Qualitative and quantitative characterization of spoilage bacteria from packed fish

Advancements in Predictive Microbiology: Integrating New Technologies for Efficient Food Safety Models

Predictive microbiology is a rapidly evolving field that has gained significant interest over the years due to its diverse application in food safety. Predictive models are widely used in food microbiology to estimate the growth of microorganisms in food products. These models represent the dynamic interactions between intrinsic and extrinsic food factors as mathematical equations and then apply these data to predict shelf life, spoilage, and microbial risk assessment. Due to their ability to predict the microbial risk, these tools are also integrated into hazard analysis critical control point (HACCP) protocols. However, like most new technologies, several limitations have been linked to their use. Predictive models have been found incapable of modeling the intricate microbial interactions in food colonized by different bacteria populations under dynamic environmental conditions. To address this issue, researchers are integrating several new technologies into predictive models to improve efficiency and accuracy. Increasingly, newer technologies such as whole genome sequencing (WGS), metagenomics, artificial intelligence, and machine learning are being rapidly adopted into newer-generation models. This has facilitated the development of devices based on robotics, the Internet of Things, and time-temperature indicators that are being incorporated into food processing both domestically and industrially globally. This study reviewed current research on predictive models, limitations, challenges, and newer technologies being integrated into developing more efficient models. Machine learning algorithms commonly employed in predictive modeling are discussed with emphasis on their application in research and industry and their advantages over traditional models.

Insight to the diversity of Photobacterium spp. isolated from European plaice (Pleuronectes platessa) based on phylogenetic analysis, phenotypic characterisation and spoilage potential

This study aimed to explore the diversity of fifty-four Photobacterium strains isolated from muscle tissue of European plaice (Pleuronectes platessa) caught at different fishing seasons and stored 14-days under various conditions. Single phylogenetic markers (16S rRNA, gapA, gyrB and recA) and multilocus sequence analysis (MLSA) were employed to classify isolates at species level. Furthermore, intra- and interspecies variability in the phenotypic traits, maximum specific growth rate (μmax) and spoilage potential of the Photobacterium isolates were investigated. The isolates were classified into the P. iliopiscarium (53.7 %), P. phosphoreum (40.7 %) and P. piscicola (5.6 %) clades using MLSA. Two housekeeping genes, gyrB and recA, exhibited a consistent phylogenetic relationship with MLSA, suggesting that they might be used as individual phylogenetic markers for the Photobacterium genus. Intra- and interspecies variability in the expression of phenotypic characteristics and the production of trimethylamine (TMA), inosine (HxR), and hypoxanthine (Hx) were observed. A growth optimum temperature for P. iliopiscarium was approximately 20 °C, while those for P. phosphoreum and P. piscicola were closer to 15 °C. All isolates exhibited the highest growth density at 1.5 % NaCl, followed by 0.5 %, 3 %, and 6 % NaCl. However, P. phosphoreum demonstrated a higher NaCl tolerance than the other two species. Although, the high CO2 atmosphere significantly inhibited the growth of all strains at 4 °C, P. phosphoreum and P. piscicola showed higher growth density at 15 °C than P. iliopiscarium. Notably, all strains demonstrated H2S production. The μmax varied considerably within each species, highlighting the significance of strain-level variability. This study demonstrates that P. iliopiscarium and P. piscicola, alongside P. phosphoreum, are efficient TMA-, HxR-, Hx-, and H2S-producers, suggesting their potential contribution to synergistic off-odour generation and spoilage. Moreover, the Photobacterium isolates seem to exhibit diverse adaptations to their environments, resulting in fluctuated growth and spoilage potential. Understanding intra- and interspecies variability will facilitate modelling seafood spoilage in microbial risk assessments and developing targeted hurdles to prolong products' shelf-life.

Extending the Shelf Life of Atlantic Salmon (Salmo salar) with Sub-Chilled Storage and Modified Atmosphere Packaging in Recyclable Mono-Material Trays

This study investigated the effect of sub-chilling whole gutted salmon and sub-chilled storage at -1 °C in modified-atmosphere packaging in two recyclable mono-material trays (CPET, HDPE). Quality parameters were measured, including water-holding properties, salt content, color, texture, lipid oxidation, and sensory and microbiological shelf life. The oxygen transmission rate was measured for the packages. Compared to traditional fish storage on ice, sub-chilling gave a 0.4% weight gain, better water-holding capacity, and higher salt content. The sub-chilled fish gave a significantly better sensory quality and microbiological shelf life of up to 49 days. Photobacterium was the dominating bacteria during storage. Salmon packaged in CPET trays had a higher drip loss than HDPE trays, but a lower rate of lipid oxidation (1-penten-3-ol). Our results showed the feasibility of significantly extending shelf life with sub-chilling, removing the need for ice. Moreover, using recyclable trays for packaging contributes to a circular economy without compromising food quality.

Deciphering the growth responses and genotypic diversity of bioluminescent Photobacterium phosphoreum on chicken meat during aerobic refrigerated storage

