Cinnamic acid–carbohydrate esters: An evaluation of a model system

Abstract Chemical esterification of cinnamic acids to cell walls has been proposed to simulate the negative effect of these acids on fibre digestion by ruminants. In this model a spontaneous reaction between an acid chloride and a free hydroxyl group forms an ester linkage between p‐coumaric acid (PCA), for example, and cell wall carbohydrates. This study was conducted to evaluate that system. Esterification of p‐coumaric acid (PCA) to switchgrass ( Panicum virgatum L) neutral detergent fibre (NDF) significantly ( P <0·05) increased PCA concentration 12·3 to 32·7 mg g −1 NDF and reduced ( P <0·05) NDF digestibility from 385·4 to 244·4 mg g −1 NDF. Treated NDF (0, 50, 100 mg PCA g −1 NDF) was sieved through screens with diminishing mesh size in order to isolate fractions with increasing surface area per unit mass. Measured PCA concentration was 50% greater in PCA‐treated fractions isolated by 0.11‐mm mesh than in fractions isolated by 0·50‐mm mesh, suggesting that esterification of PCA occurred predominantly at the surface of the particle. In addition, at the highest level of PCA treatment, digestibility of the fraction isolated from the smallest mesh sieves was 17% greater than that of the material from the largest mesh sieve. The predominant placement of PCA on the particle surface constitutes a major limitation of this technique. Other serious limitations include the potential for polymerisation of cinnamic acids with free hydroxyl groups prior to esterification to fibre, and the non‐specific placement of the ester linkages on the fibre. All three factors cause the model fibre to depart substantially from the characteristics of plant cell wall.