Pei Gao

The contribution of autochthonous microflora on free fatty acids release and flavor development in low-salt fermented fish

To investigate the contribution of autochthonous microflora on free fatty acids (FFA) release and flavor development in low-salt fermented fish, three groups of processed fish, including bacteriostatic-acidification group (BAG), bacteriostatic group (BG), and spontaneous fermented fish (CG) were established. Results showed that addition of NaN3 reduced microbial load in BAG and BG below 3.5 log CFU/g after 3 weeks of incubation. Activities of lipases and lipoxygenase declined markedly with increasing time, where BG had the highest activities, followed by CG and BAG. There is a 36.3% higher in the total FFA content in CG than that in BAG, indicating both microbial and endogenous lipases contributed to the FFA liberation in fermented fish while endogenous lipases play a major role. However, compared to BAG and BG, largely higher levels of volatile compounds were observed in CG, suggesting that autochthonous microflora dominated the generation of volatile flavor compounds in fermented fish.

Biosynthesis of acetate esters by dominate strains, isolated from Chinese traditional fermented fish (Suan yu)

Biosynthesis of acetate esters by Lactobacillus plantarum 120, Staphylococcus xylosus 135 and Saccharomyces cerevisiae 31, isolated from Chinese traditional fermented fish (Suan yu) were studied. A buffer system containing acyl donors (acetic acid/glyceryl triacetate/acetyl-CoA) and aliphatic alcohols (C2-C6) was established, inoculated with intracellular and extracellular extracts of the three strains. The results showed that the biosynthesis pathway of L. plantarum 120 was esterification and alcoholysis, while the biosynthesis pathway of S. xylosus 135 and S. cerevisiae 31 was hydrolysis and esterification, rather than alcoholysis. The ester-synthesizing activity of L. plantarum 120 via alcoholysis was higher than that via esterification at high pH value, while an opposite result for each strain was observed at low pH value. Moreover, the ester-synthesis activity of L. plantarum 120 was higher than that of S. xylosus 135 and S. cerevisiae 31. In addition, microbial ester-synthesis activity increased with the increase of aliphatic alcohol carbon number.

Esterase activities of autochthonous starter cultures to increase volatile flavour compounds in Chinese traditional fermented fish (Suan yu)

ABSTRACT Suan yu, a type of Chinese traditional fermented freshwater fish in solid form, is cheered among consumers due to its characteristic sensory flavour, primarily taste and smell. The purpose of this study was to investigate the presence of esterase activity of Lactobacillus plantarum 120, Staphylococcus xylosus 135, and Saccharomyces cerevisiae 31, in selecting appropriate processing means to accelerate the formation of the desired flavour in Suan yu. The results showed that all strains exhibited esterase activities in both intracellular and extracellular fractions against p-nitrophenyl (p-NP)-acetate, p-NP-butyrate, and p-NP-octanoate, and also that the maximum activity was observed in the intracellular fraction with p-NP-octanoate. Esterase activities of all strains were inhibited by low pH and low water activity (Aw), with the exception of L. plantarum 120 exhibiting an increase in intracellular activity with decreasing Aw. Moreover, L. plantarum 120 and S. xylosus 135 esterase were negatively affected by decreasing temperature, while a positive influence was observed on the intracellular esterase of S. cerevisiae 31. The results suggested that a low pH, a low Aw and a high temperature would be more appropriate for Suan yu processing to produce more flavour compounds which are acceptable by consumers.

Quality of giant freshwater prawn (Macrobrachium rosenbergii) during the storage at −18°C as affected by different methods of freezing

ABSTRACT This study investigated the effects of liquid nitrogen quick-freezing, −35°C air-blast freezing, and −18°C refrigerator direct-freezing on quality retention of frozen giant freshwater prawns using various indicators: microscopic structure, physicochemical, texture, and color properties. The results showed that the shelf lives of prawns treated with liquid nitrogen quick-freezing, −35°C air-blast freezing, and −18°C refrigerator direct-freezing were about 20, 12, and 8 weeks, respectively, in terms of total volatile basic nitrogen content and K-value. Meanwhile, liquid nitrogen quick-freezing could effectively improve texture property of frozen giant freshwater prawn. These findings confirm that liquid nitrogen quick-freezing is a promising method to extend the shelf life of frozen prawns.

