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Role of cell walls in the bioaccessibility of lipids in almond seeds^1,2,3

¹ From the Biopolymers Group, Department of Life Sciences, King’s College London, University of London (PRE, YR, JFP); the Department of Nutritional Sciences, Faculty of Medicine, University of Toronto (CWCK and DJAJ); and the BBSRC Institute of Food Research, Norwich, United Kingdom (CP and KWW)

Background: Certain nutrients and phytochemicals in almonds may confer protection against cardiovascular disease, but little is known about factors that influence their bioavailability. A crucial and relevant aspect is the amount of these dietary components available for absorption in the intestine, which is a concept referred to as bioaccessibility.

Objective: We investigated the role played by cell walls in influencing the bioaccessibility of intracellular lipid from almond seeds.

Design: Quantitative analyses of nonstarch polysaccharides (NSPs) and phenolic compounds of cell walls were performed by gas-liquid chromatography and HPLC, respectively. In a series of experiments, the effects of mechanical disruption, chewing, and digestion on almond seed microstructure and intracellular lipid release were determined. In the digestibility study, fecal samples were collected from healthy subjects who had consumed diets with or without almonds. Almond seeds and fecal samples were examined by microscopy to identify cell walls and intracellular lipid.

Results: Cell walls were found to be rich in NSPs, particularly arabinose-rich polysaccharides, with a high concentration of phenolic compounds detected in the seed coat cell wall. During disruption of almond tissue by mechanical methods or chewing, only the first layer of cells at the fractured surface was ruptured and able to release lipid. In fecal samples collected from subjects consuming the almond diet, we observed intact cotyledonary cells, in which the cell walls encapsulated intracellular lipid. This lipid appeared susceptible to colonic fermentation once the cotyledonary cell walls were breached by bacterial degradation.

Conclusion: The cell walls of almond seeds reduce lipid bioaccessibility by hindering the release of lipid available for digestion.

Key Words: Almond seeds • plant cell wall • nonstarch polysaccharides • dietary fiber • phenolics • lipid digestion • bioavailability • microstructure • microscopy • chewing • fecal fat

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