Co-fermentation of Lactiplantibacillus plantarum and Pediococcus acidilactici induces non-thermal gel of tilapia surimi: Quantitative assessment of microbial contribution to gel improvement

Abstract

Fermentation by lactic acid bacteria (LAB) is an innovative method to improve the gel properties of surimi by induction of non-thermal gel. However, the improvement of gel and inhibition of spoilage microorganisms are limited by the usage of single starter. In this study, the changes in the non-thermal gel properties and microbial community were studied after co-fermentation with Lactiplantibacillus plantarum and Pediococcus acidilactici at low and high concentrations, followed by the clarification of gel change mechanisms. The texture properties, gel strength, and whiteness of tilapia surimi were markedly improved after co-fermentation with starters, while the pH value obviously decreased. The total plate count significantly increased during fermentation in all groups, but remained lower in the co-fermentation groups at 12 and 24 h compared to the natural fermentation group. During co-fermentation, LAB rapidly became the dominant position, and both the starters could adapt well to the fermentation environment. High concentrations of starters not only shortened the gel time from 24 h to 12 h but also obviously suppressed the spoilage microorganisms. Group-dimension correlation analysis at different fermentation time showed that the decreasing richness and evenness of microbial community by starters mainly resulted from the decrease of Lactococcus, Aeromonas, and Streptococcus. The good synergistic effect of starters during fermentation was responsible for the formation of acid environment and consequently improved the non-thermal gel of surimi, and L. plantarum showed higher influence contribution to the non-thermal gel improvement than P. acidilactici. The suppression of spoilage microorganisms by starters also contributed to the non-thermal gel improvement of tilapia surimi. Co-fermentation with high concentration of L. plantarum and P. acidilactici is expected to be used for production of high-quality tilapia surimi products.

Graphical abstract

Highlights

• •Co-fermentation of starters improved gel properties and whiteness of tilapia surimi.
• •High starters obviously shortened self-gel time and inhibited spoilage microorganisms.
• •High acidification induced by starters was responsible for self-gel improvement.
• •L. plantarum had higher contribution to self-gel improvement than P. acidilactici.
• •Inhibition of spoilage microorganisms contributed to self-gel improvement of surimi.

1. Introduction

Tilapia has become the main farmed freshwater fish in the world because of its high feed efficiency and yield. However, there are relatively few technologies for the high-value utilization of tilapia, leading to the low economic value of tilapia. Surimi is famous for the gel properties which make it possible to produce products with various shapes (Li et al., 2023), obviously increasing the added value of fish. Tilapia has low price, high meat yield and no intermuscular bones, making it a potential raw material for the production of surimi. However, as a kind of freshwater fish surimi, tilapia surimi has low gel strength and easy gel deterioration which restrict the development of high-quality surimi products. Traditional gel improvement techniques for freshwater fish surimi mainly focus on the addition of additives, such as polysaccharides (Lin et al., 2025), proteins (Wang et al., 2023), and transglutaminase (Tong et al., 2023). However, their usage is generally large in order to achieve significant improvement results. Therefore, there is a great need for an alternative technique that is effective and low-cost.

As an innovative gel improvement method, the application of fermentation technology can effectively elevate the gel properties of surimi by inducing the non-thermal gel of surimi (Zhao et al., 2021). Lactic acid bacteria (LAB) are generally used as the starters, and the organic acids they produce are responsible for non-thermal gel of surimi (Li et al., 2024b; Yuan et al., 2024). Under the metabolism of starters, the flavor of surimi can be improved by reducing the substances with fishy flavor and producing the substances with pleasant odor (Li et al., 2024a). During the fermentation, there is no need to wash the surimi, which will not cause the loss of nutrients in the surimi, consequently reducing the discharge of washing wastewater and the pollution to the environment (Li et al., 2025a). Most studies focus on the changes in gel properties of freshwater fish surimi by starters of LAB but ignore the changes in the microbial community during the fermentation (Nie et al., 2014; Yuan et al., 2024; Zhang et al., 2022). Poor control of spoilage microorganisms will not only produce food safety problems but also affect the whole flavor of surimi products. Various strains of LAB have been found to be able to induce the non-thermal gel of tilapia surimi, but the gel improvement effect is limited by the usage of single starter. Moreover, although LAB play a dominant role in the microbial community after fermentation of tilapia surimi, the high abundance of spoilage microorganisms has not yet been resolved.