The advent of high-throughput sequencing technologies in recent years has revealed the unexpected presence of genus Photobacterium within the chicken meat spoilage ecosystem. This study was undertaken to decipher the occurrence, the growth patterns and the genotypic biodiversity of Photobacterium phosphoreum on chicken breast fillets stored aerobically at 4 °C through conventional microbiological methods and molecular techniques. Samples were periodically cultured on marine broth agar (MA; supplemented with meat extract and vancomycin) for the enumeration of presumptive bioluminescent Photobacterium spp. In total, 90 bioluminescent bacteria were recovered from the initial (time of first appearance), middle and end stages of storage. Concomitantly, 95 total psychrotrophic/psychrophilic bacteria were isolated from the same medium to assess the presence and diversity of non-luminous photobacteria. Genetic diversity between bioluminescent isolates was assessed with two PCR-based DNA fingerprinting methods, i.e. RAPD and rep-PCR. Moreover, the characterization of selected bacterial isolates at the genus and/or species level was performed by sequencing of the 16S rRNA and/or gyrB gene. Bioluminescent bacteria were scarcely encountered in fresh samples at population levels of ca. 2.0 log CFU/g, whilst total psychrotrophic/psychrophilic bacteria were found at levels of ca. 4.4 log CFU/g. As time proceeded and close to shelf-life end, bioluminescent bacteria were encountered at higher populations, and were found at levels of 5.3 and 7.0 log CFU/g in samples from the second and third batch, respectively. In the first batch their presence was occasional and at levels up to 3.9 log CFU/g. Accordingly, total psychrotrophic/psychrophilic bacteria exceeded 8.4 log CFU/g at the end of storage, suggesting the possible underestimation of bioluminescent populations following the specific cultivation conditions. Sequence analysis assigned bioluminescent isolates to Photobacterium phosphoreum, while genetic fingerprinting revealed high intra-species variability. Respectively, total psychrotrophs/psychrophiles were assigned to genera Pseudomonas, Shewanella, Psychrobacter, Acinetobacter, Vibrio and Photobacterium. Non-luminous photobacteria were not identified within the psychrotrophs/psychrophiles. Results of the present study reveal the intra- and inter-batch variability on the occurrence and growth responses of P. phosphoreum and highlight its potential role in the chicken meat spoilage consortium.

Potassium Chloride, Sodium Lactate and Sodium Citrate Impaired the Antimicrobial Resistance and Virulence of Pseudomonas aeruginosa NT06 Isolated from Fish

Sodium chloride (NaCl) is a commonly used additive in minimally processed fish-based products. The addition of NaCl to fish products and packaging in a modified atmosphere is usually efficient with regard to limiting the occurrence of the aquatic environmental pathogen Pseudomonas aeruginosa. Given the negative effects of excess NaCl in the diet, there is a growing demand to reduce NaCl in food products with safer substituents, but the knowledge of their impact on antibiotic resistant P. aeruginosa is limited. This study aimed to evaluate the physiological and transcriptome characteristics of P. aeruginosa NT06 isolated from fish and to determine the effect of selected concentrations of alternative NaCl compounds (KCl/NaL/NaC) on the P. aeruginosa NT06 virulence phenotype and genotype. In the study, among the isolated microorganisms, P. aeruginosa NT06 showed the highest antibiotic resistance (to ampicillin, ceftriaxone, nalidixic acid, and norfloxacin) and the ability to grow at 4 °C. The Comprehensive Antibiotic Resistance Database (CARD) and the Virulence Factor Database (VFDB) revealed the presence of 24 and 134 gene products assigned to AMR and VF in the P. aeruginosa NT06 transcriptome, respectively. KCl, KCl/NaL and KCl/NaL/NaC inhibited pyocyanin biosynthesis, elastase activity, and protease activity from 40 to 77%. The above virulence phenotypic observations were confirmed via RT-qPCR analyses, which showed that all tested AMR and VF genes were the most downregulated due to KCl/NaL/NaC treatment. In conclusion, this study provides insight into the potential AMR and VF among foodborne P. aeruginosa and the possible impairment of those features by KCl, NaL, and NaC, which exert synergistic effects and can be used in minimally processed fish-based products.

Bacterial Changes in Boiled Crayfish between Different Storage Periods and Characterizations of the Specific Spoilage Bacteria

This study investigated changes in the microbial compositions of crayfish tails during storage at 4 °C (for 0-12 days) as measured using high-throughput sequencing (HTS). The specific spoilage organisms (SSOs) in the crayfish tails were isolated using culture-dependent cultivation methods, and they were identified by 16S rRNA and characterized for their enzymatic spoilage potentials (e.g., protease, lipase, phospholipase, and amylase). The spoilage abilities of the selected strains in the crayfish tails were assessed by inoculating them into real food. Moreover, the microbial growth and the volatile basic nitrogen (TVB-N) changes were monitored during the storage period. The results from the HTS showed that the dominant genus of shrimp tails evolved from Streptococcus (D0) to Pseudomonas (D4) and, finally, to Paenisporosarcina (D12) during storage. Seven bacterial species (Acinetobacter lwoffii, Aeromonas veronii, Kurthia gibsonii, Pseudomonas sp., Exiguobacterium aurantiacum, Lelliottia amnigena, and Citrobacter freundii) were screened from the spoiled shrimp tails by the culture-dependent method, among which Aeromonas veronii had the strongest spoilage ability.

Photobacterium predominate the microbial communities of muscle of European plaice (Pleuronectes platessa) caught in the Norwegian sea independent of skin and gills microbiota, fishing season, and storage conditions

The aim of this study was to investigate seasonal variations (September, December and April) in the initial microbial communities of skin and gills' external mucosal tissues (EMT) and muscle of European plaice (Pleuronectes platessa). Moreover, a potential relationship between EMT and fresh muscle microbiota was examined. The microbial community succession in plaice muscle as a function of fishing season and storage conditions was also investigated. The selected seasons for the storage experiment were September and April. Investigated storage conditions were; fillets packaged in either vacuum or modified atmosphere (70 % CO2, 20 % N2, 10 % O2) and chilled/refrigerated conditions (4 °C). Whole fish stored on ice (0 °C) was selected as a commercial standard. Seasonal variations were detected in the initial microbial communities of EMT and plaice muscle. The highest microbial diversity was found in EMT and muscle of April-caught plaice, followed by December and September catch indicating the important role of environmental factors in shaping the initial EMT and muscle microbial communities. The EMT microbial communities were more diverse than fresh muscle samples. The low number of shared taxa between EMT and initial muscle microbial communities indicates that only a minor part of the muscle microbiota came from the EMT. Psychrobacter and Photobacterium were the predominant genera in the EMT microbial communities in all seasons. Photobacterium dominated the initial muscle microbial communities with a gradual seasonal reduction of its abundance from September to April. Storage time and storage conditions shaped a less diverse and distinct community compared to the fresh muscle. However, no clear separation was seen between the communities at the middle and end of storage time. Regardless of EMT microbiota, fishing season and storage conditions, Photobacterium dominated the microbial communities of stored muscle samples. The Photobacterium prevalence as the primary specific spoilage organism (SSO) could be attributed to its high relative abundance in the initial microbiota of muscle and its CO2-tolerance. The findings of this study indicate the important contribution of Photobacterium to the microbial spoilage of plaice. Thus, the development of innovative preservation techniques addressing the rapid growth of Photobacterium could contribute to the production of high-quality and shelf-stable convenient retail plaice products.

