Freezing: an underutilized food safety technology?

Furcellaran: Impact of Concentration, Rheological Properties, and Structure on Ice Recrystallization Inhibition Activity

Recrystallization is considered the main damaging mechanism during the frozen storage of biologic materials. In this study, furcellaran, a polysaccharide related to κ-carrageenan, was studied for its concentration-dependent effect on ice crystal growth and recrystallization. The structure and sulfate content of the utilized furcellaran was analyzed by 1H nuclear magnetic resonance spectroscopy, ion chromatography, and high-performance size-exclusion chromatography. Additionally, the rheological properties of furcellaran solutions were investigated. Our findings demonstrate that furcellaran inhibits ice growth as effectively as κ-carrageenan. Furthermore, the rheological properties change with increasing furcellaran concentration, resulting in a gel-like consistency at 5 g/L, which coincides with decreased recrystallization inhibition activity and larger crystals. This suggests that gel formation or a gel-like consistency has to be avoided for optimal recrystallization inhibition activity.

Evaluation of the flavor profiles of Yanbian-style sauced beef from differently treated raw beef samples

In this study, we investigated the volatile flavor compounds and sensory perceptions of Yanbian-style sauced beef prepared from raw meats subjected to different treatments (hot fresh, chilled, and frozen beef). The results indicated that the treatment of raw beef significantly impacted the quality and flavor of sauced beef. Sauced chilled beef (CRSB) exhibited the highest content of fatty acids and total amino acids. A total of 48 volatile compounds were identified. Moreover, a relative odor activity value analysis identified hexanal, nonanal, heptanal, 1-octen-3-ol, and 2,3-octanedione as the characteristic flavor compounds in Yanbian-style sauced beef. The sensory evaluation demonstrated that CRSB was the most palatable and flavorful. Additionally, correlation loading plot analysis indicated strong correlations between sensory evaluation, fatty acids, amino acids, and volatile flavor compounds. These results suggest that chilled beef meat is the best raw material for the production of Yanbian-style sauced beef.

Vendors' handling practices of edible long-horned grasshoppers (Ruspolia differens) products and implications on microbial safety

Edible grasshopper, Ruspolia ruspolia, has nutritional and cherished cultural and economic importance to people from diverse cultures, particularly in over 20 African countries. It is consumed at home or commercially traded as sautéed, deep-fried, or boiled products. However, there is limited information on the hygiene practices of the vendors and the implications on the microbial safety of the final product. This research aimed at assessing the food safety knowledge, handling practices and shelf life of edible long-horned grasshopper products among vendors and the microbial safety of ready-to-eat products sold in 12 different markets in Uganda. Samples of raw, deep-fried and boiled grasshoppers were randomly collected from 74 vendors (62% street and 38% market vendors) and subjected to microbial analysis. Over 85% of the vendors surveyed had no public health food handler's certificate and >95% had limited post-harvest handling knowledge. Total aerobic bacteria (7.30-10.49 Log10 cfu/g), Enterobacteriaceae (5.53-8.56 Log10 cfu/g), yeasts and molds (4.96-6.01 Log10 cfu/g) total counts were significantly high and above the acceptable Codex Alimentarius Commission and Food Safety Authority of Ireland (FSAI) limits for ready-to-eat food products. Eight key pathogenic bacteria responsible for foodborne diseases were detected and these isolates were characterized as Bacillus cereus, Hafnia alvei, Serratia marcescens, Staphylococcus aureus, S. xylosus, S. scuiri, S. haemolyticus, and Pseudomonas aeruginosa. Findings from this study highlight the urgent need to create local and national food safety policies for the edible grasshopper "nsenene" subsector to regulate and guide street and market vending along the value chain, to prevent the transmission of foodborne diseases to consumers.

Challenges and processing strategies to produce high quality frozen meat

Freezing is an effective means to extend the shelf-life of meat products. However, freezing and thawing processes lead to physical (e.g., ice crystals formation and freezer burn) and biochemical changes (e.g., protein denaturation and lipid oxidation) in meat resulting in loss of quality. Over the last two decades, several attempts have been made to produce thawed meat with qualities similar to that of fresh meat to no avail. This is due to the fact that no single technique exists to date that can mitigate all the quality challenges caused by freezing and thawing. This is further confounded by the consumer perception of frozen meat as lower quality compared to equivalent fresh-never-frozen meat cuts. Therefore, it remains challenging for the meat industry to produce high quality frozen meat and increase consumer acceptability of frozen products. This review aimed to provide an overview of the applications of novel freezing and thawing technologies that could improve the quality of thawed meat including deep freezing, high pressure, radiofrequency, electro-magnetic resonance, electrostatic field, immersion solution, microwave, ohmic heating, and ultrasound. This review will also discuss the development in processing strategies such as optimising the ageing of meat pre- or post-freezing, and the integration of freezing and thawing in one process/regime to collapse the difference in quality between thawed meat and fresh-never-frozen equivalents.

Fungal Gut Microbiome in Myasthenia Gravis: A Sub-Analysis of the MYBIOM Study

An altered gut microbiota is a possible contributing pathogenic factor in myasthenia gravis (MG), an autoimmune neuromuscular disease. However, the significance of the fungal microbiome is an understudied and neglected part of the intestinal microbiome in MG. We performed a sub-analysis of the MYBIOM study including faecal samples from patients with MG (n = 41), non-inflammatory neurological disorder (NIND, n = 18), chronic inflammatory demyelinating polyradiculoneuropathy (CIDP, n = 6) and healthy volunteers (n = 12) by sequencing the internal transcribed spacer 2 (ITS2). Fungal reads were obtained in 51 out of 77 samples. No differences were found in alpha-diversity indices computed between the MG, NIND, CIDP and HV groups, indicating an unaltered fungal diversity and structure. Overall, four mould species (Penicillium aurantiogriseum, Mycosphaerella tassiana, Cladosporium ramonetellum and Alternaria betae-kenyensis) and five yeast species (Candida. albicans, Candida. sake, Candida. dubliniensis, Pichia deserticola and Kregervanrija delftensis) were identified. Besides one MG patient with abundant Ca. albicans, no prominent dysbiosis in the MG group of the mycobiome was found. Not all fungal sequences within all groups were successfully assigned, so further sub-analysis was withdrawn, limiting robust conclusions.

