| Casein was important and used widely in food industry as one of functional additives. Its functions, such as gelling property, water holding capacity, attracted great interest and attention. But its solubility was poor under low p H conditions, which impacted applications of other functional properties. So it was necessary to modify it to enhance its properties, widen applied field, decrease used amount and lower cost. The Maillard reaction(MR) was one of the major protein modifying reactions and an effective and simple method to improve functional properties. Compared with other chemically modi?ed methods, MR had fewer safety issues. However, there were still some shortcomings, such as poor efficiency, long reaction time and high energy consumption. Additionally, ultrasound(US) was a environmentally friendly new processing technology and had diverse applications in many ?elds. US produced the acoustic cavitation which altered material properties through immense pressure, shear stresses, turbulence, dynamic agitation. US could be used to promote certain reactions.The aim of this paper was to investigate MR under neutral conditions at 95 ℃ for 2.5 h under ultrasonic pretreatment. Casein was first sonicated in the absence of glucose and then reacted with glucose in aqueous solution. Evolvement of MR with ultrasonic pretreatment was studied by browning, A294, degree of graft, SDS-PAGE and fluorescence spectroscopy. Then structure of Maillard reaction products(MRPs) pretreated with ultrasound was determined by intrinsic fluorescence spectroscopy and Ultraviolet–visible spectra. At last, some functional properties of MPRs pretreated with ultrasound were estimated. Specific contents and results are as follows:(1) Effect of ultrasonic time and power on development of casein-glucose system MR, the results showed that browning, A294 and degree of graft increased and then decreased with ultrasonic time extending, and all reached the maximum(0.5327, 0.54830, 42.40%) after ultrasonic time 20 min, respectively. Compared with those without ultrasonic treatment, there were 17%, 12.13% and 34.35% increases, respectively(P< 0.05). While browning, A294 and degree of graft gradually increased with ultrasonic power increasing. At ultrasonic power 450 W, browning, A294 and degree of graft got maximum(0.5763, 0.5852, 46.48%) and were 1.26 times, 1.23 times, 1.47 times of those without ultrasonic treatment. The results of fluorescence spectroscopy indicated there were more MRPs produced through ultrasonic pretreatment. The results of SDS-PAGE showed that the amount of casein decreased and more MRPs were produced(2) Effect of ultrasonic time and power on tertiary structure of casein-glucose MPRs, the results showed that absorbance peaks of UV–vis spectra were observed at ultrasonic power 450 W and for ultrasonic time 20 min, respectively. Every peak was within the range of melanoidin(260-320nm), which suggested that there were more MRPs with ultrasonic pretreatment. The results of intrinsic fluorescence spectroscopy showed that conformation of conjugates changed after ultrasonic pretreatment. Tertiary structure got looser. So there was effect of ultrasound on tertiary structure of MRPs.(3) Effect of ultrasonic time and power on functional properties of casein-glucose MPRs, the results showed thatWhen ultrasonic power was fixed and time increased, reducing power and inhibitory effect on lipid peroxidation were maximum after ultrasonic time 20 min, scavenging effect against ·OH reached 13.19% after ultrasonic time 30 min. When ultrasonic time was fixed and power increased to 450 W, reducing power, inhibitory effect on lipid peroxidation and scavenging effect against ·OH of MRPs were 0.5930, 73.16% and 12.14%, respectively. There were all significant increases compared with those without ultrasound(P < 0.05). So ultrasound could improve antioxidation of MRPs.When ultrasonic power was fixed, EAI and ESI of MRPs got 73.92% and 20.02 min after ultrasonic time 10 min and 20 min(P < 0.05). When ultrasonic time was fixed, EAI and ESI of MRPs got maximum at ultrasonic power 300 W and 450 W, respectively, which had 1.5 times EAI of those without ultrasonic pretreatment and 1.38 times ESI of those without ultrasonic pretreatment(P < 0.05). Hence, EAI and ESI of MRPs were improved with ultrasonic pretreatment.When ultrasonic power was fixed, gel strength and water holding capacity(WHC) reached the maximum(152.35 g and 62.55%) after ultrasonic time 20 min, which increased by 39.42% and 32.43%, respectively(P < 0.05). When ultrasonic time was fixed and ultrasonic power increased to 450 W, gel strength and WHC reached the maximum(153.34 g and 67.80%), which increased by 29% and 39.42%, respectively(P < 0.05). The results of SEM showed that microstructure of gel weas denser and more uniform after ultrasonic pretreatment.When ultrasonic time was fixed and power increased to 450 W and ultrasonic power was fixed and time was 20 min, foaming capacity(FC) and foaming stability(FS) of MRPs were the maximum(P < 0.05). |
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