Commissioned by
Joe McNaughton
EkoSolutions Pty Ltd
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Report on a comparison of C-Fresh to Everfresh for black spot inhibition
Aim
The client wants to determine the effectiveness of a new prawn black spot formulation called C-Fresh. The performance of the treatment was compared with Everfresh and a control of no chemical treatment.
Methods
Prawns freshly caught from a beam trawler in the Logan River were held under ice and dipped within 30 minutes of landing. The prawns were juvenile banana prawns, a good choice for this experiment as small prawns moult frequently and thus are more prone to black spot. The solutions of C-Fresh and Everfresh were made up per directions on the sachets using fresh town water. The Everfresh powder had a slight orange tinge when removed from the foil sachet, which is not uncommon. The C-Fresh looked quite different when poured into the dip tank water and took longer than the Everfresh to dissolve. After 10 minutes there were still some large white flakes present which may have been the citric acid added to this new formulation of 4-hexylresorcinol.
75 prawns were counted out into 6 individual batches for duplicates of the three treatments. The control treatment consisted of immersion of two batches of prawns for 2 minutes in clean fresh water and lifted up and down in the water to ensure good exposure. The remaining four batches of prawns were placed in the two different treatment solutions for 2 minutes and lifted up and down in the water to ensure good exposure to the treatments. Samples for 4-hexylresorcinol residue testing were taken and the remaining batches containing 50 prawns were stored under ice in a commercial seafood cold room operating at 0°C for up to 8 days. The numbers of prawns exhibiting black spot (melanosis) were recorded every 2 days. Representative prawns were then photographed before all were returned to cold storage.
Results
The table below shows the counts of prawns exhibiting black spot for each treatment on each storage day.
Treatment / Replicate / Day 0 / Day 2 / Day 4 / Day 6Control / 1 / 0 / 29 / 48 / 50
Control / 2 / 0 / 26 / 48 / 50
C-Fresh / 1 / 0 / 2 / 30 / 50
C-Fresh / 2 / 0 / 5 / 32 / 50
Everfresh / 1 / 0 / 13 / 29 / 49
Everfresh / 2 / 0 / 17 / 36 / 49
The following figure shows the incidence of black spot as a percentage of the amount stored.
The following figures show representative samples of prawns from each treatment after 2 and 4 days’ storage.
Discussion
As found with previous studies (Slattery et. al, 2009; Slattery, 1995) untreated prawns rapidly developed black spot. In this case the small banana prawns did exhibit lower levels after 2 days storage than that present on the black tiger prawns but there was much more black spot present this trial than on the larger sized banana prawns used in this earlier study.
There were significant differences (P<0.05) between all three treatment after 2 days storage, with the C-Fresh treated prawns exhibiting the least amount of black spot while the Everfresh treated prawns showed significantly more black spot than these prawns but significantly less than the control. Both replicates of each treatment were similar on every sampling day indicating a consistent effect. The reason for using 50 prawn batches was to minimise any chance of uneven moult stages in each replicate.
By day 4 there was no difference in the amount of protection from black spot provided by the C-Fresh and Everfresh treatments. One reason could be the presence of citric acid in the C-Fresh formulation which may have provided better initial protection of the prawns until it was washed off the prawns by the melting ice. By the end of the trial all of the prawns were exhibiting black spot in some form.
Conclusion
C-Fresh does provide protection from black spot as effectively as Everfresh. The presence of citric acid in the formulation does appear to provide an additional benefit early on during storage but is washed out by the melting ice. The level of protection then becomes similar to that provided by Everfresh. The extra funding that was sought from FRDC was intended to assist with further validation of the outcomes identified here and should be pursued in the future.
Slattery, S.L . 1995. A sulphite-free treatment inhibits blackspot formation in prawns. Food Australia 47(11):509-514.
Slattery, S.L., Williams, D.J. and Torrisi, C. 2009. New modified dipping method using 4-hexylresorcinol for preventing blackspot formation in prawns. Journal of Aquatic Food Product Technology 18(3):284-293.
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