you can  go with one of those two methods :1- weight 5 g of homoginzed sample , reflux in about 100 ml methanol 80% at  70 c  then centrifuge at 3000 rpm collect the supernatant and recentrifuge the residue with methanol 80% , evaoprate the superntant for dryness and redisolve in suitable solvent for your column , detector and mobile phase ,filter through   a 0.45-μm poresize filter                                                         2- extract  potato juice by a mixer then 

 centrifuge the juice at 3000 rpm for 30 min, dilute the superntant  1:5 by

adding HPLC grade water and  passe through

a 0.45-μm poresize filter.

which column and detector do you use ?