8400, FOSS Kjeltec 8400, Höganäs, Sweden) after acid digestion. Crude lipid was extracted using petroleum ether (FOSS-Soxtec 2050, Höganäs, Sweden). Glycogen content was determined using the Glycogen Content Kit, following the manufacturer’s instructions (Jiancheng Biological Engineering Institute, Nanjing, China). All samples were analyzed in triplicate.

2.4. Amino Acid Analysis

The amino acid content of all samples was measured according to the previously described method [25]. Samples were hydrolyzed at 110 °C for 24 h with 10 mL 6 M HCl in sealed glass tubes filled with nitrogen. After hydrolysis, the hydrolysate was dried in a vacuum oven, dissolved in sodium citrate solution (pH 2.2), and filtered with a 0.45-μm Millipore nylon membrane filter. Amino acids were determined using a Biochrom 20 Automatic Amino Acid Analyzer (Holliston, MA, USA) and their concentrations were expressed as g/100 g wet weight. Samples were analyzed in triplicate.

2.5. Fatty Acid Analysis

Total fatty acids were extracted according to the method of Folch [26]. In detail, total lipids were extracted from 2 g of homogenised oyster soft tissues with chloroform–methanol (2:1, V/V), then saponified, followed by esterification, and, finally, extraction of fatty acid methyl esters (FAMEs) in hexane. The FAMEs were analyzed by gas chromatography (Hewlett-Packard model HP 5890, Palo Alto, CA, USA) using the method of Liu et al. [27]. Fatty acids were identified after a comparison of their retention times with Supelco^TM 37 component FAME mixture (Bellefonte, PA, USA). Quantitative data were calculated using the peak area ratio (% total fatty acids). All samples were analyzed in triplicate.

2.6. Free Amino Acid (FAA) Analysis

FAAs were evaluated as described previously [28]. The samples (~2.5 g) was homogenized in three volumes of 10% trichloroacetic acid (TCA) and centrifuged at 10,000× g for 15 min at 4 °C to obtain supernatants that were separated into 25-μL aliquots. The FAAs in the samples were then analyzed by high-performance liquid chromatography (HPLC) in a Waters 2996 (Waters Corporation, Milford, MA, USA) equipped with a Waters Pico-Tag-C18 column (3.9 mm × 150 mm). All samples were analyzed in triplicate. The qualitative and quantitative analysis of each amino acid was performed by comparing the retention times and peak areas of each amino acid standard (Sigma-Aldrich, St. Louis, MO, USA).

2.7. 5′-Nucleotide Analysis

The 5′-nucleotide compounds were measured according to the previously described method [28]. The HPLC conditions were as follows: injection volumes were 20 μL; eluents A and B were methanol and 0.05% phosphoric acid, respectively; flow rate was 1.0 mL/min; the temperature was 30 °C; detector wavelength was 260 nm. All samples were analyzed in triplicate. The identity and quantity of each nucleotide were assessed by comparing the retention times and the peak areas of each nucleotide standard (Sigma-Aldrich, St. Louis, MO, USA).

2.8. Organic Acid and Betaine Analysis

Organic acid was extracted and analyzed as described previously in [28]. Samples (~2 g) were homogenized in 10 mL of purified water for 5 min, centrifuged at 10,000× g for 20 min at 4 °C, and filtered through a 0.45-μm filter prior to HPLC analysis. The HPLC conditions were similar to those mentioned in Section 2.7, except the detector wavelength was 215 nm. All samples were analyzed in triplicate. Malic acid, lactic acid, citric acid, and succinic acid were analyzed by comparing the retention times and peak areas of standards (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).

Betaine in crayfish was extracted and analyzed according to the previously described method [26]. Samples (~5 g) were homogenized in 10 mL of purified water for 1 min, heated to 75 °C for 1 h, and centrifuged at 5000× g for 10 min. The supernatants were then filtered through a 0.45 μm filter prior to HPLC analysis. A total of 10 μL of sample was injected to the HPLC (Waters 2996 HPLC) equipped with an Agilent ZORBAX NH2 column (4.6 mm × 250 mm) (Agilent Technologies, Beijing, China). HPLC conditions were as follows: injection volumes: 20 µL; mobile phase: 83% aqueous acetonitrile; flow rate: 0.7 mL/min; column temperature: 20 °C; detector wavelength: 260 nm. All samples were analyzed in triplicate. Betaine was measured by comparing the retention time and peak area of standard (Sigma-Aldrich, St. Louis, MO, USA).