The strains of LAB, Lactiplantibacillus plantarum H30-2 and Pediococcus acidilactici H30-21 isolated from tilapia surimi have been found to possess good acidification capacity, adaptability to surimi system, and gel-strengthening property after addition of single starter in the tilapia surimi (Li et al., 2024b). However, high abundance of spoilage microorganisms is observed and the formation of non-thermal gel is relatively long during single starter fermentation. In this study, the co-fermentation with these two starters at different concentrations was used to improve the gel properties of tilapia surimi and inhibit the spoilage microorganisms in the microbial community. The group-dimension correlation analysis at different fermentation time followed by the calculation of influence contribution was used to analyze the non-thermal gel improvement mechanisms of tilapia surimi induced by starters. This study is expected to provide an effective method for the development of high-quality tilapia surimi products.

2. Material and methods

2.1. Preparation of starters

Lactiplantibacillus plantarum H30-2 and Pediococcus acidilactici H30-21 were obtained from fermented tilapia surimi. The strains were activated by anaerobic incubation in MRS medium (10.0 g peptone, 5.0 g beef powder, 20.0 g glucose, 4.0 g yeast extract powder, 5.0 g CH₃COONa, 2.0 g Na₂HPO₄, 0.2 g MgSO₄, 2.0 g triammonium citrate, 0.05 g MnSO₄, and 1 mL Tween 80 with 1.0 L water) at 30 °C for 24 h, respectively. After centrifugation at 4 °C and 12,000 r/min for 10 min, their microbial precipitation was then resuspended into the 0.9% NaCl solution as starters, respectively.

2.2. Tilapia surimi fermentation

The dorsal muscle of tilapia was obtained from a local aquatic product market (Guangzhou, China) and was minced into surimi. The surimi was mixed with 5% water, 2% salt, 0.5% sucrose, and 0.5% glucose. The starters (L. plantarum:P. acidilactici = 2:1) were then added into the surimi at the total final concentration of 10⁶ CFU/g (ZRL group) and 10⁷ CFU/g (ZRH group), while the surimi mixed without starters was used as the natural fermentation group (CK group). The mixture was filled in casings (Φ = 30 mm) and fermented for 12 and 24 h at 25 °C.

2.3. Determination of pH value and whiteness

The fermented surimi (5.0 g) was homogenized using distilled water (45 mL) to determine the pH value. After measurement of L∗, a∗, and b∗ by the CR-400 Chroma meter (Konica Minolta, Japan) with the CIE color system, the whiteness of fermented surimi was determined through equation (1):

$$\text{Whiteness}=100-\sqrt{{\left(100-L^{\ast }\right)}^2+{a^{\ast }}^2+{b^{\ast }}^2}$$ (1)

2.4. Determination of gel strength and texture

The fermented surimi without heating was cut into 2.5 cm height. After measurement of the breaking force (W) and breaking distance (L) through a CT3 texture analyzer (Brookfield, USA) equipped with spherical probe (Φ = 5 mm), the gel strength (X) was calculated using equation (2):

$$X=W\times L$$ (2)

where X is the gel strength of surimi (g·cm), W is the breaking force (g), and L is the breaking distance (cm).

The texture properties of fermented surimi without heating were also measured by a CT3 texture analyzer (Brookfield, USA) equipped with flat bottom probe (Φ = 4 mm). Force-time deformation curve was first constructed, and the texture properties were then detected through the TexturePro CT software.

2.5. Determination of total plate count and microbial community

The fermented surimi (3.0 g) was homogenized by 0.9 % NaCl solution (27 mL). The solution with microorganisms after gradient dilution was mixed with the plate count agar at 46 °C. After the ager was solidified, the plate was turned over and cultivated at 37 °C for 48 h to calculate the total plate count (TPC). The rest solution with microorganisms was collected and centrifuged for 10 min at 4 °C and 12000 r/min to obtain the microbial precipitation. After the DNA extraction from microbial precipitation, the MiSeq PE300 platform (Illumina, USA) was used to perform the16S rRNA gene high-throughput sequencing. The operational taxonomic units (OTUs) were clustered with 97% similarity cutoff through the UPARSE v7.0.1090, and then were used to study the taxonomy by the RDP Classifier v2.11.

