Influence of ultrasonic treatment on functional properties and structure of tussah pupa protein isolate

Tussah pupa protein is one of the high-quality insect proteins with high nutritional value and has been gradually recognised by the public, which can be used as a substitute for traditional animal-derived food proteins. But in contrast, the processing potential of it is much worse. Given this, this study took tussah silkworm chrysalis as the research object, and the isolated protein was extracted by alkali dissolution and acid precipitation and treated by ultrasonic wave (suspension concentration 9%, ultrasonic time 100 min, ultrasonic amplitude 40%), the effects of ultrasound were evaluated by determining the tussah pupa protein isolate (TPPI) functionalities and its structure changes. The results showed that compared with the untreated TPPI, the ultrasound-treated TPPI solubility significantly increased 4.57 times, water-holding capacity increased 1.06 times, emulsifying activity and emulsifying stability increased 2.10 times and 1.23 times respectively, foaming capacity as well as foaming stability increased 1.83 times and 1.05 times respectively (P<0.05). Besides, the protein surface hydrophobicity and flexibility were also strongly improved (P<0.05). Protein structure analysis showed that the disulphide bonds content reduced 1.21 times, sulfhydryl content increased 0.82 times, alpha-helices and random coils content in protein secondary structures increased significantly, beta-sheets and beta-turns content significantly decreased (P<0.05) after ultrasonic treatment, indicating that ultrasound caused the transformation of some ordered structures into irregular structures. According to Fourier transform infrared spectrum, some protein characteristic peaks changed, including O-H stretching vibration intensity increased, and the amide I band moved to the low wavenumber direction, which indicated that the spatial structure of TPPI had been extended. Ultrasonic treatment can expose the internal polar groups and increasing the surface active sites of protein, to improve some functional properties of TPPI to a certain extent, which provided a theoretical foundation for developing new types of tussah pupa protein foods.