Bacterial concentration and Campylobacter spp. quantification differ when fresh or ultra-frozen samples are analysed over time using molecular biology and culture-based methods

The study aimed to delineate the robustness of the culture-based and molecular biology methods to assess the total bacterial concentration and Campylobacter jejuni (C. jejuni) quantification in caecal content, analysed as fresh or after being stored immediately at ultra-low (-80°C) temperature at different time points (for 3, 7, 14, 28 and 62 days post collection). The caecal content was collected from birds that were artificially colonised with C. jejuni (in-vivo), and quantification was performed using both colony-forming unit (CFU) and qPCR. The results showed that storage time affected the output of culture-based analyses but mostly did not alter concentration retrieved via qPCR. After an initial ~4.5 log₁₀ reduction in CFU observed from fresh (day 0) to frozen samples, bacterial concentration retrieved with culture-based methods seemed to be constant in samples frozen for 3 to 62 days, indicating a possible threshold for C. jejuni loss of viability due to effect of storage temperature. Ranking order analyses, revealed that the molecular biology technique was able to attribute somewhat the same relative C. jejuni concentrations to the samples analysed via qPCR. However, day 0 measurements from culture-based methods were associated with the absence of or negatively weak correlations with the rest of the time points, but ranking order was maintained from day 3 onwards. On the other hand, ranking order correlations were less constant when measuring total bacterial concentration through qPCR. The study suggests that if biological samples can’t be analysed as fresh (immediately after collection) and have to be stored prior to analysis, then storage at -80°C samples be recommended to avoid the temporal-dependent effects on C. jejuni concentrations. In addition, irrespective of the method of analysis, an initial loss of CFU must be factored in when interpreting the results obtained from frozen samples.

Infectivity and Transmissibility of Acute Hepatopancreatic Necrosis Disease Associated Vibrio parahaemolyticus in Frozen Shrimp Archived at −80 °C

Acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio parahaemolyticus (VpAHPND) has been reported in commodity shrimp, but the potential risk of its global spread via frozen shrimp in the shrimp trade is yet to be fully explored. We hypothesized that frozen shrimp with AHPND could be a source of VpAHPND transmission; thus, the infectivity of frozen shrimp with AHPND was evaluated using a shrimp bioassay. To prepare infected frozen shrimp, 12 Penaeus vannamei (average weight, 2 g) were exposed to VpAHPND by immersion in water with a VpAHPND concentration of 1.55 × 107 CFU mL−1; once dead, the shrimp were stored at −80 °C for further analysis. After two weeks, a PCR assay was used to confirm AHPND positivity in frozen shrimp (n = 2), and VpAHPND was reisolated from the hepatopancreases of these shrimp. For the infectivity test, 10 P. vannamei (average weight, 4 g) were fed with the hepatopancreases of VpAHPND -infected frozen shrimp (n = 10). After feeding, 70% of the shrimp died within 118 h, and the presence of VpAHPND was confirmed using a PCR assay and histopathology examination; moreover, VpAHPND was successfully reisolated from the hepatopancreases of the dead shrimp. We are the first to evaluate the potential transmissibility of VpAHPND in frozen shrimp, and our results suggest that frozen shrimp with AHPND are a potential source of disease spreading between countries during international trade.

Analyzing Consumer Perception on Quality and Safety of Frozen Foods in Emerging Economies: Evidence from Albania and Kosovo

Freezing technology is one of the most well established long-term preservation techniques for producing high-quality, nutritious foods with prolonged shelf-life. Frozen foods (FFs) are a significant section of the global food market experiencing rapid growth. It also represents an alternative to small producers in developing countries to add value to their products in a competitive market. However, unfairly, FFs are often perceived as less qualitative than fresh produce, although studies have shown that some FFs have higher nutritional values than fresh products. This study’s aim is to analyze consumers’ perceptions in the two Balkan countries towards FFs. A total of 380 questionnaires were completed in both countries (182 in Kosovo and 198 in Albania). Consumers’ perceptions towards FFs were measured through eleven items using a five-point Likert scale. The items addressed issues related to the quality and safety of FFs, information on FFs, and the impact of origin on the perception of FFs. The differences between populations were tested with the t-test and correlation analysis with the bootstrapping method for sociodemographic factors. The results show that Kosovo consumers generally show a higher positive attitude toward FFs than Albanian consumers. Albanian consumers prefer fresh foods over FFs. The lack of trust in food safety institutions was expressed with concern for the conditions of the frozen chain applied both on the imported and domestic frozen products. Similarly, the findings show that Albanian consumers are willing to pay more than the baseline price to obtain fresh products instead of frozen compared with Kosovo consumers. Additional studies are needed to explore whether the lack of trust in food safety institutions inhibits the successful development of FFs in Albania and Kosovo. In both countries, responsible authorities should help consumers to have a more profound knowledge of the quality of FFs and boost these activities to increase farmers’ incomes and play an active role in reducing food loss and waste.

