Color
Besides a bland odor and taste, a light color is also a quality
parameter for most refined oils. In the refining process, the color
components can be removed during the bleaching process by adsorption
onto a suitable bleaching earth or by thermal degradation during
deodorization. Heat bleaching is a purely time-temperature-dependent
reaction. Literature data on heat bleaching of palm oil indicate that
the rate of carotene breakdown doubles per 20°C increase in temperature
and heat degradation of carotene takes a few hours at 210°C but only a
few minutes at 270°C (Shahidi, 2005). Similarly, to conform to the trend
of minimal processing, specific restrictions of R value have been
revoked and replaced with descriptive words in the newest Chinese
national standards for these refined oils.
Fig. 8 shows the comparisons of R-values of four oils (soybean oil,
rapeseed oil, maize oil and sunflower seed oil) between conventional
high-temperature deodorization and the DCDT deodorization. The majority
of oils have an R-value below 1.5R. For DCDT deodorized oils the R-value
of sunflower seed oil was the lowest (≤1.0R), followed by soybean oil,
rapeseed oil and maize oil. Except sunflower seed oil, R values of the
other zero-TFA oils were all slightly higher than corresponding
conventional ones.