Volatilome Analysis and Evolution in the Headspace of Packed Refrigerated Fish

Fresh fish is a perishable food in which chemical (namely oxidation) and microbiological degradation result in undesirable odor. Non-processed fish (i.e., raw fish) is increasingly commercialized in packaging systems which are convenient for its retailing and/or which can promote an extension of its shelf-life. Compared to fish sent to its retail unpackaged, fish packaging results in a modification of the gaseous composition of the atmosphere surrounding it. These modifications of atmosphere composition may affect both chemical and microbiological degradation pathways of fish constituents and thereby the volatile organic compounds produced. In addition to monitoring Total Volatile Basic Nitrogen (TVB-N), which is a common indicator to estimate non-processed fish freshness, analytical techniques such as gas chromatography coupled to mass spectrometry or techniques referred to as "electronic nose" allow either the identification of the entire set of these volatile compounds (the volatilome) and/or to selectively monitor some of them, respectively. Interestingly, monitoring these volatile organic compounds along fish storage might allow the identification of early-stage markers of fish alteration. In this context, to provide relevant information for the identification of volatile markers of non-processed packaged fish quality evolution during its storage, the following items have been successively reviewed: (1) inner atmosphere gaseous composition and evolution as a function of fish packaging systems; (2) fish constituents degradation pathways and analytical methods to monitor fish degradation with a focus on volatilome analysis; and (3) the effect of different factors affecting fish preservation (temperature, inner atmosphere composition, application of hurdle technology) on volatilome composition.

Isolation and characterization of the new isolated bacteriophage YZU-L1 against Citrobacter freundii from a package-swelling of meat product

Citrobacter freundii is an important foodborne pathogen that can cause urethritis, bacteremia, necrotizing abscess, and meningitis in infants. In this study, a gas-producing isolate from vacuum-packed meat products was identified as C. freundii by 16S rDNA. In addition, a new virulent phage YZU-L1, which could specifically lyse C. freundii, was isolated from sewage samples in Yangzhou. Transmission electron microscopy showed that phage YZU-L1 had a polyhedral head of 73.51 nm in diameter and a long tail of 161.15 nm in length. According to phylogenetic analysis employing the terminase large subunit, phage YZU-L1 belonged to the Demerecviridae family and the Markadamsvirinae subfamily. The burst size was 96 PFU/cell after 30 min of latent period and 90 min of rising period. Phage YZU-L1 could maintain high activity at pH of 4-13, and resist 50 °C for up to 60 min. The complete genome of YZU-L1 was 115,014 bp double-stranded DNA with 39.94% G + C content, encoding 164 open reading frames (ORFs), without genes encoding for virulence, antibiotic resistance, or lysogenicity. Phage YZU-L1 treatment significantly reduced the viable bacterial count of C. freundii in a sterile fish juice model, which is expected to be a natural agent for the biocontrol of C. freundii in foods.

Study on spoilage potential and its molecular basis of Shewanella putrefaciens in response to cold conditions by Label-free quantitative proteomic analysis

To elucidate how Shewanella putrefaciens survives and produces spoilage products in response to cold conditions, the metabolic and protease activity of S. putrefaciens DSM6067 cultured at three different temperatures (30 °C, 10 °C, and 4 °C) was studied by determining the bacterial growth, total volatile basic nitrogen (TVB-N), biogenic amines, extracellular protease activity, as well as the differential expressed proteins via Label-free quantitative proteomics analysis. The lag phase of the strain cultured at 10 °C and 4 °C was about 20 h and 120 h longer than at 30 °C, respectively. The TVB-N increased to 89.23 mg N/100 g within 28 h at 30 °C, and it needed at least 72 h and 224 h at 10 °C and 4 °C, respectively. Cold temperatures (10 °C and 4 °C) also inhibited the yield factors and the extracellular protease activity per cell at the lag phase. However, the protease activity per cell and the yield factors of the sample cultivated at 10 °C and 4 °C well recovered, especially at the mid and latter stages of the log phase. The further quantitative proteomic analysis displayed a complex biological network to tackle cold stress: cold stress responses, nutrient uptake, and energy conservation strategy. It was observed that the protease and peptidase were upregulated, so as to the degradation pathways of serine, arginine, and aspartate, which might lead to the accumulation of spoilage products. This study highlighted the spoilage potential of S. putrefaciens still should be concerned even at low temperatures.

Mechanism of Inosine Monophosphate Degradation by Specific Spoilage Organism from Grass Carp in Fish Juice System

Microbial growth strongly affects the quality and flavor of fish and fish products. This study aimed to explore the role and function of grass carp-borne microorganisms in the degradation of inosine monophosphate (IMP) related compounds in a fish juice system during chill storage (4 °C. Prokaryotic transcriptomic analysis was used to explore the microbial contribution to metabolic pathways and related enzymes. The degree of microbial contribution was verified by the activity of enzymes and metabolite content. Collectively, there were multiple IMP relative product degradation pathways. A. rivipollensis degraded IMP by producing 5'-nucleotidase (5'-NT) while S. putrefaciens degraded IMP mainly by alkaline phosphatase (ALP). Hypoxanthine (Hx) was degraded to uric acid (Ua) induced by P. putida and S. putrefaciens mainly with producing xanthine oxidase (XOD), while A. rivipollensis almost could not produce XOD. This work can used as a guide and provide basic knowledge for the quality and flavor control of aquatic products.