2.9. Taste Activity Value (TAV) Calculation

The TAV values of FFAs, 5′-nucleotides, organic acids, and betaine were calculated as the ratio between the concentrations of component molecules and their threshold value measured in a sample matrix [27]. The compounds with a TAV > 1 were considered as active compounds in P. f. martensii taste analysis.

2.10. Statistical Analysis

Values are expressed as mean ± standard deviation (S.D.). Data on environmental parameters were analyzed by using one-way analysis of variance (ANOVA), followed by the least significant difference (LSD) test. Data on biological characteristics, proximate compositions, contents of amino acid, FAAs, 5′-nucleotides, organic acids and betaine, and fatty acid profiles for all groups were analyzed by two-way analysis of variance (ANOVA), and means were subsequently separated by Tukey’s test. Prior to ANOVA, homogeneity of variances was tested using the Levene’s Test. All statistical analyses were performed using Data Processing System (DPS) statistical software. A value of p < 0.05 was considered statistically significant. The significance levels of Two-way ANOVAs between strain and sampling season for P. f. martensii biological characteristics, biochemical composition and non-volatile taste active compounds are shown in Table S1.

3. Results

3.1. Environmental Conditions

As shown in

Table 2. Environmental conditions at different sampling seasons.

Season	T (°C)	Salinity	DO (mg/L)	pH	Chl-a (μg/L)	Turbidity (FTU)
February	15.93 ± 0.45 b	30.80 ± 0.27 a	5.03 ± 0.21 a	7.86 ± 0.07 a	1.26 ± 0.38 b	5.06 ± 0.33 b
June	29.86 ± 0.59 a	30.65 ± 0.46 a	5.29 ± 0.68 a	8.01 ± 0.15 a	2.32 ± 0.35 a	6.65 ± 0.61 a

Data are mean ± standard deviation (n = 3). Data in the same column with different superscripts are significantly different (p < 0.05).

, the water temperature, Chl-a, and turbidity in February and June were 15.93 °C vs. 29.86 °C, 1.26 μg/L vs. 2.32 μg/L, and 5.06 FTU vs. 6.65 FTU, respectively, and significant seasonal changes in these environmental parameters were observed between February and June (p < 0.05), while the salinity, DO, and pH were nearly the same in the two sampling seasons.

3.2. Proximate Compositions

Table 3. Proximate compositions (% wet weight) of two P. f. martensii selective strains at different seasons.

Compositions	PP-Feb	PE-Feb	PP-Jun	PE-Jun
Moisture	79.39 ± 0.71 a	79.85 ± 0.18 a	80.76 ± 0.30 a	79.50 ± 0.15 a
Ash	2.43 ± 0.17 a	2.12 ± 0.08 b	2.42 ± 0.09 a	2.05 ± 0.19 b
Crude protein	14.02 ± 0.35 a	13.94 ± 0.09 a	13.27 ± 0.65 a	12.27 ± 0.43 b
Crude lipid	1.86 ± 0.05 a	1.68 ± 0.07 ab	1.45 ± 0.05 bc	1.29 ± 0.09 c
Glycogen	0.66 ± 0.05 b	0.82 ± 0.07 a	0.38 ± 0.01 c	0.31 ± 0.03 c

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05).

showed the proximate compositions of two P. f. martensii strains in different seasons. The water contents in P. f. martensii were 79.39–80.76% wet weight, and no significant differences were observed among four pearl oyster groups (p > 0.05). The ash contents in the PP strain were significantly higher than in the PE strain in the two seasons (p < 0.05). The crude protein, crude lipid, and glycogen contents in both strains significantly decreased in June compared to those in February, except for crude protein in the PP strain, (p < 0.05). Furthermore, in February, the glycogen content in the PE strain was significantly higher than that in the PP strain (p < 0.05).

3.3. Amino Acids Compositions

The concentrations of amino acids in different groups were shown in

Table 4. The contents of amino acids of two P. f. martensii selective strains in different seasons (g/100 g wet weight).