2.6. Statistical analysis

The data were expressed as mean ± SD after triplicate experiments. The statistical analysis was studied by one-way analysis of variance with multiple comparison Tukey test. The similarity in the physical characteristics among different groups was studied by principal component analysis (PCA). The α diversity of microbial community was studied by the Mothur v1.30.2, and the β diversity of microbial community was studied using the principal co-ordinates analysis (PCoA) and hierarchical clustering based on Bray-Curtis dissimilarity. The correlation heatmaps were built by the MetaboAnalyst v6.0. The correlation networks were constructed by the Cytoscape v.3.8.1. The influence contribution of genus (IC_g) to the quality indicator was calculated by equation (3):

$${IC}_g=\frac{{r\times A}_g}{|\sum {r\times A}_g|}\times 100\%$$ (3)

where IC_g is the influence contribution of genus to the quality indicator (%), r is the Pearson's correlation coefficient between quality indicator and genus, A_g is the mean abundance of genus for correlation analysis (%), and Σ r × A_g is the sum of genus influence for positive or negative correlation with indicator.

3. Results and discussion

3.1. Change in physical property of tilapia surimi after co-fermentation with starters

The changes in physical properties of tilapia surimi after co-fermentation are shown in Fig. 1. The texture properties are the key indicators to assess the gel property of surimi (Lan et al., 2023). With the increasing fermentation time, the hardness, springiness, chewiness, and adhesiveness of surimi without heating significantly increased in all fermentation groups, while there was no significant difference in the cohesiveness of surimi (Fig. 1A). Compared with those in the CK group, the hardness, springiness, chewiness, and adhesiveness of surimi were all markedly enhanced after co-fermentation. Meanwhile, the higher the concentration of starters, the more these texture properties increased. Gel strength is regarded as an evaluation index that can directly reflect the gel (Liu et al., 2025). As shown in Fig. 1B, the gel strength of surimi significantly increased in all fermentation groups with the fermentation time increasing from 12 h to 24 h, and was obviously higher in the ZRL and ZRH groups than the CK group. Interestingly, the gel strength of surimi without heating in the ZRH group at 12 h could reach 251.8 g cm, suggesting the good non-thermal gel. When fermentation with starters in both the ZRL and ZRH groups, the hardness, chewiness, and gel strength at 12 h were even higher than those in the CK group at 24 h, suggesting co-fermentation with starters could accelerate the non-thermal gel formation of tilapia surimi. Whiteness is also an important physical indicator to evaluate surimi, and high whiteness is more likely to be accepted by consumers (Zhao et al., 2021). In this study, the whiteness of surimi significantly increased as fermentation proceeded (Fig. 1C). Moreover, co-fermentation with starters could obtain higher whiteness, especially in the ZRH group. As one of the key physical properties, low pH value usually corresponds to better gel property in the fermented surimi (Li et al., 2024b; Yuan et al., 2024). Similar result was also found in this work that the pH values of surimi in all groups were all reduced along with fermentation, completely contrary to the gel strength (Fig. 1D). The pH value in the ZRH group was markedly lower than that in the other groups at 12 h, but showed no significant difference with that in the ZRL group at 24 h, consistent with the variation trend of gel strength. The PCA results showed that the physical properties in the CK12 group were obviously different from those in other groups (Fig. 1E). The physical properties in the CK12 group were similar to those in the ZRL12 group, while the properties among the ZRH12, ZRL24, and ZRH24 groups got closer to each other.

Fig. 1.

Change in physical properties of tilapia surimi after co-fermentation with starters at different concentrations. (A) Texture properties, (B) gel strength, (C) whiteness, and (D) pH value during surimi fermentation. (E) PCA of physical properties among different fermentation groups. Bars labeled with different letters indicate significantly different at p < 0.05.

