|MadSci Network: Cell Biology|
The pigments in beetroot are called betalaines and are found in all plants belonging to a taxonomic group known as "Caryophyllales". There they replace the xantophylles as pigments found in the vacuole and have a color-span from yellow to orange. In contrast to the xantophylles they do not show a pH-dependent change of color. The amount of pigment extracted from beetroot pieces is only indirectly dependent on the concentration of detergent in the solution you use. Detergents help bring fat and lipids into solution. For this reason they are used in many biochemical methods to help break down membranes. In your experiment higher detergent concentration leads to a more efficient membrane destruction, opening more vacuoles and so setting free more pigment. Information on this can be found in every biochemistry textbook ("Biochemistry", Stryer) and most chemistry textbooks.
Try the links in the MadSci Library for more information on Cell Biology.