Supplementary Information

1. Materials and methods

1.1 Weight loss

The weights of the fillets were recorded before packaging and after storage for

the given period. The weight loss (%) was calculated as:

Weight loss (%) = (W0 - Wt)/W0×100

Where W0 and Wt are the weight of the sample before and after storage,

respectively.

1.2 pH value

Approximately 10 g sample was stirred and dissolved in 90 mL distilled water

for 30 min. Then, the mixture was filtered. The pH of the sample was measured using

a digital pH meter (MP511, Sanxin Instrument, Shanghai, China).

1.3 Microbial correlation

Bacterial DNA was extracted as described in a previous study (Zhang, Li, Lv, Li,

Kong, & Luo, 2017). PCR was used to amplify the V3-V4 region of bacterial 16S

rRNA gene using primers 338f (5'- ACTCCTACGGGAGGCAGCA -3') and 806r (5'-

GGACTACHVGGGTWTCTAAT -3'). The amplified products were purified by 2%

agarose gel electrophoresis and tested using an Agilent Bioanalyzer (2100, Agilent

Technologies，Santa Clara，CA，USA). Subsequently, 2 * 300 bp dual terminal

sequencing was performed using the Illumina MiSeq platform at Personal

Biotechnology Co. Ltd. (Shanghai, China).

The chimera sequences were evaluated and eliminated by the QIIME software

(v1.8.0). QIIME was used to merge and classify the sequences above according to

97% sequence similarity. The most abundant sequence in each operational taxonomic

unit (OTU) was selected as the representative sequence. By comparing the OTU

representative sequence with the template sequence of the corresponding database

(Greengenes database, Release 13.8), the taxonomic information corresponding to

each OTU was obtained. Sequence data analyses were mainly performed using

QIIME and R project (v3.2.0).

2. Results and discussion

2.1 Weight loss

Weight loss is an indicator for assessing the quality of fish fillets that have been

reported to cause texture change (Hong, Luo, Zhou, Bao, Lu, & Shen, 2013). The

weight loss in all groups significantly increased (p < 0.05) to 11-15% as a function of

time (Supplementary Table). The weight loss in CON declined to 10.43% and at the

fastest rate among the four groups on 9 d. However, the weight in the EGT group

exceeded 10% on 18 d. As the freshness of the fillets decreased, water release resulted

from the disintegration of the proteins in myofibrillar fibers (Ocaño-Higuera,

Marquez-Ríos, Canizales-Dávila, Castillo-Yáñez, Pacheco-Aguilar, Lugo-Sánchez, et

al., 2009). Lower weight losses were observed in the gelatin treated samples (GT and

EGT) throughout storage. The barrier function of gelatin, which hinders the

evaporation of water (Tongnuanchan, Benjakul, Prodpran, Pisuchpen, & Osako,

2016). However, the value of the EGT group was lower than that for the GT group

because EGCG protected the microstructures of fish fillets from bacteria by delaying

the degradation of fish myofibril (Feng, Ng, Mikš-Krajnik, & Yang, 2017).

2.2 Change in pH

The change in pH value is related to the biological and chemical reactions in the

samples. The pH of different groups showed the same trends with an initial decrease

followed by an increased at the end of the experiment (Supplementary Table). The

initial decrease was due to the decomposition of glycogen, ATP, and creatine

phosphate in fish muscle. The subsequent increase was due to the production of

alkaline substances. Its accumulation from the protein degradation caused by bacteria

and endogenous enzymes (Remya, Mohan, Venkateshwarlu, Sivaraman, &

Ravishankar, 2017). The pH of fish fillets was approximately 6.23 on 0 d and was not

affected by different treatments. Then, the pH of the CON group decreased faster than

that of the other three groups and dropped to the lowest value (6.13) on 3 d, while the

lowest pH of the other groups was detected on 6 d. During the subsequent period, the

pH gradually increased to 7.02 - 7.32. According to these results, the pH of fish fillets

in EGT was lower than that in the other groups, indicating that the production of

alkaline compounds related to spoilage was delayed by EGCG and gelatin (Hong,

Luo, Zhou, Bao, Lu, & Shen, 2013).

2.3 Microbial correlation

Recently, network inference analysis based on the relationship between microbial

members has become popular (Faust & Raes, 2012). The fundamental purpose of this

kind of analysis is to investigate the interaction among the members of different

communities and to elucidate the interaction patterns of co-occurrence or co-exclusion

among the members of the communities in different habitats by using correlation

analysis. The possible cooperation or competition among the different microbial

groups was inferred as shown in Supplementary Figure 1. Among the 40 genera, an

unclassified - solirubrobacterales had 26 matching relationships with other genera and

promoted 17 genera such as Acinetobacter, and inhibited 9 genera such as

Lactococcus. Gluconacetobacter always played the most important role in

competition with other genera, and had inhibitory effects on 13 genera, including

Chryseobacterium (Supplementary Figure 1).

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Supplementary Table Change in the pH and weight loss of tilapia during cold storage. Means ± SD were used to describe the results (n = 4).

Different superscripts of values within a column are significantly different (p < 0.05). GT = gelatin treatment, ET = EGCG treatment, EGT =

EGCG-gelatin biofilm treatment.

parameters group 0 d 3 d 6 d 9 d 12 d 15 d 18 d 21 d

pH value

CON 6.23±0.012a 6.13±0.006a 6.44±0.006a 6.65±0.038a 6.69±0.015a 7.06±0.045a 7.11±0.032a 7.32±0.026a

GT 6.24±0.00a 6.34±0.011b 6.12±0.017b 6.45±0.051b 6.63±0.031a 6.75±0.045b 6.95± 0.06b 7.08±0.038b

ET 6.23±0.011ab 6.34±0.012b 6.22±0.025c 6.32±0.031c 6.44±0.047b 6.45±0.076c 6.87± 0.015b 7.13±0.029b

EGT 6.22±0.006b 6.45±0.015c 6.32±0.026d 6.41±0.026bc 6.52±0.026c 6.45±0.042c 6.64± 0.055c 7.02±0.036c

Weight

loss (%)

CON - 6.38±0.70 9.80±1.33a 10.43±0.55a 11.24±0.21a 12.99±0.81a 14.26±0.97a 15.47±0.62a

GT - 5.76±0.42 6.73±0.78c 9.03±0.60b 9.48±0.43c 10.25±0.17c 10.65±0.47c 13.23±0.21b

ET - 6.38±0.95 8.28±0.35b 9.46±0.34b 10.58±0.32b 11.00±0.12b 12.97±1.35b 13.02±0.95b

EGT - 5.77±0.69 6.87±0.54c 7.17±0.43c 8.03±0.65d 8.25±0.52d 10.37±0.51c 11.48±0.23c

Supplementary Figure 1 Microbial correlation at the genus level in tilapia fillets during storage

(n=5). Red indicated positive correlation while green indicated negative correlation.

Supplementary Figure 2 The chromatograms of 8 standard biogenic amines.