Advances in the Formation and Control Methods of Undesirable Flavors in Fish

Undesirable flavor formation in fish is a dynamic biological process, decreasing the overall flavor quality of fish products and impeding the sale of fresh fish. This review extensively summarizes chemical compounds contributing to undesirable flavors and their sources or formation. Specifically, hexanal, heptanal, nonanal, 1−octen−3−ol, 1−penten−3−ol, (E,E)−2,4−heptadienal, (E,E)−2,4−decadienal, trimethylamine, dimethyl sulfide, 2−methyl−butanol, etc., are characteristic compounds causing off−odors. These volatile compounds are mainly generated via enzymatic reactions, lipid autoxidation, environmentally derived reactions, and microbial actions. A brief description of progress in existing deodorization methods for controlling undesirable flavors in fish, e.g., proper fermenting, defatting, appropriate use of food additives, and packaging, is also presented. Lastly, we propose a developmental method regarding the multifunctional natural active substances made available during fish processing or packaging, which hold great potential in controlling undesirable flavors in fish due to their safety and efficiency in deodorization.

Impact of Modified Atmospheres on Growth and Metabolism of Meat-Spoilage Relevant Photobacterium spp. as Predicted by Comparative Proteomics

Modified atmosphere packaging (MAP) is a common strategy to selectively prevent the growth of certain species of meat spoiling bacteria. This study aimed to determine the impact of high oxygen MAP (70% O₂, 30% CO₂, red and white meats) and oxygen-free MAP (70% N₂, 30% CO₂, also white meat and seafood) on preventing the growth of spoiling photobacteria on meat. Growth of Photobacterium carnosum and P. phosphoreum was monitored in a meat simulation media under different gas mixtures of nitrogen, oxygen, and carbon dioxide, and samples were taken during exponential growth for a comparative proteomic analysis. Growth under air atmosphere appears optimal, particularly for P. carnosum. Enhanced protein accumulation affected energy metabolism, respiration, oxygen consuming reactions, and lipid usage. However, all the other atmospheres show some degree of growth reduction. An increase in oxygen concentration leads to an increase in enzymes counteracting oxidative stress for both species and enhancement of heme utilization and iron-sulfur cluster assembly proteins for P. phosphoreum. Absence of oxygen appears to switch the metabolism toward fermentative pathways where either ribose (P. phosphoreum) or glycogen (P. carnosum) appear to be the preferred substrates. Additionally, it promotes the use of alternative electron donors/acceptors, mainly formate and nitrate/nitrite. Stress response is manifested as an enhanced accumulation of enzymes that is able to produce ammonia (e.g., carbonic anhydrase, hydroxylamine reductase) and regulate osmotic stress. Our results suggest that photobacteria do not sense the environmental levels of carbon dioxide, but rather adapt to their own anaerobic metabolism. The regulation in presence of carbon dioxide is limited and strain-specific under anaerobic conditions. However, when oxygen at air-like concentration (21%) is present together with carbon dioxide (30%), the oxidative stress appears enhanced compared to air conditions (very low carbon dioxide), as explained if both gases have a synergistic effect. This is further supported by the increase in oxygen concentration in the presence of carbon dioxide. The atmosphere is able to fully inhibit P. carnosum, heavily reduce P. phosphoreum growth in vitro, and trigger diversification of energy production with higher energetic cost, highlighting the importance of concomitant bacteria for their growth on raw meat under said atmosphere.

Use of Food Spoilage and Safety Predictor for an “A Priori” Modeling of the Growth of Lactic Acid Bacteria in Fermented Smoked Fish Products

Fermentation is one of the oldest methods to assure the safety and quality of foods, and to prolong their shelf life. However, a successful fermentation relies on the correct kinetics depending on some factors (i.e., ingredients, preservatives, temperature, inoculum of starter cultures). Predictive microbiology is a precious tool in modern food safety and quality management; based on the product characteristics and the conditions occurring in food processing, the inactivation of or increase in microbial populations could be accurately predicted as a function of the relevant intrinsic or extrinsic variables. The main aim of this study was the optimization of the formula of a smoked fermented fish product using predictive modeling tools (tertiary and secondary models) in order to define the role of each factor involved in the formulation and assure a correct course of fermentation. Product optimization was conducted through the software Food Spoilage and Safety Predictor (FSSP), by modeling the growth of lactic acid bacteria (LAB) as a function of some key parameters such as temperature, pH, salt, liquid smoke, carbon dioxide, and nitrites. The variables were combined through a fractional design of experiments (DoE) (3^k-p), and the outputs of the software, i.e., the maximal growth rate (μ_max) and the time to attain the critical threshold (t_crit), were modeled through a multiple regression procedure. The simulation, through FSSP and DoE, showed that liquid smoke is the most critical factor affecting fermentation, followed by temperature and salt. Concerning temperature, fermentation at 20–25 °C is advisable, although a low fermentation temperature is also possible. Other parameters are not significant.

Evaluation of Different Techniques, including Modified Atmosphere, under Vacuum Packaging, Washing, and Latilactobacillus sakei as a Bioprotective Agent, to Increase the Shelf-Life of Fresh Gutted Sea Bass (Dicentrarchus labrax) and Sea Bream (Sparus aurata) Stored at 6 ± 2 °C

Simple Summary

A method of increasing the shelf life of gutted sea bass and sea bream packaged under vacuum and stored at 6 ± 2 °C (simulating supermarkets and consumer fridges) through the use of bioprotective starter cultures was studied. An increase in the shelf life up until 14 days was observed by washing the gutted fish in water supplemented with a culture of Latilactobacillus sakei and dextrose (0.1%). L. sakei reduced the growth of specific spoilage microorganisms and consequently reduced the production of total volatile nitrogen and oxidized compounds.

