Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/18605
The main purpose of this project was to study how different processing and packaging methods affect the rate of lipid oxidation, microbiological spoilage, and therefore the shelf life of fresh redfish fillets. The project was divided into three experiments performed in May, June, and October 2013.
In experiment I, the fish was evaluated in terms of lipid oxidation. Two bleeding methods, including bleeding by gill cut and bleeding by throat cut, were compared to the traditional unbled fish. All fish was packed the same way into expanded polystyrene (EPS) boxes with ice on top and stored in a cooling chamber at -1 °C.
In experiment II, the fish was evaluated in terms of lipid oxidation, microbiological spoilage, and sensory acceptance. Three different packaging methods were compared, including the traditional method of air packaging using EPS boxes with the addition of ice (Air), modified atmosphere packaging (MAP, 40% CO2: 60% N2) using plastic boxes (M1), and modified atmosphere with the addition of CO2 emitting pads using plastic boxes (M2). All fish was stored in a cooling chamber with the initial temperature of -1 °C. To imitate temperature fluctuations during a sea freight transport, temperature of the cooling chamber was increased to +1 °C on day 6, and to +2 °C on day 10.
In experiment III, the fish was evaluated in terms of lipid oxidation, microbiological spoilage, and sensory acceptance of the fish. Four different groups were compared, including bled fish by gill cut placed into a box with the skin side up (H1), the traditional unbled fish placed into a box with the skin side up (H2), unbled fish with sandwich placement into a box (H3), and unbled fish with sandwich placement into a box and the addition of CO2 emitting pads (H4). All fish in this experiment was packed into EPS boxes and stored in a cooling chamber with the initial temperature of -1 °C. As in experiment II, the temperature of the cooling chamber was increased to +1 °C on day 6, and further to +2 °C on day 10.
Lipid oxidation measurements in experiment I showed some conflicting results as primary and tertiary oxidation measurements showed no benefit of bleeding whereas secondary measurements showed no secondary oxidation in either of the bled groups. However, as heme iron content measured higher in both bled groups it can be assumed that the bleeding was not effective. The colour measurements showed that bleeding the fish did not significantly affect the color of the fillets. However, visual observation of the fillets showed a clear difference between the groups on day 0, as the bled fillets were lighter in colour than the unbled fillets. This colour difference evened out during storage.
MAP proved to be a useful packaging method in reducing lipid oxidation. In terms of microbes, MAP fish favored the growth of P. phosphoreum as Pseudomonads count was reduced. However, total viable count (TVC) was not reduced in the MAP fish. An increase in drip was observed in the M1 and M2 groups as MAP fish had a lower pH than the air packed fish. In fact the highest drip was observed in the M2 group, and the lowest in the Air group. The addition of CO2 emitting pads did not appear to increase the benefits of a regular gas packaging any further. As colour measurements showed no significant difference between the three groups, it can be concluded that MAP did not affect the colour. Freshness period of the MAP fish was increased by 1.5 days, with no difference between the two MAP groups.
Results of experiment III indicated that the different treatments and packaging methods did not prove to be beneficial in increasing the quality of the fillets compared with traditional methods. The addition of the CO2 pads did not prove to be effective in reducing the pH much, which was probably caused by the gas leaking out of the boxes. Therefore, EPS boxes without an airtight inside seal cannot be recommended when using CO2 pads. Bleeding the fish by gill cut, did not prove effective in the reduction of lipid oxidation, or microbiological spoilage. Heme iron content proved to be just as high in the bled group as in the unbled group indicating that the bleeding was not efficient enough. Another factor, which may have affected the outcome of the bled group (H1), is that the fillets were much smaller than in the unbled groups (H2, H3, H4). The sandwich placement of the fish in the box (H3) did not affect the spoilage factors. The colour analysis showed no significant difference between the groups on any of the sampling days. However, bled fish had somewhat more lightness than the unbled groups and sandwich placed fish was somewhat lighter than fish with skin side up placement. Darker spots were more obvious in unbled groups than in the bled group in the beginning of storage, however, the colour difference evened out as storage prolonged.
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