Addressing quality deficits in farmed barramundi: optimising flavour and quality through pre-harvest practices

Jones, Ben C. (2016) Addressing quality deficits in farmed barramundi: optimising flavour and quality through pre-harvest practices. PhD thesis, James Cook University.

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This thesis identifies factors resulting in quality deficits in Australian farmed barramundi and generates data on which to develop management practices that optimise flavour and quality. Five interrelated studies were undertaken focussing respectively on:

• The occurrence of earthy-muddy tainting of cultured barramundi linked to geosmin in tropical northern Australia.

• The controlled off-flavour tainting of cultured fish using the geosminproducing cyanobacterium, Anabaena circinalis.

• The uptake, depuration and spatial distribution of the off-flavour tainting compound geosmin in farmed barramundi, Lates calcarifer.

• The flavour enhancement of freshwater farmed barramundi, Lates calcarifer, through dietary enrichment with cultivated sea lettuce, Ulva ohnoi.

• The effects of dietary enrichment with alpha-tocopherol acetate and postharvest filleting on lipid oxidation and flesh quality of tropical farmed barramundi Lates calcarifer.

Three of these studies have been published in the scientific literature and two have been submitted for publication. These scientific papers are presented in chapters 2-6. Chapter 1 reviews the scientific literature on flavour and quality of aquaculture fish and explains the rationale for the studies undertaken. Chapter 7 summarises research findings and reviews the significance of the body of research for: aquaculture research; aquaculture production and product quality globally; and the Australian barramundi aquaculture industry.

The findings from this body of research relate primarily to three topics: off-flavour tainting; a lack of flavour complexity in farmed fish; and deterioration in product quality during the post-harvest storage period.

Water samples from outdoor barramundi rearing ponds were analysed for the presence of geosmin (GSM) and 2-methylisoborneol (MIB). GSM was deemed to be the compound responsible for off-flavour tainting in pond-reared barramundi, persisting at moderate (~2.00 μg L⁻¹) to extreme levels (~14.36 μg L⁻¹), while MIB was never detected during the study. The accumulation of GSM in the flesh of barramundi was directly related to GSM levels of the holding water. Elevated levels of GSM in fish-flesh resulted in increases in the intensity of off- flavour tainting. The uptake of GSM by barramundi exposed to an extreme concentration of GSM (15.1μg L⁻¹) was extremely rapid; a significant increase in flesh GSM was observed after three minutes of exposure, with GSM concentration reaching 0.98±0.54μg kg⁻¹. GSM continued to accumulate in flesh reaching a maximum concentration of 8.8μg±1.88μg kg⁻¹ after 3 hours of exposure. GSM deposition within the fillet was spatially variable with the ventral belly region containing approximately three times more GSM than either the dorsal shoulder or posterior tail regions. When returned to untainted water, the concentration of GSM in flesh declined exponentially, with a half-life of 99 hours at 27°C although GSM was still present (0.77±0.32μg kg⁻¹) in muscle tissue after 14 days of depuration. The potential to recover flavour quality was assessed for fish exposed to a moderate level of GSM (2.15μg L⁻¹) by depurating them in untainted water. Human sensory assessment revealed that off-flavour tainting was eliminated after 8 days of depuration.

As part of the investigation of off-flavour tainting in farmed barramundi, a new technique for producing natural GSM, for the purpose of intentionally imparting offflavour tainting in fish, was developed. This technique is more precise and lower cost than existing methods, ensures authenticity of the organoleptic nature of off-flavour taint and is suitable for use on large cohorts of fish.

The potential to add flavour complexity to farmed barramundi was assessed by feeding the marine macroalgae Ulva ohnoi to cultured freshwater barramundi (1800- 2000 g). When barramundi were fed diets containing ≥20% inclusion level of U. ohnoi for 7-21 days fish developed stronger crab-like/seafood flavour, cooked crab aroma and sweetness resulting in increased desirability and flavour complexity compared to fish fed a standard rearing diet. The potent flavour compound dimethylsulfide (DMS) was found to be more elevated (~8 fold) in fish fed U. ohnoi and appears to be a key flavour compound in this instance.

