Effects of simulated digestion in vitro on cell wall polysaccharides from kiwifruit (Actinidia spp.)
Highlights
► In vitro upper-intestinal tract digestion of green and gold kiwifruit pulp. ► Yields of insoluble fibre decreased; soluble fibre increased. ► Changes in molecular weight profiles of pectin-rich soluble fibre fractions. ► Decreased methylesterification of galacturonic acid in soluble fibre fractions.
Introduction
Kiwifruit contain about 2–3% non-starch polysaccharides (Ferguson & Ferguson, 2003) that make up the fruit cell walls and are considered a good source of both soluble and insoluble dietary fibre. Both green (Actinidia deliciosa [A. Chev.] C.F. Liang and A.R. Ferguson) and gold (Actinidia chinensis Planch.) kiwifruit contain pectic polysaccharides, hemicelluloses and cellulose in varying proportions (Dawson and Melton, 1991, Redgwell et al., 1988, Redgwell et al., 1992, Sauvageau et al., 2010, Schröder et al., 2001). The major pectic polysaccharides include homogalacturonans and rhamnogalacturonans substituted with galactan and arabinogalactan side-chains, while the hemicellulosic polysaccharides include xyloglucan, glucuronoarabinoxylan and galacto-(gluco)-mannan.
The polysaccharides of plant cell walls are resistant to digestion by human enzymes in the small intestine and are delivered to the colon in what has been assumed to be a chemically unaltered state, where they are fermented by intestinal microbiota. However, there are very few data confirming the validity of this assumption. Whether or not chemical or structural changes occur when fruit cell walls are exposed to gastric acidity, followed by an influx of alkali during entry into the small intestine is uncertain, but important, because even minor chemical or structural changes in polysaccharides can substantially change the physicochemical properties that determine their impact on health. For instance, the viscosity of polysaccharides depends on the logarithm of chain length, so a single chain cleavage could greatly affect their digesta properties. Even under neutral conditions pectins can undergo depolymerisation by β-elimination (Voragen, Pilnik, Thibault, Axelos, & Renard, 1995).
There are some reports detailing the amount of dietary fibre reaching the terminal ileum of human subjects. For example, Englyst and Cummings, 1985, Englyst and Cummings, 1987 found more than 90% of the non-starch polysaccharides of some cereals and potatoes were recovered in the ileostomy fluid, whereas most of the starch was digested. Similarly, Saito et al. (2005) showed that about 90% of the pectin fed to male volunteers reached the terminal ileum. However, the methods used for measuring dietary fibre and total pectin could not detect the subtle changes that may, nonetheless, alter polysaccharide structure and physical properties.
Methods for studying carbohydrate digestion in vivo are, generally, time-consuming and costly for routine analyses of digestion products. There are a large number of methods for simulating human carbohydrate digestion in vitro that vary in complexity (Woolnough, Monro, Brennan, & Bird, 2008). The TNO-Intestinal model of the stomach and small intestine is perhaps the most elaborate and enables manipulation of many parameters, including regulation of gastric and intestinal pH, flow of gastric and pancreatic juice including digestive enzymes, peristalsis for mixing, gastrointestinal transit times and continuous removal of digested compounds (Minekus, Marteau, Havenaar, & Huis in’t Veld, 1995). Monro, Mishra, and Venn (2010) have used a much simpler model that allows multiple treatments to be conducted in parallel, to monitor the digestibility of foods. This model is appropriate for an initial investigation of the effects of exposing fruit polysaccharides to conditions that would be typically encountered in vivo. In this study we have subjected green and gold kiwifruit to a similar in vitro digestion and report on the physical and chemical changes that occur in the cell wall polysaccharides.
Section snippets
Materials
ZESPRI® GREEN (Actinidia deliciosa ‘Hayward’; green kiwifruit) and ZESPRI® GOLD kiwifruit (Actinidia chinensis ‘Hort16A’; gold kiwifruit) were harvested at maturity, in May 2010, from a commercial orchard in the Gisborne growing area of the East Coast, New Zealand, and ripened to a ready-to-eat state (0.5–0.8 kgf; Hopkirk, Maindonald, & White, 1996) under ambient conditions. The fruit were peeled, pulped by briefly blending in a laboratory blender (waring) on a low speed until all lumps of
In vitro digestion of kiwifruit
Changes in the proportions of the soluble and insoluble dietary fibre fractions with in vitro digestion are shown in Fig. 2. The presence of a substantial proportion of seed in the insoluble material would have had a significant influence on the results of the carbohydrate analysis, had the seeds been included in the analysis, because the seed coat consists of carbohydrates resistant to digestion and also encapsulates storage polysaccharides, protecting them against digestion. As the seed coat
Discussion
The aim of this work was to measure the effects of exposing kiwifruit cell wall polysaccharides (dietary fibre) to simulated upper-intestinal tract digestion, in order to provide insights into the composition and structure of the polysaccharides that would be available for bacterial fermentation in the large intestine. Our data indicate that, although there was a small decrease in the yields of insoluble fibre with simulated gastric and gastrointestinal digestion, the types and chemical
Funding Sources
This work was funded by Zespri International Limited (Contract No. 25534).
Acknowledgements
We gratefully acknowledge the expert advice of Lynley Drummond (Zespri International Limited, New Zealand) and Juliet Ansell (Plant and Food Research Ltd., Palmerston North New Zealand).
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