Jette Nielsen

Methods to evaluate fish freshness in research and industry

Current work in a European concerted action project ‘Evaluation of Fish Freshness’ (AIR3 CT94-2283) focuses on harmonizing research activities in the area of fish freshness evaluation in leading fish laboratories in Europe (see Box 1). The overall aim of the concerted action project is to validate methods for the assessment of fish freshness and to discuss the freshness criteria for fish commercialized within the European Union. The projects participants are working in subgroups studying sensory analysis, microbiology, volatile compounds, proteins, lipids, adenosine triphosphate and physical measurements with respect to fish freshness evaluation. In this article, the different subgroups have summarized changes that occur in fish and methods to evaluate fish freshness as a first step towards the definition of criteria for fish freshness.

Thawed cod fillets spoil less rapidly than unfrozen fillets when stored under modified atmosphere at 2°C

The effect of two months of frozen storage at -20°C on the spoilage characteristics and shelf life of thawed and modified atmosphere packed (MAP) cod fillets stored at 2°C was studied. Thawed MAP cod fillets were compared with fresh cod fillets stored in CO2-containing modified atmospheres with and without added oxygen. The shelf life of 11 to 12 days in the fresh MAP cod was extended to more than 20 days in the thawed MAP cod at 2°C. This shelf life extension was most likely due to the inactivation of the spoilage bacterium Photobacterium phosphoreum during frozen storage as reflected both in Chemical analyses and sensory evaluation. In contrast to fresh MAP cod fillets no significant production of trimethylamine occurred and almost no amine odor and taste were detected during 20 days of chill storage of thawed MAP cod fillets. The use of frozen fillets as raw material not only provides a more stable product in MAP but also allows much greater flexibility for production and distribution. However, a slightly increased concentration of dimethylamine, a larger drip loss, and detection of weak frozen storage flavor were observed in the thawed MAP cod fillets.

'Eating Quality' of Fish- A Review

Abstract The quality of fish is a very complex concept. Quality is frequently described using terms related to nutritional, microbiological, biochemical and physiochemical characteristics alone, but none of these terms serve as adequate indices of quality-sensory perception and consumer acceptability must be included. This paper discusses some of the sensory methods that can be used for fish as objective markers of quality in the chain from catch to consumer with focus on methods used for evaluation of whole raw fish and suggest methods and systems that can build a bridge between research and development, industry, marketing and consumer.

Low-temperature transitions in cod and tuna determined by differential scanning calorimetry

Differential scanning calorimetry measurements have revealed different thermal transitions in cod and tuna samples. Transition temperatures detected at −11°C, −15°C and −21°C were highly dependent on the annealing temperature. In tuna muscle an additional transition was observed at −72°C. This transition appeared differently than the thermal events observed at higher temperatures, as it spanned a broad temperature interval of 25°C. The transition was comparable to low-temperature glass transitions reported in protein-rich systems. No transition at this low temperature was detected in cod samples. The transitions observed at higher temperatures (−11°C to −21°C) may possibly stem from a glassy matrix containing muscle proteins. However, the presence of a glass transition at −11°C was in disagreement with the low storage stability at −18°C during practical time scales. It was proposed that freezing of cod could be associated with more than one glass transition, with a glass transition at a temperature lower than −11°C being too small to be detectable with instrument, yet governing important deterioration processes. In order to optimize frozen storage conditions, the relationship between deterioration processes important for preservation of quality and glass transition temperatures still needs to be established.

Aqueous solutions of proline and NaCl studied by differential scanning calorimetry at subzero temperatures

Abstract The hydration properties of proline are studied by differential scanning calorimetry (DSC) in aqueous solutions during freezing to −60°C and subsequent heating to +20°C. The concentration of proline in the freeze concentrated solution was estimated to approximately 50 wt% (w/w) indicating a high water solubility of proline at subzero temperatures. No glass transition was observed within the concentration range 0.9–40.1 wt% (w/w), neither at a low scanning rate of 2.5°C/min nor at a higher scanning rate of 10°C/min. Eutectic crystallization of proline was not observed during freezing or melting which shows that proline has the ability to stay in solution at subzero temperatures. Samples containing proline-NaCl-water were also investigated by DSC and it was shown that the solubility of proline is maintained in aqueous salt solutions at temperatures as low as −60°C. From DSC measurements it was found that the eutectic crystallization of NaCl is prevented by the presence of proline, even when NaCl (initially) is present in molar excess ( [ NaCl ] [ proline ] = 2.6 ). The possible association of these findings with the occurrence of proline accumulation in some plants and insects living under water stress conditions is discussed.

