L*, a* and b *value of EWP were measured with TES-135
material color analyzer, which L* represents transparency, a*
(positive value) represents red degree and b* (positive value)
represents yellow degree [1, 19, 20]. The bigger the L* value
is, the shallower the color is, the lighter the browning is. A*
value and b* value represent internal browning degree and
surface yellowing degree respectively. The theory was identical
to the method of Wardy [21]. Every sample was determined
in triplicate. Take the average as the result.