Efficacy of freeze-chilled storage combined with tea polyphenol for controlling melanosis, quality deterioration, and spoilage bacterial growth of Pacific white shrimp (Litopenaeus vannamei)

This study aimed to investigate melanosis, quality attributes, and bacterial growth of freeze-chilled Pacific white shrimp (Litopenaeus vannamei) during 6 days of chilled storage, as well as the preservative effects of tea polyphenol on shrimp. The results showed that freeze-chilled storage retarded the growth of bacteria and the accumulation of putrescine in shrimp. The growth of spoilage bacteria Photobacterium and Shewanella were inhibited. However, freeze-chilled storage aggravated melanosis and lipid oxidation. The total volatile basic nitrogen (TVB-N) slightly accumulated in the thawed shrimp. The incorporation of tea polyphenol preserved freeze-chilled shrimp. Melanosis and lipid oxidation of shrimp were alleviated. The accumulation of biogenic amines, TVB-N, hypoxanthine riboside, and hypoxanthine were retarded. Meanwhile, the growth of spoilage bacteria Pseudoalteromonas, Photobacterium, Psychrobacter, and Carnobacterium were inhibited. Based on sensory analysis, the shelf-life of chilled, freeze-chilled, and freeze-chilled tea polyphenol shrimp were 4 days, 3 days, and 6 days, respectively.

Impact of Freezing on the Microbiological Quality and Physical Characteristics of Buffalo Mozzarella Cheese

Simple Summary

Buffalo Mozzarella cheese from Campania is made from the fresh milk of the Italian Mediterranean buffalo. In 1996, thanks to its distinctive characteristics (specific environmental conditions and production method), Regulation (EC) No. 1107/96 recognized it as a Protected Designation of Origin product. The limited availability of milk and the increased demand for buffalo mozzarella cheese, especially during the spring-summer period (when milk production is lowest), have induced the use of frozen milk or curd for its production. The aim of this research was to provide preliminary results about the effect of freezing on microbial communities of fresh buffalo milk, curd and Buffalo Mozzarella cheese, and on physical properties (whiteness, hardness, and oxidation state) of Buffalo Mozzarella cheese. The preliminary results obtained have allowed us to conclude that the freezing process if properly carried out, does not compromise the microbiological quality of the products but produces only slight changes of some physical properties.

Abstract

Buffalo Mozzarella cheese from Campania is one of the most worldwide appreciated Italian dairy products. The increased demand for buffalo dairy products and the limited availability of the finest buffalo milk has prompted the diffusion of illicit practices, such as the use of milk, curd, or other products that are frozen or bought at low cost. The aim of this research was to provide preliminary results about the trend of the microbial communities of buffalo milk, curd and Buffalo Mozzarella cheese, during freezing storage of eleven months. At the same time, the alterations of physical properties and the presence of the molecular marker “γ4-casein”, have been investigated. The results showed that freezing reduced the concentrations of the total bacterial count, Enterobacteriaceae, coliforms, Escherichia coli and yeasts in fresh milk and, the concentrations of the total bacterial count, coliforms, lactic acid bacteria and yeasts in mature curd. In the finished product, no notable decreases were observed, except for lactic acid bacteria. About the γ4-casein, no increase was observed in all matrices. These preliminary results allow us to conclude that the freezing process if properly carried out, does not compromise the microbiological quality and the physical properties of the Buffalo Mozzarella cheese.

Novel perspective on a conventional technique: Impact of ultra-low temperature on bacterial viability and protein extraction

Ultra-low temperature (ULT) storage of microbial biomass is routinely practiced in biological laboratories. However, there is very little insight regarding the effects of biomass storage at ULT and the structure of the cell envelope, on cell viability. Eventually, these aspects influence bacterial cell lysis which is one of the critical steps for biomolecular extraction, especially protein extraction. Therefore, we studied the effects of ULT-storage (-80°C) on three different bacterial platforms: Escherichia coli, Bacillus subtilis and the cyanobacterium Synechocystis sp. PCC 6803. By using a propidium iodide assay and a modified MTT assay we determined the impact of ULT storage on cellular viability. Subsequently, the protein extraction efficiency was determined by analyzing the amount of protein released following the storage. The results successfully established that longer the ULT-storage time lower is the cell viability and larger is the protein extraction efficiency. Interestingly, E. coli and B. subtilis exhibited significant reduction in cell viability over Synechocystis 6803. This indicates that the cell membrane structure and composition may play a major role on cell viability in ULT storage. Interestingly, E. coli exhibited concomitant increase in cell lysis efficiency resulting in a 4.5-fold increase (from 109 μg/ml of protein on day 0 to 464 μg/ml of protein on day 2) in the extracted protein titer following ULT storage. Furthermore, our investigations confirmed that the protein function, tested through the extraction of fluorescent proteins from cells stored at ULT, remained unaltered. These results established the plausibility of using ULT storage to improve protein extraction efficiency. Towards this, the impact of shorter ULT storage time was investigated to make the strategy more time efficient to be adopted into protocols. Interestingly, E. coli transformants expressing mCherry yielded 2.7-fold increase (93 μg/mL to 254 μg/mL) after 10 mins, while 4-fold increase (380 μg/mL) after 120 mins of ULT storage in the extracted soluble protein. We thereby substantiate that: (1) the storage time of bacterial cells in -80°C affect cell viability and can alter protein extraction efficiency; and (2) exercising a simple ULT-storage prior to bacterial cell lysis can improve the desired protein yield without impacting its function.