Pre-harvest dietary enrichment with α-tocopherol acetate in combination with different post-harvest processing techniques was investigated with regard to quality deterioration of farmed barramundi, during chilled storage. Fish were fed commercial rearing diets supplemented with two levels of α-tocopherol acetate (standard level 192mg kg⁻¹ and enriched level 628 mg kg⁻¹) for a period of 5 months. Dietary α-tocopherol enrichment in combination with storing fish whole and ungutted constrained lipid oxidation over 14 days of chilled storage when compared to fish fed the standard diet and filleted prior to storage. Filleting also resulted in significant colour changes, with reddening and yellowing of the flesh.

These findings have considerable significance for aquaculture research. Researchers should be aware that GSM can persist at moderate to extreme levels, for prolonged periods in tropical aquaculture ponds and that uptake by fish can occur extremely rapidly. The spatial distribution of GSM in fish fillets should also be considered as this has serious implications for the design of future research and sampling procedures. The potential to enhance flavour through the short-term application of diets that incorporate marine algae is critical information for researchers and further studies should be undertaken to build on this finding. This thesis also provides the first report of the potential association between the flavour of farmed fish and DMS. The observation that flavour and quality can be optimised during the post-harvest period by feeding diets enriched with α-tocopherol acetate and ensuring that fish are stored whole and ungutted also has implications for aquaculture research. Future studies should consider not only the benefits of pre-harvest and post-harvest strategies to optimise storage stability but also possible interactive effects between the two.

The findings also have practical significance for aquaculture producers, including in the Australian barramundi aquaculture industry. Aquaculture producers, especially those in tropical locations, should be aware of the potential for GSM to persist in pond water for extended periods of time and reach exceptionally high concentrations. This thesis also reveals the potential to recover flavour quality by depurating fish in clean water prior to slaughter. The spatial distribution of GSM within the fillet has further implications and may enable producers to more accurately determine the presence of off-flavour taint during sensory assessment or may provide an opportunity to reduce off-flavour tainting by removing the most heavily tainted fillet regions prior to human consumption. Diets that include a significant fraction of marine algae may additionally be used by producers to optimise flavour and quality. Aquaculture producers can also benefit from the use of diets enriched with α-tocopherol acetate. This can limit lipid oxidation during storage. Products can be further fortified against quality deterioration by storing fish whole and ungutted.

Item ID: 47453
Item Type: Thesis (PhD)
Keywords: alpha-tocopherol acetate, aquaculture, barramundi fisheries, barramundi, dimethylsulfide, flavour taint, flavour, functional feed, geosmin, giant perch, Lates calcarifer, lipid oxidation, post-harvest processing, quality
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Jones, B., Fuller, S., and Carton, A.G. (2013) Earthy-muddy tainting of cultured barramundi linked to geosmin in tropical northern Australia. Aquaculture Environment Interactions , 3 (2). pp. 117-124.

Chapter 5: Jones, B., Smullen, R., and Carton, A.G. (2016) Flavour enhancement of freshwater farmed barramundi (Lates calcarifer), through dietary enrichment with cultivated sea lettuce, Ulva ohnoi. Aquaculture, 454. pp. 192-198.

Chapter 6: Jones, Ben C., and Carton, Alexander G. (2015) Effects of dietary enrichment with alpha-tocopherol acetate and post-harvest filleting on lipid oxidation and flesh quality of tropical farmed barramundi (Lates calcarifer). Aquaculture, 448. pp. 280-287.

Date Deposited: 26 Feb 2017 23:50
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070406 Post-Harvest Fisheries Technologies (incl Transportation) @ 100%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830102 Aquaculture Fin Fish (excl. Tuna) @ 100%
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