Background: Soybean is rich in dietary fibers; consequently, soybean ingestion considerably
increases the breath level of hydrogen molecules via anaerobic colonic fermentation. However,
the influence of cooking methods on this effect, which can affect the overall health benefits of soybean,
Objectives: The aim is to examine whether different methods of cooking soybean affect the colonic
Methods: Nine healthy adult volunteers participated in the study; they ingested either roasted soybean
flour (kinako) or well-boiled soybean (BS). Differences in their breath components were compared.
Both test meals were cooked using 80 g of soybeans per individual. After a 12 h fast, the participants
ate the test meals, and their breath hydrogen level was analyzed every 1 h for 9 h by using a
gas chromatograph with a semiconductor detector. In addition, particle size distribution and soluble/
insoluble fibers in the feces were examined.
Results: The oro-cecal transit time did not significantly differ between individuals who ingested
kinako and BS. However, the area under the curve between 7 and 9 h after the ingestion of BS was
significantly increased compared with that after the ingestion of kinako. The nutritional analysis indicated
that the content of both soluble and insoluble fibers in BS was higher than that in kinako. In
addition, the levels of unfermented fragments and soluble/insoluble fibers in the feces were increased
after the ingestion of kinako compared with those after the ingestion of kinako.
Conclusion: Cooking methods alter the composition of non-digestible fibers in soybean, and this can
result in the lack of fermentative particles in the feces, thereby causing alterations in the breath level
of hydrogen via colonic fermentation.