Amino Acids	PP-Feb	PE-Feb	PP-Jun	PE-Jun
Lysine *	0.72 ± 0.11 b	1.04 ± 0.08 a	0.91 ± 0.09 ab	0.83 ± 0.06 b
Valine *	0.38 ± 0.01 b	0.56 ± 0.01 a	0.48 ± 0.01 a	0.43 ± 0.09 ab
Phenylalanine *	0.47 ± 0.07 a	0.49 ± 0.00 a	0.40 ± 0.00 a	0.36 ± 0.01 a
Methionine *	0.81 ± 0.00 a	0.33 ± 0.00 b	0.26 ± 0.04 b	0.25 ± 0.07 b
Leucine *	0.60 ± 0.04 c	0.94 ± 0.01 a	0.77 ± 0.02 b	0.73 ± 0.01 bc
Isoleucine *	0.33 ± 0.00 b	0.48 ± 0.00 a	0.39 ± 0.02 ab	0.36 ± 0.07 b
Threonine *	0.94 ± 0.10 a	0.69 ± 0.06 b	0.55 ± 0.05 c	0.55 ± 0.04 c
Tryptophan *	0.30 ± 0.01 a	0.16 ± 0.00 b	0.05 ± 0.00 c	0.04 ± 0.01 c
EAA	4.55 ± 0.13 a	4.69 ± 0.16 a	3.81 ± 0.20 b	3.55 ± 0.19 b
Glutamic acid	2.07 ± 0.01 a	2.28 ± 0.01 a	1.81 ± 0.04 b	1.80 ± 0.04 b
Glycine	0.36 ± 0.01 b	0.99 ± 0.05 a	0.85 ± 0.04 a	0.79 ± 0.05 a
Aspartic acid	1.44 ± 0.01 a	1.64 ± 0.03 a	1.31 ± 0.01 b	1.26 ± 0.02 b
Alanine	0.56 ± 0.01 c	0.79 ± 0.02 a	0.68 ± 0.06 b	0.61 ± 0.07 bc
Tyrosine	1.14 ± 0.09 a	0.44 ± 0.01 b	0.36 ± 0.02 b	0.34 ± 0.05 b
Serine	0.53 ± 0.01 b	0.81 ± 0.03 a	0.62 ± 0.01 b	0.56 ± 0.01 b
Histidine	0.16 ± 0.01 a	0.22 ± 0.00 a	0.18 ± 0.00 a	0.16 ± 0.00 a
Arginine	0.82 ± 0.05 b	1.13 ± 0.14 a	0.95 ± 0.14 ab	0.82 ± 0.09 b
Proline	0.88 ± 0.03 a	0.59 ± 0.00 bc	0.44 ± 0.01 c	0.43 ± 0.00 c
Cysteine	0.39 ± 0.00 a	0.27 ± 0.00 b	0.16 ± 0.00 b	0.14 ± 0.03 b
NEAA	8.35 ± 0.23 a	9.16 ± 0.29 a	7.36 ± 0.33 b	6.91 ± 0.33 b
TAA	12.90 ± 0.46 a	13.85 ± 0.45 a	11.17 ± 0.56 b	10.46 ± 0.69 b
EAA/TAA	0.35	0.34	0.34	0.34

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05). EAA: essential amino acids; NEAA: non-essential amino acid; TAA: total amino acid; *: EAA.

. The glutamic acid (Glu) and aspartic acid (Asp) were the main hydrolyzed amino acids in four peal oyster groups, accounting for 1.80–2.28 and 1.26–1.64 g/100 g wet weight, respectively. The essential amino acid (EAA) and total amino acid (TAA) contents of these four pearl oyster groups were 3.55–4.68 and 10.46–13.83 g/100 g wet weight, respectively. When comparing the differences in this pearl oyster species caused by strains and harvest seasons, the Glu, Asp, EAA, and TAA contents of PP and PE sampled in February were shown to be significantly higher than those in June (p < 0.05), while no significant differences were observed between the two pearl oyster stains in the same season (p > 0.05). The EAA/TAA values of the four pearl oyster groups were almost the same, maintaining at 0.33–0.35.

3.4. Fatty Acid Profiles

The fatty acid profiles of four pearl oyster groups are displayed in

Table 5. Fatty acid profiles of two P. f. martensii selective strains it different seasons (%).

