From bolus to digesta: How structural disintegration affects starch hydrolysis during oral-gastro-intestinal digestion of bread
Introduction
It is well accepted that food structure strongly affects the rate of starch digestion. Studies have demonstrated the important role of food structure in people's glycaemic response after eating bread (Lau et al., 2015) and their satiety after eating a mixed meal (Moorhead et al., 2006), highlighting the importance of food structure on carbohydrate digestion. Solid food requires considerable physical transformation during digestion. The extensive disruption of the original food structure at each step of digestion diminish potential differences derived from food structural properties. There is a lack of understanding of how structure transformation is linked to starch hydrolysis at various stage of digestion.
Oral processing is the first step of physical disruption of food structure during digestion (Chen, 2009). A study has shown that particle size of rice bolus, varied by the level of mastication (15 vs. 30 times), significantly changed people's glycaemic response (Ranawana et al., 2014). Food bolus is transferred into the stomach where gastric juice penetrates into the bolus, causing enzymatic digestion and acid hydrolysis (Bornhorst and Singh, 2012). Physical characteristics of food bolus and its disintegration in the stomach affect the amount of acid and enzymes penetrates into the food matrix and therefore, the accessibility of starch granule during this hydrolysis process (Bornhorst et al., 2015). Bornhorst and Singh (2013)'s study showed that the disintegration of whole-wheat, barley, rye, sourdough and bread boluses during gastric digestion was significantly affected by bolus cohesive force, which was determined by the initial bread structure and moisture content. The physical characteristics of food chyme modulate gastric emptying and lead to differences in nutrient digestion and absorption kinetics during the intestinal digestion (Kong and Singh, 2008).
It is reasonable to postulate that both bolus formation and disintegration have a significant influence on starch hydrolysis. However, very few studies have been done to correlate these two processes, despite the close relationship have been suggested by in vivo studies (Lau et al., 2015; Ranawana et al., 2014). Also, it remains unclear whether the level of food breakdown at the oral phase will affect the digestion kinetics predicted by the in vitro digestion models. In particular, little information is available on the relationship between the initial bolus structure, its breakdown and the degree of enzymatic degradation at various stage of digestion that interlinked with each other.
The preparation step prior to in vitro starch digestion, either by human chewing, grinding, or cutting, significantly affected the predicted glycaemic response of pasta (Foschia et al., 2014). Our previous study of sandwich bread also showed that human mastication and in vitro oral processing methods affect the predicted digestibility, largely depending on the particle size of the bolus (Gao et al., 2019). However, it is remains unclear how structure transformation at the gastric and intestinal digestion led to such differences in predicted starch digestibility.
This study aimed to understand the relationship between bread structure and its disintegration and digestion in the oral, gastric and intestinal phases using an in vitro model digestion model. Three types of bread with distinctly different structure but the same formulation were used as the model food. In vitro oral processing method varied in the mode of comminution and mixing were compared. The harmonised INFOGEST in vitro protocol was used to serve as a standardised platform to simulate the gastrointestinal digestion. We hypothesised that the physical characteristics of bread bolus significantly affect its structural disintegration at the gastrointestinal digestion and subsequently, the kinetics of starch hydrolysis. By understanding the correlation between the physical and chemical aspects of the food digestion process, food functionality can be optimized for specific digestive outcomes for consumers.
Section snippets
Materials
Bread flour (13.7% protein, Prima, Singapore), vegetable shortening (Redman, Singapore) and instant dry yeast (Saccharomyces cerevisiae, Saf-instant, France), fine salt and sugar were purchased from NTUC FairPrice (Singapore). α-Amylase (from porcine pancreas, 12 units/mg solid), pepsin (from porcine gastric mucosa, 727 units/mg solid), pancreatin (from porcine pancreas, 8 × USP), amyloglucosidase (Aspergillus Niger, > 260U/mL), porcine bile extract and dialysis tubing (cellulose membrane,
Bread characteristics
Baked bread (BB), steamed bread (SB) and French baguette (FB) were prepared using the same formulation but different processing methods, which resulted in very different physical and structural characteristics (Table 1). The FB has the most porous structure (26.7% porosity) with large pores (average pore size 0.87 mm) whereas the SB has the most compact structure (18.2% porosity) with small pores (average pore size 0.79 mm).
Bread crumb of the FB has significantly lower moisture content than
Conclusion
The present study has demonstrated that different in vitro oral processing methods created bread particles with significantly different size and physical properties. Blended particles had a small particle size and a high SSF impregnation efficiency, resulting in a high starch hydrolysis at the oral phase. However, such differences created by oral processing method were gradually diminished along the digestion process. By the end of intestinal digestion, such difference was only observed for the
CRediT authorship contribution statement
Jing Gao: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing - original draft. Eileen Yi Ning Tan: Investigation, Formal analysis, Data curation. Sherlene Hui Ling Low: Investigation, Formal analysis, Data curation. Yong Wang: Conceptualization, Writing - review & editing, Funding acquisition. Jian Ying: Conceptualization, Writing - review & editing, Funding acquisition. Zhizhong Dong: Conceptualization, Writing - review & editing, Funding acquisition.
Acknowledgement
The authors declare that they do not have any conflict of interest. The authors gratefully acknowledge the research funding from COFCO Corporation through Project 101011201600614 at the National University of Singapore (Suzhou) Research Institute, Singapore Ministry of Education Academic Research Fund Tier 1 R-143-000-677-114 and the Agency for Science, Technology and Research (A*STAR), Singapore through IAF-PP FSENH grant H18/01/a0/G11.
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