Extrusion Effects on Fiber: Challenges and Opportunities
Extrusion cooking is a highly-versatile, globally-accepted method for processing and improving food and feed production. The effects of extrusion on improving starch digestibility, decreasing antinutritional factors (which inhibit overall digestion), and increasing energy utilization by increasing oil digestibility in ingredients, are well understood. Less-well understood are the effects of extrusion on the variety of molecules that make up dietary fiber.
Dietary fibers consist of the portion of foods and feeds that play structural roles in all plants and a few animals (such as crustaceans). As a result, dietary fiber resists digestion. This inert material is beneficial as it provides bulk for laxation. Also, it resists hydrolytic digestion by enzymes in the upper digestive tract, and travels to the large intestine, where it may undergo fermentation. During the fermentation process, microbes can use certain fibers to produce short-chain fatty acids, which have been shown to promote gut health. It should be noted here that the rumen of dairy and beef cattle, sheep, and goats also contains microbial populations that produce short-chain fatty acids as part of normal digestion.
So, the effects of extrusion cooking on dietary fibers are worth exploring. An extensive study examined this very topic. For our purposes, the effects of extrusion (from no extrusion, to high temperature and pressure conditions, which is shown on the graphs as “high” on the far right) on resistant starch (which acts as dietary fiber) and fiber in cornmeal and oat bran are shown.
*Relative, and not actual, data values were used for ease of viewing on the graphs.
Cornmeal and oat bran were chosen to illustrate a point. The two ingredients exhibited similar nutrient contents, including total fiber, starch, and dry matter. Also, the ratio of insoluble fiber to soluble fiber changed somewhat similarly as extrusion conditions increased to “high”. None of this is shone on the graphs above.
What did not change in a similar manner is resistant starch, which declined in cornmeal as extrusion conditions increased. However, in oat bran, resistant starch was created as extrusion conditions increased. See the graphs above.
Also, cornmeal was much less fermentable (as measured by short-chain fatty acid production) than oat bran, which became more fermentable as extrusion conditions went from “moderate” to “high”. This is also shown in the graphs.
So, the fiber component of foods and feeds can be manipulated by extrusion. The challenge is that these changes appear to be very specific to each ingredient. Therefore, opportunities exist to create improved ingredients with more resistant starch and higher fermentability (i.e., more short-chain fatty acid produced), both of which may improve gut health when consumed in food products.