Many species of cacti contain a mucopolysaccharide that has two orders of
branching and binds water very efficiently. I suspect that the mucus
binds other things as well; organic acid titrations have a lot of noise
if the mucilage isn't dealt with in some manner.
Some things I've tried in the past, and which you may consider: cactus
mucilage can be precipitated with isopropanol, although this may
interfere with your enzyme assays. I've also found that letting the
extract sit on the bench for a few minutes causes the mucilage to settle
to the bottom of your solution; centrifugation may work even better. You
may also want to try filtration with nylon netting, which seemed to
separate mucilage from the rest of the extract solution.
Michael E. Loik
Rocky Mountain Biological Laboratory,
Crested Butte, CO