Changes Of Lipids In Fermented Whipping Cream Products And The Influence On Flavor Formation

As the main component of cream and its products,lipids not only affect the overall quality and nutritional value,but also affect the sensory attributes of an important component.In particular,the free fatty acids produced by lipid degradation during fermentation had an important effect on the formation of the unique flavor of dairy products.Therefore,identifying the contribution of lipids to flavor formation is of great significance for improving the quality of fermented whipping cream and regulating its flavor.In this study,the changes of lipolysis,oxidation and related enzyme activities before and after the optimal fermentation time of cream were explored to reveal the changing rules of the volatile components of cream under different fermentation times and the relationship between the volatile components and lipoxidation.At the same time,the lipid composition and changes in the cream before,during and after fermentation were systematically studied using lipidomics technology combined with multivariate statistical analysis methods to identify significantly different lipids and differential lipid metabolism pathways,and preliminarily explore the relationship between lipid metabolism and the formation of volatile compounds,which provided a theoretical basis for elucidating the characteristic flavor formation mechanism of fermented cream products with lipids as the precursor.The main contents and results are as follows:(1)Study on optimization of whipping cream fermentation process and fat changeThe evaluation model based on sensory score was established,and the optimum fermentation process of whipping cream was determined as follows:fermentation temperature 34℃,fermentation time 15 h,and the addition of starter 0.004%.After fermentation,the physical and chemical properties of whipping cream have changed significantly,and the products are mainly natural and rich in vinegar flavor and slight milk flavor.Under different time conditions,the total number of colonies increased first and then decreased,reaching the maximum of 2.40 × 107 CFU/mL at 15 h and then gradually decreasing to 2.20×107 CFU/mL.The free fatty acids in fermented cream are mainly myristic acid and palmitic acid,and the content of most fatty acids increases after fermentation.The acidity value and thiobarbituric acid value showed a significant increase trend(P<0.05),which reaching 2.53 g/100g and 0.052 mg/kg respectively at the later stage of fermentation.While the peroxide value began to decrease from 1.07 meq/kg to 0.74 meq/kg after 15 h.The activities of lipase and lipoxygenase increased first and then decreased,while the activities of alcohol dehydrogenase,hydroperoxide lyase and acyltransferase increased significantly after fermentation(P<0.05).(2)Changes of volatile compounds in the process of whipping cream fermentationThe change law of volatile compounds during the fermentation of whipping cream was comprehensively analyzed by HS-SPME-GC-MS combined with HS-GC-IMS technology.Among them,36 kinds of volatiles were identified by GC-MS,and 31 kinds of volatiles were found by GC-IMS.The contents of 3-hydroxy-2-butanone,propyl hexanoate,acetic acid and 2-heptanol increased after fermentation.By comparing the detection results of the two methods,it was found that there were 6 kinds of substances detected by the two methods because of their different detection principles and working conditions.2-heptanol,hexanoic acid,decanoic acid,valeric acid,heptanal and ethyl acetate were selected as key flavor substances by VIP>1 and threshold size.The results of correlation analysis showed that the content of some flavor substances was significantly correlated with the content of free fatty acids,and most lactones and ketones were strongly correlated with the sensory properties of the product,such as milk flavor and fruit flavor.(3)Lipidomics analysis of whipping cream products in different fermentation periods.Lipid composition and content changes of whipping cream before,during and after fermentation were determined by the method of lipidomics,in which GPs is the most diverse lipid and GLs is the most abundant lipid,and their changing trends were opposite,indicating that GPs might be transformed into GLs.Combined with multivariate statistical analysis,PCA and OPLS-DA evaluation models were established.According to VIP>1,P<0.05 and FC≥1.5 or FC≤0.67,significantly different lipid molecules were screened,including SL,LPC,SM,CL,FFA,LPE,Cer,DG,LPG and LPI.Map it into KEGG database,it is found that that metabolism of glycerol and phospholipids is the most significant metabolic pathway,and PS,PC,PE,PI,CL,LPS,LPG,LPC,LPE and LPI are important lipid involved in it,which may become important precursors for the formation of volatile compounds in the metabolic process.