Why is the β-Glucan Assay Kit (Yeast and Mushroom) (K-YBGL) not suitable for measuring yeast β-glucan in the presence of cellulose (1,4-β-D-glucan)?

In this procedure total glucan is measured by hydrolysis with H₂SO_4, to glucose. 

α-Glucan is measured specifically using a starch assay procedure. α-glucans found in maltodextrins, sucrose and trehalose are also included. β-glucan is then determined by difference. 

Under the assay conditions (acid hydrolysis) beta-glucans present in the sample will be hydrolysed, including 1,3, 1,3:1,6, 1,3:1,4-β-glucan and most of 1,4-β-D-glucan (cellulose), which will lead to overestimation of 1,3:1,6-β-glucan.

Product Page (K-YBGL)