Biochemical composition, volatile organic compounds, sensory and consumer evaluation of Pacific cupped oysters during land-based refinement

Abstract

Fresh oysters are considered a delicacy in Western Europe. In the Netherlands two species are cultivated; the Pacific cupped oyster (Crassostrea gigas) and the European flat oyster (Ostrea edulis). The Pacific cupped oyster is the main product of Dutch producers with a total production of 2300 tons in 2014. In order to increase the value of the Dutch Pacific cupped oyster product innovations, such as oyster refinement are necessary. Oyster refinement is a relatively new process in the Dutch oyster sector. The refinement process usually consists of keeping live market-sized oysters in land-based systems for a period varying from one to four months in which the oysters are fed with (naturally occurring) microalgae. The main aims of this thesis were to study the impact of feed on the biochemical composition, flavor profile and sensory characteristics during the cultivation and refinement of Pacific cupped oysters. Additionally, to gain insight into Dutch consumers’ preferences, quality perception and factors influencing their evaluation of Pacific cupped oysters. The presence of identified odor- and taste active compounds present in fresh oysters has been the subject of a review (Chapter 2). The main odor- and taste active compounds found in oysters are Volatile Organic Compounds (VOC’s), Free Amino Acids (FAA), ‘5 nucleotides and bromophenols. Furthermore, the occurrence of many of these compounds has been found to be linked to the diet of the oysters. In this thesis the main focus is on VOC’s and FAA. The VOCs found in oysters (aldehydes, alcohols, ketones, alkenes, cyclenes, alkanes, cyclanes, furans, acids and aromatic compounds) originate from n-3 PUFA, n-6 PUFA, n-9 MUFA oxidation or carotenoid, amino acid or polysaccharide degradation. The VOCs are known to be odor active compounds and are linked to odor descriptors in oysters such as mushroom, grass, green, cucumber, marine, citrus and fresh. The FAA however, are known to be taste active compounds. The FAA known to have an impact on the taste profile of oysters are glutamate, glycine and alanine. Glutamte has been reported to increase both the umami taste and sweetness of oysters. Both glycine and alanine influence the perceived sweetness of the oysters. The next step in this research was to evaluate Dutch consumer preference for either the Pacific cupped oyster (Crassostrea gigas) or the European flat oyster (Ostrea edulis)(Chapter 3). Consumer preference was evaluated using two-Alternative Forced Choice (2-AFC) tests while the sensory properties were assessed using a nine attribute consumer evaluation. In total 74 naive Dutch consumers participated in this study. No significant differences in Dutch consumer preference for Pacific cupped or European flat oysters was shown. Furthermore Dutch consumer preference for Pacific cupped or European flat oysters was unchanged when presenting oysters without visual cues. In the consumer evaluation Pacific cupped oysters were perceived as being saltier and more intense in greenness in comparison with European flat oysters. No significant differences were found for the attributes overall odor intensity, sea odor, mud odor, sweetness, pungency, firmness and creaminess. A salt content analysis of the oyster tissues confirmed a higher salt content (1.07% NaCl) in the Pacific cupped oyster compared to the European flat oyster (0.82% NaCl). In Chapter 4 consumers used the temporal dominance of sensations method in order to compare the dynamic sensory perception of the Pacific cupped oyster and European flat oyster. This sensory method was used in order to assess flavor differences of oysters during consumption. The attributes salt, sweet, creamy, earthy, pungent and firm were used in this evaluation. Both Pacific cupped oysters and European flat oysters were evaluated by 31 naive Dutch consumers. Most pronounced differences between the two oyster species were found for the attributes creamy, sweet and pungent. The dynamic sensory perception of the Pacific cupped oyster evolved from salty at the beginning of consumption to creamy at the middle of consumption proceeding to sweet at the end of consumption. The dynamic sensory perception of the European flat oyster evolved from salt at the beginning of consumption to pungent at the middle of consumption and sweet at the end of consumption. In Chapter 5 of this thesis the differences in biochemical composition and VOC content of the Pacific cupped oyster and the European flat oyster from the main Dutch cultivation grounds were investigated. Standard analytical methods were used in order to analyze the biochemical composition of the oysters. The VOC content was analyzed using headspace