Hydrothermal Cooking (HTC) is a steam infusion process used to continuously cook slurries at high temperatures for short periods of time. The objectives of present studies were to investigate the effects of HTC processing on functional properties of soy protein products and to contribute to a better understanding of protein ultrastructure changes induced by heat;In the first study, the effects of HTC processing on the functional properties of soy flour, soy protein concentrate (aqueous-alcohol-washed) and soy protein isolate were studied. HTC processing increased the NSI of soy protein concentrate by approximately four times (from 15 to 56%). HTC changed the solubility profile of soy protein concentrate from a flat profile to one typical of undenatured soy protein. It also improved foaming and emulsifying properties. For soy protein isolates, HTC also improved NSI, and foaming and emulsifying properties, although the effects were less dramatic than for soy protein concentrate. NSI and emulsifying properties of soy flour were also improved when processed for longer times than 19 sec. The foaming properties of soy flour were not improved by HTC;In the second study, the effects of HTC on coagulation properties of soymilk were investigated. HTC-processed soymilk, especially those processed at high temperature (162°C), gave lower tofu yields and lower solids and protein recoveries in tofu. HTC-processed soymilk, especially those processed at high temperature (132, 154, and 162°C), resulted in tofu with inferior texture characteristics, being very soft, fragile, and adhesive;In the final study, light microscopy, transmission electron microscopy, gel filtration chromatography, immuno rocket electrophoresis and differential scanning calorimetry (DSC) were used to investigate ultrastructure changes of soy products during HTC processing. HTC processing denatured soy proteins, but they existed as highly dispersable aggregates;The general conclusions were that HTC processing was a very effective means to improve functional properties of soy protein products, but has little potential in tofu production. HTC processing transformed proteins into smaller and more water dispersable aggregates than was achieved by traditional heating.