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  • Spectrometer Measures Salt in Salmon Caviar
Feb 2001
For years, the preparation of ikura -- also known as salmon caviar -- was something of an art form. Producers soaked the harvested eggs in a brine solution that extended the final product's shelf life and, if done correctly, preserved taste and texture. But preparation of ikura has undergone a sea change as modern diets have begun demanding lower salt content. Coincident with this demand, markets for the roe have expanded eastward across the Atlantic, which have brought about more stringent export regulations on ikura's shelf life.
   Combined, these trends have made it more and more important to determine the roe's salt content quickly and nondestructively. A group of researchers in the Pacific Northwest may have found a solution in the form of a fiber optic spectrometer from Dsquared Development Inc.
   Before the days of refrigeration, shelf-stable ikura required salt content to be in the neighborhood of 12 percent to slow the growth of pathogens. Refrigeration allows the amount of salt to be reduced to 3 or 4 percent. But aside from tasting ikura -- sometimes costing more than $15 a lb wholesale -- the only method for judging its saltiness relies on slow and destructive wet chemistry techniques.
   "If we can get a better handle on salt content, we can give people a better idea of shelf life," said Barbara Rasco, a researcher at Washington State University's department of food science and human nutrition. "What sent us in this direction is increased federal regulations on food safety. [Making ikura] has been an art, but the feds aren't interested in art."

Diffuse reflectance spectrometry using a fiber optic probe could fish up a nondestructive way to evaluate salt content in salmon caviar. Courtesy of Eastern Oregon University.

   Dsquared's solid-state spectrometer measures diffuse reflectance from a tungsten lightbulb using a linear variable filter and an imaging array. "You could take any near-infrared technology to analyze salmon roe, but the linear variable filter, together with the array, basically gives you a system that's very small, rugged and stable against temperature and vibration," said David Mayes, president of Dsquared Development.
   The fiber optic probe is integral to the instrument's design, according to Anna Cavinato, an assistant professor of chemistry at Eastern Oregon University and one of the researchers. "The idea was to have a large number of fibers to send a lot of light into the sample," she explained. Thirty-two illuminating fibers are arranged in a concentric circle around a single pickup fiber that leads to the charge-coupled device detector.
   Obtaining measurements through diffuse reflection in ikura requires an abundance of light from a strong illumination source. One benefit of the probe is that it's fast, allowing 200-ms acquisition times because of the powerful light source, and the fiber optic delivery prevents the lamp's heat from denaturing the sample.
   Dsquared's spectrometer uses standard 600-µm-core low-OH fiber, said Mayes, to transmit light between 600 and 1100 nm. "We used the low OH because it has better throughput than your garden-variety fibers -- plus it's not very expensive," he explained.

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