Abstract

Fish meat is very perishable because of indigenous and microbial enzymes, which determine spoilage and shelf life. The deterioration processes, which lead to an important, sequential, and progressive modification of the initial state of freshness, are fast and depend on rearing, harvesting, slaughtering, handling, and storage conditions. Usually, the shelf life of gutted fish stored at 4 ± 2 °C under vacuum packaging (VP—1.0 bar) and modified atmosphere packaging (MAP, 70% N₂, <1% O₂, 30% CO₂) is approximately 9 days. The aim of this work was to improve the shelf life and preserve the microbiological and sensory quality of farmed gutted sea bass (Dicentrarchus labrax) and sea bream (Sparus aurata) using different methods, including VP, MAP, and bioprotective culture containing Latilactobacillus sakei, until 12–14 days. Microbiological, physicochemical, and sensory quality indices were monitored to confirm the effectiveness of biopreservation on product quality during proper refrigeration (4 ± 2 °C) or abuse (6 ± 2 °C, simulating supermarkets and consumer fridges) storage period. Considering the quality indexes represented by Enterobacteriaceae, total volatile nitrogen (TVB-N), and malonaldehyde concentrations (TBARS) and the sensorial analysis, the VP samples were more acceptable than the MAP fish, even though the shelf-life of the VP and MAP fish was similar at about 12 days. The second phase of the work was to evaluate the shelf-life of both VP fish stored at 6 ± 2 °C, which simulates the normal abuse temperature of supermarkets or consumer fridges. Data confirmed the previous results and demonstrated, despite the abuse temperature of storage, a shelf-life of about 12 days. Finally, the third phase consisted of prolonging the shelf life until 14 days of storage at 6 ± 2 °C by washing the gutted sea bass and sea bream in a suspension of bioprotective starter (7 log CFU/mL) with or without the addition of dextrose (0.1%) and by VP packaging. The bioprotective culture reduced the growth of spoilage microorganisms. Consequently, the total volatile nitrogen (TVB-N) concentration in both fish species was low (<35 mg N/100 g). Nonprofessional and untrained evaluators confirmed the acceptability of the inoculated samples by sensorial analysis.

An Active Peptide-Based Packaging System to Improve the Freshness and Safety of Fish Products: A Case Study

Fresh fish are highly perishable, owing mainly to their moisture content, high amount of free amino acids and polyunsaturated fatty acids. Microorganisms and chemical reactions cause the spoilage, leading to loss in quality, human health risks and a market value reduction. Therefore, the fishing industry has always been willing to explore new technologies to increase quality and safety of fish products through a decrease of the microbiological and biochemical damage. In this context, antimicrobial active packaging is one such promising solution to meet consumer demands. The main objective of this study was to evaluate the effects of an active polypropylene-based packaging functionalized with the antimicrobial peptide 1018K6 on microbial growth, physicochemical properties and the sensory attributes of raw salmon fillets. The results showed that application of 1018K6-polypropylene strongly inhibited the microbial growth of both pathogenic and specific spoilage organisms (SSOs) on fish fillets after 7 days. Moreover, salmon also kept its freshness as per volatile chemical spoilage indices (CSIs) during storage. Similar results were obtained on hamburgers of Sarda sarda performing the same analyses. This work provides further evidence that 1018K6-polymers have good potential as antimicrobial packaging for application in the food market to enhance quality and preserve the sensorial properties of fish products.

Development of a rapid qPCR method to quantify lactic acid bacteria in cold-smoked salmon

Quantification of lactic acid bacteria (LAB) is essential to control quality of seafood products like cold-smoked salmon (CSS). In the present study, we report the design and optimization of a dual-labelled TaqMan ™ probe targeting the V7 region of 16S rRNA gene for the detection of LAB in CSS. This quantitative PCR (qPCR) assays is useful for the simultaneous detection of the ten LAB genera communally encountered in CSS as Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Macrococcus, Streptococcus, Vagococcus and Weissella. The specificity of this method was demonstrated against 14 genera (44 isolates, 35 species) of Gram-positive bacteria and 19 genera of Gram-negative (40 isolates, 34 species). Calibration of the method was performed in CSS matrix using a mix of equimolar cultured solution of five LAB. Quantification with the qPCR method range from 3.5 to 8.5 Log CFU/g in CSS matrix, covering 5 orders of magnitude. On these artificially contaminated CSS slices, PCR method results correlated successfully (R² = 0.9945) with the conventional enumeration on Elliker medium. In addition, the new method was successful on commercial CSS from five different origins with a quantification range from 3.7 Log CFU/g to 8.0 Log CFU/g. This one-step quantitative methodology is proposed as a rapid and complementary tool of the cultural methods to investigate the LAB microbiota and biodiversity of CSS.

Bacterial Diversity of Sun-Dried Spanish Mackerel in Dalian and Application of Lactobacillus plantarum X23 as a Biopreservative

ABSTRACT

Sun-dried Spanish mackerel is a common food in Dalian and made by adding salt and sun drying, which has special physical, chemical, and microbiological properties. In this study, the physicochemical properties and microbial composition of commercially available sun-dried Spanish mackerel in Dalian were assessed, and some Lactobacillus strains were screened as a biopreservative for sun-dried Spanish mackerel preparation. The results showed that the total volatile base nitrogen content in the traditional sun-dried Spanish mackerel samples from Dalian was within 30 mg/100 g, the histamine content was 7 to 17 mg/kg, and the dominant bacteria at the genus level were Lactobacillus, Psychrobacter, and Ralstonia. A strain with biopreservative potential was isolated from a sun-dried Spanish mackerel sample, identified as L. plantarum species by 16S rDNA sequencing, and assigned as L. plantarum X23. Fresh Spanish mackerel flesh was treated with 16% brine and L. plantarum X23 at a dose of 107 CFU/mL and then dried in the sun. The sun-dried Spanish mackerel flesh treated with 16% brine and L. plantarum X23 showed a decreased histamine and acid value, increased free amino acid content, and a higher sensory score compared with the sun-dried Spanish mackerel without L. plantarum X23 treatment (P < 0.05). In conclusion, the sun-dried Spanish mackerel purchased from the supermarkets in Dalian were safely edible, and L. plantarum X23 can significantly reduce the content of histamine and putrescine in self-made, low-salt, sun-dried Spanish mackerel and has potential as a biopreservative for sun-dried Spanish mackerel preparation.

