| The aroma quality is a crucial factor affecting the flavour and typicality of cider, and it is a significant index for evaluating cider qualities. Therefore, the metabolism and regulation of aroma compounds have always been the research hotspot. In this study, the changes of physicochemical indexes and aroma compounds were monitored throughout the fermentation process, and the aroma compounds were regulated and controlled by means of adding nutrients, conducting mixed fermentation and accelerating the aging of cider and so on, in order to provide some new ideas and effective methods to improve the aroma quality of cider. In addition, the rapid detection methods of aroma compounds and physicochemical indexes of cider were constructed based on the Fourier transform near infrared spectroscopy,which may provide a good way of real time and online monitoring and control of cider during the fermentation process. The main research contents and conclusions were as follows:(1) The changing profile of physicochemical indexes and aroma compounds were monitored throughout the cider fermentation process. The total sugar content and assimilable nitrogen content experienced rapid decrease once the fermentation began, and the free amino acids also showed a downward trend, whereas the p H value increased on the whole. The total polyphenols content experienced large fluctuation during fermentation, but the content did not change significantly after fermentation. The contents of all the polyphenols were much higher at 108 h, and they decreased significantly after fermentation except protocatechuic acid, which increased significantly compared with that in the apple juice. The content of organic acids was much higher at 84 h except succinic acid. The organic acids obtained increase after fermentation except succinic acid, which decreased after fermentation. As a whole, during 72 h~120 h and 144 h~216 h, most of the minerals contents underwent great fluctuation. Especially when fermented for 192 h, most of the minerals reached peak value or valley value. During the fermentation process, thirty-six volatile aroma compounds were monitored, including eight higher alcohols, fourteen esters, six fatty acids and eight other carbonyl compounds. They were mainly synthesized during fermentation by yeasts, and fluctuated greatly after fermenting for 48 h. But there were also some aroma compounds which were metabolized by yeasts during the fermentation process, leading to their decrease after fermentation.(2) The factors which affected the production of esters significantly, i.e. leucine, glutamine, pyridoxine and zincchloride, were selected by conducting the Plackett-Burman experiment design. Subsequently, the central points of the four selected factors were determined by Steepest Accent Design. Based on the central points, the factors were optimized further by conducting the quadratic rotating combined design experiment and ridge analysis. The optimizing condition was that adding 34.8 mg/L of leucine, 84 mg/L of glutamine, 0.24 mg/L of pyridoxine and 1.30 mg/L zincchloride to the fermentation liquor. Under this optimizing condition, the content of esters in the final cider could achieve 5.29 g/L. In addition, the sensory quality was also improved.(3) Eighteen yeast strains were separated from apple samples and air samples, and then six yeast strains were further selected according to their ester-producing ability. The colony morphology, cell morphology, liquid cultural characteristics, physiological and biochemical characteristics and ITS complete sequences of the six tested yeasts were analyzed. The six strains were identified by sequence homology comparison with known sequences in Gen Bank and by constructing phylogenetic tree. The results showed that the three strains of them were identified as Wickerhamomyces anomalus, two were Saccharomyces cerevisiae, and one was Pichia guilliermondii. By conducting the fermentation trails, the Wickerhamomyces anomalus YN6 were selected for its best ester-producing ability. Fermenting with Wickerhamomyces anomalus YN6, both the kinds and the content of volatile compounds of cider increased, suggesting that YN6 was a good aroma-producing yeast.(4) The mixed fermentation was conducted by Wickerhamomyces anomalus YN6 and Saccharomyces cerevisiae WLS21 to investigate the production of volatile compounds as well as the fermentation performance of apple cider. The two yeasts were inoculated simultaneously or sequentially. The results showed that the growth of two yeast strains during fermentation were restrained by each other, but the effect of the existence of Saccharomyces cerevisiae WLS21 on the growth of Wickerhamomyces anomalus YN6 was much greater. The mixed fermentation with the two strains of different species could increase both the kind and the content of volatile aroma compounds, resulting in the improvement of the aroma complexity. Modulation of the inoculation time of these yeasts was crucial for the optimization of the desired style of cider. Only 3 days of juice fermentation with W. anomalus in sequential mixtures, i.e. mixed fermentation WS3, was enough to improve the quality of cider. Results from this study evidence the fact that apple cider fermented with mixed culture of W. anomalus and S. cerevisiae provides another possible strategy of modulating cider quality.(5) The ultrasonic technology was used to accelerate the aging process of cider. The cider without ultrasonic processing and the cider which aged for one year conventionally were kept as control. The effects of different ultrasonic processing time, ultrasonic power, processing temperature and ultrasonic frequency on the quality of cider were investigated. The results showed that, ultrasonication did accelerate the aging process of cider to some extent. It could affect the total acidity, p H value, soluble solid content(SSC), total polyphenols content and volatile aroma compounds. Some components could achieve the aging level after ultrasonic processing, and different ultrasonic conditions could bring the cider different effect. As a whole, the cider processed under the condition of low ultrasonic frequency, high ultrasonic power and high temperature would be closer to the one year conventionally aged cider.(6) The Fourier transform near infrared spectroscopy(FT-NIR) was used to determine and analyze the SSC, total acidity, total ester content and p H value of cider. By comparing different spectra pretreatment methods and wave number ranges, the best pretreatment method of spectra and the effective wave number ranges were picked out. The best models obtained the accuracies with R2 values of 0.91, 0.93, 0.98, and 0.91; and RMSECV values of 0.85 °Brix, 0.08, 0.02 g/100 m L and 0.12 g/L for SSC, p H, total acidity, and total ester, respectively. The R2 values for prediction were 0.91, 0.93, 0.98, and 0.92; and RMSEP values were 0.60 °Brix, 0.08, 0.02 g/100 m L and 0.10 g/L for SSC, p H, total acidity, and total ester, respectively. This work demonstrated that NIR spectroscopy was a rapid and convenient technique for analyzing and predicting SSC, p H, total acidity, and total ester content simultaneously during apple wine fermentation process.(7) The Fourier transform near infrared spectroscopy(FT-NIR) was used to determine and analyze the volatile aroma compounds in cider, including esters(ethyl acetate, ethyl octanoate, ethyl butyrate, ethyl hexanoate, phenethyl acetate, ethyl lactate, ethyl decanoate and hexyl acetate), higher alcohols(isobutanol, hexanol, 2, 3-butanediol, 2-phenyl ethanol and 3,4,5-trimethyl-4-heptanol) and fatty acids(decanoic acid, hexanoic acid and octanoic acid). All the calibration models obtained the high R2 values, which were all higher than 0.83, and the RPD values were between 2.43 and 3.02. All the prediction models obtained much higher R2 values, which were all higher than 0.88, and the RPD values were between 2.90 and 3.50. These results demonstrated that NIR spectroscopy was a rapid and convenient technique for analyzing and predicting volatile aroma compounds in ciders. |
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