| Zein is a co-product of the corn starch industry. It is insoluble in water due to high proportion of nonpolar amino acid residues. In addition, the notable absence of tryptophane and lysine in zein accounts for its lower nutritional value and limits its application in food industry. Zein hydrolysate showed a strong ACE-inhibitory activity. However, traditional enzymolysis has many disadvantages, such as long enzymolysis time, low utilization rate of the enzyme, and low conversion rate of the substrate. The objective of this study was to enhance the efficiency and productivity by preparation of ACE-inhibitory peptides on enzymatic from zein with sweeping frequency ultrasond (SFU) pretreatment. The characteristics of ultrasonic field of water was studied first The objective of this research was to observe the impact of SFU pretreatment on physicochemical properties of zein, were precisely studied by the methods of solubility, surface hydrophobicity and free sulfhydryl content. The structure of zein after SFU pretreatment was investigated by the methods of CD and FT-IR spectra and change of surface morphology, particle size was also invested to assess the enzymatic properties of zein for preparation of ACE inhibitory peptides after ultrasonic pretreatment. Thermodynamic parameters of zein changed with different sweeping frequency ultrasonic treatments. It proved that ultrasonic could promote enzymatic hydrolysis. The main achievements are summarized as follows:(1) Characteristics of ultrasonic field of water were studied by high speed photography and iodine release method.The ultrasounic cavitation yield at40kHz was remained highest for the range of ultrasounic frequency and the cavitation phenomenon observed for40kHz by high speed photography also more obvious than others The effect ultrasounic cavitation for sweeping model is better than fixed model.(2) Effects of SFU pretreatment on physicochemical properties of zein was studied. After SFU pretreatment significant increase was observed in increased in solubility and surface hydrophobicity which indicates the polypeptide chains of zein extended and denaturation temperatures were changed. The change for some ultrasonic parameters indicated that ultrasonic pretreatment was beneficial to protein hydrolysis reaction. Specialy an increase for surface hydrophobicity indicated ultrasonic pretreatment help the enzyme to attack the interior of zein aggregate easily which resulted in the release of hydrophobic amino acids from zein aggregate. These changes play an important role in improvement of ACE-inhibitory activity of hydrolysates.(4) Effects of SFU pretreatment on physical microstructure of zein were studied. Microstructure analysis by AFM revealed that upon SFU pretreatment, the fine meshwork structure of zein was ruptured and there were many dissociated protein globules. As compared to control the particle size of ultrasound-pretreated zein was smaller as well as more homogeneous. Analysis of SEM showed that compared with the smooth surface of untreated zein, the surface structures of the ultrasonic-treated zein became rough and their specific surface area was larger. Interestingly, several micro-holes appeared in the zein following ultrasound pretreatment. The results suggested that ultrasound might loosen the tissue of zein aggregate, and help the enzyme to attack the interior of zein aggregate easily which resulted in the release of hydrophobic amino acids from zein aggregate. The particles size of zein increased upon the higher ultrasounic frequency showed sweeping frequency ultrasounic pretreatment induced polarization of zein and dissociation of sub-units and molecular unfolding; changed molecular conformation and caused hydrophobic groups and free sulfhydryls buried inside the molecules to expose.(5) Effect of SFU pretreatment on enzymatic characteristics of zein was studied. Significant increase was observed in degree of hydrolysates of zein and hydrolysates of ACE inhibition.The degree of hydrolysates of zein was significantly increased under the sweeping ultrasonic and fixed ultrasonic pretreatment, and hydrolysates of ACE inhibition were also increased. Compared to control, the40±2-kHz SFU pretreatment increased by21.7%and hydrolysate ACE-inhibitory activity was46.1%representing by1.33times increase over the control. Hydrolysate ACE-inhibitory activity was43.7%after dual sweeping frequency ultrasounic pretreatment (40±2kHz/68±2kHz) pretreatment, representing by1.21times increase over the control. Amino acid composition and molecular weight profile revealed that hydrolysates of sweeping frequency ultrasounic pretreatment zein had a higher relative concentration of low molecular weight peptides, hydrophobic amino acid and branched-chain amino acid as compared to control. These changes play an important role in improvement of ACE-inhibitory activity of hydrolysates(6) Analyzing results of thermodynamic parameters for zein showed that the values of Ea, AH, AS and AG could be decreased after sweeping frequency ultrasonic pretreatment. Single frequency sweeping and dual frequency ultrasonic pretreatment decreased thermodynamic parameters Ea from5.84kJ/mol to4.70and5.74kJ/mol;ΔH from46.05kJ/mol to36.55and45.20kJ/mol.It indicated the digestion of zein become more facility and benefit to release more activity ACE-inhibitory peptides. |
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