HIGHLIGHTS

Investigation of the pectin grafting with gallic acid and propyl gallate and their antioxidant activities, antibacterial activities and fresh keeping performance

In this paper, a method for the enzymatic modification of pectin, in which gallic acid (GA) and propyl gallate (PG) were grafted onto pectin molecules in an aqueous/organic two-phase system catalyzed by lipase, was proposed. The potential reaction mechanism was explored through UV-Vis, FTIR and ¹H NMR spectroscopic methods and density functional theory. Results suggested that the lipase played a dual role during the modification by catalyzing the hydrolysis of methyl ester bonds of pectin in the aqueous phase and the esterification between the 4-OH of GA and PG and the -COOH of pectin in the organic phase. Moreover, the effects of GA and PG on the antioxidant and the antibacterial activities of pectin were evaluated, and results showed that the antioxidant and the antibacterial activities of modified pectin were better than those of native pectin. The effect of modified pectin on the quality of fresh bass (Lateolabrax maculatus) was further studied. Results suggested that, compared to control group, the total viable count, histamine level, malondialdehyde content and acid value of bass fillets treated with modified pectin were significantly reduced, whereas the sensory score was significantly increased.

Evidence for biosurfactant-induced flow in corners and bacterial spreading in unsaturated porous media

The spread of pathogenic bacteria in unsaturated porous media, where air and liquid coexist in pore spaces, is the major cause of soil contamination by pathogens, soft rot in plants, food spoilage, and many pulmonary diseases. However, visualization and fundamental understanding of bacterial transport in unsaturated porous media are currently lacking, limiting the ability to address the above contamination- and disease-related issues. Here, we demonstrate a previously unreported mechanism by which bacterial cells are transported in unsaturated porous media. We discover that surfactant-producing bacteria can generate flows along corners through surfactant production that changes the wettability of the solid surface. The corner flow velocity is on the order of several millimeters per hour, which is the same order of magnitude as bacterial swarming, one of the fastest known modes of bacterial surface translocation. We successfully predict the critical corner angle for bacterial corner flow to occur based on the biosurfactant-induced change in the contact angle of the bacterial solution on the solid surface. Furthermore, we demonstrate that bacteria can indeed spread by producing biosurfactants in a model soil, which consists of packed angular grains. In addition, we demonstrate that bacterial corner flow is controlled by quorum sensing, the cell-cell communication process that regulates biosurfactant production. Understanding this previously unappreciated bacterial transport mechanism will enable more accurate predictions of bacterial spreading in soil and other unsaturated porous media.

Identification of the Specific Spoilage Organism in Farmed Sturgeon (Acipenser baerii) Fillets and Its Associated Quality and Flavour Change during Ice Storage

Hybrid sturgeon, a popular commercial fish, plays important role in the aquaculture in China, while its spoilage during storage significantly limits the commercial value. In this study, the specific spoilage organisms (SSOs) from ice stored-sturgeon fillet were isolated and identified by analyzing their spoilage related on sensory change, microbial growth, and biochemical properties, including total volatile base nitrogen (TVBN), thiobarbituric acid reactive substances (TBARS), and proteolytic degradation. In addition, the effect of the SSOs on the change of volatile flavor compounds was evaluated by solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). The results showed that the Pseudomonas fluorescens, Pseudomonas mandelii, and Shewanella putrefaciens were the main SSOs in the ice stored-sturgeon fillet, and significantly affect the odors by changing the volatile compounds in the sturgeon. Compared with the fresh sturgeon, the appreciable increase of polycyclic aromatic hydrocarbons and tetramethyl-pyrazine might be the spoilage indicators of the sturgeon contaminated by P. fluorescens; the appreciable increase of 1-octen-3-ol and (z)-2-penten-1-o might be the potential marker of the sturgeon contaminated by P. mandelii; and the appreciable increase of 1-(3,3-dimethylbicyclo [2.2.1] hept-2-yl)-ethanon and butylated hydroxytoluene were associated with S. putrefaciens. This study reveals the relationship between the SSOs and flavor changes in sturgeon fillets, which will contribute to the sturgeon preservation and shelf-life extension.

High Oxygen Packaging of Atlantic Cod Fillets Inhibits Known Spoilage Organisms, but Sensory Quality Is Not Improved Due to the Growth of Carnobacterium/Carnobacteriaceae

Improved quality control and prolonged shelf life are important actions in preventing food waste. To get an overview of the bacterial diversity of fillets from live stored mature Atlantic cod, bacterial isolates were identified before and after storage (air and vacuum) and freezing/thawing. Based on the load of dominating bacteria, the effect of different packaging methods and a short freezing/thawing process on prolonged shelf-life was evaluated (total viable counts, bacteriota, sensory attributes, and volatile components). Hand filleted (strict hygiene) cod fillets had a low initial bacterial load dominated by the spoilage organism Photobacterium, whereas industrially produced fillets had higher bacterial loads and diversity (Pseudomonas, Arthrobacter, Psychrobacter, Shewanella). The identified bacteria after storage in vacuum or air were similar to the initially identified bacteria. Bacteriota analysis showed that a short time freezing/thawing process reduced Photobacterium while modified atmosphere packaging (MAP; 60% CO₂/40% O₂ or 60% CO₂/40% N₂) inhibited the growth of important spoilage bacteria (Photobacterium,Shewanella, Pseudomonas) and allowed the growth of Carnobacterium/Carnobacteriaceae and Acinetobacter. Despite being dominated by Photobacterium, fresh fillets stored in MAP 60% CO₂/40% N₂ demonstrated better sensory quality after 13 days of storage than fillets stored in MAP 60% CO₂/40% O₂ (dominated by Carnobacterium/Carnobacteriaceae). Carnobacterium spp. or other members of Carnobacteriaceae may therefore be potential spoilage organisms in cod when other spoilage bacteria are reduced or inhibited.

Characterization of Microbial Community in Cold-Chain Hairtail Fish by High-Throughput Sequencing Technology

ABSTRACT

High-throughput DNA sequencing with the Illumina MiSeq platform was used to analyze the microbial communities in hairtail (Trichiurus haumela) muscle samples to study the diversity and dynamic changes in these communities during cold-chain circulation of these fish. The richness and diversity of the microbial community in hairtail muscle had a transient decline from 0 to 24 h and decreased after the first rise from 24 to 216 h. The diversity and richness of bacteria in cold-chain hairtail reached maximum at 168 h. The Shannon and Simpson diversity indices of the bacteria were 2.96 and 0.16, respectively, and their ACE and Chao1 richness indices were 254.84 and 155.10, respectively. The dominant bacteria belonged to phylum Proteobacteria, class Gammaproteobacteria, order Pseudomonadales, family Pseudomonadaceae, and genus Pseudomonas, and their relative abundances were 80.52, 72.11, 76.68, 23.25, and 53.50%, respectively. These results provide a basis for exploring how to maintain the freshness and predict the shelf life of hairtail.