Fatty Acids	PP-Feb	PE-Feb	PP-Jun	PE-Jun
C12:0	0.28 ± 0.05 b	0.26 ± 0.02 b	0.61 ± 0.01 a	0.61 ± 0.03 a
C14:0	1.56 ± 0.02 b	1.89 ± 0.02 a	1.93 ± 0.04 a	1.41 ± 0.06 b
C15:0	0.65 ± 0.00 b	0.67 ± 0.02 b	0.74 ± 0.02 ab	0.98 ± 0.01 a
C16:0	19.25 ± 0.98 a	19.07 ± 0.32 a	20.31 ± 0.48 a	15.61 ± 0.57 b
C17:0	1.58 ± 0.04 a	1.53 ± 0.02 a	1.66 ± 0.10 a	1.78 ± 0.08 a
C18:0	7.41 ± 0.03 b	7.86 ± 0.06 b	10.46 ± 0.11 a	9.77 ± 0.13 a
C20:0	2.88 ± 0.00 d	4.65 ± 0.04 c	7.34 ± 0.09 b	9.62 ± 0.06 a
∑SFA	33.61 ± 0.44 c	35.93 ± 0.58 c	43.05 ± 3.76 a	39.78 ± 2.69 b
C16:1n-9	2.01 ± 0.02 a	2.81 ± 0.09 a	2.27 ± 0.00 a	1.36 ± 0.03 b
C18:1n-9	1.79 ± 0.02 b	2.06 ± 0.09 ab	2.29 ± 0.11 a	2.41 ± 0.20 a
C20:1n-9	1.33 ± 0.08 c	1.41 ± 0.09 c	2.35 ± 0.04 b	4.63 ± 0.17 a
∑MUFA	5.13 ± 0.34 c	6.28 ± 0.79 b	6.91 ± 0.19 b	8.40 ± 0.11 a
C18:2n-6	1.02 ± 0.04 a	1.08 ± 0.04 a	0.81 ± 0.00 b	0.69 ± 0.01 c
C20:2n-6	0.50 ± 0.00 a	0.56 ± 0.02 a	0.65 ± 0.01 a	0.61 ± 0.00 a
C20:2n-7	2.79 ± 0.08 c	2.97 ± 0.07 c	3.59 ± 0.05 b	5.13 ± 0.03 a
C18:3n-3	1.14 ± 0.02 a	1.15 ± 0.09 a	0.97 ± 0.04 a	0.97 ± 0.03 a
C18:3n-6	0.20 ± 0.00 a	0.25 ± 0.02 a	0.21 ± 0.00 a	0.33 ± 0.06 a
C20:4n6 ARA	9.43 ± 0.14 b	9.02 ± 0.09 b	13.73 ± 0.19 a	12.85 ± 0.21 a
C20:5n-3 EPA	13.55 ± 0.15 a	12.30 ± 0.32 a	7.30 ± 0.08 b	7.33 ± 0.16 b
C22:5n3	1.86 ± 0.11 a	1.73 ± 0.18 a	1.43 ± 0.00 a	1.66 ± 0.21 a
C22:6n3 DHA	30.77 ± 0.65 a	28.73 ± 1.87 a	21.33 ± 1.27 b	22.26 ± 2.43 b
∑PUFA	61.26 ± 3.19 a	57.79 ± 4.18 a	50.02 ± 4.76 b	51.83 ± 3.54 b
Σn-3	47.32 ± 3.28 a	43.91 ± 4.11 b	31.03 ± 0.43 c	32.22 ± 2.98 c
Σn-6	11.15 ± 0.44 b	10.91 ± 0.67 b	15.4 ± 0.12 a	14.48 ± 0.31 a
Σn-3/Σn-6	4.24	4.02	2.01	2.23

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05). SFA: saturated fatty acids; MUFA: monounsaturated fatty acids; PUFA: polyunsaturated fatty acids.

. In this study, docosahexaenoic acid (DHA, C22:6n-3) was the major fatty acid, accounting for 21.33–30.77% of total fatty acid, followed by palmitic acid (C16:0) (15.61–20.31%), arachidonic acid (ARA, C20:4n6) (9.02–13.73%), and eicosapentaenoic acid (EPA, C20:5n-3) (7.30–13.55%) in all P. f. martensii groups. When comparing the differences between groups, the profiles of polyunsaturated fatty acids (PUFA), DHA, and EPA of the two P. f. martensii strains sampled in February were significantly higher than those sampled in June (p < 0.05), while no significant differences were observed between the two P. f. martensii strains in the same season (p > 0.05). Furthermore, the profiles of Σn-3 of the two P. f. martensii strains sampled in February were 43.91–47.32%, which was significantly higher than those sampled in June (31.03–32.22%) (p < 0.05), while the opposite results were observed in Σn-6. Therefore, the Σn-3/Σn-6 values of the two P. f. martensii strains sampled in June sharply decreased when compared to those sampled in February (from 4.02–4.24 to 2.01–2.23).

3.5. Comparison of Free Amino Acids (FAAs) and Flavor 5′-Nucleotide

The contents, taste attributes, and taste activity values (TAV) of 17 FAAs in pearl oysters with different groups are shown in

Table 6. The contents, taste attributes (+ pleasant, − unpleasant), taste thresholds, and TAVs of free amino acids (FAA) of two P. f. martensii selective strains in different seasons.

