| Citrus fruit holds the highest production in fruit kingdom and is the second largest growing fruit in China, where citrus production is up to the second in world. In recent years, citrus processing has been developing in a high speed, which correspondingly produces large quantities of by-products with widely use. However, most of citrus by-products in conventional food processing owing to lack of knowledge and lag of technology were wasted, resulting in a certain environmental pollution. Previous investigations have found that citrus peels are rich in numerous biologically active compounds with medicinal effects, which were associated with healthy properties. Therefore, extraction and isolation of naturally bioactive components from citrus by-products were regarded as one of available approaches.In recently, ultrasound-assisted extraction (UAE) has been widely applied in extraction of a variety of biologically active compounds from plants, the advantages of which can significantly reduce extraction times, decrease extraction, temperature, increase extraction yields and reduce environmental pollution compared to classically extraction methods. In this research, low-frequency high-power ultrasound were applied to extract flavanoid and phenolic acids from citrus peels, the effects of UAE on the yields of extracts and relationship between total phenolic contents (TPC) and antioxidant activity (ATT) were evaluated, the simultaneous extraction efficiency of multi-phenolic compounds from citrus peels by UAE was esitmated, and it was analyzed by their chemical structures, the properties of ultrasonic parameters in local ultrasound field were discussed. The objection is to obtain the inner and outer information of ultrasound-assisted extraction of bioactive compounds from plants, to provide available parameters for manufacturing industrial ultrasound equipment, and to establish possible model in industrial point of view.The main results and conclusions were listed as follows:1. The effect of ultrasonic parameters including solvent, ultrasonic time, temperature, ultrasonic frequency, and ultrasonic power on the yields of hesperidin from Penggan (Citrus reticulata) peel was estimated. The resulted showed that solvent, temperature and ultrasonic frequency were the most important parameters, but ultrasonic power has a little effect on the yields of hesperidin. With increase of ultrasonic time and temperature, the yields of hesperidin also increased. Moreover, extended extraction time did not resulted in the degradation of heperidin. UAE gave higher extraction efficiencies at lower temperature for shorter time when compared to soxhlet method. At the same time, the samples of absorption-uniformed irradiation power contribute to higher yields of extraction. The most optimum ultrasonic conditions of extraction hesperidin from Penggan peels were determined as:methanol, frequency of 60 kHz, temperature of 40℃, extraction time of 60 min in experimental range under study.2. The effects of ultrasonic parameters (ultrasonic time, temperature, ultrasonic power) on the extracts from Satsuma mandarin (Citrus unshiu Marc) peels (SMP) were analyzed. Through the compared analysis, the resulted presented that the effect of ultrasonic frequencies such as 20,60,100 kHz at same ultrasonic conditions on the yields of phenolics were similar. For example, the yields of phenolic acids increased significantly at lower temperature (15,30℃) when ultrasonic time increased, whereas a relatively higher temperature (40℃) resulted in vigorously decrease in extraction yields with extended extraction time. Ultrasound with 60 kHz has more highly effect on the yields of phenolic acids in comparison to 20,100 kHz. It was observed that the degradation of phenolic compounds depended on the number and type of substituents in their aromatic ring, the greater number of hydroxylic-type substituents and the smaller number of methoxylic-type substituents in aromatic ring of phenolics is easy to be degraded; however, cinnamic acids of extracts from SMP by UAE were susceptible to be degraded in contrastion to benzoic acids. Furthermore, the yields of two flavanone glycosides were presented distinctly increased trend.The optimal ultrasonic conditons in the range under study for phenlic acids from SMP were determined as:extraction time of 20 min, temperature of 30℃, ultrasonic power of 8 W; and for flavanone glycosides:extraction time of 60 min, temperature of 40℃, ultrasonic power of 8 W. At last, the correlation between TPC and ATT, after ultrasonic treatment, is evaluated. The results indicated that both TPC and ATT have a linear relationship. For example, R2 of correlation coefficient for 20,60,100 kHz is 0.8288,0.8354,0.8435 at 15℃; 0.7706,0.8735,0.8895 at 30℃; 0.8626,0.9248,0.8895 at 40℃, and 0.9143,0.9958,0.7971 at ultrasonic powers of 3.2,8,30,56 W, respectively. This meant that TPC was the major factor accounting for ATT of SMP extracts.3. The effect of UAE on the yields of flavanoid, phenolic acids and ATT of extracts from Guanxi pomelo peel (GPP) was evaluated. The results showed that ultrasonic frequency, ultrasonic time and extraction temperature have significantly effects on the extraction efficiency, while ultrasonic power has a weak effect