HIGHLIGHTS

Involvement of RpoN in Regulating Motility, Biofilm, Resistance, and Spoilage Potential of Pseudomonas fluorescens

Pseudomonas fluorescens is a typical spoiler of proteinaceous foods, and it is characterized by high spoilage activity. The sigma factor RpoN is a well-known regulator controlling nitrogen assimilation and virulence in many pathogens. However, its exact role in regulating the spoilage caused by P. fluorescens is unknown. Here, an in-frame deletion mutation of rpoN was constructed to investigate its global regulatory function through phenotypic and RNA-seq analysis. The results of phenotypic assays showed that the rpoN mutant was deficient in swimming motility, biofilm formation, and resistance to heat and nine antibiotics, while the mutant increased the resistance to H₂O₂. Moreover, the rpoN mutant markedly reduced extracellular protease and total volatile basic nitrogen (TVB-N) production in sterilized fish juice at 4°C; meanwhile, the juice with the rpoN mutant showed significantly higher sensory scores than that with the wild-type strain. To identify RpoN-controlled genes, RNA-seq-dependent transcriptomics analysis of the wild-type strain and the rpoN mutant was performed. A total of 1224 genes were significantly downregulated, and 474 genes were significantly upregulated by at least two folds at the RNA level in the rpoN mutant compared with the wild-type strain, revealing the involvement of RpoN in several cellular processes, mainly flagellar mobility, adhesion, polysaccharide metabolism, resistance, and amino acid transport and metabolism; this may contribute to the swimming motility, biofilm formation, stress and antibiotic resistance, and spoilage activities of P. fluorescens. Our results provide insights into the regulatory role of RpoN of P. fluorescens in food spoilage, which can be valuable to ensure food quality and safety.

Glycerol-Based Dendrimer Nanocomposite Film as a Tunable pH-Sensor for Food Packaging

Large amounts of food are wasted during the food supply chain. This loss is in part due to consumer confusion over dates on food packages that can indicate a variety of quality indicators in the product (e.g., expiration date, "best by" date, "sell by" dates, etc.). To reduce this food loss, much research has been focused on the films that offer simple and easily manipulated indication systems to detect food spoilage. However, these materials are usually hydrophilic biopolymers that can detect the food spoilage in a wide pH range but do not provide highly sensitive real-time measurements. In this work, a glycerol-based nanocomposite core-shell latex film was synthesized to create a responsive packaging material that can provide real-time pH detection of food with high sensitivity. First, the pH-responsive dendrimer comonomer was synthesized from glycerol and diamine. Then, the nanoencapsulation polymerization process via miniemulsion was conducted to form a core-shell structure with tunable nanoshell thickness for a sensible pH-responsive release (<0.5 pH change). Next, the flexible film encapsulated a color-indicative dye that provided highly sensitive and visible color changes as both the pH dropped and the time elapsed in the food. This film also provided a barrier to water and heat and resisted deformation. Ultimately, this nanocomposite flexible film pending a pH sensor has the potential as an intelligent food packaging material for a universal, accurate, easy-to-use, and real-time food spoilage monitoring system to reduce food waste.

Enhanced preservation of vacuum-packaged Atlantic salmon by hyperbaric storage at room temperature versus refrigeration

Hyperbaric storage at room temperature (HS/RT: 75 MPa/25 °C) of vacuum-packaged fresh Atlantic salmon (Salmo salar) loins was studied for 30 days and compared to atmospheric pressure at refrigerated temperatures (AP/5 °C, 30 days) and RT (AP/25 °C, 5 days). Most of the fatty acids were not affected by storage conditions, with only a slight decrease of docosahexaenoic acid (DHA) content (n-3 polyunsaturated fatty acid) for AP samples, reflected in the lower polyene index values obtained and higher oxidation extent. For HS, a lower lipid oxidation extension and a slower increase of myofibrillar fragmentation index values were observed, when compared to AP samples. The volatile profile was similar for the HS and fresh samples, with the HS samples retaining fresh-like alcohols and aldehydes components, which disappeared in AP samples, mainly in AP/25 °C samples. The volatile profile for AP samples (5 and 25 °C) revealed mostly spoilage-like compounds due to microbial activity. Drip loss increased progressively during the 30 days of storage under HS, while a slight decrease of water holding capacity after 5 days was observed, increasing further after 30 days. Regarding textural properties, only resilience was affected by HS, decreasing after 30 days. So, HS/RT could represent an interesting extended preservation methodology of fresh salmon loins, since allows retaining important physicochemical properties for at least 15 days, while refrigeration after 5 days showed already volatile spoilage-like compounds due to microbial activity. Furthermore, this methodology allows additional considerable energy savings when compared to refrigeration.

Impact of onboard chitosan treatment of whole cod (Gadus morhua) on the shelf life and spoilage bacteria of loins stored superchilled under different atmospheres

The initial handling of marine fish on board fishing vessels is crucial to retain freshness and ensure an extended shelf life of the resulting fresh products. Here the effect of onboard chitosan treatment of whole, gutted Atlantic cod (Gadus morhua) was studied by evaluating the quality and shelf life of loins processed six days post-catch and packaged in air or modified atmosphere (% CO₂/O₂/N₂: 55/5/40) and stored superchilled for 11 and 16 days, respectively. Sensory evaluation did not reveal a clear effect of chitosan treatment on sensory characteristics, length of freshness period or shelf life of loins under either packaging conditions throughout the storage period. However, directly after loin processing, microbiological analysis of loins showed that onboard chitosan treatment led to significantly lower total viable counts as well as lower counts of specific spoilage organisms (SSO), such as H₂S-producers and Pseudomonas spp., compared to the untreated group. In addition, the culture-independent approach revealed a lower bacterial diversity in the chitosan-treated groups compared to the untreated groups, independently of packaging method. Partial 16S rRNA gene sequences belonging to Photobacterium dominated all sample groups, indicating that this genus was likely the main contributor to the spoilage process.

