Evaluation of microalgae diets on flavor characteristics of Pacific oysters (Crassostrea gigas) during fattening

Effect of sous-vide processing duration on flavor and taste variations of oyster (Crassostrea gigas)

Sous-vide (SV), as a mild processing technique, exhibits some potential for keeping the original flavor of oyster. The dynamic changes mechanism of flavor and taste in oyster during SV processing (0-30 min/75 °C) were investigated. SV processing for 10-15 min improved the umami of oysters, likely due to the increase in adenosine monophosphate and glutamate, while processing for 20-30 min resulted in a significant loss of "grassy" flavor. GC-MS and GC-IMS analysis showed that the loss of short-chain aldehydes, such as (E)-2-pentenal, (E)-2-hexenal and (E, E)-2,4-hexadienal may be related to the weakening of the "grassy" flavor, and the formation of 2,3-diethylpyrazine and octanal produced a "cooked" and "fatty" flavor. The analysis of lipidomics indicated that phosphatidylethanolamine, lysophosphatidylcholine and sphingomyelin, synthesized mainly through glycerophospholipid and sphingolipid metabolism, were key precursors for aldehyde formation. This study provides a theoretical basis for controlling the flavor quality of oyster during mild processing.

Growth Performance, Dietary Enzyme Profiling, and Antioxidant-Induced Immunity of Crassostrea belcheri Fed With Microalgal Diet

Diseases, overfishing, and habitat loss are constantly reducing oyster populations. Moreover, environmental pollution and natural disasters also hinder offshore shellfish cultivation. Therefore, filter-feeding bivalves could be cultivated on land-based system to maintain the culture condition and secure food safety. Four different diets including Chaetoceros gracilis (CG), Tetraselmis chuii (TC), mixture of CG and TC (CG/TC), and fresh seawater without feed (Con) were trialed for Crassostrea belcheri spat in this research. After 35 days of culture, the highest survival rate (SR), volume, and weight increment (WI) of oyster and improved water quality appeared in CG/TC. In the same manner, CG/TC diet exhibited greater lipase (LPS), pepsin (PES), catalase (CAT), alkaline phosphatase (AKP), and hydrogen peroxide (H2O2) activity. Conversely, amylase (AMS), acid phosphatase (ACP), superoxide dimutase (SOD), malondialdehyde (MDA), and lysozymes (LZMs) activities were substantially higher in CG diet compared to TC, CG/TC, and Con. No significant differences were observed among CG, TC, and CG/TC for antioxidant capacity (AOC). In this investigation, mixed algal diet had excellent results for the growth and development of oyster, whereas unialgal diet improved immunity and AOC to survive in unfavorable conditions. This observation help elucidates the knowledge on microalgal diet influenced immune modulation and health of marine bivalves in the scenario of land-based farming.

Effect of Different Regional Characteristics of Spawning and Growing Sites on Growth and Taste of Pacific Oyster, Crassostrea gigas

While Pacific oysters are important commercial aquaculture species worldwide, the effect of hormonal regulation and environmental conditions on growth and taste profile have not been fully known. Insulin-like growth factor (IGF) systems are known to play a major role in regulating neuroendocrine functions across various physiological processes and are particularly involved in growth. IGFs expression also is directly related to the nutritional status of vertebrates, however, full mechanism has not been clearly identified in bivalves. In this study, differences in growth, IGFs expression, and taste according to cultivation site of Pacific oysters were investigated. Oysters were collected in three different spawning sites located on the south coast of Korea in July 2022 and hardened until June 2023. Then, the oysters were cultured in two different growing sites (Tongyeong site, TS; Geoje site, GS) for six months. The total weight of oysters, along with their condition index and tissue weight rate, was significantly higher in TS. Additionally, IGF expression was higher in TS during most of the sampled months. However, oysters from the GS scored higher in taste evaluations. The IGFs system in oysters shows a similar trend to previous studies, with higher levels in faster-growing individuals, suggesting oysters in TS were more adequately nourished by the surrounding environment in this research. However, in taste evaluation, oysters from the GS showed better results than those from the TS. Despite these results, determining whether one site is superior in certain aspects is still not fully possible, which warrants further investigations.

