Development Of The Set-type Yogurt Enriched With C9,t11-conjugated Linoleic Acid

Linoleic acid（LA）is a precursor of n-6 polyunsaturated fatty acids,and its derivatives are involved in complex metabolic pathways in biological processes and are essential for human.Conjugated linoleic acid（CLA）is a general term for the mixture of positional and geometric isomers of LA,which is beneficial to human health and has effects on immunomodulatory activity,cancer,obesity,atherosclerosis,as well as diabetes and hypertension.Add LA to skim milk,and ferment to obtain the set-type yogurt enriched with c9,t11-CLA.This increases the health care function of yogurt.In this paper,first,the effect of LA on the fermentation characteristics of yogurt was studied.Then,the complex system of LA with casein,β-lactoglobulin and bovine serum albumin was used as the research object,and the interaction mechanism between LA and the three proteins was explored.The effects of LA on the whey separation,texture properties and microstructure of fermented yogurt were also studied,which provided the basis for the development of functional fermented milk enriched with c9,t11-CLA.The main findings are as follows:1.The effects of LA,fermentation time,and concentration of skim milk on properties of yogurt and the synthesis of c9,t11-CLA were mainly studied.Add LA to skim milk,and ferment to obtain the set-type yogurt enriched with c9,t11-CLA.The titration acidity,p H,water holding capacity,changes of lactic acid bacteria and c9,t11-CLA content of yogurt were measured.It was found that adding LA to enrich fermented yogurt with c9,t11-CLA,a beneficial substance with unique efficacy,could improve the quality of yogurt to a certain extent..It was found that the fermentation conditions were LA concentration of 0.1%（v/v）,skim milk concentration of 11%（w/w）and fermentation time of 36 h.The fermented milk under this condition had suitable acidity and p H,which is around 113°T,the water holding capacity reached 71%,the number of viable bacteria reached 2.5×10⁸ cfu/m L,the less whey separation,and the yield of c9,t11-CLA reached 873.12μg/m L.The yogurt had better quality and less whey separation,which may be due to the interaction between LA and milk protein.2.The interaction of LA with casein,β-lactoglobulin and bovine serum albumin and the protein conformation were studied by UV-Vis absorption spectra,fluorescence quenching spectrum,synchronous fluorescence spectrum,three-dimensional fluorescence spectrum,and circular dichroism spectrum.It found that LA interacted with the three proteins and formed a complex via the UV-Vis absorption spectrum.Fluorescence quenching spectrum showed that LA can quench the endogenous fluorescence of three proteins according to the changes of fluorescence intensity after binding of LA to three proteins and the calculation of Stern-Volmer constant(K_sv)and quenching bimolecular rate constant（K_q）.The reaction process is spontaneous,and the quenching mechanism is static quenching.The binding constant（K_a）,the number of binding sites（n）and the type of interaction force between LA and the three proteins were further determined by double logarithmic equation and Van’t Hoff equation（Van’t Hoff formula）.It found that the interaction between LA and the three proteins is mainly based on hydrogen bonding and Van der Waals force,the binding ability is strong,and there is only one binding site between them.Synchronous fluorescence spectrum showed that the action of LA on the three proteins would cause changes in the amino acid residue microenvironment of the proteins.Three-dimensional fluorescence spectrum and circular dichroism spectrum found that the interaction of LA with three proteins could change the conformation of proteins.3.By observing the changes of curd status,whey separation,texture,and gel microstructure of the fermented set-type yogurt,it was found that after adding LA,the whey separation of fermented milk decreased,and it was positively correlated with the LA concentration.Whey separation reduced from 2.70%to 2.11%The addition of LA reduces the hardness value of the yogurt gel from 27.27 g to 17.87 g,and the yogurt exhibits a denser and homogeneous microstructure.