| Fruit and vegetable juice are rich in nutrienst such as vitamin, dietary fibre and minerals. Production of vegetable and fruit juice has played an important role in food industry. In the conventional heating sterilization processing, the thermosensitive ingredients in vegetable and fruit juice are easily destroyed, resulting in the loss of nutrition and quality. However, the oxidases related to the enzyme browning of vegetable and fruit juice are insufficiently inactivated in low temperature. In this research, the ultrasound in combination with heat was introduced to the sterilization of fruit and vegetable juices. The sterilization process was carried out from laboratory scale to pilot scale, and a pilot sterilization equipment with ultrasound and heat treatment was designed and established. Meanwhile, the influences of the ultrasound on the properties of the catalase (CAT) and polyphenol oxidase (PPO) in vegetables and fruits such as the enzyme activity, enzyme stability, enzyme secondary structure and residues in the enzyme molecule, were investigated. The results are shown as following:(1) The optimum sterilization parameters of ultrasonic treatment are sterilization time of 10 min, ultrasound power density of 200 W/cm2 and the duty cycle of ultrasound of 2:2. The sterilization rate was just 70.45% after thermal treatment in 15 oC with the optimal condition, while the commercial sterile requirement could be achieved in 60 oC. Microorganism in carrot juice sterilized with ultrasound power of 1732 W for 10 min at 60 oC by the pilot equipment was 73 cfu/mL, less than the requirement in the the national standard of the vegetable and fruit juices as 100 cfu/mL.(2) The effects of ultrasound on enzyme characteristics and structure of CAT were studied. The results indicated that CAT activity decreased with the increase of ultrasonic power. However, the activity reduced slightly while extending the treatment time alone. The stable tempreture and pH for CAT were respectively 37 oC and 7.4 in the presence or absence of ultrasound. The secondary structure of CAT presented remarkable variation after ultrasonic treatment.α-helix reduced mildly, andβ-fold reduced by 12.3%. On the contrary,β-turn and random coil increased about 4.89% and 7.9%. Overall, the ultrasound aroused disorder, which deduced that ultrasound could lead to the structure change and affect the CAT activity. (3) This part discussed the effect of ultrasound on enzyme properties and structure of PPO. PPO activity decreased with prolonged ultrasonic treatment time, while it increased slightly at first and then gradually reduced when increasing ultrasonic power. The stable temperature and pH for PPO treated by ultrasound were 40°C and 6.8. The content ofβ-turn in PPO secondary structure relatively lessened. Fluorescence spectrum analysis showed that the endogenous fluorescent decreased after ultrasonic treatment which could be due to the the mechanical effect of ultrasound, resulting in the damage of tyrosine and tryptophan.(4) Polyphenol oxidase was treated with ascorbic acid, citric acid, sodium sulfite, potassium sorbate in combination with ultrasound. The effect of these treatments on the activity of PPO was studied. Results indicated that the separate treatment of ascorbic acid, citric acid, sodium sulfite could obviously inhibit PPO activity and could make a completely inhibition when the concentration was respectively 1 mM, 1 mM and 10 mM. Ascorbic acid or citric acid in combination with ultrasound had a certain synergistic effect for PPO inactivation. Whether treated alone or combined with ultrasound, potassium sorbate would promote PPO activity when the concentration was less than 2000 mM...
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