The high-speed spectrophotometer used a D65 light
source as the standard light, had a 2° field of view and
3×6 mm measured surface, and removed specular
reflection when measuring the a⁎ value of samples. A
reference value was created by taking the average a⁎ of
10 oysters in which no adductor coloration at all was
observed. From the difference between this reference
value and the values measured from samples Δa⁎ was
determined and used as an indicator for the degree of
adductor coloration. When Δa⁎ is higher than 3, it is
possible to confirm reddening even by visual observations