Food Structure


Heat-induced structural changes of faba bean starch dispersions were examined at a concentration range of 8-10% as a function of heat treatment. Faba bean starch was isolated from raw (RF-starch) and steamed (SF-starch) faba bean seeds. Hydrothermal treatment resulted in an increase in the amount of non-starch components in the isolated starch. Microstructure of low-sheared 8% starch suspensions heated at 75, 90 and 98°C (for 10 minutes) was studied by light microscopy. Heating induced huge swelling of RF-starch. Even at 75°C, a considerable amount of amylose was found outside the granule. The amylopectin in starch paste heated to 98°C was highly swollen. Steaming of faba bean seeds had a marked effect on the pasting behaviour of the isolated starch. At 75°C the swelling of the granules and the release of amylose was limited. At 90°C, some of the granules stained blue, some brown and a weak amylose phase could be seen outside the granules. At 98°C, the granules stained brown, were more swollen, and amylose surrounded the intact swollen granules.

Rheological properties were studied during cooling of10% starch dispersions heated to 90°C and 98°C. Storage modulus increased and phase angle decreased below 50°C as a result of amylose gelation. RF-starch dispersion heated at 90°C formed a gel with high storage modulus and low phase angle; the SF-starch dispersion formed a less elastic gel (higher phase angle) . Light and scanning electron micrographs showed that, the RF-starch dispersion preheated at 90°C, cooled and stored in a refrigerator for one day consisted of an amylose network, whereas the amylose was deposited on the surface of the granules in the SF-starch dispersion.