The Effect of Heat Treatments on Dietary Fiber As Assessed by Chemical Analysis and Scanning Electron Microscopy

Today it is generally accepted by many nutritionists that dietary fiber plays an important role in human health. Increased consumption of dietary fiber is advocated for beneficial effects on human health; thus food technologists and the food industry are interested in the use of fiber as an ingredient in food products and in the manufacture of high fiber foods. Numerous researchers have concentrated on the physiological effects of dietary fiber, while few have investigated the effect different processing parameters have on dietary fiber.

Apple fiber, corn fiber, oat bran and soy fiber were prepared and analyzed after further processing (autoclaving at 121°C for 15 minutes, 100°C for 30 minutes, and microwave heating for 5 and 10 minutes) to study the effect heat processing has on dietary fiber fractions of the products. Unprocessed samples were analyzed as controls. The samples were analyzed for insoluble, soluble, and total dietary fiber by an enzymatic-gravimetric method. Photomicrographs were taken with a scanning electron microscope.

Autoclaving significantly reduced insoluble dietary fiber of apple fiber, and total dietary fiber of apple fiber and oat bran. Microwave heat treatment resulted in a significant reduction of total dietary fiber in apple fiber and oat bran. The microwave heat also caused a decrease in the insoluble dietary fiber of oat bran, but caused an increase in the soluble dietary fiber of apple fiber. All processing treatments appeared to decrease the soluble dietary fiber content in corn fiber. The scanning electron micrographs showed structure differences between processed and unprocessed apple, corn, oat and soy samples. Increased processing generally produced more cracking and furrowing on the surface of the fiber which resulted in an increase in the fiber's surface area. The increased surface area was indicated in the photomicrographs.

This study demonstrates the effects heat processing has on different fiber sources. Results indicate the effect of processing on dietary fiber is dependent upon the fiber source and the processing conditions.