FAAs	Content (mg/g Wet Weight)				Taste Attribute	Threshold (mg/100 mg)	TAV
	PP-Feb	PE-Feb	PP-Jun	PE-Jun			PP-Feb	PE-Feb	PP-Jun	PE-Jun
Aspartic acid	0.59 ± 0.01 a	0.51 ± 0.02 a	0.50 ± 0.03 a	0.43 ± 0.03 a	Umami (+)	100	0.59	0.51	0.50	0.43
Glutamic acid	1.40 ± 0.03 a	1.16 ± 0.05 b	1.12 ± 0.01 b	0.83 ± 0.01 c	Umami (+)	30	4.67	3.87	3.73	2.77
Umami FAA	1.99 ± 0.04 a	1.67 ± 0.07 b	1.62 ± 0.01 b	1.26 ± 0.01 c
Serine	0.07 ± 0.01 a	0.08 ± 0.02 a	0.09 ± 0.01 a	0.06 ± 0.02 a	Sweet (+)	150	0.05	0.05	0.06	0.04
Glycine	2.93 ± 0.19 a	1.91 ± 0.32 b	2.41 ± 0.20 a	1.02 ± 0.03 c	Sweet (+)	130	2.25	1.47	1.85	0.78
Threonine	0.10 ± 0.02 a	0.08 ± 0.01 a	0.08 ± 0.02 a	0.07 ± 0.00 a	Sweet (+)	260	0.04	0.03	0.03	0.03
Alanine	0.89 ± 0.01 a	0.53 ± 0.04 c	0.69 ± 0.03 b	0.59 ± 0.04 c	Sweet (+)	60	1.48	0.88	1.15	0.98
Argnine	0.79 ± 0.04 a	0.52 ± 0.04 c	0.63 ± 0.02 b	0.39 ± 0.02 d	Sweet/bitter (+)	50	1.58	1.04	1.26	0.78
Valine	0.08 ± 0.01 a	0.05 ± 0.00 a	0.06 ± 0.00 a	0.04 ± 0.01 a	Sweet/bitter (+)	40	0.20	0.13	0.15	0.10
Lysine	0.17 ± 0.01 a	0.10 ± 0.01 b	0.07 ± 0.00 b	0.05 ± 0.00 c	Sweet/bitter (+)	50	0.34	0.20	0.14	0.10
Proline	0.52 ± 0.05 a	0.25 ± 0.04 b	0.08 c ± 0.03 c	0.06 c ± 0.00 c	Sweet/bitter (+)	300	0.17	0.08	0.03	0.02
Sweet FAA	5.55 ± 0.34 a	3.52 ± 0.28 c	4.11 ± 0.30 b	2.28 ± 0.12 d
Histidine	0.09 ± 0.01 a	0.08 ± 0.00 a	0.08 ± 0.00 a	0.05 ± 0.00 b	Bitter (−)	20	0.45	0.40	0.40	0.25
Phenylalanine	0.06 ± 0.00 a	0.06 ± 0.00 a	0.06 ± 0.01 a	0.05 ± 0.01 a	Bitter (−)	90	0.07	0.07	0.07	0.06
Isoleucine	0.05 ± 0.01 a	0.03 ± 0.00 a	0.04 ± 0.00 a	0.03 ± 0.00 a	Bitter (−)	90	0.06	0.03	0.04	0.03
Leucine	0.08 ± 0.01 a	0.05 ± 0.01 b	0.05 ± 0.01 b	0.04 ± 0.00 b	Bitter (−)	190	0.08	0.05	0.05	0.04
Methionine	0.03 ± 0.00 a	0.02 ± 0.00 a	0.02 ± 0.00 a	0.02 ± 0.00 a	Bitter (−)	30	0.10	0.07	0.07	0.07
Tryptophan	0.04 ± 0.00 a	0.04 ± 0.01 a	0.04 ± 0.00 a	0.04 ± 0.00 a	Bitter (−)	90	0.04	0.04	0.04	0.04
Bitter FAA	0.35 ± 0.03 a	0.28 ± 0.02 b	0.29 ± 0.02 b	0.23 ± 0.01 c
Taurine	17.14 ± 0.12 b	19.58 ± 0.76 a	17.84 ± 0.71 b	17.10 ± 0.23 b	Flat/tasteless	N/A
Total	25.03 ± 0.53 a	25.05 ± 1.10 a	23.86 ± 1.05 b	20.87 ± 0.37 c

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05). N/A: not available; +—pleasant taste; −—unpleasant taste.

