Determination of phenolic acids in plant cell walls by microwave digestion

Microwave digestion (750 W for 90 s) with 4 M NaOH was used to release esterified and etherified hydroxycinnamic acids from cell walls of maize (Zea mays L), wheat (Triticum aestivum L), barley (Hordeum vulgare L) and oilseed rape (Brassica napus L) stems. Subtraction of values for saponifiable phenolic acids obtained after treatment with I M NaOH at room temperature from digest results provided a measure of β‐ether linked units. These were exclusively (E + Z)‐ferulic acid in the cereal straws. Only trace amounts of ether‐ and ester‐linked hydroxycinnamic acids were released from the dicotyledon, ripe straw. Microwave digestion was shown to be an order of magnitude more effective than dioxane‐HCl at liberating β‐ether bound phenolic acids and as effective, but substantially quicker, than previously described high‐temperature alkaline digestions.