Carbohydrate Polymers 87 (2012) 1678–1688 Contents lists available at SciVerse ScienceDirect Carbohydrate Polymers j ourna l ho me pag e: www.elsevier.com/locate/carbpol The role of cotyledon cell structure during in vitro digestion of starch in navy beans Thilo Berg a , Jaspreet Singh a,∗ , Allan Hardacre b , Mike J. Boland a a Riddet Institute, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand b Institute of Food Nutrition and Human Health, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand a r t i c l e i n f o Article history: Received 28 July 2011 Received in revised form 19 September 2011 Accepted 26 September 2011 Available online 1 October 2011 Keywords: Navy beans Microstructure Cotyledon cells Cooking In vitro starch hydrolysis Beans digesta Storage High pressure treatment a b s t r a c t Studies on the physico-chemical, microstructural characteristics and in vitro (under simulated gastric and small intestine conditions) starch digestibility of navy beans were carried out. The microstructure of raw and cooked beans observed through scanning electron microscopy (SEM) showed the presence of hexag- onal or angular shaped cotyledon cells (50–100 m size) containing starch granules with a size ranging between 10 and 50 m. The extent of starch hydrolysis (%) after 120 min of in vitro gastro-intestinal digestion differed between whole navy beans (∼60%) and milled bean flour and bean starch (85–90%) after they were cooked under similar conditions. Starch hydrolysis (%) increased significantly when the cotyledon cells in the cooked whole navy beans were disrupted using high pressure treatment (French press). The storage of freshly cooked whole beans resulted in a lower (40–45%) starch hydrolysis whereas re-heating increased the same to 70–80% during in vitro small intestinal digestion. The SEM pictures of cooked navy bean digesta after different intervals of in vitro gastric and small intestinal digestion showed that the cotyledon cell structure is maintained well throughout the digestion period. However cotyledon cells appear shrunken and developed wrinkles during in vitro digestion. Particle size analysis of cooked bean paste taken before and after the in vitro gastro-intestinal digestion showed similar particle size distributions. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Beans (Phaseolus vulgaris), a good source of protein and car- bohydrates in human diets, are widely grown and consumed in developed as well as developing nations of the world. Apart from proteins (20–38%) and complex carbohydrates (50–60%), beans are rich in minerals, vitamins and polyunsaturated free fatty acids (Rehman & Shah, 2005). The glycemic index of beans is generally low and postprandial glucose response is moderate after inges- tion which makes them a preferred source of energy (Jenkins et al., 1981). Furthermore they contain high levels of starch that escapes hydrolysis in the small intestine (resistant starch) and is also known for its prebiotic properties (Rehman et al., 2001; Vargas-Torres et al., 2004). Starch in legumes is naturally situated inside the living cotyledon cells (Hahn, Jones, Akhavan, & Rockland, 1977). Primary cell walls of growing and fleshy tissues have a conserved general composition of cellulose, hemicelluloses and pectin (Chanda, 2005). The non-cellulosic material acts as a “glue” that holds the microfib- rils of cellulose together which in turn is responsible for the stability of cell walls (Carpita & Gibeaut, 1993). The starch granules in beans ∗ Corresponding author. Tel.: +64 6 3505062; fax: +64 6 3505655. E-mail address: J.X.Singh@massey.ac.nz (J. Singh). are present in the cotyledon cells and are embedded in the protein matrix of the cellular contents (Daussant, Mosse, & Vaughan, 1983). This situation might restrict the complete swelling of the bean starch during gelatinization due to steric hindrance and other limit- ing effects including restricted water availability. Hahn et al. (1977) and Kon, Wagner, Becker, Booth, and Robbins (1971) observed birefringence of intracellular starch granules when microscopi- cally examining cooked beans using plain polarized light. Wursch, Delvedovo, and Koellreutter (1986) pointed out that the thick and mechanically resistant nature of the cotyledon cell walls in legumes prevent complete swelling of starch granules during gelatinization which may restrict their interaction with digestive enzymes. Starch degrading enzymes are present in digestive fluids as well as in the brush border of the small intestine (Smith & Morton, 2001). The enzymes present in the human body are difficult to extract or expensive to buy, therefore enzymes from other mammals or from microorganisms are usually used in in vitro systems that attempt to simulate the digestive process in the gastro-intestinal tract of human beings. The mammalian enzymes are very similar to human enzymes and are therefore preferred for this work whereas the enzymes from microorganisms may work differently even though they are similarly classified. -Amylase inhibitors present in raw beans are known to inhibit the activity of porcine pancreatic amylase, however these inhibitors 0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2011.09.075