Prediction of chemical, physical and sensory data from process parameters for frozen cod using multivariate analysis

Physical, chemical and sensory quality parameters were determined for 115 cod (Gadus morhua) samples stored under varying frozen storage conditions. Five different process parameters (period of frozen storage, frozen storage temperature, place of catch, season for catching and state of rigor) were varied systematically at two levels. The data obtained were evaluated using the multivariate methods, principal component analysis (PCA) and partial least squares (PLS) regression. The PCA models were used to identify which process parameters were actually most important for the quality of the frozen cod. PLS models that were able to predict the physical, chemical and sensory quality parameters from the process parameters of the frozen raw material were generated. The prediction abilities of the PLS models were good enough to give reasonable results even when the process parameters were characterised by ones and zeroes only. These results illustrate the application of multivariate analysis as an effective strategy for improving the quality of frozen fish products.

The fish industry - toward supply chain modeling.

Mathematical models for simulating and optimizing aspects of supply chains such as distribution, planning, and optimal handling of raw materials are widely used. However, modeling based on a holistic chain view including several or all supply chain agents is less studied, and food-related aspects such as quality and shelf-life issues enforce additional requirements onto the chains. In this article, we consider the supply chain structure of the fish industry. We discuss and illustrate the potential of using mathematical models to identify quality and value-adding activities. The article provides a first step toward innovative supply chain modeling aimed to identify benefits for all agents along chains in the fish industry.

Changes in liquid-holding capacity, water distribution and microstructure during chill storage of smoked salmon.

BACKGROUND During recent years, increases in liquid loss and soft texture in cold smoked salmon have been reported, possibly due to increased lipid content and a change in muscle structure. These problems can lead to downgrading of the product. This study aimed to investigate the influence of raw material composition and chill storage on quality parameters of cold smoked salmon. RESULTS Initial smoked fish had a higher liquid-holding capacity (LHC) than samples stored for 20 days. Large fish lost more liquid than small fish. The difference in LHC was consistent with a change in water distribution, which could indicate denaturation of muscle protein. Studies of the microstructure showed the influence of both processing and chill storage. An indication of lipid released from the cells was seen after 20 days of chill storage, which could be related to the reduced LHC at that time. CONCLUSION Both raw material composition and chill storage affected the quality parameters of smoked salmon. This study has improved knowledge about the relationships between muscle structure, liquid-holding properties and water distribution in smoked salmon. Copyright

Production of High Quality Frozen Cod (Gadus morhua) Fillets and Portions on a Freezer Trawler

Abstract Five batches of “frozen at sea” cod products processed on a freezer trawler in the Barents Sea were studied: (i) glazed portions, (ii) glazed vacuum packed portions, (iii) unglazed portions, (iv) interleaved packed fillets, and (v) double frozen cod fillets. All batches were produced from a single catch on a freezer trawler. Changes in product quality during frozen storage were evaluated by measuring water holding capacity, water loss on cooking and sensory quality indices of cooked samples. The glazed cod portions, glaze vacuum packed cod portions and interleaved packed cod fillets remained with high sensory and physical quality after 46 weeks of frozen storage. In contrast, unglazed cod portionsand double frozen cod fillets developed cold storage odor and flavor coupled with very dry and fibrous texture during frozen storage. It was concluded the improved handling and processing technique used in this present study made it possible to produce high quality “frozen at sea” cod products from the Barents Sea on freezer trawlers over a storage period of 46 weeks. This provides the opportunity to introduce high quality “frozen at sea” cod products in place of the traditional single or double frozen cod products on the retail or catering markets.

Sensory properties of marinated herring ( Clupea harengus ) - influence of fishing ground and season

ABSTRACT The sensory properties of marinated herring produced immediately post mortem of raw material from different fishing ground and seasons were described and related to biological, biochemical and functional properties. Subtle variation was encountered in the appearance of whole marinated herring fillets. Fishing ground did not influence the odor, flavor or texture, but there was an apparent effect of season on the sensory profile. The sensory properties were influenced by body weight, but not by age, sex and gonad maturity. The influence of varying lipid content, water content and liquid holding capacity resulted in similar effects showing the high correlation between these properties. The results indicated that variation in sensory quality observed by the industry is not primarily due to the parameters fishing ground and season.