In search of the perfect oyster

What is actually a “good” oyster? How can we evaluate and grade ”quality”? And how can we produce the quality we wish?

The European flat oyster has been used as food as long as man has inhabited the European coastline. The ancient Romans established oyster farms, and oyster culture and harvest gradually developed as an important activity along the coasts of Europe. Today, the oyster industry is important. The Pacific oyster, which is now the dominant species, is cultivated all over the world at the incredible rate of more than three million tonnes a year.

All over the world, oysters are a popular seafood, which are presented in a variety of species, forms and qualities. Many oyster consumers have strong opinions of the quality and origin of their preferred oysters. But, in spite of the long history of oyster culture and the high status of this seafood product, there are surprisingly enough, neither standardised quality grades nor defined, applicable quality parameters . . . and very few scientific papers describing the sensory quality of oysters have been written. This field is a wide-open challenge . . .

Stein Mortensen (Institute of Marine Research) and Arne Duinker (NIFES) have taken up the challenge. Mortensen has strong opinions about oysters and is convinced that Norwegian oyster growers are capable of producing ”the perfect oyster” – if they can learn how to optimise quality through a perfect production chain, including grading and correct handling. Without measurable quality standard parameters the industry will never attain this goal.

The first step of the work has been to define some sensory (appearance, taste, smell, texture) parameters to be used to evaluate oyster quality. Chef Morten Schakenda joined a team from the sensory panel at NORCONSERV in Stavanger, under the supervision of colleagues from the Norwegian University of Agriculture at Ås, in order to define some recognisable sensory attributes.

These were used in an experiment in which oysters were stored live under various conditions for several weeks. A panel of experts with different backgrounds and expertise then sampled and analysed the oysters. Did the sensory profile of the oysters change? How? How fast? Did they die as a result of prolonged storage times and unsuitable conditions? Did the bacterial load increase? And were there changes in the different oyster tissues? The answers to these questions have been published in the August 2004 issue of the Journal of Food Science.

Storage in fresh seawater best

The results obtained by the research team showed that the sensory profile of live oysters stored on ice and in a cool storage room was significantly different from that of live oysters stored in seawater. Oysters kept in freshwater ice suffered the most pronounced changes, and also displayed detrimental tissue alterations caused by the low storage temperature. Dead and live oysters could easily be distinguished by measuring the pH of their flesh with a probe pressed directly into the adductor muscle.

A gradual death

Lisbeth Harkestad from IMR and Kristin Hopkins from the National Veterinary Institute open and measure oisters. Studies of death and autolysis of oyster tissues during the experiment also produced some interesting results. As the bivalves lack a central nervous system, individual organs gradually stop functioning one by one. We may say that the oyster dies “a bit at a time”. If the temperature is low – as it should be during storage – the death process is slow. Moribund oysters may in fact respond to a stimulus and be considered ”live” at the same time as an autolytic process is going on inside their ”dead” digestive tissues. Such oysters, of course, are not edible.

The experiments showed that the human nose may be trained to become a very sensitive sensory instrument to evaluate oyster quality. Positive olfactory parameters such as ”sea”, ”fresh fish” and ”shellfish”, and negative parameters like ”mud”, ”rotten seaweed” ”ammonia” and ”spoiled shellfish” are relatively easy to learn. The results also verified the important and practical point that oysters should be kept cool – but not on ice.

Oysters do have a remarkable ability to survive if they are stored correctly – that means cool and humid and with a light pressure on top of them, which keeps them from opening. It is therefore tempting to expect that they can also retain a satisfactory quality for a long period of time. Unfortunately, they do not! If we want “the perfect oyster”, we have to accept that the fresh “sea” taste disappears rather quickly. The oyster stays fresh for only a few days after it has been harvested. It may seem to be a paradox, but professional logistics and live storage facilities are essential – even in the production of a bivalve that can be kept alive for weeks!

Now, Mortensen and Duinker are starting work on the second step in their process – using the experience they have gained from the storage experiments and the establishment of a sensory profile in order to develop a quality grading system that can be used by producers, wholesalers and chefs. “The perfect oyster” has to be good enough. If not, it will soon be “dead” in the market. In collaboration with Eivind Bergtun at Bømlo Skjell and a network of oyster growers south of Bergen, they are collecting and evaluating the quality of oysters in the entire region. Measure, weigh, look and taste . . . According to Mortensen the results are promising. “The perfect oyster” should not be too far away . . .