Diverse uses of valuable seafood processing industry waste for sustainability: a review

Seafoods are rich in untapped bioactive compounds that have the potential to provide novel ingredients for the development of commercial functional foods and pharmaceuticals. Unfortunately, a large portion of waste or discards is generated in commercial processing setups (50-80%), which is wasted or underutilized. These by-products are a rich source of novel and valuable biomolecules, including bioactive peptides, collagen and gelatin, oligosaccharides, fatty acids, enzymes, calcium, water-soluble minerals, vitamins, carotenoids, chitin, chitosan and biopolymers. These fish components may be used in the food, cosmetic, pharmaceutical, environmental, biomedical and other industries. Furthermore, they provide a viable source for the production of biofuels. As a result, the current review emphasizes the importance of effective by-product and discard reduction techniques that can provide practical and profitable solutions. Recognizing this, many initiatives have been initiated to effectively use them and generate income for the long-term sustainability of the environment and economic framework of the processing industry. This comprehensive review summarizes the current state of the art in the sustainable valorisation of seafood by-products for human consumption. The review can generate a better understanding of the techniques for seafood waste valorisation to accelerate the sector while providing significant benefits.

Sustainable aquaculture and seafood production using microalgal technology - A circular bioeconomy perspective

The aquaculture industry is under the framework of the food-water-energy nexus due to the extensive use of water and energy. Sustainable practices are required to support the tremendous growth of this sector. Currently, the aquaculture industry is challenged by its reliance on capture fisheries for feed, increased use of pharmaceuticals, infectious outbreaks, and solid/liquid waste management. This review posits microalgal technology as a comprehensive solution for the current predicaments in aquaculture in a sustainable way. Microalgae are microscopic, freshwater and marine photosynthetic organisms, capable of carbon mitigation and bioremediation. They are indispensable in aquaculture due to their key role in marine productivity and their position in the marine food chain. Microalgae are nutritious and are currently used as feed in specific sectors of aquaculture. Due to their bioremediation potential, direct application of microalgae in shellfish ponds and in recirculating systems have been adopted to improve water quality and aquatic animal health. The potential of microalgae for integration into various aspects of aquaculture processes, namely hatcheries, feed, and waste management has been critically analyzed. Seamless integration of microalgal technology in aquaculture is feasible, and this review will provide new insights into using microalgal technology for sustainable aquaculture.

Changes in Flavor-Related Biomarkers in Pacific Oysters (Crassostrea gigas) Following Microplastic Exposure

Microplastics have been an emerging threat to filtering species and the ingestion and impacts of microplastics on oysters are a cause for concern. However, much remains unknown about the effects of microplastics on flavor-related biomarkers in oysters. Herein, a laboratory microplastic exposure with concentrations of 1, 10, and 100 mg/L for 15 days was performed to investigate the impacts of microplastics on the flavor parameters of oysters. Exposure to microplastics changed the odor characteristics of oysters. Microplastic exposure had minor effects on the fatty acid composition; however, significant alterations in free amino acids and nucleotides were observed under the 1 and 10 mg/L exposure groups, respectively. The overall results indicated 10 mg/L of microplastic exposure significantly increased the equivalent umami value of oysters. These findings stressed the effects of microplastics on oysters and would be an important reference for the assessment of the potential risks associated with microplastics in marine edible species.

The dynamic change of flavor characteristics in Pacific oyster (Crassostrea gigas) during depuration uncovered by mass spectrometry-based metabolomics combined with gas chromatography-ion mobility spectrometry (GC-IMS)

The flavor of Pacific oyster (Crassostrea gigas) significantly changed during the depuration process. This work aimed to explore the mechanism of flavor changes during the 72 h depuration by metabolomics combined with gas chromatography-ion mobility spectrometry (GC-IMS). The metabolomics analysis indicated that carbohydrate metabolism was more affected in the early stage of depuration, including the citrate cycle, glyoxylae and dicarboxylate metabolism, etc. After 72 h depuration, it affected mainly the metabolism of global and overview maps and nucleoside metabolism, etc. The equivalent umami concentration (EUC) value was calculated and exhibited a gradual increase following a 48 h depuration. The GC-MS results revealed that the content of furans was the highest, and the content of aldehydes, ketones, and alcohols was the lowest after 48 h depuration, while the content of aldehydes, ketones, and alcohols increased after 72 h depuration. All these results suggested the depuration period was recommended to be controlled within 48 h.

