Effect of thermal compression on the mechanical fusion of soy protein powder

A plant-based analog of katsuobushi (boiled and dried bonito fish stick), which is made via the thermal compression of soy protein powder, is not an aggregated powder but rather a smooth solid. The mechanism by which the compressed soy protein powder transforms to a smooth solid, however, has not yet been clarified. The present work assessed the associated mechanism while varying the water activity ( a w ), compression stress ( σ c ) and compression temperature ( T c ) based on consideration of the glass transition temperature ( T g ) of soy protein. Soy protein powders ( a w 0.689–0.973) were compressed at T c 25–100 °C and σ c 12.7–63.7 MPa. The compressed products were categorized as beige, brown or transparent brown solids. The degree of transparency increased with increases in the ( T c – T g ) value, which reflected molecular mobility, and σ c . The protein particles were found to fuse at particle interfaces and also internally, and so underwent volume reduction at suitably high σ c . From the practical standpoint of production cost and storage stability, a soy protein solid having an a w of 0.843 and compressed 80 °C ( T c – T g = 45.5 °C) and 38.2 MPa would likely be optimal. These results will be helpful for the future production of various plant-based dry foods and for the texturization of other powdered food items. • • Thermally compressed soy protein was a beige, brown or transparent brown solid. • • The transition of soy protein to transparency was denoted as mechanical fusion. • • The transparency increased with increases in compressive temperature and pressure.