. Taurine contents comprised more than 65% of the total FAAs. Other major FAAs were glycine, alanine and arginine (belonging to sweet FAA), and glutamic acid (belonging to umami FAA), because the TAVs of these four FAAs were more than 1. The contents of umami, sweet, and bitter FAAs in P. f. martensii were 1.26–1.99, 2.28–5.55, and 0.23–0.35 mg/g wet weight, respectively. The taurine content in PE-Feb was 19.58 mg/g wet weight, which was significantly higher than in the other three groups (p < 0.05). The contents of umami and sweet FAAs were significantly higher in the PP group compared to the PE group in the same season (p < 0.05). Furthermore, the umami and sweet FAA contents of the same P. f. martensii strain in February were also significantly higher than those in June (p < 0.05).

As shown in

Table 7. The concentrations, taste thresholds and TAVs of 5′-nucleotides of two P. f. martensii selective strains at different seasons.

Nucleotides	Content (mg/100 g)				Taste Attribute	Threshold (mg/100 mg)	TAV
	PP-Feb	PE-Feb	PP-Jun	PE-Jun			PP-Feb	PE-Feb	PP-Jun	PE-Jun
CMP	2.33 ± 0.05 b	2.43 ± 0.01 b	2.90 ± 0.08 a	2.64 ± 0.05 ab	Umami (+)	12.5	0.19	0.19	0.23	0.21
GMP	0.27 ± 0.00 c	0.58 ± 0.00 b	1.34 ± 0.00 a	0.75 ± 0.00 b	Umami (+)	12.5	0.02	0.05	0.11	0.06
AMP	46.32 ± 0.69 b	39.49 ± 3.24 c	64.17 ± 0.62 a	45.43 ± 2.18 b	Umami/Sweet (+)	50	0.93	0.79	1.28	0.91
Total	48.92 ± 0.74 b	42.50 ± 3.25 c	68.41 ± 0.70 a	48.82 ± 2.23 b

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05). +—pleasant taste.

, AMP was the predominant 5′-nucleotide in P. f. martensii, with 39.49–64.17 mg/100 g wet weight, and its TAVs were 0.79–1.28, while the contents of CMP and GMP were much less than their threshold values. The AMP content in PP-Jun was the highest, with 64.17 mg/100 g wet weight, followed by PP-Feb, PE-Jun, and PE-Feb, and significant differences were observed in PP-Jun vs. PP-Feb and PE-Jun vs. PE-Feb (p < 0.05).

3.6. Comparison of Organic Acids and Betaine

As shown in

Table 8. The concentrations, taste thresholds, and TAVs of organic acids and betaine of two P. f. martensii selective strains at different seasons.

Organic Acids and Betaine	Content (mg/g Wet Weight)				Taste Attribute	Threshold (mg/100 g)	TAV
	PP-Feb	PE-Feb	PP-Jun	PE-Jun			PP-Feb	PE-Feb	PP-Jun	PE-Jun
Succinic acid	0.64 ± 0.08 a	0.70 ± 0.04 a	0.40 ± 0.07 b	0.50 ± 0.03 b	Sour/Umami	10	6.40	7.00	4.00	5.00
Malic acid	0.15 ± 0.01 c	0.20 ± 0.01 c	0.53 ± 0.07 b	0.85 ± 0.05 a	Sour/Bitter	50	0.30	0.40	1.06	1.70
Citric acid	0.33 ± 0.05 a	0.18 ± 0.06 b	0.33 ± 0.03 a	0.34 ± 0.04 a	Acidic/Mild	45	0.73	0.40	0.73	0.76
Betaine	23.02 ± 0.25 a	20.43 ± 1.53 b	16.28 ± 0.11 c	12.33 ± 0.30 d	Sweet	860	2.68	2.38	1.89	1.43

Data are mean ± standard deviation (n = 3). Different letters within a row indicate significant difference (p < 0.05).

, three organic acids, succinic acid, malic acid, and lactic acid, were detected in P. f. martensii. According to the threshold values, succinic acid was the major organic acid in P. f. martensii. Its contents in PP-Feb and PE-Feb was 0.70 and 0.64 mg/g wet weight, respectively, which was significantly higher than those in PP-Jun and PE-Jun (p < 0.05). Furthermore, the betaine content in PP-Feb was the highest (23.02 mg/g wet weight), followed by PE-Feb (20.43 mg/g wet weight), PP-Jun (16.28 mg/g wet weight), and PE-Jun (12.33 mg/g wet weight), and significant differences were observed among these four groups (p < 0.05) (Table 8).

