Scientists start unlocking slowly digestible starch secrets

The researchers indicate that the studies are a fundamental advance toward eventually improving food quality with higher amounts of SDS, which could lead to low-glycaemic index foods.

The glycaemic index measures how quickly certain foods release carbohydrates into the body, which then raise consumers' blood glucose levels. High GI foods cause blood sugar levels to rise more rapidly.

There is growing concern about the role of inappropriate diet in chronic non-communicable conditions has prompted campaigns promoting increased consumption of fruits, vegetables and other fibre-rich foods.

At the same time, there has been considerable interest in the role of dietary carbohydrates, which has led to wider use of the glycaemic index to describe the impact that consumption of particular products will have on blood sugar and to provide consumers with information.

The new research, scheduled for the November 13 issue of the American Chemical Society's journal Biomacromolecules​ (doi: 10.1021/bm060343a, 10.1021/bm060342i), reports fundamental research on the structural basis of the slow digestion properties of the natural cereal starches.

Starches can be divided into three groups: rapidly digestible starch (RDS, digested within 20 minutes), slowly digestible starch (SDS, digested between 20 and 120 minutes), and resistant starch (RS). The latter is not digested but is fermented in the large intestine and has 'prebiotic' properties.

Lead author of the two studies, Genyi Zhang from the Southern Yangtze University in China, reports that maize starch (Tate & Lyle) consists of 53 per cent slowly digestible starches, with 22.4 per cent RDS, and 22.6 per cent RS. Potato starch consists of only 15 per cent SDS, he said.

Using X-ray powder diffraction and scanning electronic microscopy (SEM) the researchers determined that high proportion in SDS in the maize was associated with the A-type crystalline structure, as opposed to the B-type in potato starches.

"These observations demonstrate that the supramolecular A-type crystalline structure determines the slow digestion property of native cereal starches,"​ wrote the Southern Yangtze University and Purdue University researchers.

In other words, it is this specific internal structure that delays digestion and conversion into glucose, and is the reason behind the slow digestion of the starches. Expansion of this criteria could lead to the identification of other SDS that could lower the GI of foods.

The researchers also suggested ways of forming slow digestible starches: "This study indicates that a SDS material can be made by encapsulating easily digestible material, for example, gelatinized starch, between layers of films that are resistant to enzyme digestion, and food products with multiple layers of this structure would likely have a slow digestion property."​

Foods with a low GI value keep the body's blood sugar levels relatively steady throughout the day, regulating appetite and reducing the tendency to snack. Although there is no evidence to show that a low-glycaemic index diet is any more effective than a low-fat, high GI diet for weight loss, low GI foods are increasingly being positioned as weight management foods, partly because they may also have benefits in preventing obesity-related disease like type 2 diabetes.