3.2. Change in microbial community in tilapia surimi after co-fermentation with starters

The total plate count is an important indicator that can reflect the growth of microorganisms in fermented foods. In this work, the TPC in the surimi significantly increased in all groups with the increasing fermentation time (Fig. 2). Although the CK group had the lowest TPC before fermentation, but its TPC was higher than that in the ZRL and ZRH groups. The increase in the inoculation concentration of starters obviously increased the TPC before 12 h, but had little effect on TPC at 24 h. The decrease in the TPC in fermented surimi after starter addition has also been observed in other studies (Cui et al., 2024; Li et al., 2024b). In order to determine which type of microorganisms caused the changes in TPC, the change in microbial community in tilapia surimi was further studied by high-throughput sequencing after co-fermentation with starters (Fig. 3). As shown in Fig. 3A, good sequencing results were observed that the Coverage during the fermentation of three groups was all over 0.999. The α diversity indexes can reflect the evenness and richness of microbial community (Aregbe et al., 2019; Serra et al., 2019). With the increase of fermentation time, the Sobs, ACE, Chao, and Shannon significantly increased in all groups, while the Simpson significantly decreased, suggesting the reduction of richness and evenness (Fig. 3B). The lower richness and evenness were found after co-fermentation, especially in the ZRH group, in which the richness and evenness of microbial community at 12 h could reach the level of those at 24 h in the other groups. The β diversity can reflect the similarity of microbial community among different groups (Li et al., 2025b). In this study, according to the results of PcoA (Fig. 3C) and hierarchical clustering (Fig. S1), three apparent clusters were observed, including CK, ZRL, and ZRH. The microbial community in the CK cluster changed more obviously during fermentation than the others, while the microbial community in three groups of ZRH cluster changed very little, indicating that the microbial community was more stable after co-fermentation with high concentration of starters.

Fig. 2.

Change in TPC in tilapia surimi after co-fermentation with starters at different concentrations. Bars labeled with different letters indicate significantly different at p < 0.05.

Fig. 3.

Change in microbial community in tilapia surimi after co-fermentation with starters at different concentrations. (A) Coverage of sequencing results, (B) α diversity, (C) β diversity, and (D) genus composition of microbial community during surimi fermentation. Bars labeled with different letters indicate significantly different at p < 0.05.

The changes in genus composition of microbial community were further studied in tilapia surimi after co-fermentation (Fig. 3D). A total of 13 genera were found with the relative abundance over 1% in the three groups. Macrococcus and Streptococcus were the dominant genera in the raw material of surimi, possessing the relative abundance of 37.4% and 35.1%, respectively (CK0). As the fermentation proceeded, these two genera significantly reduced and their relative abundance reached only 0.1% and 3.5% at 24 h, respectively. At the end of fermentation (CK24), Lactococcus became the most abundant genus, possessing the relative abundance of 86.1%, followed by Aeromonas (8.2%). After addition of low concentration of starters, besides Macrococcus (15.7%) and Streptococcus (12.1%), Lactobacillus and Pediococcus had the dominant position in the microbial community of ZRL0 group, with the relative abundance of 34.8% and 14.2%, respectively. At the end of fermentation (ZRL24), the LAB including Lactobacillus, Pediococcus, and Lactococcus had the most abundance of 46.3%, 25.2%, and 23.2%, respectively. After addition of high concentration of starters (ZRH0), only Lactobacillus and Pediococcus had the dominant position in the microbial community, possessing the relative abundance of 59.6% and 24.7%, respectively. As the fermentation proceeded, these two genera significantly increased and reached 65.6% and 29.2% at the end of fermentation (ZRL24). Although Lactococcus also increased during the fermentation of ZRH group, its relative abundance only reached 2.0% at 24 h, obviously lower than that in the CK24 and ZRL24 groups. The absolutely high abundance of LAB with the fermentation proceeding is the main reason of the decrease in richness and evenness of microbial community.