4. Discussion

The biochemical compositions and flavor compounds of aquatic animals are affected by their inherited characteristics and cultured environmental factors [22,25,29,30]. In the present study, some significant differences were observed between two P. f. martensii strains in biological parameters, biochemical compositions, and non-volatile taste active compounds. Furthermore, the biochemical compositions and non-volatile taste active compounds in the same strain also showed obvious changes in different seasons. These results indicated a genetic difference between pearl-production and edible P. f. martensii strains, and changes in environmental factors (such as temperature, Chl-a and turbidity) caused the differences in biochemical compositions and flavor compounds in four P. f. martensii groups.

Edible bivalves, such as oysters, scallops, mussels, and clams, can provide plenty of nutritional components, and are consumed by humans worldwide [31,32,33,34]. In our study, the crude protein contents of P. f. martensii were 12.27–14.02% wet weight (59.85–69.18% of dry weight), which is similar to results (69.42–71.60% dry weight) reported by Wang et al. [17], while slightly lower than the results reported by Zhang et al. (74.9% dry weight) [7]; the lipid contents of P. f. martensii in our study were higher than previous reports; the glycogen contents were almost the same as in the previous studies by Zhang et al. [7]. When compared, the proximate composition of P. f. martensii with other edible oyster species, such as C. hongkongensis (widely cultured and consumed in south Chinese provinces), the contents of crude protein in P. f. martensii were obviously higher than those in C. hongkongensis (more than 12.27% vs. 8.43% wet weight), the crude lipid contents were almost the same in these two oyster species (1.29–1.86% vs. 1.39% wet weight), and the glycogen contents were lower (less than 0.82% vs. 2.67%) [27].

Glu and Asp were the top two amino acids in our study, and this result was also observed in juvenile P. f. martensii [17]. Similarly, in oyster C. hongkongensis, Asp and Glu were also the main amino acids [22], while the top two amino acids in mussels were glycine (Gly) and alanine (Ala) for M. edulis [35], and Glu and Gly for Perna viridis [36]. It has been reported that Asp, Glu, Gly, and Asp are four delicious amino acids, which means all four of these bivalve species taste wonderful [37]. Marine-sourced omega-3 long chain PUFAs, including DHA and EPA, are important fatty acids due toproviding human health benefits [38]. Previous studies reported that the DHA profiles in C. hongkongensis, male mussel M. galloprovincialis, and Asian hard clam Meretrix lusoria were 9.05–19.95%, 11.31–17.52%, and 13.33–16.47%, respectively [22,39,40]. In P. f. martensii, both DHA and EPA were the major PUFAs, which indicated that P. f. martensii could supply more health DHA and EPA as seafood.

Amino acids and fatty acids, especially EAA and PUFA, are not only two major necessary nutrients, but they also play important roles in maintaining the basic physiological functions of organisms [41,42,43]. As shown in our study, the content of major amino acids (Glu and Asp), EAAs, and TAAs in two P. f. martensii strains were almost the same in the same season. Similarly, the major fatty acids (such as DHA, ARA, EPA, and C16:0) also showed the same tendency. These results mean that the different selected objectives (for pearl production and as an edible bivalve) of P. f. martensii could not cause obvious changes in amino acid and fatty acid compositions, though the ratio of soft tissues of PE-Feb was significantly higher than that of PP-Feb (p < 0.05).

Until now, nutritional qualities of many bivalves have been reported, and their variations in amino acid and fatty acid composition are attributed to species, growth stage, and environmental factors (such as diet, season, and harvest area) [22,35,42,44]. In the present study, significant decreases of almost all amino acids, and DHA, EPA, Σn-3/Σn-6, and Σn-3 profiles of PP and PE strains, were observed in June compared to February (p < 0.05). According to our experiment, three environmental factors, temperature, Chl-a, and turbidity, showed significantly changes in the two sampling seasons, which might cause obvious changes in amino acid and fatty acid compositions. Moreover, P. f. martensii, in February, were filled with fully-developed gonads, and most of them had been spawned in June in Beihai, Guangxi Province. In previous studies, the seasonal variation of protein and lipid composition has been observed in many other bivalve species, such as M. edulis, C. gigas, and C. hongkongensis [22,23,35]. Taking C. hongkongensis as an example, its C22:6n-3 (DHA) and C20:5n-3 (EPA) profiles reached the peak at gonad maturation season, and then decreased to the lowest concentration when gonads were inactive [22]. The C22:6n-3 (DHA) and C20:5n-3 (EPA) play important roles in the structure and function of cell membranes and energy metabolism during oogenesis and embryogenesis [22,45]. Similarly, significantly higher contents of C22:6n-3 (DHA) and C20:5n-3 (EPA) were also observed in P. f. martensii during breeding season in our study.

