Study On Structure,Function And Surface Properties Of Protein From Milk Dregs At Different Altitudes And Its Application In Probiotic Yogurt

Yak milk and milk products(milk residue)are important sources of Tibetan food protein.There are significant differences in quality and processing characteristics of yak milk and milk residue at different elevations.However,due to the serious lack of understanding of milk residue obtained from different altitude,poor quality,few kinds and low added value of milk residue food was produced.Thus,the application of milk dregs is limited.For this reason,the purpose of this project is tantamount to understand yak milk residue protein,develop yak milk residue protein products and improve the added value of yak milk dregs.This topic takes the water-soluble protein and alkali-extracted protein were obtained by water-dissolving method and alkali extraction-acid precipitation method from yak milk residue,respectively.The molecular structure,biological function and protein surface properties of milk residue protein[water-soluble Protein(WP)and alkali-extracted protein(AP)]at three elevations(Nyingzhi,2800 m;Gali County,4500 m and Amdo,5200 m)were studied,and the application of yak milk residue protein in yoghurt processing was explored.It serves as a theoretical basis for the development and application of yak milk dregs.The results of the paper are as following.The secondary structure,tertiary structure,carbonyl content,sensitivity of pepsin and trypsin,antioxidant activity and other structural and functional characteristics of yak milk residue protein at three elevations were investigated and analyzed.The results showed that,for the secondary structure of milk residue protein,Amdo WP had the highest α-helix content of(16.1%),followed by Gali County(6.3%),and Nyingzhi had the lowest(0%).For AP,all yak milk residue proteins at three elevations had aβ-corner structure.For the tertiary structure of milk dregs,the contents of tryptophan,tyrosine and phenylalanine residues were the highest in Nyingzhi milk dregs,followed by Ando milk dregs,and the lowest in Gali County milk dregs.For the content and distribution of AP tryptophan,tyrosine and phenylalanine residues,the protein contents of milk dregs in the three elevations were less.Among the protein carbonyl content of milk dregs,for WP,Nyingchi milk dregs had the highest carbonyl content(3.88 mol/mg)and Gali County milk dregs had the lowest carbonyl content(1.34 mol/mg).For AP,Amdo milk dregs had the highest carbonyl content(2.03 mol/mg)and Gali County milk dregs had the lowest carbonyl content(1.24 mol/mg).In view of the sensitivity of milk dregs to pepsin and trypsin,WP and AP of low altitude milk dregs had good sensitivity to pepsin,while the WP and AP of high-altitude milk dregs were more sensitive to trypsin.The results of antioxidant activity showed that the antioxidant capacity of WP was generally stronger than that of the AP at the same elevations,and that of Nyingzhi and Amdo milk dregs was stronger than that of Gali County milk dregs.Comprehensive analysis showed that altitude had significant effects(p<0.05)on the resultant structure,tertiary structure,carbonyl content,sensitivity of pepsin and trypsin and antioxidant activity of yak milk residue protein.The emulsions prepared from yak milk dregs at three elevations were studied based on their particle size distribution,zeta potential,turbidity,emulsification index,microstructure,rheological properties,free sulfhydryl groups,disulfide bonds,surface hydrophobicity,and the surface stable emulsions of yak milk dregs at three elevations.The results showed that the emulsion prepared by water-soluble protein(MWPE)and emulsion prepared by alkali extraction of protein(MAPE)of Nyingchi has the smallest particle size(about 40 μm and 70 μm,respectively)and more uniform distribution for the particle size of emulsion prepared from milk residue WP and emulsion prepared by milk residue AP.For the zeta potential of MWPE,there were no significant differences in the zeta potential of MWPE at three elevations.For the zeta potential of MAPE at different elevations,Gali County was the highest(up to 26 mV),Nyingchi was the second,and Amdo was the smallest.For the turbidity of MWPE at different elevations,the turbidity of Gali County is the highest(57.32 nm/m),53.51 nm/m is the highest,and Nyingzhi is the lowest(49.47 nm/m);for the turbidity of MAPE at different elevations,Amdo is the highest(74.81 nm/m),Nyingchi is the second(60.62 nm/m),and Gali County is the smallest(55.62 nm/m).For the