HEAT-INDUCED STRUCTURAL-CHANGES IN FABA BEAN STARCH PASTE - THE EFFECT OF STEAMING OF FABA BEAN-SEEDS

Heat-induced structural changes of faba bean starch dispersions were e xamined at a concentration range of 8-10% as a function of heat treatm ent. Faba bean starch was isolated from raw (RF-starch) and steamed (S F-starch) faba bean seeds. Hydrothermal treatment resulted in an incre ase in the amount of non-starch components in the isolated starch. Mic rostructure of low-sheared 8% starch suspensions heated at 75, 90 and 98-degrees-C (for 10 minutes) was studied by light microscopy. Heating induced huge swelling of RF-starch. Even at 75-degrees-C, a considera ble amount of amylose was found outside the granule. The amylopectin i n starch paste heated to 98-degrees-C was highly swollen. Steaming of faba bean seeds had a marked effect on the pasting behaviour of the is olated starch. At 75-degrees-C the swelling of the granules and the re lease of amylose was limited. At 90-degrees-C, some of the granules st ained blue, some brown and a weak amylose phase could be seen outside the granules. At 98-degrees-C, the granules stained brown, were more s wollen, and amylose surrounded the intact swollen granules. Rheologica l properties were studied during cooling of 10% starch dispersions hea ted to 90-degrees-C and 98-degrees-C. Storage modulus increased and ph ase angle decreased below 50-degrees-C as a result of amylose gelation . RF-starch dispersion heated at 90-degrees-C formed a gel with high s torage modulus and low phase angle; the SF-starch dispersion formed a less elastic gel (higher phase angle). Light and scanning electron mic rographs showed that, the RF-starch dispersion preheated at 90-degrees -C, cooled and stored in a refrigerator for one day consisted of an am ylose network, whereas the amylose was deposited on the surface of the granules in the SF-starch dispersion.