From a nutritional standpoint the most desirable starch ingredients are believed to be slowly digestible. In recent years there has been much research into the development of new techniques to reduce the digestibility of starches in food products, in order to create products that may be nutritionally beneficial.
The new study, published in Food Chemistry, reported that adding non-starch polysaccharides (NSP) – the major part of dietary fibre – to rice starch gels may suppress digestion and alter its rheological properties.
The researchers, led by Dr Tomoko Sasaki from the National Agriculture and Food Research Organization, Japan, noted that the results of the study indicate that the suppressive effect of non-starch polysaccharides on rice starch digestibility.
Rice starch is known to have the whitest colour of all starches. The fine white powder is known to produce a smooth, glossy finish which can be used a coating for confectionery products, as well as in food production and for use as tablets coatings.
The ingredient is also hypoallergenic, and as such is widely used in baby food and as the main starch ingredient in other special dietary foods, such as gluten free products. It also has a potential in fat replacement for low fat products, and is used in soups, dressings, meat preparations, and in white sauces that are widely used frozen food products.
Starch and non-starch polysaccharide (NSP) mixtures are often used to modify the texture of food products and, according to the researchers much work has focused on blending cereal starch (such as corn starch) and NSP in order to modify the digestibility and rheological behaviour of the starches. The researchers said that such interactions between cereal starches and NSP have a nutritionally beneficial impact, which may be applicable to other types of starches.
“In this study we focused on a mixed system of high-amylose rice starch and NSP and prepared a concentrated rice starch gel mixed with NSP to examine how the presence of NSP modifies starch digestibility and the rheological properties of gels,” said Sasaki and colleagues.
The researchers selected three types of NSP commonly used in foods: Konjac glucomannan (KG);Xanthan gum; and Agar.
High amylose rice starch was mixed with the non-starch polysaccharide (KG, xanthan gum, and agar) and concentrated starch gels were prepared. The in vitro digestibility of the starch gels and their rheological properties were determined, and the relationship between starch digestibility and rheological parameters was evaluated.
Sasaki and co workers reported that the effects of mixed non-starch polysaccharide on the rheological properties varied, depending on the type of NSP.
They said that xanthan gum and agar increased measures of visco-elasticity (a combination viscous and elastic characteristics that effects the flow, and stretch of an ingredient), while KG was found to decrease it.
Xanthan gum, KG, and agar were observed to reduce the break down of starch, with the extent of starch digestion decreasing with increased concentrations of each NSP.
The authors reported that no correlation was observed between the extent of starch digestion and the rheological properties of starch gels.
They said that the lack of significant correlation between digestion and rheological properties suggest that the suppressing effects of NSP on starch digestion is not due to increased rigidity of the gel. They said that other effects of NSP, such as increased viscosity and restriction of water movement, appear to work in reducing starch digestion.
Source: Food Chemistry
Published online ahead of print, doi: 10.1016/j.foodchem.2011.01.038
“Effect of non-starch polysaccharides on the in vitro digestibility and rheological properties of rice starch gel”
uthors: T. Sasaki, K. Kohyama