The Effects Of Freezing-thawing On The Characteristics And Flavor Of Frozen Blueberries Studied By Metabonomics Methods

In recent years,blueberry has been widely planted in China as a high-grade fruit,and its frozen fruit production has been increasing year by year.In the process of cold chain storage and transportation and processing of blueberry jelly,blueberries will be repeatedly frozen and thawed,which will affect their sensory quality and nutritional content.The analysis method based on metabolomics is a powerful method to study the change of the composition of secondary metabolites.The application of this method in the study of changes in food components during processing is still rare.Based on liquid chromatography-mass spectrometry(HPLC-MS)and gas chromatography-mass spectrometry(GC-MS)combined with multivariate statistical analysis methods,this paper systematically studied the effects of repeated freezing and thawing on the sensory quality,anthocyanins and aromatic substances of blueberries.The results provide a certain theoretical reference for the storage,transportation and processing of blueberries.Firstly,the thesis studied the effects of different freezing and thawing times on the juice yield,p H value,soluble solids,juice color difference,oxidation resistance(DPPH method)and sensory flavor of blueberries.The results showed that the blueberry juice yield gradually increased with the increase of freezing and thawing times,from 50.9±0.5% to 61.5±0.5%;The p H and soluble solids of frozen juice fluctuate around 3.7 and 12.5 respectively;The color parameter a* value of frozen juice is significantly lower than the value b*,indicating that the red color of juice is reduced the most,and the brightness L* is basically constant;After freezing and thawing,the antioxidant capacity of blueberries is significantly reduced,and is maintained at about 50% of fresh blueberries,indicating that freezing and thawing will reduce the content of substances with antioxidant capacity;The results of the electronic nose showed that freezing and thawing significantly changed the blueberry flavor.Evaluation by sensory ranking method showed that the fragrance of grass(reference:(E)-2-hexenal)gradually weakened with the increase of freezing and thawing times,and the flavor such as hexanol gradually increased.The results show that repeated freezing and thawing have a significant effect on the sensory properties of blueberries.Secondly,this article uses UV p H differential method and HPLC-MS method to study the effects of repeated freezing and thawing on the content of blueberry total anthocyanins and the types and contents of anthocyanin monomers.The results showed that the total anthocyanin content in the blueberry jelly fruit that was repeatedly frozen and thawed gradually decreased,and the fresh blueberry decreased by 9.7%,19.3% and 40.8% after three freeze-thaw cycles.This result is basically consistent with the trend of blueberry total anthocyanin content determined by HPLC method.Through the HPLC-MS method,9 kinds of anthocyanin monomers were identified in blueberries,and the four major anthocyanin aglycones were Delphinidin,Petunidin,Cyanidin,and Malvidin.Compared with fresh blueberries after repeated freezing and thawing three times,the content of Delphinidin decreased by-5.1%,47.2%,and 51.0%;The content of Petunidin decreased by-5.8%,25.4%,and33.7%;the content of Cyanidin decreased by-0.7%,25.7%,and 33.6%;The content of Malvidin decreased by 8.4%,19.0%,24.9%;The results showed that the content of the four types of anthocyanin aglycones showed a gradual decline with repeated freezing and thawing of blueberries.This experiment proved that repeated freezing and thawing will reduce the anthocyanin content in blueberry jelly.Finally,the GC-MS omics technology combined with multivariate statistical analysis was applied to study the effects of repeated freezing and thawing on the volatile compounds of blueberries.The results showed that the volatile metabolites of blueberries were mainly concentrated in terpenes(22.22%),aldehydes(11.11%)and ketones(8.33%).The characteristic substances of blueberry freezing and thawing are(E)-2-hexenal,ethanol and1-hexanol.The content of(E)-2-hexenal decreases gradually with the increase of freezing and thawing times,from 45.84% to 5.99%,indicating that the flavor of blueberry is gradually weakening;The content of ethanol and 1-hexanol gradually increased with the increase of freezing and thawing times,the content of ethanol rose from 0.97% to 38.17%,and the content of 1-hexanol rose from 0.27% to 6.14%,indicating that the alcohol flavor in blueberries increased.The results show that repeated freezing and thawing is not conducive to the retention of blueberry volatile flavor compounds.