Reviewing the effect of cooking, tempering and extrusion on the resistant starch (RS) content of cereals and cereal products, Turkey based scientist Ayten Alsaffar, notes that RS can function as a prebiotic and can also act as a satiety agent.
“There is a vast amount of information about the RS contents of starches, flours and gel systems but there are a few studies that concentrated on the whole cereal grains,” said the researcher in regards to the prompt for his review.
Writing up his findings in the International Journal of Food Science and Technology, Alsaffar reports that changes in moisture, temperature, the duration of heating and subsequent cooling can influence the RS content of foods.
The article defines four types of resistant starch:
- RS type 1 (RS1) is physically inaccessible starch, which is entrapped within whole or partly milled grains or seeds.
- RS type 2 (RS2) refers to native starch granules that resist digestion due to the conformation or structure of the starch granule. These types of starches may be added to a wide variety of foods.
- RS type 3 (RS3) is mainly retrograded starch, usually obtained as a result of food processing).
- RS type 4 (RS4) describes a group of starches that have been chemically modified for their technological (but not nutritional) attributes.
“Food processing, which involves heat and moisture, can destroy RS1 and RS2 but it may form RS3,” comments Alsaffar.
And he said that while retrogradation during tempering may lead to RS formation in foods, the current RS levels in commercially available breakfast cereals are not high enough to deliver its beneficial effects.
“The highest RS value, 3.6 g/100 g food (as eaten), is found in corn flakes. These RS values, however, does not appear to be nutritionally significant (Englyst & Cummings, 1987).
The challenge is to generate breakfast cereals with much higher RS contents (such as 10–20 per cent) to make a substantial difference to the physiological properties of these foods,” observes the reviewer.
Extrusion impact
Evaluating the impact of extrusion, in particular, on starch digestibility in cereals, the Turkish reviewer reported conflicting findings.
He found that numerous studies reported a decrease in the RS content (or increased digestibility) after extrusion and ascribed this to the destruction of granular structure, due to thermal treatment, high pressure and shear forces(Parchure & Kulkarni, 1997; Unlu & Faller, 1998; Vasanthan & Bhatty, 1998; Farhat et al., 2001; Wolf, 2010).
Similarly, he notes that Mahasukhonthachat et al. (2010) reported that the rate of starch digestion of sorghum was increased by about ten times after extrusion when compared with non-extrudates.
Chanvrier et al. (2007), reports Alsaffar, observed a slight increase in enzyme resistant starch (ERS) content for wheat starch (from 0.8 per cent to 2.8 per cent) and normal maize starch (from 1.5 per cent to 2.1 per cent) after extrusion.
Huth et al. (2000), he continued, obtained 2–3 per cent of RS after the extrusion of barley. The values rose up to 6 per cent when the samples were freeze-stored after extrusion, he added.
The Turkish reviewer also reports on the findings of Englyst et al. (2003) who concluded that the starch–sugar ratio used in the formulation affected the digestibility of cereal products together with the cereal type and the degree of food processing.
Alsaffar observes that the inconclusive trends could be due to possible differences in the nature of the gelatinised systems used.
Source: International Journal of Food Science and Technology
Published online ahead on print: doi:10.1111/j.1365-2621.2010.02529.x
Title: Effect of food processing on the resistant starch content of cereals and cereal products – a review
Author: Ayten Aylin Alsaffar