Effect of diet and environment on the volatile flavour components of crustaceans
Final report
The flavour of a seafood is one of several sensory properties that determines whether it is marketable and at what market price. Those species of crustaceans that possess characteristic flavours, such as the Endeavour prawn, the Royal Red prawn and the Balmain Bug have from time to time been unpopular in markets because of the occurrence of these natural flavours. Other species with bland or little flavour also draw critical comment from consumers paying high prices in restaurants. Although some evidence has indicated that diet and environment are major contributing factors in the determination of the flavours of crustaceans, no definite link has been established. The current study was accordingly undertaken to identify the sources of the compounds responsible for such characteristic flavours, and to provide an explanation for the occasional outbreaks of strong flavours in wild-harvested prawns, and the absence of natural flavours in cultivated animals. The technological aim of the work was to improve the flavour quality of Australian produced prawns for both domestic and overseas consumption. The ultimate return to the fishing industry from this research would be products of reliable flavour quality capable of yielding high market prices.
Evidence obtained from the chemical and sensory analyses of nine species of wild-harvested prawns and two species of cultivated prawns showed that bromophenols, particularly 2-bromophenol, 4-bromophenol, 2,6-dibromophenol and 2,4,6-tribromophenol, enhanced the desirable seafood flavours of wild-harvested animals. Conversely, the near absence of these compounds from cultivated prawns left the flesh bland and lacking prawn-like flavours. In addition, these analyses showed that the bromophenol content of prawn heads (which includes the gut) was seven times that found in the tails for wild-harvested prawns and three times that found in cultivated animals. As an adjunct to this work 31 species of ocean fish were analysed for their bromophenol content. Results from these analyses showed that the average total bromophenol content of benthic carnivores and omnivores was 100 times greater than that of piscivorous carnivores. The analyses also showed that the bromophenol content in the animal’s gut was greater than that found in the flesh. These findings supported the belief that bromophenols are derived from the diet of marine animals.
Following detailed surveys of literature pertaining to the dietary intake of prawns and fish, the likely sources of bromophenols in these animals appeared to be polychaetes and marine algae. Analyses of 16 species of polychaetes showed that these soft bodied organisms were a major source of bromophenols in the marine environment. Analyses of 50 species of marine algae showed that these plants were probably the world’s major repository of bromophenols in the marine environment, because of the wide occurrence of such plants along the nations’ coastlines.
Modified prawn feeds were prepared in which bromophenols both in the free form and as their sulfate esters were added to a CSIRO formulation. The concentrations of these compounds were similar to the highest levels found in commercial feeds. Results from feeding trials showed that the prawns did not discriminate between the modified feeds and the control. Furthermore, sensory analyses carried out on these prawns showed that the modified feeds enhanced the natural flavour of prawn meat. Of equal importance, it was found prawns fed on modified feed containing free bromophenols retained more of these compounds than prawns fed on feed containing the sulfate esters.