on the yields of extracts. It was found that the effects of ultrasonic frequency and temperature on the extracts from SMP and GPP presented similar trend. For example, the highest extraction efficiency was obtained using ultrasound with 60 kHz, and phenolic acids were susceptible to be degraded at higher temperature. The optimal ultrasonic conditions of extracts from GPP were determined as:for phenlic acids, extraction time of 30 min, temperature of 30℃, ultrasonic power of 30 W; and for naringin:extraction time of 30 min, temperature of 40℃, ultrasonic power of 8 W. At last, the correlation between TPC and ATT of extracts from GPP after UAE are analyzed. A linear relationship between TPC and ATT was observed. For example, R~2 of correlation coefficient for 20,60,100 kHz is 0.7814,0.7648,0.8832 at 30℃; 0.7686,0.8847,0.8874 at 40℃, and 0.8752, 0.8829,0.9397 at ultrasonic powers of 3.2,8,30,56 W, respectively. This meant that TPC was the major factor accounting for ATT of the GPP extracts.Comparison of the effect of UAE on the extracts from both SMP and GPP, the main compounds in flavanoids in two citrus species peel were entirely different, the former mainly contained narirutin and hesperidin by HPLC, naringin for the latter; likewise, the yields of the cited seven phenolic acids also different each other, ferulic acids was the main phenolic compounds for the former, sinapic for the latter, indicating that the nature of plant materials have the most significantly effect on the yields of extracts. The extraction efficiency of the same phenolic compounds in the two citrus species peels under same ultrasonic conditions also was different, which indicated that the nature of plant material probably affected the propagation of ultrasonic wave directly related to the extraction efficiency. In addition, after UAE, it was observed that there were the well linear correlations between TPC and ATT of extracts from the two citrus species peels. The results emphasized the application of UAE in the efficient extraction of biologically active compounds and the enhancement of ATT of extracts from citrus peels is considerably potential, which might be beneficial to effectively use citrus by-products.4. Based on the above results, it was generally concluded that ultrasonic power has a weak effect on the yields of extracts from citrus peels. The earlier works reported that ultrasonic power attenuated with increase of distance of ultrasonic irradiation surface, whether the property of ultrasonic power produced an effect on extraction efficiency or not. Therefore, it is necessary that confirm this assumption by experimental data. In the paper, combination with previous investigation, the effect of ultrasonic parameters at local ultrasound field on the yields of hesperidin and TPC, ATT of extracts from penggan peel was analyzed by response surface methodology (RSM). The results showed that ultrasonic power has a little effect on the yields of penggan peel when the distance between samples and ultrasonic generator is 13.4 cm; in contrary to, ultrasonic power has a significantly effect when the distance is 2 cm, and by analyses of the quadratic multiple regression models at this ultrasonic irradiation surface, it was observed that the effect of the interactive terms on responses was significant, which possibly indicated highly ultrasonic power in combination with other extraction variables resulted in more complicate ultrasonic behavior.RSM was used to evaluate the effects of independent variables (ultrasonic time, power and temperature) on responses, and mathematical regression models were established. Analysis of variance (ANOVA) showed that the quadratic multiple regression models adequately represented the experimental values by the coefficients of determination (R2), which were observed to be 0.9889, 0.9854,0.9929 for hesperidin, TPC and ATT at high irradiation surface, and 0.9578,0.9601,0.9713 for hesperidin, TPC and ATT at low irradiation surface, respectively. This indicated the good agreement between the observed and predicated responses. The optimal ultrasonic conditions in range under study by RSM for the highest yields of hesperidin, TPC and ATT of extracts were simultaneously determined as:ultrasonic power of 42～45 W, ultrasonic time of 23～25 min, and extraction temperature of 31～3434℃. Furthermore, the higher extraction efficiency was obtained at the vicinity of ultrasonic generator. This was a valuable reference for design the special ultrasound equipment on a large scale.5. Morphological properties of tissue of citrus peels by scanning electron microscopy (SEM) were observed. Cell structures of the fresh peels were found to present an arrangement regularly and clearly, and to hold thicker cell wall. Moreovere, the cell structure differed with difference of citrus species. However, after UAE, The morphological properties of cell arrangement of penggan peel became loose and were difficult to be discerned, the cell structure of samples was distinctly destroyed to some certain. These suggested that UAE broke up the cell wall of plant materials due to cavitation energy, facilitating the release of cell contents and enhancing the yields of extracts. 0...
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