Guidance on date marking and related food information: part 2 (food information)

A risk‐based approach was used to develop guidance to be followed by food business operators (FBOs) when deciding on food information relating to storage conditions and/or time limits for consumption after opening a food package and thawing of frozen foods. After opening the package, contamination may occur, introducing new pathogens into the food and the intrinsic (e.g. pH and a_w), extrinsic (e.g. temperature and gas atmosphere) and implicit (e.g. interactions with competing background microbiota) factors may change, affecting microbiological food safety. Setting a time limit for consumption after opening the package (secondary shelf‐life) is complex in view of the many influencing factors and information gaps. A decision tree (DT) was developed to assist FBOs in deciding whether the time limit for consumption after opening, due to safety reasons, is potentially shorter than the initial ‘best before’ or ‘use by’ date of the product in its unopened package. For products where opening the package leads to a change of the type of pathogenic microorganisms present in the food and/or factors increasing their growth compared to the unopened product, a shorter time limit for consumption after opening would be appropriate. Freezing prevents the growth of pathogens, however, most pathogenic microorganisms may survive frozen storage, recover during thawing and then grow and/or produce toxins in the food, if conditions are favourable. Moreover, additional contamination may occur from hands, contact surfaces or contamination from other foods and utensils. Good practices for thawing should, from a food safety point of view, minimise growth of and contamination by pathogens between the food being thawed and other foods and/or contact surfaces, especially when removing the food from the package during thawing. Best practices for thawing foods are presented to support FBOs.

The effects of the novel home freezing system on microstructure, color, antioxidant activity, and microbiological properties of strawberries

In these days people are more interested in frozen foods, especially home freezing fruits and vegetables. In this study, the effects of a new developed quick freezer system for home-type refrigerator on the freezing rates and some quality properties of strawberries were investigated. The freezing cabinet (at -30 °C with 1.2 ms^-1 air) was designed and manufactured by Bosch und Siemens Hausgerate GmbH (Çerkezköy, Turkey) then the strawberrries were frozen in a novel quick freezer and compared with the samples frozen statically at classic home type refrigerator (at -18 °C without any air blown) via the freezing times at three different (bottom, middle and top) position. Microstructure, color, antioxidant content and microbiological quality during storage of 4 months at -25 °C compared. Strawberries reached -15 °C approximately 234 min shorter in the novel system. Freezing rates were determined as 0.32 cm h^-1 for the static freezer and 1.51 cm h^-1 for the quick freezer. Unlike the classic home-type refrigerator, quick freezing process took place in the novel home freezing system. SEM images showed that higher freezing rate in the novel system provides better protection in tissue structure and cell walls. The antioxidant activity of the strawberries frozen in the novel home freezing system was 8.96% higher after the freezing process. Brightness and redness of samples were protected better after quick freezing during storage. Nonetheless, no evident differences were observed in microbiological criteria.

The effects of active double-layered furcellaran/gelatin hydrolysate film system with Ala-Tyr peptide on fresh Atlantic mackerel stored at -18 °C

This is the first time that active double-layered furcellaran/gelatin hydrolysate films containing Ala-Tyr peptide were developed and characterised for their properties. Afterwards, films were used on Atlantic mackerel stored at -18 °C for 4 months and samples were analysed for changes in their microbiological quality, TVB-N, biogenic amine content, fatty acid composition and TBARS. Active films had higher TS (13.4 MPa) and lower WS (62.8%). The films showed no DPPH radical scavenging properties but high FRAP (6.6 mMol Trolox/mg). No significant effects on the oxidation of fish samples were observed with TBARS increasing from 12.04 to 22.50 mg/kg. Freezing successfully inhibited the growth of microorganisms and no differences in microbiological growth or biogenic amine formation were observed. However, the application of films inhibited the formation of TVB-N. Antimicrobiological properties of the film should be further investigated during storage of perishable food products at temperatures above 0 °C.

Survival of Methicillin-Resistant Staphylococcus aureus in Fish and Shrimp under Different Storage Conditions

ABSTRACT

Foods that are extensively handled during preparation and stored without refrigeration are often associated with staphylococcal food poisoning. This problem is more confounding when contaminating strains belong to the methicillin-resistant Staphylococcus aureus (MRSA) group. In this study, we investigated the survivability of MRSA in two seafood matrices under different storage conditions. MRSA was inoculated at 6 and 3 log CFU/g into all sample groups of peeled shrimp (Parapeneopsis stylifera) stored at -20°C, Bombay duck fish (Harpadon nehereus) stored in ice, and dried Bombay duck fish stored at 30 ± 2°C. The populations of MRSA in frozen peeled shrimp inoculated with MRSA at 6 log CFU/g were reduced by 1.52 log CFU/g, whereas in samples inoculated with 3 log CFU/g levels remained stable after 60 days of storage. In fresh Bombay duck fish inoculated with 6 log CFU/g and stored in ice for 18 days, MRSA levels decreased by 2.75 log CFU/g. In contrast, in fresh fish inoculated with 3 log CFU/g the total viable count increased by 3.02 log CFU/g over 16 days of ice storage. In dried fish stored at 30 ± 2°C, MRSA levels declined by 3.27 log CFU/g in samples inoculated with 6 log CFU/g and by 0.91 log CFU/g in samples inoculated with 3 log CFU/g. These results suggest that the survival of MRSA depends on the temperature of storage and the inoculum level. In our study, MRSA survival was higher when inoculated at 3 log CFU/g regardless of the seafood matrix and storage temperature.

HIGHLIGHTS

Salmonella in captive and wild Tasmanian devils (Sarcophilus harrisii) in Tasmania

Translocation of Tasmanian devils (Sarcophilus harrisii) is a common strategy for recovery of the species as carried out by the Save the Tasmanian Devil Program. Dasyurids including the endangered Tasmanian devil are well known to asymptomatically harbour the zoonotic bacteria Salmonella enterica in their intestinal tracts. Testing for Salmonella is a routine component of pretranslocation health testing, so a statewide microbiological survey of captive and wild devils was implemented in order to understand prevalence and common Salmonella serotypes, and inform decision-making when positive cultures are identified. This preliminary study identified a significantly higher proportion of Salmonella isolations in wild compared with captive devils. Mississippi and Typhimurium were the most common serotypes, followed by Lexington, Bovismorbificans, Kottbus and Amsterdam. Given the common finding of Salmonella in wild devils and the range of serotypes involved, in addition to numerous isolations in domestic species and humans, it is unlikely that the release of small numbers of captive devils to the wild in Tasmania poses a significant risk to the destination ecosystem. Ongoing monitoring of devils is required as the stress of acclimatisation could predispose devils to clinical disease. Appropriate personal protective attire is pertinent to protect personnel handling animals from this zoonotic infection.

