The β-Glucan Assay Kit (Mixed Linkage), K-BGLU, measures all beta-glucan, even if it is hydrolysed down as far as disaccharide when the streamlined procedure is followed for powder samples.
In a modified procedure for liquid samples, with ethanol precipitation beta-glucan oligosaccharides will not be measured. We cannot provide the information about molecular weight cut off point, as we have not measured it. It would be also quite difficult due to lack of relevant standards and their costs.
Alternatively, you can use borohydride reduction instead of ethanol precipitation. This method can be found where K-BGLU appears on the Megazyme website under the "Documentation" tab. Please see the link below.
https://www.megazyme.com/documents/Other_automated_assay_procedures/K-BGLU_ALT.pdf
Please note that the borohydride reduction method will also underestimate the oligosaccharide content. This underestimation is due to the reduction of terminal reducing sugars in the oligosaccharides to sugar alcohols (sorbitol), which are not measured in the current assay.
The underestimation will depend on the degree of hydrolysis of the original beta-glucan.