solid phase micro extraction. Marked differences were found between the two oyster species investigated. The fatty acid profile of the European flat oyster was found to contain higher C15:0, C20:0, C20:4n6, C22:5n6 and an unknown fatty acid content and lower C18:1, C18:1n7, C184n3 and C20:1n9 content as compared with Pacific cupped oysters. The main VOC found in the Pacific cupped oyster was 1,5-octadien-3-ol while the main VOC in the European flat oyster was 3-cyclohexene-1-ethanol. The effects of refinement with different microalgae (Rhodomonas baltica and Skeletonema costatum) on the biochemical composition, volatile organic compounds, sensory and consumer evaluation of Pacific cupped oysters were studied in Chapter 6. Pacific cupped oysters were fed for a period of seven weeks with both microalgae diets while being kept under controlled conditions in land-based basins. The composition of the oysters was influenced by the microalgae diet fed. Higher ∑PUFA, ∑n-3, ∑n-6, C18:2n6, C18:3n3, C183:4n3 and C22:6n3 content was found in Rhodomonas fed Pacific cupped oysters in comparison with Skeletonema fed oysters which is coherent with the differences in the fatty acid profiles of the two microalgae. The total VOC content decreased during refinement, with lowest content found for the Skeletonema refined oysters. Furthermore, threonine, glutamine, leucine, histidine, (E)-2-hexenal, (E)-2-octenal, (E)-2-octen-1-ol, (E,E)-2,4-octadien-1-ol, (E,Z)-3,6-nonadien-1-ol and (Z,E)-2,6-nonadienal content were found to be higher in Skeletonema fed oysters compared to Rhodomonas fed oysters. The differences in the biochemical composition (incl. fatty acid, FAA and VOC content) led to perceived sensory differences in trained panelists evaluations. Skeletonema fed oyster were perceived as having a stronger seaweed flavor, higher perceived sweetness and a firmer texture in comparison with Rhodomonas fed oysters. Reference oysters from the Eastern Scheldt were characterized by a higher overall odor intensity, higher saltiness and a stronger marine flavor in comparison with both refined oysters. In the differentiation tests naive Dutch consumers were only able to differentiate Rhodomonas fed oysters from reference oysters. Dutch consumers’ preferences, quality perception and factors influencing their evaluation of Pacific cupped oysters were investigated in Chapter 7. Using questionnaires the importance of oyster quality parameters to Dutch consumers and the willingness to buy-pay in relation to purchase intention drivers, such as refinement, were evaluated. Furthermore, the effect of product information on consumer evaluation of refined and non-refined oysters was studied. Consumers evaluated taste, texture and odor as the most important quality characteristics in oysters. Consumers also considered sweetness as the most important taste characteristic while saltiness was considered the least important. Whether oysters were refined or non-refined did not have any influence on the willingness to buy and willingness to pay by Dutch consumers. Other factors such as flavor profile, country of origin and cultivation area did influence both willingness to buy and willingness to pay by consumers. Consumers preferred to buy native oysters, oyster cultivated in natural waters and oysters with the attribute sweet included in the flavor profile. When actual products, refined and non-refined oysters, were evaluated Dutch consumers perceived refined oysters as being sweeter. Providing information on the refinement process increased overall appreciation of the refined oysters by consumers. In Chapter 8 practical considerations on the implementation of refinement for the Dutch oyster sector, options for microalgae cultivation and the costs and benefits of refinement are discussed. After some small changes the traditional oyster stocking basins in the Netherlands could be used as refinement basins. The additional costs of required microalgae as refinement diet could be as low as € 0.03 per oyster. The research presented in this thesis shows that the feed given during refinement may alter the biochemical composition, volatile organic compounds, sensory and consumer evaluation of Pacific cupped oysters. Furthermore, it was shown that Dutch consumers do not have a negative attitude towards refined oysters. Dutch consumers might even prefer refined oysters as refined oysters are characterized as being sweeter in comparison with non-refined oysters. Dutch consumers showed no preference for either the Pacific cupped oyster or the European flat oyster. Dutch consumers rank sweetness as the most important taste characteristic of oysters. Consumers perceive refined oysters as being sweeter in comparison with reference oysters. Also, Dutch consumers indicated that they are willing to pay more for sweet oysters.