Freezing point temperature is in favor of not-from-concentrate apple juice storage

The short storage period is still a problem hindering the promotion of not-from-concentrate (NFC) apple juice, despite the fact it possesses higher nutritional value and more attractive taste compared with its concentrated counterparts. In this study, we compared the effects of temperature range including room temperature (25°C), refrigerator temperature (4°C), freezing point temperature (-1.5°C), and frozen temperature (-18°C), respectively, on the quality of NFC apple juices during a long storage period (150 days). The results suggested that all the juices exhibited good safety during the storage, and the juice stored at -1.5°C possessed higher polyphenol contents, physicochemical properties, less color alteration, and less loss of aroma and taste than 25 and 4°C. Besides, although an exceedingly low temperature (-18°C) could greatly retard the juice deterioration, the loss of aroma and taste was significant. Overall, our results indicated that the NFC juice was most favored by storage at freezing point temperature (-1.5°C), with the highest similarity to the freshly squeezed apple juice.

Prevalence, Levels, and Viability of Salmonella in and on Raw Chicken Livers

We surveyed chicken livers from various sources for the presence and levels of Salmonella. The pathogen was recovered from 148 (59.4%) of 249 chicken livers purchased at retail stores in Delaware, New Jersey, and Pennsylvania over about a 9-month period. Positive samples harbored Salmonella at levels of 6.4 most probable number (MPN)/g to 2.4 log CFU/g. The percentage of Salmonella-positive livers purchased at retail outlets in New Jersey (72%, 59 of 82 livers) was significantly higher (P < 0.05) than the percentage for livers purchased in Delaware (48%, 36 of 75 livers); however, this percentage was not significantly different (P > 0.05) from that for livers purchased in Pennsylvania (57.6%, 53 of 92 livers). The pathogen was also recovered more often (P = 0.019) from livers that were packaged by retailers (81 of 121 livers, 66.9%) than from livers packaged directly by processors (67 of 128 livers, 52.3%). In related studies, 12 (5.8%) of 207 chicken livers harvested from birds on a research farm tested positive for Salmonella at levels of 0.4 to 2.2 MPN/g. The recovery rate of Salmonella was 4.4% (6 of 135 livers) from livers with the gall bladder attached and 8.3% (6 of 72 livers) from livers when the gall bladder was removed at harvest on the research farm. We also quantified the levels of a nine-strain cocktail (ca. 6.5 log CFU/g) of Salmonella strains inoculated externally onto or internally into livers both before and after extended cold storage. Storage for at least 2 days at 4°C or 15 days at -20°C resulted in a decrease of about 1.0 log CFU/g in pathogen levels. Given the relatively high recovery rate (ca. 6.0 to 60.0%) and high (possibly illness causing) levels (0.4 MPN/g to 2.4 CFU/g) of Salmonella associated with chicken livers in the present study, further interventions for processors are needed to lower the prevalence and levels of this pathogen on poultry liver.

Development of Freeze-Thaw Tolerant Lactobacillus rhamnosus GG by Adaptive Laboratory Evolution

The industrial application of microorganisms as starters or probiotics requires their preservation to assure viability and metabolic activity. Freezing is routinely used for this purpose, but the cold damage caused by ice crystal formation may result in severe decrease in microbial activity. In this study, adaptive laboratory evolution (ALE) technique was applied to a lactic acid bacterium to select tolerant strains against freezing and thawing stresses. Lactobacillus rhamnosus GG was subjected to freeze-thaw-growth (FTG) for 150 cycles with four replicates. After 150 cycles, FTG-evolved mutants showed improved fitness (survival rates), faster growth rate, and shortened lag phase than those of the ancestor. Genome sequencing analysis of two evolved mutants showed genetic variants at distant loci in six genes and one intergenic space. Loss-of-function mutations were thought to alter the structure of the microbial cell membrane (one insertion in cls), peptidoglycan (two missense mutations in dacA and murQ), and capsular polysaccharides (one missense mutation in wze), resulting in an increase in cellular fluidity. Consequently, L. rhamnosus GG was successfully evolved into stress-tolerant mutants using FTG-ALE in a concerted mode at distal loci of DNA. This study reports for the first time the functioning of dacA and murQ in freeze-thaw sensitivity of cells and demonstrates that simple treatment of ALE designed appropriately can lead to an intelligent genetic changes at multiple target genes in the host microbial cell.

Display stability of fresh and thawed lamb supplemented with vitamin E or sprayed with an antioxidant borage seed extract

BACKGROUND

The commercialization of thawed lamb packaged in modified atmosphere and maintained on display could serve as an alternative capable of satisfying the requirements of both customers and distributors. However, previous studies have suggested that lipid oxidation may accelerate post-thawing because peroxidation occurs during frozen storage, thereby leading to rapid and severe secondary lipid oxidation. The addition of an antioxidant compound either in the lamb diet or in the packaged meat could resolve this problem. Therefore, the present study aimed to compare the effect of dietary vitamin E (1000 mg of dl-α-tocopheryl acetate per kg of basal diet) and the spraying of borage seed aqueous extract (10% p/v) on the quality of fresh and thawed lamb leg chops.

RESULTS

Both borage extract and vitamin E improved colour (as measured via instrumental and visual assessment of colour) and lipid stability (thiobarbituric acid reactive substances) of fresh and thawed lamb throughout display, although neither of them had any antimicrobial effect. Freezing/thawing accelerated bone marrow darkening and reduced redness but delayed microbial growth.

CONCLUSION

Both of these antioxidant strategies would be very profitable for the preservation of lamb meat, allowing thawed meat packaged in a modified atmosphere to be commercialized. However, additional studies should be carried out to determine how bone darkening in thawed chops can be avoided. © 2017 Society of Chemical Industry.

Salmonella Cold Stress Response: Mechanisms and Occurrence in Foods

Since bacteria in foods often encounter various cold environments during food processing, such as chilling, cold chain distribution, and cold storage, lower temperatures can become a major stress environment for foodborne pathogens. Bacterial responses in stressful environments have been considered in the past, but now the importance of stress responses at the molecular level is becoming recognized. Documenting how bacterial changes occur at the molecular level may help to achieve the in-depth understanding of stress responses, to predict microbial fate when they encounter cold temperatures, and to design and develop more effective strategies to control pathogens in food for ensuring food safety. Microorganisms differ in responding to a sudden downshift in temperature and this, in turn, impacts their metabolic processes and can cause various structural modifications. In this review, the fundamental aspects of bacterial cold stress responses focused on cell membrane modification, DNA supercoiling modification, transcriptional and translational responses, cold-induced protein synthesis including CspA, CsdA, NusA, DnaA, RecA, RbfA, PNPase, KsgA, SrmB, trigger factors, and initiation factors are discussed. In this context, specific Salmonella responses to cold temperature including growth, injury, and survival and their physiological and genetic responses to cold environments with a focus on cross-protection, different gene expression levels, and virulence factors will be discussed.

Comparative Proteomic and Morphological Change Analyses of Staphylococcus aureus During Resuscitation From Prolonged Freezing

When frozen, Staphylococcus aureus survives in a sublethally injured state. However, S. aureus can recover at a suitable temperature, which poses a threat to food safety. To elucidate the resuscitation mechanism of freezing survived S. aureus, we used cells stored at -18°C for 90 days as controls. After resuscitating the survived cells at 37°C, the viable cell numbers were determined on tryptic soy agar with 0.6% yeast extract (TSAYE), and the non-injured-cell numbers were determined on TSAYE supplemented with 10% NaCl. The results showed that the total viable cell number did not increase within the first 3 h of resuscitation, but the osmotic regulation ability of freezing survived cells gradually recovered to the level of healthy cells, which was evidenced by the lack of difference between the two samples seen by differential cell enumeration. Scanning electron microscopy (SEM) showed that, compared to late exponential stage cells, some frozen survived cells underwent splitting and cell lysis due to deep distortion and membrane rupture. Transmission electron microscopy (TEM) showed that, in most of the frozen survived cells, the nucleoids (low electronic density area) were loose, and the cytoplasmic matrices (high electronic density area) were sparse. Additionally, a gap was seen to form between the cytoplasmic membranes and the cell walls in the frozen survived cells. The morphological changes were restored when the survived cells were resuscitated at 37°C. We also analyzed the differential proteome after resuscitation using non-labeled high-performance liquid chromatography–mass spectrometry (HPLC-MS). The results showed that, compared with freezing survived S. aureus cells, the cells resuscitated for 1 h had 45 upregulated and 73 downregulated proteins. The differentially expressed proteins were functionally categorized by gene ontology enrichment, KEGG pathway, and STRING analyses. Cell membrane synthesis-related proteins, oxidative stress resistance-related proteins, metabolism-related proteins, and virulence factors exhibited distinct expression patterns during resuscitation. These findings have implications in the understanding of the resuscitation mechanism of freezing survived S. aureus, which may facilitate the development of novel technologies for improved detection and control of foodborne pathogens in frozen food.

Recent developments in novel freezing and thawing technologies applied to foods

This article reviews the recent developments in novel freezing and thawing technologies applied to foods. These novel technologies improve the quality of frozen and thawed foods and are energy efficient. The novel technologies applied to freezing include pulsed electric field pre-treatment, ultra-low temperature, ultra-rapid freezing, ultra-high pressure and ultrasound. The novel technologies applied to thawing include ultra-high pressure, ultrasound, high voltage electrostatic field (HVEF), and radio frequency. Ultra-low temperature and ultra-rapid freezing promote the formation and uniform distribution of small ice crystals throughout frozen foods. Ultra-high pressure and ultrasound assisted freezing are non-thermal methods and shorten the freezing time and improve product quality. Ultra-high pressure and HVEF thawing generate high heat transfer rates and accelerate the thawing process. Ultrasound and radio frequency thawing can facilitate thawing process by volumetrically generating heat within frozen foods. It is anticipated that these novel technologies will be increasingly used in food industries in the future.

Growth of Stressed Strains of Four Non-O157 Shiga Toxin-Producing Escherichia coli Serogroups in Five Enrichment Broths

The purpose of this study was to evaluate (i) the behavior of several strains of non-O157 Shiga toxin-producing Escherichia coli (STEC) serogroups (O26, O103, O111, and O145) exposed to different stress conditions and (ii) the growth dynamics of stressed and nonstressed non-O157 STEC cells in five enrichment media. STEC strains were exposed to acid, cold, and freeze stresses. Lethal and sublethal injuries were determined by plating in parallel on selective and nonselective agar media. Freeze stress (8 days, 20°C) caused the most lethal (95.3% ± 2.5%) injury, as well as the most sublethal (89.1% ± 8.8%) injury in the surviving population. Growth of stressed and nonstressed pure cultures of non-O157 STEC on modified tryptic soy broth, buffered peptone water (BPW), BPW with sodium pyruvate, Brila, and STEC enrichment broth (SEB) was determined using total viable counts. To compare growth capacities, growth after 7 and 24 h of enrichment was measured; lag phases and maximum growth rates were also calculated. In general, growth on BPW resulted in a short lag phase followed by a high maximum growth rate during the enrichment of all tested strains when using all three stress types. Furthermore, BPW ensured the highest STEC count after 7 h of growth. Supplementing the medium with sodium pyruvate did not improve the growth dynamics. The two selective media, Brila and SEB, were less efficient than BPW, but Brila's enrichment performance was remarkably better than that of SEB. This study shows that irrespective of the effect of background flora, BPW is still recommended for resuscitation of non-O157 STEC.

Growth potential of exponential- and stationary-phase Salmonella Typhimurium during sausage fermentation

Raw meat for sausage production can be contaminated with Salmonella. For technical reasons, meat is often frozen prior to mincing but it is unknown how growth of Salmonella in meat prior to freezing affects its growth potential during sausage fermentation. We investigated survival of exponential- and stationary-phase Salmonella Typhimurium (DT12 and DTU292) during freezing at -18°C and their subsequent growth potential during 72h sausage fermentation at 25°C. After 0, 7 and >35d of frozen storage, sausage batters were prepared with NaCl (3%) and NaNO2 (0, 100ppm) and fermented with and without starter culture. With no starter culture, both strains grew in both growth phases. In general, a functional starter culture abolished S. Typhimurium growth independent of growth phase and we concluded that ensuring correct fermentation is important for sausage safety. However, despite efficient fermentation, sporadic growth of exponential-phase cells of S. Typhimurium was observed drawing attention to the handling and storage of sausage meat.

Detection of viable Salmonella in ice cream by TaqMan real-time polymerase chain reaction assay combining propidium monoazide

Real-time polymerase chain reaction (PCR) allows rapid detection of Salmonella in frozen dairy products, but it might cause a false positive detection result because it might amplify DNA from dead target cells as well. In this study, Salmonella-free frozen ice cream was initially inoculated with heat-killed Salmonella Typhimurium cells and stored at −18°C. Bacterial DNA extracted from the sample was amplified using TaqMan probe-based real-time PCR targeting the invA gene. Our results indicated that DNA from the dead cells remained stable in frozen ice cream for at least 20 days, and could produce fluorescence signal for real-time PCR as well. To overcome this limitation, propidium monoazide (PMA) was combined with real-time PCR. PMA treatment can effectively prevent PCR amplification from heat-killed Salmonella cells in frozen ice cream. The PMA real-time PCR assay can selectively detect viable Salmonella at as low as 10³ CFU/mL. Combining 18 hours of pre-enrichment with the assay allows for the detection of viable Salmonella at 10⁰ CFU/mL and avoiding the false-positive result of dead cells. The PMA real-time PCR assay provides an alternative specifically for detection of viable Salmonella in ice cream. However, when the PMA real-time PCR assay was evaluated in ice cream subjected to frozen storage, it obviously underestimated the contamination situation of viable Salmonella, which might lead to a false negative result. According to this result, the use of enrichment prior to PMA real-time PCR analysis remains as the more appropriate approach.

Assessing the survival of Listeria monocytogenes in a domestic freezer by analyzing subsequent growth at 30°C using a novel reference method

Listeria monocytogenes is a serious pathogen capable of extensive survival under frozen storage. Using optical density and multiple initial inocula in multiple identically prepared microtiter plates, the effect of storage time at -22°C on the subsequent growth at 30°C of the organism when defrosted was studied using a technique that compared the growth (through time to detection) of a test plate (previously frozen) with that of an identically prepared control plate, analyzed at the start of the experiment. Experiments were carried out using tryptic soy broth (TSB) or TSB supplemented with 3% salt. Plates were stored and frozen for up to 6 months (10 days, 20 days, 2 months, and 6 months). As storage time increased, there was only a small relative increase in the lag and the variance in the time to detection observed. When compared with storage in 3% salt TSB, which reduced the specific growth rate relative to growth in standard TSB, there were only marginally greater increases in lag and data variance. After 6 months storage in 3% salt TSB, there were some indications of inactivation (observed as small reductions of the initial optical density (equal to 1 × 10(9) CFU/ml) equivalent to a 50% inactivation. The method and the analyses suggest that this technique could allow easy examination of the effect of frozen storage on given cultures, with respect to the effects of pH, water activity, and also the effect of preservatives commonly used as extra hurdles in foods.

Effect of temperature on the survival of Alaria alata mesocercariae

Recent findings of Alaria alata mesocercariae in wild boars and other animals in Europe reinforced the concern about the public health risk posed by this parasite especially if the game meat is insufficiently heated during preparation. Cooking and freezing are effective methods for the inactivation of parasites in meat whereas refrigeration is considered as an essential part of the Good Hygiene Practice. Additionally, microwave dielectric heating may represent an equally effective tool for parasite inactivation. Therefore, isolated vital mesocercariae were examined with respect to their resilience against heating, refrigeration, freezing, and microwave heating. A. alata mesocercariae stored in Ringer's solution do not survive heating temperatures that exceed 60.0 °C. Similarly, exposure to microwave heating ensured an inactivation of all parasite developmental stages after 90 s of treatment. In contrast, the parasites' tolerance towards cold is far higher as the mesocercariae survived refrigeration temperatures (4.0 ± 2 °C) in Ringer's solution for up to 13 days. An effective inactivation by cold is therefore only guaranteed if the infested game meat is frozen to a core temperature of -13.7 °C for a minimum of 2 h at least. Game meat should be handled with the same or even higher caution than meat of husbandry animals since wild animals may be infected with parasites or other zoonotic agents that are not common in livestock. It is therefore of crucial importance that appropriate temperature time protocols are used for the reliable inactivation of these zoonotic agents.

Pressure-Inactivated Virus: A Promising Alternative for Vaccine Production

In recent years, many applications in diverse scientific fields with various purposes have examined pressure as a thermodynamic parameter. Pressure studies on viruses have direct biotechnological applications. Currently, most studies that involve viral inactivation by HHP are found in the area of food engineering and focus on the inactivation of foodborne viruses. Nevertheless, studies of viral inactivation for other purposes have also been conducted. HHP has been shown to be efficient in the inactivation of many viruses of clinical importance and the use of HHP approach has been proposed for the development of animal and human vaccines. Several studies have demonstrated that pressure can result in virus inactivation while preserving immunogenic properties. Viruses contain several components that can be susceptible to the effects of pressure. HHP has been a valuable tool for assessing viral structure function relationships because the viral structure is highly dependent on protein-protein interactions. In the case of small icosahedral viruses, incremental increases in pressure produce a progressive decrease in the folding structure when moving from assembled capsids to ribonucleoprotein intermediates (in RNA viruses), free dissociated units (dimers and/or monomers) and denatured monomers. High pressure inactivates enveloped viruses by trapping their particles in a fusion-like intermediate state. The fusogenic state, which is characterized by a smaller viral volume, is the final conformation promoted by HHP, in contrast with the metastable native state, which is characterized by a larger volume. The combined effects of high pressure with other factors, such as low or subzero temperature, pH and agents in sub-denaturing conditions (urea), have been a formidable tool in the assessment of the component's structure, as well as pathogen inactivation. HHP is a technology for the production of inactivated vaccines that are free of chemicals, safe and capable of inducing strong humoral and cellular immune responses. Here we present a current overview about the pressure-induced viral inactivation and the production of inactivated viral vaccines.

Inactivation and sublethal injury kinetics of Staphylococcus aureus in broth at low temperature storage

Low temperatures are widely used to ensure food quality and safety. However, sublethally injured Staphylococcus aureus is an important microbiological safety concern in low temperature food. The objective of this study was to develop predictive inactivation kinetic models for the inactivation and sublethal injury of S. aureus in broth at different temperatures (4 to -18°C) and time points. S. aureus was diluted in tryptic soy broth plus 0.6% (wt/vol) yeast extract (TSBYE) to obtain approximately 10(8) CFU/ml and was stored separately at 4, -3, -11, and -18°C. After specific time points within 96 days, survival of S. aureus was determined on TSBYE and TSBYE agar plus 10% NaCl for enumeration of the total viable and noninjured cell numbers, respectively. Linear, Weibull, and modified Gompertz models were applied to determine survival curve regression. The combination of low temperature and time resulted in S. aureus inactivation, although the cells were able to survive in this sublethal state. Storage temperature was the critical parameter in survival of S. aureus. The modified Weibull model successfully described a second model of noninjured S. aureus cell survival at different low temperatures, whereas only the linear model was able to fit the total viable cells. The predictive model may be used to estimate the level of S. aureus contamination in food at low storage temperatures and times, and it provides new insight into the sublethally injured survival state of S. aureus in low temperature food.

Improving ice nucleation activity of zein film through layer-by-layer deposition of extracellular ice nucleators

Zein protein has been of scientific interest in the development of biodegradable functional food packaging. This study aimed at developing a novel zein-based biopolymer film with ice nucleation activity through layer-by-layer deposition of biogenic ice nucleators, that is, extracellular ice nucleators (ECINs) isolated from Erwinia herbicola , onto zein film surface. The adsorption behaviors and mechanisms were investigated using quartz crystal microbalance with dissipation monitoring (QCM-D). On unmodified zein surface, the highest ECINs adsorption occurred at pH 5.0; on UV/ozone treated zein surface followed by deposition of poly(diallyldimethylammonium chloride) (PDADMAC) layer, the optimum condition for ECINs adsorption occurred at pH 7.0 and I 0.05 M, where the amount of ECINs adsorbed was also higher than that on unmodified zein surface. QCM-D analyses further revealed a two-step adsorption process on unmodified zein surfaces, compared to a one-step adsorption process on PDADMAC-modified zein surface. Also, significantly, in order to quantify the ice nucleation activity of ECINs-coated zein films, an empirical method was developed to correlate the number of ice nucleators with the ice nucleation temperature measured by differential scanning calorimetry. Calculated using this empirical method, the highest ice nucleation activity of ECINs on ECINs-modified zein film reached 64.1 units/mm(2), which was able to elevate the ice nucleation temperature of distilled water from -15.5 °C to -7.3 °C.

Validation of a predictive model for survival and growth of Salmonella typhimurium DT104 on chicken skin for extrapolation to a previous history of frozen storage

The U.S. Department of Agriculture's tertiary Pathogen Modeling Program (PMP) model for survival and growth of Salmonella enterica ser. Typhimurium definitive type 104 (DT104) on chicken skin stored for 0 to 8 h at 5 to 50°C was evaluated for its ability to predict survival and growth of the same organism on chicken skin after frozen storage for 6 days at -20°C. Experimental design and methods used to collect data for model development (dependent data) were the same as those used to collect data for survival and growth after frozen storage (independent data for extrapolation). This was done to provide a valid comparison of observed and predicted values. The model was classified as providing acceptable predictions of the test data when the proportion of residuals in an acceptable prediction zone (pAPZ) from -1 log (fail-safe) to 0.5 log (fail-dangerous) was ≥0.7. The pAPZ for dependent data, independent data for interpolation, and independent data for extrapolation to a new independent variable of previous frozen storage were all acceptable (pAPZ ≥0.7), with the exception of the pAPZ for dependent data at 50°C, where an unacceptable pAPZ of 0.625 was obtained. Although a majority of observed log counts were less than predicted log counts, indicating that frozen storage of chicken skin for 6 days at -20°C had injured some Salmonella Typhimurium DT104, the injury was not large enough to cause the tertiary PMP model to provide unacceptable predictions. Thus, it was concluded that the tertiary PMP model provided valid predictions of survival and growth of Salmonella Typhimurium DT104 on chicken skin that had a previous history of frozen storage for 6 days at -20°C. Additional research is needed to determine how broadly the model can be applied to other conditions of previous frozen storage.