3.3. Group-dimension correlation analysis among the microbial community and physical properties

During the fermentation of fermented foods, the quality properties and microbial community will change after addition of starter, and the group-dimension correlation analysis can reveal the quality change mechanisms induced by microbial community (Li et al., 2025b). In this work, the group-dimension correlation analysis among the physical properties and microbial community was studied at different fermentation time through Pearson's correlation (Fig. 4, Fig. 5). After group-dimension correlation analysis at 12 h (Fig. 4A), two apparent microbial clusters were discovered including cluster 12 h-1 (Lactococcus, Aeromonas, Streptococcus, Macrococcus, Acinetobacter, Enterobacter, Enhydrobacter, Vagococcus, Staphylococcus, Psychrobacter, and Weissella) and cluster 12 h-2 (Lactobacillus and Pediococcus). The genera in cluster 12 h-1 showed positive correlation with Sobs, ACE, Chao, Shannon, cohesiveness, pH and TPC, while the genera in cluster 12 h-2 was positively correlated with hardness, springiness, chewiness, adhesiveness, gel strength, whiteness, and Simpson. As shown in Fig. 4B, the correlation network was then built based on the significant correlation (p < 0.05). The TPC and α diversity indexes except Simpson were significantly positively correlated with most of the genera in cluster 12 h-1. In fermented foods, the microorganisms with high abundance are more likely to affect the formation of food quality (Li et al., 2022). Therefore, in this work, the influence contribution of each genus was further calculated based on the correlation and microbial abundance (Fig. 4C). Among the genera in cluster 12 h-1, the decrease of Lactococcus, Aeromonas, and Streptococcus had the highest influence contribution to the decreasing richness and evenness of microbial community as well as the TPC after co-fermentation with starters for 12 h. Lactobacillus and Pediococcus were significantly positively correlated with hardness (r = 0.965 and 0.836), springiness (r = 0.867 and 0.888), chewiness (r = 0.913 and 0.795), adhesiveness (r = 0.807 and 0.658), gel strength (r = 0.918 and 0.776), and whiteness (r = 0.968 and 0.870). These improved physical properties of surimi were mainly due to the increasing Lactobacillus and Pediococcus after co-fermentation with the influence contribution of over 68.6% and 26.7%, respectively. The pH value were significantly negatively correlated with Lactobacillus (r = −0.995) and Pediococcus (r = −0.923), leading to the increase of acidity after co-fermentation with starters for 12 h.

Fig. 4.

Group-dimension correlation analysis among the physical properties and microbial community at 12 h. (A) Correlation heatmap, (B) correlation network, and (C) influence contribution of genus to the change in different indexes.

Fig. 5.

Group-dimension correlation analysis among the physical properties and microbial community at 24 h. (A) Correlation heatmap, (B) correlation network, and (C) influence contribution of genus to the change in different indexes.

After group-dimension correlation analysis at 24 h (Fig. 5A), two apparent microbial clusters were discovered including cluster 24 h-1 (Lactococcus, Aeromonas, Streptococcus, Macrococcus, Enterobacter, Vagococcus, Staphylococcus, and Weissella) and cluster 24 h-2 (Lactobacillus, Pediococcus, Acinetobacter, Enhydrobacter, and Psychrobacter). The genera in cluster 24 h-1 showed positive correlation with Sobs, ACE, Chao, Shannon, pH and TPC, while the genera in cluster 12 h-2 was positively correlated with hardness, springiness, chewiness, adhesiveness, cohesiveness, gel strength, whiteness, and Simpson. As shown in Fig. 5B, the correlation network was then built based on the significant correlation (p < 0.05). Similarly, the TPC and α diversity indexes except Simpson showed significantly positive correlation with most of the genera in cluster 24 h-1, and the decrease of Lactococcus, Aeromonas, and Streptococcus had the highest influence contribution to the reduction of diversity of microbial community as well as the TPC after co-fermentation with starters for 24 h. Besides Lactobacillus and Pediococcus, Acinetobacter and Psychrobacter also significantly positively correlated with some texture properties and gel strength. However, their influence could be ignored because their abundance was extremely low at the end of fermentation. The obviously improved texture properties, gel strength, and whiteness after co-fermentation with starters for 24 h mainly resulted from the increasing Lactobacillus and Pediococcus with the total influence contribution over 99%. The metabolism of Lactobacillus and Pediococcus also played the key role in the increase of acidity after co-fermentation for 24.

