A new approach for eliminating off-flavors from RAS fish, as part of the normal grow-out period.

A new concept is presented for eliminating off-flavor from cold-water RAS-grown fish, while feeding, and as a part of the normal grow-out period. The technology is based on disconnecting the nitrification biofilter, and instead passing the water through an electrolysis system, which both oxidizes the ammonia and disinfects the water, while also removing the off-flavor compounds from the water, which thereby results in the purging of the fish. The purging period was expected to last up to 2 weeks and the fish are fed throughout it. Laboratory and pilot plant experiments were performed to prove the new concept. Lab experiments included quantification of the removal of MIB and geosmin by electrooxidation and stripping, together and separately, in the presence and absence of organic matter. A pilot plant experiment was performed using Rainbow trout to determine the rate at which the off-flavor compounds were removed from the water and the fish flesh (both skin and muscle were tested). The results show that the treatment process eliminated off-flavors in the water after ∼7 days and that the fish were below taste and odor threshold for geosmin and MIB after a maximum of 11 days. Detachment from the biofilter and the fact that the water was vigorously disinfected during the electrooxidation step guaranteed that no further off-flavor compounds would be generated during the operation. Aquacultural-management assessment indicates that RAS farms can increase both their annual production and their income by more than 10%, by implementing the suggested concept as part of the grow-out period.

Synthesis and characterization of zinc-hexacyanoferrate composite beads for controlling the ammonia concentration in low-temperature live seafood transports.

A new water treatment technology is presented for extending the longevity and increasing the maximal bio-load of container-bound, lucrative live seafood transportations. The technology is designed for removing ammonia and minimizing the bacterial concentration that develop in the water during the transport. This paper focuses on the characteristics of self-synthesized polyether-sulfone (PES) coated Zn-HCF composite beads, which have a high adsorbing capacity for NH4+ in seawater and constitute the heart of the developed technology. Adsorption isotherms show that the operational capacity of the composite material (PES = 20% w/w) at NH4+ concentration of 10 mgN/L at 3.5 °C is ∼3 mgN/g Zn-HCF. The kinetics of the PES-coated beads were shown to be considerably slower than the bare Zn-HCF, but since the retention time in the transport is long (many days), this does not detract from the effectiveness of the adsorption. Simulation experiments with and without live fish showed that the adsorbing material behaved as expected during a 21-d trip and that it did not have any effect on the fish. Repeated adsorption/regeneration (3 and 6 M NaCl) tests proved the composite material's stability and ion-exchange robustness. Electrooxidation of the ammonia in the exhausted regeneration solution was carried out with high efficiency and the treated solution could be used effectively in the following chemical regeneration step. The cost of a treatment unit installed in a 40-foot container was estimated at $40,000 and the ROI at 6 to 12 months.

Guanine and 7,8-Dihydroxanthopterin Reflecting Crystals in the Zander Fish Eye: Crystal Locations, Compositions, and Structures.

The eyes of many fish contain a reflecting layer of organic crystals partially surrounding the photoreceptors of the retina, which are commonly believed to be composed of guanine. Here we study an unusual fish eye from Stizostedion lucioperca that contains two layers of organic crystals. The crystals in the outer layer are thin plates, whereas the crystals in the inner tapetum layer are block-shaped. We show that the outer layer indeed contains guanine crystals. Analyses of solutions of crystals from the inner layer indicated that the block-shaped crystals are composed of xanthopterin. A model of the structure of the block-shaped crystals was produced using symmetry arguments based on electron diffraction data followed by dispersion-augmented DFT calculations. The resulting crystal structure of xanthopterin included, however, a problematic repulsive interaction between C═O and N of two adjacent molecules. Knowing that dissolved 7,8-dihydroxanthopterin can oxidize to xanthopterin, we replaced xanthopterin with 7,8-dihydroxanthopterin in the model. An excellent fit was obtained with the powder X-ray diffraction pattern of the biogenic crystals. We then analyzed the biogenic block-shaped crystals in their solid state, using MALDI-TOF and Raman spectroscopy. All three methods unequivocally prove that the block-shaped crystals in the eye of S. lucioperca are crystals of 7,8-dihydroxanthopterin. On the basis of the eye anatomy, we deduce that the guanine crystals form a reflective layer producing the silvery color present on part of the eye surface, whereas the block-shaped crystals backscatter light into the retina in order to increase the light sensitivity of the eye.