THE SHELLFISH CORNER

mand very premium prices due to their reputations, which have built Merrior up over time as a result of the ter- rior of their grapes. In French, the word mer means In French, the word mer means sea, so the portmanteau term – The Good sea, so the portmanteau term merroir was coined to describe a sense of merroir was coined to describe a terroir for oysters, and the term has sense of terroir for oysters, and become popular around oyster bars, Flavors of particularly in North America.Each the term has become popular oyster is intimately impacted by the around oyster bars, body of water it comes from, the al- Oysters gae it feeds on, the strength of cur- particularly in North America. rents and tides, the mineral content of the seafloor, rainfall, temperature, By Michael A. Rice* season and more. Although oysters can be the same species and grown Figure 2. Oyster claires near the village of La Cayenne, in the Marennes-Oléron region of France. in a similar manner, just a difference It has been known for eons by ostreophiles (lovers of oysters) that of a few hundred meters in location ences in taste of oysters at different ing days and warming waters. These ecule called glycogen that is a com- oysters of the same species will taste very differently depending upon can have a big effect on their flavor. times of year has to do with how conditions are favorable for spring plex carbohydrate consisting of a The flavors of oysters are most ripe the gonads of the oysters might phytoplankton blooms and intense string of glucose (sugar) molecules where they are grown and the season of the year. often described as having three phas- be during a particular season. Dur- filter feeding by the oysters. During in a chain. es: an initial first impression stage ing the spring season in the north- this period of abundant food and Oysters are the fattest just prior involving saltiness, a second stage ern hemisphere for example, oysters relatively cool waters, oysters un- to their late spring or early summer involving body and sweetness, and a emerge from inactivity during the dergo rapid gonad maturation and spawning period, with ripe gonads, final third stage, often described in cold winter and are met by lengthen- build-up of an energy storage mol- displaying a creamy-colored appear- terms like floral, fruity, or metallic aftertastes or finishes. But what are the factors that affect the flavors of oysters and how? Of course the spe- cies of an oyster has a great influence upon its taste. For instance, various popular oysters such as the Eastern oyster, Crassostrea virginica, the Pacific oyster Magallana (Crassostrea) gigas, the Olympia oyster, Ostrea lurida and the European flat oyster, Ostrea edu- lis, all have distinctive flavors that are characteristic to the species. Pacific oysters have what is described as a robust full bodied flavor, whereas Eastern oysters are often described as having a salty taste of the sea and Figure 1. A green-gilled Crassostrea gigas oyster grown in the Marennes Oléron Bay Region and finished in claires for greening of the gills with marennine algal pigment. Photo courtesy of Cagette Canteen and Deli, Bangkok, Thailand. a more delicate, sweeter flavor. The Olympia and European oysters are frequently described as having a full- his variation in taste in istics, terrain and drainage, sunlight, bodied flavor but with a slight metal- oysters parallels very water quality, microclimate, etc. all lic finish. closely a similar phenom- contribute to a unique flavor that The merrior of an oyster is most enon that occurs with va- encapsulates a particular place and prominently experienced when eat- rietalT wine grapes. In varietal wines, time. Those who market wines are ing oysters of the same species, but terroir is the concept that their fla- well-accustomed to the value of ter- harvested at different times of the vors are derived from a sense of rior, as wines from particular vine- year or harvested from different wa- place. The vineyard’s soil character- yards and chateau wineries can com- ter bodies. One of the largest differ-

