The RP-HPLC-fluorescence method developed in this preliminary
study allows for the first time to directly measure steviol
equivalents in commercial steviol glycoside mixtures and different
food matrices. The application of the long-known observation that
all steviol glycosides are degraded to a single compound (isosteviol)
upon acid hydrolysis allows for a more accurate determination
of the total amount of steviol equivalents added to foods.
Furthermore, the use of an internal standard that can easily be prepared
chemically significantly reduces analytical errors. By using
4-(bromomethyl)-7-methoxycoumarin as a derivatizing agent for
aromatic and heterocyclic acids, isosteviol and dihydroisosteviol
can be measured by a highly specific, fluorometric detection which
is significantly more sensitive than detection methods based on UV
(Minne et al., 2004). Detection by MS on the other hand is reportedly
either more sensitive (Well et al., 2013) or less sensitive
(Morlock et al., 2014; Shah, De Jager, & Begley, 2012) by
comparison.