FAA is one kind of non-volatile taste compound, which could provide different flavor tastes, such as umami, sweet, and bitter [27]. In the present study, the taurine contents of P. f. martensii were 17.10–17.14 mg/g wet weight, which is significantly higher than seen in other marine bivalve species such as oyster C. gigas and C. rivularis (7.38–9.02 and 6.13 mg/g wet weight, respectively), paphia Paphia papilionacea (6.60 mg/g wet weight), and clam M. meretrix (4.47 mg/g wet weight) [46,47]. Furthermore, in P. f. martensii, the taurine content in PE-Feb was the highest compared to the other three groups. In marine invertebrates, taurine is the dominant FAA and plays an important role in osmoregulation [48]. As an important amino acid in human nutrition and health, taurine plays crucial roles in protecting cells from oxidative stress and injury, and is considered as an essential amino acid for children (particularly preterm infants) and a conditionally essential amino acid for adults [49,50]. The high content of taurine in P. f. martensii, especially at PE-Feb, can be a benefit to human health. On the contrary, other mainly-positive tasteful FAAs (TAV > 1), such as Glu (belonging to umami FAAs), and Gly, Ala, and Arg (belonging to sweet FAAs), were significantly higher in PP compared to PE in the same seasons (p < 0.05), and in February compared to June of the same P. f. martensii strain (p < 0.05). Therefore, artificial selection of the edible P. f. martensii strain increased the taurine contents, while it decreased the umami and sweet FAAs.

AMP, IMP, and GMP are three umami-taste 5′-nucleotides [27]. Our results indicated that AMP was the main nucleotide component in P. f. martensii. In the previous study, AMP was also the major flavor-contributing nucleotide in oyster Ostrea rivularis, and its content of AMP was more than 10 times (674.3 mg/100 g wet weight) that in P. f. martensii [51]. In C. hongkongensis, IMP was the major nucleotide, and its total 5′-nucleotides content was 58 mg/100 g wet weight, which was almost the same as P. f. martensii [27]. When comparing the seasonal changes of total nucleotide concentration in P. f. martensii, significantly higher contents were observed in June than in February. In blue mussels M. edulis, the highest total 5′-nucleotides in summer was also observed, and a high seawater temperature and high metabolic capacity were recognized as the key causes [24].

Succinic acid was the major organic acid in P. f. martensii, which was the same as other edible bivalves [27,51]. Succinic acid is a taste active compound and can contribute to a strong salty and bitter flavor at different concentrations [52,53]. However, much lower succinic acid content was reported in P. f. martensii compared to other common edible bivalves, such as C. hongkongensis (12.23 and 1.17 mg/g wet weight), C. ariakensis (4.52 mg/g wet weight), and O. rivularis (1.26 mg/g wet weight) [27,51,52]. In seafood, betaine has a refreshing sweet taste, and is one of the main flavor components [27]. Moreover, betaine also plays an important role in physiological regulation as an osmolyte and methyl donor [53]. Plenty of evidence shows that betaine is an important nutrient for the prevention of human chronic disease [53,54]. Our results showed that betaine was abundant in P. f. martensii, and its content was significantly higher in February compared to June. When compared to other edible species, betaine content in P. f. martensii was slightly lower than that in C. hongkongensis (26.69 mg/g wet weight) [27], and much higher than in mud crab Scylla paramamosain (2.3–6.98 mg/g wet weight) [55] and squid Todarodes pacificus (17.6 mg/g dry weight) [56].

5. Conclusions

In the present study, the contents of crude lipid, glycogen, EAA, and TAA, and profiles of PUFA (most contributed by C22:6n3 and C20:5n-3), were significantly higher in two P. f. martensii strains sampled in February compared to the same strains sampled in June. Similarly, the significantly higher contents of umami FAA, sweet FAA, bitter FAA, total FAA, succinic acid, and betaine of two P. f. martensii strains sampled in February were also observed, while opposite results were observed in nucleotide contents. When the differences between two P. f. martensii strains sampled in February were compared, the PE strain showed a higher ratio of soft tissues, glycogen, and taurine contents compared to the PP strain, while the contents of ash, umami FAA, sweet FAA, bitter FAA, nucleotides and betaine, and Σn-3 profile in the PP strain were obviously higher than those in the PE strain.