Will samples containing high levels of starch (e.g. mushrooms grown on brown rice, harvested and milled together) cause problems when measuring β-glucan using the β-Glucan Assay Kit (Yeast & Mushroom) (K-YBGL)?

No.

In this method total glucan (plus free glucose and glucose from sucrose) is measured using controlled acid hydrolysis with H₂SO₄ and the glucose released is measured using glucose oxidase/peroxidase reagent. α-Glucan (starch/glycogen) plus free glucose and glucose from sucrose are measured after hydrolysis of starch/glycogen to glucose with glucoamylase and sucrose to glucose plus fructose with invertase and the glucose measured with GOPOD reagent. β-Glucan is determined by the difference. 

The updated K-YBGL kit address any concerns with high starch samples. There should be no issue measuring β-Glucan in high starch samples. 

Product Page (K-YBGL)