MWPE emulsification index at different altitudes,Amdo was the largest(16.11%),Gali County was the second(10.93%),and Nyingchi was the smallest(9.21%);for different elevations,the MAPE emulsification index was the largest in Gali County(8.45%),Amdo the second(8.08%),and Nyingchi the smallest(6.86%).For the microstructure of MWPE and MAPE at separate elevations,the size distribution of milk droplets in Gali County was the most uniform,followed by Nyingchi,and Amdo was the worst.For the rheological properties of MWPE at different elevations,the initial viscosity of Gali County is the highest(3.00 Pa.s),the shear stress is also the largest(0.97 Pa),the initial viscosity of Amdo is the second,and the initial viscosity of Nyingchi is the lowest.For the rheological properties of MAPE at different elevations,the initial viscosity of Nyingchi is the highest(0.25 Pa.s),the initial viscosity of Gali County is the second,and the initial viscosity of Amdo is the lowest.For the contents of free sulfhydryl and disulfide bonds in MWPE at different elevations,the content of Amdo distribution was the highest(5.40 0.442 nmol/mg and 3.40 0.144 mol/mg),while that of Gali County was the lowest.For the surface hydrophobicity of MWPE and MAPE at different elevations,the surface hydrophobicity of Gali County was the highest(1761.60 α mL/mg and 2057.5 αmL/mg),Amdo was the second(1559.00 α mL/mg and 1679.3 α mL/mg),and Nyingchi was the lowest(554.25 α mL/mg and 860.05 α mL/mg).The comprehensive results demonstrate that Amdo and Gali County milk residue protein is more favorable for the formation of a stable emulsion.Yak milk dregs at three elevations were invoked as important components to prepare probiotic yogurt,and the corresponding characteristics of yogurt were studied.The sensory score,texture,rheological properties,water content,pH,dripping acidity,the total number of probiotic colonies,living bacteria,and antioxidant activity of yogurt were analyzed.The results showed that the sensory evaluation of yogurt prepared from the water-soluble protein of milk dregs(MWPY)and yogurt prepared by alkali extraction of milk dregs(MAPY)at different elevations showed that the sensory scores of MWPY and MAPY were the highest(the overall acceptability was 19.10±0.83and 18.60±1.28,respectively),followed by Nyingchi and the worst in Gali County.For the rheological properties of MWPY at different elevations,the initial viscosity of Gali County is the highest(14.3 Pa.s),followed by Amdo,and Nyingchi is the lowest.All probiotic yogurt produced from MWPY is plastic fluids.For the rheological properties of MAPY at different elevations,the initial viscosity of Gali County is the highest(7200 Pa.s),followed by Nyingchi and Amdo.All probiotic yogurt produced from MAPY is pseudoplastic fluids.For the texture properties of MWPY and MAPY at different elevations,the hardness of Amdo MWPY was the highest,followed by Gali County,and Nyingchi was the lowest,and its adhesion showed the opposite trend.The hardness and adhesion of yogurt decreased gradually during storage.For the initial pH and titratable acidity of MWPY at different altitudes,Amdo pH was the highest,Nyingzhi was the highest,pH and titratable acidity were the second,and there was no significant difference between the initial pH and titratable acidity of MAPY at different altitude.The pH of most yogurt reduced and the titratable acidity of all yogurt increased during 10 d storage of MWPY and MAPY at different altitudes.For the water content of MWPY and MAPY at different altitudes,there was no significant difference in the initial water content of MWPY and MAPY of the three elevations.During the storage,except MWPY and MAPY of Amdo,the MWPY and MAPY of other elevations showed an upward tendency.For MWPY and MAPY at different elevations,Amdo MWPY and MAPY had complete probiotic morphology,the highest preliminary probiotic colony count,living bacteria number,and the highest value-added during 10 d of storage.For the antioxidant activity of MWPY and MAPY at different elevations,the supernatants of MWPY and MAPY from Gali County and Ando milk had better hydroxyl group radical scavenging activity.To sum up,compared with Nyingchi and Gali County milkresidue protein,Amdo milk residue protein has better molecular structure,biological activity,and surface function,and has more advantages in the processing and application of probiotic yogurt.This study provides basic research for the application of Tibetan yak milk residue and is beneficial to improve the utilization and added value of yak milk residue.