Changes in biochemical metabolites in manila clam after a temporary culture with high-quality microalgal feed mixed with the dinoflagellate species Karlodinium veneficum and K. zhouanum

Phytoplankton composition is an important factor affecting the growth and physiological biochemical characteristics of filter-feeding bivalves. With the increasing trend in dinoflagellate biomass and blooms in mariculture areas, how the physio-biochemical traits and seafood quality of the mariculture organism are affected by the dinoflagellates, especially those at nonfatal levels, is not well understood. Different densities of two Karlodinium species, namely K. veneficum (KV) and K. zhouanum (KZ), mixed with high quality microalgal food Isochrysis galbana was applied in feeding manila clam Ruditapes philippinarum in a 14-day temporary culture, to comparatively study how the critical biochemical metabolites such as glycogen, free amino acids (FAAs), fatty acids (FAs), volatile organic compounds (VOCs) in the clam were affected. The survival rate of the clam showed dinoflagellate density and species specificity. The high-density KV group inhibited survival to 32% lower than that of the pure I. galbana control, respectively, while KZ at low concentrations did not significantly affect the survival compared with the control. In the high-density KV group, the glycogen and FAA contents decreased (p < 0.05), indicating that energy and protein metabolism were significantly affected. Amount of carnosine (49.91 ± 14.64 to 84.74 ± 8.59 μg/g of muscle wet weight) was detected in all the dinoflagellate-mixed groups, while it was not present in the field samples or in the pure I. galbana control, showing that carnosine participated in the anti-stress activities when the clam was exposed to the dinoflagellates. The global composition of FAs did not significantly vary among the groups. However, contents of the endogenous C₁₈ PUFA precursors linoleic acid and α-linolenic acid significantly decreased in the high-density KV group compared to all the other groups, indicating that high density of KV affected the metabolisms of fatty acids. From the results of the changed VOC composition, oxidation of fatty acids and degradation of free amino acids might occur in the clams exposed to dinoflagellates. The increased VOCs, such as aldehydes, and decreased 1-octen-3-ol probably produced a more fishy taste and reduced food flavor quality when the clam was exposed to the dinoflagellates. This present study demonstrated that the biochemical metabolism and seafood qulity of the clam were affected. However, KZ with moderate density in the feed seemed to be beneficial in aquaculture for increasing the content of carnosine, a high-valued substance with multiple bioactivities.

Effect of Drying Process on the Formation of the Characteristic Flavor of Oyster (Crassostrea hongkongensis)

Oysters are nutritious and tasty but difficult to store. Drying can extend the storage period of oysters and give them a unique flavor. In this study, the effects of four drying procedures, namely, vacuum freeze drying (VFD), vacuum drying (VD), natural sun-drying (NSD), and hot air drying (HAD), on the flavor characteristics of oysters (Crassostrea hongkongensis) were investigated using blanched oysters as a control (CK). Results showed that HAD produced more free amino acids than the other methods, but VFD retained the most flavor nucleotides. Compared with cold drying (VFD), hot drying (VD, NSD, and HAD) increased the abundance of organic acids, betaine, and aroma substances. Glutamic acid, alanine, AMP, hexanal, octanal, heptanal, (E, E)-2,4-heptadienal, (E)-2-decenal, nonanal, etc., are defined as the characteristic flavor compounds of dried oysters, with umami, sweet, green, fatty, and fruity aromas being the main organoleptic attributes of dried oysters. Glutamic acid, glycine, betaine, IMP, pentanal, ethyl heptanoate, (E, Z)-2,4-nonadienal, 1-octen-3-one, 2-hexenal, 2-octenal, hexanal, decanal were defined as markers to distinguish different drying methods. Overall, HAD showed improved flavor qualities and characteristics and was better suited for the highly commercialized production of dried oysters.

Effects of liquid nitrogen freezing at different temperatures on the quality and flavor of Pacific oyster (Crassostrea gigas)

This study aimed to evaluate the effects of different liquid nitrogen freezing (LNF) temperatures (-20, -40, -60, -80, and -100 °C) on the water holding capacity, texture, microstructure, and flavor of Crassostrea gigas (C. gigas). The results showed that -40 °C LNF, -60 °C LNF, and -80 °C LNF improved the water holding capacity of C. gigas (P < 0.05); -60 °C LNF and -80 °C LNF could effectively maintain the hardness of the body trunk and adductor muscles. Compared with -20 °C refrigerator freezing (RF), the LNF group could form smaller ice crystals and thus reduce the damage to the muscle cell structure damage, especially LNF at -80 °C. Gas chromatography-ion mobility spectrometry (GC-IMS) and e-nose results indicated that -80 °C LNF maintained the flavor profile of few aldehydes and alcohols compared to other freezing groups. Therefore, -80 °C LNF effectively improved the quality and maintain the flavor characteristics of frozen C. gigas.