The proportion of lipid released during mastication of nuts is strongly influenced by particle size, due to natural encapsulation of the lipid by the walls of intact cells. The cell walls may act as barriers to digestion and may partially explain why nuts have reduced metabolizable energy versus the energy content predicted by Atwater factors. The lipid released from masticated nuts can be calculated using a mathematical model which has the cell diameter and the particle size distribution (PSD) of the bolus as variables. This study measured the cell size and PSD of four tree nuts (raw cashews, raw walnuts, roasted pistachios and raw Brazil nuts) to predict the lipid released due to their mastication.

To predict the proportion of lipid released from masticated tree nuts, using measurements of cell size and PSD of masticated tree nuts (including cashews, walnuts, pistachios and Brazil nuts). To compare the results for these nuts to those already published for almonds.

Transverse and longitudinal sections were cut from each nut (including almonds) and micrographs processed using image analysis software to calculate the average cell diameter. Two randomized, un-blinded, cross-over trials were conducted. In each trial, 10 healthy volunteers attended two sessions, at which they chewed eight samples of a randomly allocated nut. For determination of PSDs, the expectorated boluses were sieved (2 boluses) or analyzed by laser diffraction (2 boluses). Initial lipid release was then predicted using the mathematical model.

The cashew cells (34.3 μm) were smaller than the almond (45.1 μm), walnut (49.4 μm), pistachio (53.1 μm), and Brazil nut cells (60.8 μm). Laser diffraction showed that masticated nut boluses had median particle sizes (± SEM) which were smaller (cashews, 178 ± 12 μm; walnuts, 179 ± 8 μm; pistachios, 123 ± 10 μm; Brazil nuts, 145 ± 8 μm) than that for almonds measured previously (550 ± 18 μm). This results in higher predicted lipid release, calculated from the mathematical model, (mean, range) for cashews (12.3%, 8.7–16.3%), walnuts (14.5%, 12.0–18.0%), pistachios (11.0%, 8.7–13.0%) and Brazil nuts (14.4%, 11.9–17.3%) than for almonds (9.5%, 7.4–11.1%).

All of the five nuts had predicted lipid releases of less than 18%, which would be expected to attenuate postprandial lipemia and total nutrient availability relative to that expected due to their total lipid content. Due to their higher lipid content and predicted lipid release, walnuts and Brazil nuts (after mastication) are likely to release more fat on mastication than the other nuts tested.