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comes through. It is for this reason their enormous popularity, at least that oysters grown in higher salinity among the French aristocracy of waters are often sought after as hav- the time. As time went on, the prac- ing good merrior. tice of finishing oysters prior to sale One of the largest differences Salinity is not the sole criteria for evolved into a process of placing in taste of oysters at different good merrior. Different trace min- them into shallow managed ponds erals, in combination with salinity, called claires in which the pond wa- times of year has to do with how temperature, and time of year will ter could be managed to at least par- ripe the gonads of the oysters determine the timing of various phy- tially control the blooms of various toplankton blooms that feed oysters. phytoplankton species (Figure 2). might be during a particular However, phytoplankton blooms It was eventually found that the season. occur in various patterns: as cycles, greening of the oyster gills was trends, fluctuations, unusual events caused by oysters eating a specific and irregular pulses. And these can opportunistic diatom, Haslea os- occur at various time scales: hourly trearia, that produces a water-sol- or less, daily, seasonally, annually or uble green pigment now known as combined amino acids that could, if over decades, and even chaotically, marennine [See: Gastineau et al. assembled, become protein chains. at varying frequencies (See: Smayda. 2014. Marine Drugs 12(6):3161- For example, the free amino acids 1998. ICES Journal of Marine Sci- 3189]. Management of the phyto- that are most abundant in high con- ence 55: 562-573). Since different plankton blooms in the claires has centrations in Pacific oyster tissues phytoplankton have differing nutri- focused upon promotion of Skel- are glycine, alanine, serine, aspartic tional value to oysters and can also etonema costatum and other diatoms acid, and glutamic acid, along with impart different flavors, it is no won- for fattening and H. ostrearia for the non-protein amino acid, tau- der that locations prone to having the “greening” of the oysters prior rine [See: Rice and Stephens. 1987. blooms of favorable phytoplankton to sale [See: Soletchnik et al. 2000. Aquaculture 66:19-31]. These free strains with greater frequency are Aquaculture 199:73-91]. Thus the amino acids impart a rich pleasant the best for good merrior. There oyster farmers of Marennes Oléron flavor to oysters, and other inver- are known species of phytoplankton Bay truly show that assuring good Figure 3. Matunuck Oyster Farm, South Kingstown, Rhode Island USA. Photo by M.A. Rice. tebrates, that the Japanese call the such as the chain-forming diatom merrior in oyster farming need not umami taste. Indeed, one free amino Skeletomema costatum and boat-shaped necessarily be a haphazard process ance to the soft tissues that fill up Oysters in the temperate zone will cally active) molecules inside the cell acid, glutamic acid, in its monoso- diatoms of the genus Navicula, of farm site selection, but it can also the shell cavity of the oyster. This typically regain some stored glyco- is higher than the concentration of dium form (MSG) has been used among others, that are known to im- be a managed process that adds con- is the time when oysters have their gen, and thus some their flavor, as dissolved osmotically active ions for many decades as a food flavor part good flavor to oysters, so oys- siderable value to the product. sweetest flavor and pleasing texture. the result of autumn phytoplankton in the seawater, then water will be enhancer. And umami taste receptors ter grounds in areas with frequent Later in the summer, when the oys- blooms and building up glycogen drawn into the oyster cells from the have been discovered on the human blooms of favored phytoplankton ters have spawned out, their meat is stores to fuel their next overwinter seawater and the cells will swell, or tongue, providing a neurophysiolog- species would have the best merrior. thinner and glycogen is expended, period of low temperature inactivity. even burst if the concentration dif- ical basis for adding umami to the list The oyster farmers likely to have so their taste becomes more bland. The salinity of growing waters ference is drastic. Conversely, if the of basic human tastes that have tra- the best understanding of the mar- also has a profound influence on the concentration of dissolved osmoti- ditionally included sweet, sour, salty keting value of merrior are those of flavors of oysters. Oysters, like many cally active ions in seawater becomes and bitter [See: Chaudhari, Landin, the Marennes Oléron Bay Region marine invertebrates, physiologically higher than the osmotic concentra- and Roper. 2000. Nature Neurosci- (45.78N, 1.11E) in the Charente- ence 3(2): 113-119]. Maritime Department of South- The flavors of oysters are most adapt to higher or lower water sa- tion in the oyster cells, the cells will linities by adjusting the concentra- shrink in volume as water is drawn The overall concentration of free western France. This region has a often described as having three tion of dissolved substances in their out of them. amino acids in oyster tissues has a long history of oyster farming dat- profound influence on oyster taste. th phases: an initial first impression cells to match the salinity or osmotic Seawater, of course, is primar- ing back to well before the 17 cen- concentration of their aquatic envi- ily made up of sodium and chloride When oysters and other osmoconform- tury, and part of the traditional fin- stage involving saltiness, a second ronment. This physiological process ions, and to a lesser extent magne- ing invertebrates adapt to low salinity ishing process was to place market Michael A. Rice, PhD, is a Professor of Fisheries, stage involving body and sweetness, of maintaining relatively constant sium, sulfate, calcium and other ions water they lose free amino acids in sized oysters into salt marshes for a Animal and Veterinary Science at the University of their cells, and as a result, they as- month or more prior to sale so that Rhode Island. He has published extensively in the cell volume by oysters in variable and molecules as well that gener- areas of physiological ecology of mollusks, shell- and a final third stage, often salinity is referred to by physiolo- ally impart a salty taste. However, sume a bland ‘washed out’ flavor. the oysters fattened and developed a fishery management, molluscan aquaculture, and aquaculture in international development. described in terms like floral, fruity, or gists as osmoconforming, or matching the primary osmotic solutes inside Conversely, when the same oysters greenish color to their gills (Figure He has served as Chairperson of his department at the osmotic pressure of fluids in- oyster cells, and the cells of many are raised in higher salinity waters, 1). King Louis XIV of France was the University of Rhode Island, and as an elected metallic aftertastes or finishes. member of the Rhode Island House side and outside of the cells. If the other delicious marine invertebrates, there is a gain in free amino acids in reported to be fond of these green- of Representatives. concentration of dissolved (osmoti- are free amino acids. These are un- their tissues and a rich umami taste gilled oysters, thus contributing to [email protected]

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