The heat transfer in plastic tubes is a lot lower than glass, as such, if using plastic tubes, we would recommend to double check that the temperature of the solution within the tubes is at right temperature with a normal thermometer, otherwise we would advise to possibly increase the extraction time to 15mins to ensure all fructan is being extracted.
We would also recommend to run the provided kit controls in this scenario to assess if the use of plastic tubes gives enough heat transfer to provide the given value.
Any deviation from the assay procedure would require validation in your laboratory.