A heat resistant dyed oil (which has the same penetration
behavior in frying products as the frying oil) was prepared
by dissolving 1 g of the fat soluble and heat resistant
stain Sudan Red B (Sigma chemicals) in 1 L of the frying
oil (Bouchon et al., 2003). The oil mixed with the stain
was heated and stirred over a hot plate at 60 C for 4 h
until the stain dissolved completely. Then, the dyed oil
was left to cool to ambient temperature.