For maintenance of general good health, no vitamin C is necessary other than that from a healthy diet. Almost all Beta glucans selling very inexpensively are either derived from oats and barley and are in essence fiber, or are very large particle sizes which have had no or minimal processing. Some are even whole yeast cell walls without the Beta glucan extracted at all. More and larger are not better in Beta glucans. You get what you pay for in Beta glucans as in most other products.
Purification is the removal of something to the extent possible.
So it is with Beta glucans.