The French consume large amounts of raw oysters. The study of the aroma of oyster Crassostrea gigas is of economic interest because it is a good method of checking the sensory quality. Aromas were extracted by vacuum steam distillation at 20°C using whole oyster flesh. This extract presented similar sensory characteristics to raw oyster. The odour-active compounds were characterised by gas chromatography coupled with olfactometry using a panel of 10 judges trained in seafood aroma recognition. Fifty-nine volatile compounds were identified in oyster aroma extract. Among these, 25 were responsible for the overall odour of raw oyster. Four compounds identified in oysters were characterised by fresh and marine odour: 3-(E)-hexen-1-ol, decanal, 2-undecanone and 3,6-(E,Z)-nonadien-1-ol. Some compounds were identified for the first time in oysters: 4-(Z)-heptenal (white boiled fish odour), which comes from n-3 polyunsaturated fatty acid oxidation, and 3-octanol (moss and sulphury odour), 2-nonanol (cucumber odour) and octanoic acid, which arise from n-6 polyunsaturated fatty acid oxidation.
Oyster farming is of real economic interest in France. Oyster farmers attach more and more importance to improving the growth and the quality of their oysters. Some fatty acids known to be aroma precursors originate from microalgae such as Skeletonema costatum and Tahitian isochrysis clone. These microalgae were used to fatten oysters in order to observe their role in the development of oysters' aroma. This study shows that the profile of fatty acids of oysters is influenced by the contribution of fatty acids from the two microalgae (as reported in the first paper in this series: Pennarun, A.-L.; Prost, C.; Haure, J.; Demaimay, M. Comparison of Two Microalgal Diets. 1. Influence on the Biochemical and Fatty Acid Compositions of Raw Oysters (Crassostrea gigas). J. Agric. Food Chem. 2003, 51, 2006-2010 (in this issue)]. As a consequence, a microalgal diet causes changes in oysters' aroma composition. Aroma concentration depends on the content of fatty acids that are aroma precursors in oysters. Some aromas are characteristic of the diet of S. costatum, such as 6-methyl-5-hepten-2-one (ether odor), and others are characteristic of T. isochrysis, such as 3-nonyne (cucumber, marine odor), 6-(E)-nonen-1-ol (green and fresh odor), and 4-ethylbenzaldehyde (aniseed odor). Moreover, the organoleptic qualities (odor, taste, and texture) of oysters are modified by the diet of microalgae.
The study of the aroma of oysters is of great economic interest in France because it enables their organoleptic quality to be verified. The aim of this study is to optimize the extraction methods of the volatile compounds of oysters Crassostrea gigas in order to obtain an extract with an odor as close as possible to that of the original oysters'. Oyster aroma is rarely studied, and its sensory profile has not been investigated to date. Two extraction methods were studied: vacuum hydrodistillation carried out at 20 degrees C with noncrushed oyster using ultrapure water and dynamic headspace carried out using noncrushed oyster during a 30 min purge. They were compared with regard to their sensory characteristics by a panel of seven judges, all trained in seafood aroma recognition. This study has shown that vacuum hydrodistillation is the better method to obtain an extract closest in aroma to the oyster reference.
Oyster farming in France is a traditional activity. Each year, 149 000 tons of oysters are fattened before being sold. More and more oyster farmers supplement the diet of oysters by microalgae to optimize the fattening process and to improve both the growth and flesh quality of oysters. In the present study, oysters were supplemented by two microalgae: Skeletonema costatum and Tahitian isochrysis clone. The ash, protein, carbohydrate (including glycogen), and lipid contents were analyzed. The growth of oysters was improved when they were fattened with microalgae, leading to an increase in the condition index. The chemical composition of oysters was influenced by the chemical composition of the microalgae, especially the level of glycogen, which was significantly increased for oysters supplemented by microalgae. The fatty acid profile of oysters fattened by microalgae was positively correlated with the fatty acid profiles of S. costatum and T. isochrysis. These results show the effectiveness of supplementation by microalgae on the growth and on the biochemical composition (glycogen and fatty acids especially) of oysters.
The present study is part of a larger project whose aim is to understand how the oyster Crassostrea gigas develops its aromas from a lipid precursor, the eicosapentaenoic acid (EPA), in glyceride form. The objective of this study is, therefore, to prepare an encapsulation process that will enable the bivalve to be supplied with this lipid precursor. The complex coacervation method was chosen as it gave the best compatible microcapsules with respect to the nutritional aspects of oyster (i.e. digestibility) and the environmental constraints (i.e. behaviour and stability in seawater). The aim of this study is to manufacture and optimize a process of complex coacervation, to obtain capsules made of gelatin and acacia gum with a size under 100 microm in diameter and containing very small drops of cod liver oil (rich in EPA). The preservation of these microcapsules in seawater has been confirmed.
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