Diversity and succession of the microbial community and its correlation with lipid oxidation in dry-cured black carp (Mylopharyngodon piceus) during storage

This study aimed to achieve deeper insights into the microbiota composition and dynamic succession of the dry-cured black carp during storage using a high-throughput sequencing technique (HTS). The effect of lipid oxidation on microorganisms was also evaluated. Over 651 bacterial genera belonging to 37 phyla were identified. Firmicutes, Proteobacteria and Actinobacteria were the main bacterial phylum, some are highly associated with meat spoilage. Staphylococcus, Macrococcus and Acinetobacter were the most three microbial genera throughout the entire storage period (30 days). Between two different storage temperature, refrigeration at 4 °C could facilitate maintaining the microbial diversity, while 25 °C storage led to the formation of dominant microflora and the reduction of community diversity. Canonical correspondence analysis (CCA) showed that acid value (AV), malondialdehyde (MDA) and 4-hydroxy-2-hexenal (HHE) contents were three key environmental factors (oxidation products) affecting the profile of the microbiota. Staphylococcus presented a positive correlation with HHE content, while Macrococcus and Acinetobacter were negatively correlated with HHE content. These results could expand our knowledge on the effect of lipid oxidation on change of microbial distribution, it could also present an guideline to develop advanced storage methods for the vacuum packed dry-cured fish products.

Transcription factors FabR and FadR regulate cold adaptability and spoilage potential of Shewanella baltica

Shewanella baltica is the specific spoilage microorganism of Pseudosciaena crocea during low-temperature storage. Exploring the correlation between cold adaptability and spoilage potential may provide a new perspective for prolonging shelf life of aquatic products. In the present study, we investigated the synthesis pathway of unsaturated fatty acid (UFA) responsible for regulating cold adaptability in Shewanella baltica and its effect on spoilage potential. FabR and FadR, as key regulators of membrane unsaturated fatty acids synthesis pathway, were identified in S. baltica. FabR was significantly down-regulated at 4 °C compared to at 30 °C, yet FadR displayed the opposite results. By overexpressing fabR and fadR genes at 4 °C, we found that FabR and FadR had negative and positive effects on UFA content and membrane fluidity as well as spoilage potential, respectively. These data indicated that FabR and FadR functioned collectively to increase the membrane fluidity for better cold adaptability at low temperature, resulting in the maintenance of spoilage potential of S. baltica.

Tarragon essential oil as a source of bioactive compounds with anti-quorum sensing and anti-proteolytic activity against Pseudomonas spp. isolated from fish - in vitro, in silico and in situ approaches

The present study aimed to evaluate the anti-quorum sensing (anti-QS) and anti-proteolytic potentials of tarragon essential oil (TEO) and its major compounds against food-associated Pseudomonas spp. The activities were verified by in vitro, in silico and in situ approaches. In this work, methyl eugenol (ME)- and β-phellandrene (β-PH)-rich TEO was investigated. TEO at subMIC increased the percentage of saturated fatty acids in the bacterial membranes (from 7 to 22%) and exhibited anti-quorum sensing via decreasing the efficiency of QS autoinducer synthesis [3-oxo-C12-HSL (from 2.028 μg/mL to <LOD), C4-HSL (from 1.312 μg/mL to <LOD) and PQS (from 0.007625 μg/mL to <LOD)]. ME and β-PH were docked into LasR, RhlR and PqsR proteins, with docking scores comparable to native autoinductors. The subMICs of TEO, ME and β-PH decreased the proteolysis in the examined bacteria by 33, 29, and 21% (in TSB medium) and by 29, 26, and 19% (in fish juice medium), respectively. Almost all genes encoding proteases were downregulated by the applied agents. The ME- and β-PH-rich TEO acts as an anti-QS agent and significantly suppresses the proteolytic activity of food-associated pseudomonads. It might therefore increase the quality of fish-based products, where Pseudomonas spp. predominate.

Growth and spoilage metabolites production of a mesophilic Aeromonas salmonicida strain in Atlantic salmon (Salmo salar L.) during cold storage in modified atmosphere

AIMS

The aim of the study was to quantify the growth kinetic parameters and spoilage-associated metabolites of an inoculated strain of Aeromonas salmonicida in pre-rigor filleted Atlantic salmon (Salmo salar L.) stored in vacuum (VP) or modified atmosphere (MAP 60/40% CO₂ /N₂ ) at 4 and 8°C.

METHODS AND RESULTS

The maximum growth rate of A. salmonicida in VP salmon stored at 4°C was 0·56 ± 0·04 day^-1 with no detectable lag-phase and the concentration of Aeromonas reached 8·33 log CFU per g after 10 days. The growth rates and maximum population density of Aeromonas in MAP salmon were lower but the applied atmosphere did not inhibit the growth. A selection of metabolites associated with fish spoilage were quantified using ¹ H nuclear magnetic resonance (NMR) spectroscopy. The concentration of trimethylamine (TMA) was significantly affected by storage time and temperature, packaging atmosphere and inoculation with A. salmonicida (General Linear Model (GLM), P < 0·001 for all factors).

CONCLUSION

The study presents preliminary results on A. salmonicida as a potential spoilage organism in vacuum-packaged salmon during cold storage. The combination of refrigeration and a packaging atmosphere consisting of 60/40 % CO₂ /N₂ did not completely inhibit the growth but prevented the formation of TMA.

SIGNIFICANCE AND IMPACT OF THE STUDY

Little information is available on the spoilage potential of Aeromonas spp. in minimally processed salmon products under different packaging conditions. The study clearly demonstrates the importance of hurdle technology and provides data to further elucidate the significance of Aeromonas spp. as a spoilage organism.