of its digestion in the small intestine and, based on an in vivo‐supported in vitro digestion method (Englyst et al. 1992), it is described by its content of rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). The latter is considered as a type of dietary fibre with a prebiotic property (Bird et al. 2010). Whole grain dietary fibre is composed of a heterogeneous class of mostly polysaccharides with high complexity in their sugar unit and linkage compositions, molecular structures, and 3‐dimensional conformations, which lead to different physiochemical properties as well as human body functions, including modulation of starch digestion and glucose absorption, and, through changes of the gut microbiota, impacting of the immune system and intestinal epithelial barrier function (Anderson et al. 2009). The physiological quality of available or glycemic carbohydrates is captured in a number of ways with the most common being glycemic index (GI) (Jenkins et al. 1981), which is based on postprandial glycemic response. Many low GI foods have a high amount of dietary fiber (Björck and Elmstathl 2003), suggesting that dietary fibres may lower glycemic response. The combination of glycemic and non‐glycemic carbohydrates, as indispensable parts of whole grains, is related to the concept of whole grain foods having good “carbohydrate quality.”
A good understanding of the mechanism and role carbohydrates play in the health benefits associated with whole grain foods is a necessary basis to develop strategies to optimize their impact in unprocessed and processed foods. This is of particular importance for the starch component, as whole grains that are further processed to commercial products often significantly increase glycemic response. Many whole grain foods, as approved in regulatory systems, are also composed of recombined flour streams. The outcome of these processes on the health‐related property of final whole grain food products is not well understood and conceivably could be neutral, negative or positive in regards to glycemic response, as well as on the action of the dietary fiber component in the gastrointestinal tract, including its effect on the gut microbiota. The study of Mozaffarian et al. (Mozaffarian et al. 2013) on whole grain products showed that a ratio of total carbohydrate (i.e., mainly glycemic carbohydrate) to dietary fibre of ≤ 10:1 was characteristic of healthy whole grain food products. In addition to the composition of dietary carbohydrates, the spatial arrangement of starch and dietary fibre in the food matrix is also important to the digestion property of starch and probably to the health benefit of whole grain foods (Heaton et al. 1988; Björck et al. 1994).
4.2 General composition of whole grain carbohydrates
There are different ways to classify whole grain carbohydrates (Table 4.1) focusing on different aspects, including chemical structure and complexity as well as nutritional bioavailability (rate, extent and location of digestion/fermentation). While simple monosaccharides can be directly absorbed, they are in very low amount in whole grain foods. The main carbohydrates are starch and non‐starch polysaccharides with highly complex structures (Table 4.2). Their contribution to carbohydrate quality in whole grain foods is the focus of this chapter.
4.3 Dietary fibre
Historically, dietary fibre was a term first used by Hipsley (1953) to describe nondigestible food constituents referring to plant cell wall materials, including cellulose, hemicelluloses and lignin. With the advancement of scientific investigation, as well as new methodologies for dietary fibre measurement, dietary fibre is essentially now considered another food macronutrient. Organizations and regulatory agencies have their own definitions, such as the American Association of Cereal Chemists (AACC 2001), CODEX Alimentarius Commission (CODEX 2009), European Food Safety Authority (EFSA 2010), US Food and Drug Administration (FDA 2016) and the Institute of Medicine (IOM 2001). Among these definitions, CODEX Alimentarius is broadly accepted and represents a relatively comprehensive and flexible definition reflecting the current knowledge of dietary fibre. Dietary fibre as defined by CODEX Alimentarius is as follows:
Table 4.1 Classification of whole grain carbohydrates.
| Carbohydrate | 2nd Level Classification | Examples | Digestibility | |
|---|---|---|---|---|
| Simple | Monosaccharide | Glucose, fructose, galactose | Direct Absorption | |
| Carbohydrate | Disaccharide | Sucrose | Rapid | |
| Complex | Starch | RDS | Cooked waxy starch | Rapid |
| Carbohydrate | (glycemic carbohydrate) | SDS | Raw corn starch | Slow |
| Non‐starch | RS | Retrograded amylose | Fermented | |
| polysaccharides | Soluble | β‐glucan, arabinoxylan, pectin | Partially | |
| (dietary fibre) | Insoluble | Cellulose, hemicelluloses | fermented | |
| Oligosaccharide | Fructans | Fermented |
Table 4.2 The composition of whole grain carbohydrates (% dry weight) (Choct 1997; Collins et al. 2010).
| Cereal | Starch | Arabinoxylan | β‐Glucan | Cellulose | Glucomannan | Galactan | |
|---|---|---|---|---|---|---|---|
| Wheat | 68 | 8.1 | 0.8 | 2.0 | –* | 0.3 | Barley |
| 60 | 7.9 | 4.3 | 3.9 | 0.2 | 0.2 | Rye | 69 |
| 8.9 | 2.0 | 1.5 | 0.3 | 0.3 | Triticale | 53 | 10.8 |
| 1.7 | 2.5 | 0.6 |
|
