The role of plant cell wall encapsulation and porosity in regulating lipolysis during the digestion of almond seeds

Abstract

Previous studies have provided evidence that the physical encapsulation of intracellular nutrients by cell walls of plant foods (i.e. dietary fibre) plays a predominant role in influencing macronutrient bioaccessibility (release) from plant foods during human digestion. One unexplored aspect of this is the extent to which digestive enzymes can pass through the cell-wall barrier and hydrolyse the intracellular lipid in almond seeds. The purpose of the present study was to assess the role played by cell walls in influencing the bioaccessibility and digestibility of almond lipid using a range of techniques. Digestibility experiments were performed on raw and roasted almond cells as well as isolated almond oil bodies using in vitro gastric and duodenal digestion models. Residual triacylglycerols and lipolysis products were extracted after 1 h of incubation and analysed by thin layer chromatography. The lipolysis kinetics of almond cells and oil bodies were also investigated using the pH-stat technique. Finally, the potential penetration of pancreatic lipase through the cell wall matrix was investigated using confocal microscopy. Differences in the rates and extent of lipolysis were clearly seen between almond cells and oil bodies, and these differences were observed regardless of the lipase(s) used. These results also showed that almond cell walls that are completely intact limit lipid digestibility, due to an encapsulation mechanism that hinders the diffusion of lipase into the intracellular environment and lipolysis products out of the cells.

A review of the impact of processing on nutrient bioaccessibility and digestion of almonds

“…This swelling of the cell wall may explain why intact cells lose lipid after longer retention times, suggesting that lipase, colipase and bile salt could diffuse into the intracellular compartment and then initiate lipolysis. However, lipase does not seem to diffuse through the intact cell walls even after prolonged incubation times (up to 20 h) as demonstrated by in vitro digestion experiments performed on laboratory-separated almond cells (Grundy et al, 2016). Nevertheless, in small particles of masticated almond, there was some evidence of rupture and fissures in 'damaged cells' underlying the fractured surface and this may account for the lipid release that occurs after prolonged incubation in the GI tract (Mandalari et al, 2008a;Grundy et al, 2015a).…”

“…These results strengthen the assumption that by increasing the number of fractured cells through either processing or mastication, the bioaccessibility of nutrients, especially lipids, is enhanced. More recent studies have confirmed that almonds consumed as the whole kernel (raw or roasted) were not fully digested, and the lipids were released slowly during the digestion process (Grassby et al ., ; Mandalari et al ., ; Grundy et al ., ,b, ). This behaviour is strongly linked to the resistance of almond tissue/cell walls to chemical and physical breakdown in the mouth, stomach and small intestine.…”

“…The roasted almonds used in our recent studies (Grassby et al ., ; Mandalari et al ., ; Grundy et al ., ,b, ) were provided by the Almond Board of California and were roasted using a two‐step standard procedure of hot air roasting with typical temperatures ranging from ~130 to 154 °C (Almond Board of California, ). The first step employed an intermediate temperature to stabilise the nut microstructure, and the second step was performed at a higher temperature in order to generate the distinctive roasted flavour and brown colour of the cotyledon.…”

“…Nonetheless, it remains unclear to what extent lipolysis occurs inside almond cells and whether the lipids are able to leave the cells as TAG molecules or hydrolysed products. Even though lipase appears to be able to penetrate inside some cells, most likely the damaged ones, much of the lipid (TAG and/or lipolytic products) remains encapsulated inside the almond cells (Grundy et al, 2016). Regardless of the mechanism involved, the rate and extent of digestion of the lipids present in these unfractured cells is significantly reduced, as that they are less accessible to emulsification and digestion by the lipases (Grundy et al, 2015b).…”

“…This behaviour is strongly linked to the resistance of almond tissue/cell walls to chemical and physical breakdown in the mouth, stomach and small intestine. When the oil bodies are released from the almond tissue, as this is the case in almond milk, they are highly digestible and the rate and extent of lipolysis is similar to emulsified almond oil (Beisson et al, 2001a;Gallier & Singh, 2012;Grundy et al, 2016). If not in the form of oil bodies, almond oil, like any other edible plant oil, is required to be emulsified and its susceptibility to digestion relies on the size and interfacial quality (e.g.…”