Effect of heating temperature and duration on the texture and protein composition of Bighead Carp (Aristichthys nobilis) muscle

ABSTRACT The effect of heating temperature and duration on the texture and protein composition of bighead carp (Aristichthys nobilis) muscle was investigated. Samples were heated at 90°C, 100°C, 110°C, and 120°C for different times. The results showed that cooking loss increased significantly with increasing temperature and heating time, and a fractional conversion model was used to describe the increase. Hardness, chewiness, and gumminess exhibited a similar four-phase change with two peaks at 90°C, 100°C, and 110°C, and the trend was not observed at 120°C. Analysis of sodium dodecyl sulfate polyacrylamide gel electropheresis (SDS-PAGE) revealed that contents of myosin heavy chain decreased entirely to disappear and contents of actin significantly decreased. Increasing the intensity of the heavy molecular weight bands indicated the aggregation of myosin. Content of water-soluble proteins presented an increasing trend, which reflected the protein degradation and the release of low-molecular-weight compounds.

Dynamics and diversity of microbial community succession during fermentation of Suan yu, a Chinese traditional fermented fish, determined by high throughput sequencing

The main goal of this study was to investigate the dynamics, diversity and succession of microbial community present during the preparation of Suan yu (fermented fish), with and without starter cultures by high-throughput sequencing of 16S rRNA and ITS1 genes. Firmicutes and Ascomycota were the predominant phyla of bacteria and fungi, respectively, in all samples. At the genus level, Lactobacillus, Macrococcus and Staphylococcus were the predominating bacteria throughout the fermentation process, regardless of the inclusion of starter cultures. Saccharomyces was the predominating fungal genus in the early-fermentation stage of samples that inoculated starter cultures (MS), while the final product was dominated by Candida and Wickerhamomyces. Compared with naturally-fermented samples (NS; no starter cultures), Lactococcus, Leuconostoc, Enterococcus, Vibrio, Fusicolla and Torulaspora were inhibited and Aureobasidium emerged in samples inoculated with starter cultures (P < .05). Unweighted pair-group and principal component analyses of bacterial and fungal compositions revealed that microbiota structures differed between NS and MS samples. Redundancy analysis indicated that water content and pH might be important factors influencing the dominant bacterial and fungal community. Results indicated that microbial community were dynamic during fermentation process and the inoculation of mixed starter culture inhibited the growth of many organisms associated with food spoilage and contributed to the improvement of the quality of Suan yu products.

Proteolysis during fermentation of Suanyu as a traditional fermented fish product of China

ABSTRACT Proteolytic changes in Suanyu, a traditional fermented fish product of China, were determined during 6 weeks of fermentation. The gradual decrease of pH value and aw was detected, accompanied with an increasing trend detected in trichloroacetic acid–soluble C-1: peptides concentration. Protein composition changed throughout the fermentation. A decrease in sarcoplasmic and myofibrillar protein fractions was accompanied by an increase in the insoluble fraction. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed most bands of sarcoplasmic, myofibrillar, and insoluble proteins lightened or disappeared due to proteolysis or denaturation. New bands emerged especially in insoluble proteins. Fluctuation of molecular weight distribution of the water-soluble nitrogen in Suanyu was found during fermentation. The highest percentage in Suanyu end-products was observed in the fraction of <0.18 kDa, followed by 0.18–0.5 kDa, 66.02 ± 0.38% and 10.79 ± 0.27%, respectively. Free amino acids analysis showed that nearly all amino acids increased, especially for Asp, Glu, Met, Phe, Leu, Val, and Ile. Overall, intense proteolysis in Suanyu brought numerous of small peptides and free amino acids, which contribute to the taste and flavor of Suanyu.