3.4. Non-thermal gel improvement mechanism of tilapia surimi after co-fermentation with starters

After co-fermentation of starters, the gel properties of tilapia surimi without heating was obviously improved. High concentrations of starters not only significantly enhanced the non-thermal gel, but also shortened the gel time. The non-thermal gel improvement mechanism of tilapia surimi were further summarized from the perspective of microbial metabolism (Fig. 6). The acid environment induced by the metabolism of LAB is considered as the main reason of the non-thermal gel of surimi (Cui et al., 2024; Li et al., 2025a; Yuan et al., 2024). Their metabolites, especially organic acids are related to the formation of gel structure in surimi without heating (Li et al., 2024b). In this study, the non-thermal gel was also found during the fermentation in all three fermentation groups, accompanied with high acidity of surimi. Moreover, higher acidity after co-fermentation with LAB led to the better gel properties than those in the natural fermentation, suggesting that the acid environment contributed the most to the non-thermal gel of tilapia surimi. In the acid environment, the α-helix structures of myofibrillar protein gradually transform into β-sheet and random coil structures (Yang et al., 2016). The changes in these secondary structures of proteins reflect the unfolded spatial structures of proteins which are more easy to form gel structure among the myosin molecules (Wang et al., 2025a; Xiong et al., 2021; Zhang et al., 2025). In this study, the LAB including Lactobacillus and Pediococcus was responsible for the formation of acid environment and consequently improved the hardness, springiness, chewiness, adhesiveness, and gel strength of tilapia surimi during fermentation for 12 h and 24 h, suggesting the good synergistic action of starters L. plantarum and P. acidilactici. Meanwhile, L. plantarum showed higher influence contribution to the non-thermal gel improvement than P. acidilactici. Similar result is found that the starter L. plantarum possesses better gel improvement of tilapia surimi than starter P. acidilactici, probably because of the better acid-producing ability of L. plantarum (Li et al., 2024b).

Fig. 6.

Non-thermal gel improvement mechanisms of tilapia surimi after co-fermentation with starters.

Microbial proteases from spoilage microorganisms are considered as a main reason of gel deterioration by damaging their protein structures (Ba et al., 2018). In this work, the relative abundance of spoilage microorganisms markedly decreased during the fermentation of surimi in all three groups. However, in the CK group, the relative abundance of spoilage microorganisms still reached 61.4% and 13.3% at 12 and 24 h, respectively, and the main composition of the spoilage microorganisms was Aeromonas and Streptococcus. The strains belonging to Aeromonas (Sodagar et al., 2024; Wang et al., 2025b) and Streptococcus (Jia et al., 2025; Liu et al., 2024) are famous for their protease production. The high abundance of these spoilage microorganisms had adverse effect on the gel formation in the CK group. After co-fermentation with LAB, the growth of spoilage microorganisms was obviously inhibited than the natural fermentation. The relative abundance of spoilage microorganisms in the ZRH group decreased to only 7.5% and 3.3% at 12 and 24 h, respectively. The spoilage microorganisms were much lower than that after fermentation with single starter of L. plantarum and P. acidilactici with the relative abundance over 30% and 40% throughout the fermentation process, respectively (Li et al., 2024b). Moreover, the TPC also significantly decreased after co-fermentation with LAB. The significant suppression of spoilage microorganisms by co-fermentation of LAB led to the obvious decrease in the secretion of proteases, thereby reducing the damage to the gel structure of surimi. Similar results are found that the decrease of spoilage microorganisms by LAB helps to improve the gel properties of surimi (Cui et al., 2024). In this study, co-fermentation with high LAB that can not only shorten the gel time from 24 h to 12 h but also significantly inhibit the spoilage microorganisms, can be applied to the production of high-quality tilapia surimi products.

4. Conclusion

The texture properties and gel strength of tilapia surimi without heating obviously increased after co-fermentation with L. plantarum and P. acidilactici, especially in the ZRH group, suggesting the improvement of non-thermal gel of surimi by starters. Co-fermentation obviously enhanced the whiteness of surimi, while the pH value and TPC obviously decreased. With the increasing fermentation time, LAB rapidly became the dominant position, and the starters could adapt well to the fermentation environment. Group-dimension correlation analysis at different fermentation time showed that co-fermentation with starters significantly reduced the diversity of microbial community, mainly resulting from the decrease of Lactococcus, Aeromonas, and Streptococcus. The good synergistic effect of starters during fermentation was responsible for the formation of acid environment and consequently improved the non-thermal gel of surimi. L. plantarum showed higher influence contribution to the gel improvement than P. acidilactici. The metabolisms of starters also avoided the destruction of gel structure of surimi by the proteases from spoilage microorganisms. Co-fermentation with starters especially at high concentration can be used in the production of high-quality tilapia surimi products.

CRediT authorship contribution statement

Chunhui Wang: Writing-original draft, Formal analysis. Hui Chen: Conceptualization. Hui Huang: Funding acquisition. Chunsheng Li: Conceptualization, Writing-review & editing, Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Handling Editor: Dr. Xing Chen

Appendix A. Supplementary data

The following is the Supplementary data to this article: