Cold-smoked Rainbow Trout Research Articles

Effectiveness of High-Pressure Processing Treatment for Inactivation of Listeria monocytogenes in Cold-Smoked and Warm-Smoked Rainbow Trout

High-pressure processing (HPP) is a promising method for preserving food, for example by inactivating pathogens and spoilage microbes. However, there is still a lack of knowledge about the optimal processing parameters for different food products. The aim of this study was to assess the effectiveness of different pressures to inactivate Listeria monocytogenes in cold-smoked and warm-smoked rainbow trout (Oncorhynchus mykiss) ready-to-eat (RTE) fish products. L. monocytogenes ATCC 7644 was inoculated into sliced cold-smoked rainbow trout fillets and whole warm-smoked rainbow trout fillets. The fish samples were pressure-treated at three different pressures, either at 200, 400, or 600 MPa, at 4 ± 1 °C for 3 min in each pressure. Bacterial enumeration of the samples and control samples were analysed 1, 14, and 28 days after the HPP treatment by using serial dilution and the spread plate technique. Based on the study results, the most effective pressure was 600 MPa and the number of L. monocytogenes colonies, both in cold-smoked and in warm-smoked fish samples, was within the official limit of the Regulation (EC) No. 2073/2005 (100 cfu/g) even after 28 days of storage.

Developing and Improving the Production Technology of Canned Cold Smoked Rainbow Trout Fillet

Fish and fish products are foods of high nutritional value. The current research aimed at improving the technology of manufacturing canned products from cold-smoked rainbow trout fillets has a certain scientific-practical significance. Physicochemical indicators of the mentioned product were studied in the experimental and control samples. It has been revealed that energy value of the product is higher in the experimental sample than in control one, due to the content of proteins and fats. HACCP system research has indicated that safety and biological indicators of the finished product meet the accepted standards. Therefore, the product is safe and can be offered to consumers.

Activity of smoke wood condensates against Aeromonas hydrophila and Listeria monocytogenes in vacuum-packaged, cold-smoked rainbow trout stored at 4 °C

The antimicrobial effect of four smoke wood condensates against the psychrotrophic pathogens Listeria monocytogenes and Aeromonas hydrophila was tested in vacuum-packed cold-smoked rainbow trout ( Oncorhynchus mykiss) stored at refrigeration temperature. Salted trout samples were surface inoculated with L. monocytogenes or A. hydrophila, cold smoked, vacuum packaged and stored at 4 °C for 21 days. At different times of storage, samples were taken and analysed for L. monocytogenes and A. hydrophila by quantitative and qualitative procedures. All smoke extracts showed activity against A. hydrophila. However, only smoke extracts L1 and L2 were effective against L. monocytogenes. Although these extracts caused a reduction in viable cell counts of L. monocytogenes below the detectable level by surface plating procedure, this organism was detected by qualitative analysis in most of the samples. Finally, extracts L1 and L2 also reduced total viable cell counts to undetectable levels.

Synergistic inhibition of Listeria monocytogenes on cold-smoked rainbow trout by nisin and sodium lactate

The inhibition of Listeria monocytogenes and mesophilic aerobic bacteria in cold-smoked rainbow trout by nisin, sodium lactate or their combination was studied. Nisin (4000–6000 IU/ml), sodium lactate (60%) or their combination (1:1) were injected into rainbow trout at an industrial scale before the smoking process, or injected into the finished smoked product. Both types of fish samples were smoked, sliced and vacuum-packed according to normal practice in the plant. Packages were opened and L. monocytogenes was inoculated (10 3–10 4 log colony forming units (cfu)/g) onto the fish samples, which were then vacuum packed again. Samples were stored at 8°C for 17 days or at 3°C for 29 days. Listeria and mesophilic aerobic bacteria counts were measured once a week. The effects of treatments on sensory characteristics and storage stability were also analyzed. Both nisin and lactate inhibited the growth of L. monocytogenes in smoked fish, but the combination of the two compounds was even more effective. The combination of nisin and sodium lactate injected into smoked fish decreased the count of L. monocytogenes from 3.26 to 1.8 log cfu/g over 16 days of storage at 8°C. The level of L. monocytogenes remained almost constant (4.66–4.92 log cfu/g) for 29 days at 3°C in the samples injected before smoking and which contained both nisin and sodium lactate. The treatments did not affect the sensory characteristics of cold-smoked rainbow trout. Based on a triangle test, the sensory quality of all test samples remained unchanged for 23 days of storage at 3°C, whereas the control fish prepared without additives or additional salt remained unchanged only for 16 days.

Characterisation of lactic acid bacteria from spoiled, vacuum-packaged, cold-smoked rainbow trout using ribotyping

A total of 405 lactic acid bacteria (LAB) isolated from spoiled, vacuum-packaged, salted, sodium nitrite- or potassium nitrate-treated, cold-smoked rainbow trout stored at 4°C or 8°C were characterised and identified using a molecular method. The isolates were initially classified according to their restriction endonuclease profiles using HindIII and EcoRI restriction endonucleases and further characterised by rRNA gene restriction patterns (ribotypes). Numerical analysis of these ribopatterns was performed together with 19 reference LAB strain patterns in order to identify the isolates to species level. The strains were divided with HindIII and EcoRI ribopatterns into ten and nine clusters at the similarity level of 65% and 50%, respectively. The Leuconostoc-clusters and the Lb. sakei/Lb. curvatus-clusters formed the two main groups. Only one isolate was identified as Lactobacillus plantarum and no Carnobacterium strains were discovered. For both enzymes, the 35 isolates possessing six individual ribotypes and forming five clusters could not be identified further with the reference strains used. The relative proportion of Leuconostoc mesenteroides subsp. mesenteroides was higher in all samples stored at 4°C. Most of the Leuconostoc citreum were found in the samples stored at 8°C, and particularly in the nitrite-treated samples.

Molecular epidemiology of an outbreak of febrile gastroenteritis caused by Listeria monocytogenes in cold-smoked rainbow trout.

Febrile gastroenteritis in five healthy persons was associated with the consumption of vacuum-packed cold-smoked rainbow trout containing Listeria monocytogenes. L. monocytogenes isolates from the incriminated fish product lot and the stool samples were all of serotype 1/2a and were indistinguishable by pulsed-field gel electrophoresis employing AscI and SmaI.

Predicted and observed growth and toxigenesis by Clostridium botulinum type E in vacuum-packaged fishery product challenge tests

The observed growth and toxigenesis by Clostridium botulinum type E in vacuum-packaged unprocessed, raw pickled and cold-smoked rainbow trout stored at slightly abusive temperatures were compared to predictions generated by two currently available predictive microbiological programs, Food MicroModel and Pathogen Modelling Program. In unprocessed fish there was only a 2 log increase in type E cell count at the time the toxicity first occurred after 2 weeks storage at 8°C. Neither growth or toxin production was observed in raw pickled fish with a NaCl concentration of 6.7% (w/v) during 6 weeks storage at 6°C. In cold-smoked fish with a NaCl level of 3.2% (w/v) toxic samples were detected after 3 and 4 weeks storage at 8°C and 4°C, respectively, without any increase in type E count. Both models were hampered by limitations to controlling environmental factors set by the programs which also had an adverse effect on the reliability of predictions. Most predictions generated by the models were inconsistent with the results observed in the challenge studies. In certain situations, the models seemed to be `fail-safe', in that, the growth rate predicted from the model was faster or a predicted time to toxicity shorter than that which actually occurred in the food. In other situations, the predictions showed the product to be safe when it was not. The results demonstrate the need for further development and rigorous validation of the models before they are accepted for wider use by inspecting officials and the food industry.

Applicability of lactic acid and nisin to improve the microbiological quality of cold-smoked rainbow trout

The effect of lactic acid and nisin whey permeate on the microbiological and sensory quality of cold-smoked rainbow trout was studied. Lactic acid and whey permeate were mixed with salt solution and injected into fish fillets in concentrations of up to 2.5 g/kg lactic acid, 30 g/kg whey permeate and 20 g/kg sodium chloride. After smoking at 28 °C for 6 h the fillets were sliced, vacuum packed and stored at 3 °C. The aerobic and anaerobic/facultative anaerobic counts were measured after 1, 8, 15, 22 and 29 days of storage. The influence of the treatment on the sensory characteristics was analysed after 8 and 22 days by sensory profiling. A triangle test was used in order to ascertain at which stage the sensory quality of the samples changed during storage. Bacterial counts of smoked fish stored for 29 days were lowest in those samples containing a combination of lactic acid, sodium chloride and nisin whey permeate. Lactic acid alone did not affect the odour or flavour characteristics of the fish but did affect their texture as determined by sensory evaluation making it less elastic when stored at +3 °C for 8 days. A combination of lactic acid and nisin whey permeate enhanced the characteristic fishy flavour of the product. Differences between treatments in elasticity and fishy flavour were not detected after 22 days of storage. According to the triangle test, the samples from all treatments remained unchanged over the first 15 days.

Sources of Listeria monocytogenes contamination in a cold-smoked rainbow trout processing plant detected by pulsed-field gel electrophoresis typing.

Sites of Listeria monocytogenes contamination in a cold-smoked rainbow trout (Oncorhynchus mykiss) processing plant were detected by sampling the production line, environment, and fish at different production stages. Two lots were monitored. The frequency of raw fish samples containing L. monocytogenes was low. During processing, the frequency of fish contaminated with L. monocytogenes clearly rose after brining, and the most contaminated sites of the processing plant were the brining and postbrining areas. A total of 303 isolates from the raw fish, product, and the environment were characterized by pulsed-field gel electrophoresis (PFGE). PFGE yielded nine pulsotypes, which formed four clusters. The predominating L. monocytogenes pulsotypes of the final product were associated with brining and slicing, whereas contaminants of raw fish were not detected in the final product. Air-mediated contamination in the plant could not be proved. In accordance with these results, an L. monocytogenes eradication program was planned. The use of hot steam, hot air, and hot water seemed to be useful in eliminating L. monocytogenes. None of the control samples taken in the 5 months after the eradication program was implemented contained L. monocytogenes.

The spoilage flora of vacuum-packaged, sodium nitrite or potassium nitrate treated, cold-smoked rainbow trout stored at 4°C or 8°C

The spoilage flora of vacuum-packaged, salted, cold-smoked rainbow trout fillets, with or without the addition of nitrate or nitrite, stored at 4°C and 8°C, was studied. Of 620 isolates, lactic acid bacteria were the major fraction (76%), predominating in all samples of spoiled product. However, the phenotypical tests used were insufficient to identify the lactic acid bacteria to the species level. Gram-positive, catalase-positive cocci, Gram-negative, oxidase-negative rods and Gram-negative, oxidase-positive rods were found in 6%, 16% and 2% of the samples, respectively. Of 39 Gram-positive, catalase-positive cocci, 29 were identified as staphylococci and 10 as micrococci. Eighty-five isolates were found to belong to the family Enterobacteriaceae, with 45 of those being Serratia plymuthica. Eleven isolates from the nitrate treated samples stored at 8°C were identified as Pseudomonas aeruginosa. The occurrence of P. aeruginosa and staphylococci in the nitrate-containing samples, stored at 8°C, may cause problems with respect to the safety of the product. The types of lactic acid and other bacteria in the spoilage flora were generally reduced by the addition of nitrate or nitrite to fillets.

An outbreak of listeriosis suspected to have been caused by rainbow trout.

An outbreak of listeriosis in Sweden, consisting of nine cases, was investigated by means of molecular typing of strains from patients and strains isolated from suspected foodstuffs, together with interviews of the patients. Listeria monocytogenes was isolated from six of the patients, and all isolates were of the same clonal type. This clonal type was also isolated from a "gravad" rainbow trout, made by producer Y, found in the refrigerator of one of the patients. Unopened packages obtained from producer Y were also found to contain the same clonal type of L. monocytogenes. Based on the interview results and the bacteriological typing, we suspect that at least six of the nine cases were caused by gravad or cold-smoked rainbow trout made by producer Y. To our knowledge, this is the first rainbow trout-borne outbreak of listeriosis ever reported.

Sodium nitrite and potassium nitrate in control of nonproteolytic Clostridium botulinum outgrowth and toxigenesis in vacuum-packed cold-smoked rainbow trout

The effect of sodium nitrite (NaNO 2) and potassium nitrate (KNO 3) on the outgrowth and toxigenesis of nonproteolytic Clostridium botulinum in vacuum-packed cold-smoked rainbow trout stored for six weeks was studied in two inoculation studies at slightly abusive storage temperatures of 4 °C and 8 °C. The depletion rate of nitrite and the reduction rate of nitrate to nitrite as well as the effect of nitrite and nitrate on the shelf-life of the product during eight weeks' storage period were also determined. The nitrite concentrations were reduced from 166 mg kg ± 9 (mean ± SE), to a final concentration of 34 mg kg ± 2 and 11 mg kg ± 2, and the nitrate concentrations from 686 mg kg ± 67 to 465 mg kg ± 140 and 427 mg kg ± 33 at 4 ° C and 8 ° C respectively. The nitrite depletion rate was more rapid at 8 °C; nitrate depletion was not significantly affected by temperature. A considerable amount of nitrite was detected in the nitrate-treated samples in the latter half of the storage period. At 4 °C the aerobic plate counts were significantly lower in the samples treated with NaNO 2 + NaCl and with KNO 3 + NaCl as compared to the NaCl-treated controls, while at 8 °C the differences were smaller. The sensorial shelf-life of the product was considerably extended by nitrite and nitrate curing. The nitrite and nitrate concentrations used in the present study did not completely inhibit the toxigenesis of nonproteolytic C. botulinum during the six-week storage period, although the number of toxic samples was considerably reduced by nitrite and nitrate curing.