牦牛初乳和常乳中乳清蛋白的热稳定性研究

摘要

乳清蛋白是从牛奶中分离出的一种可溶性蛋白，包含人类饮食中必需的重要营养成分。然而在生产过程中的热处理会使起乳清蛋白发生变性和聚合反应，进而制约了乳清蛋白在食品工业中应用。为了研究牦牛初乳和常乳中乳清蛋白的热稳定性，本研究测定并比较了牦牛和荷斯坦奶牛初乳和常乳中乳清蛋白的组成，并分析了不同热处理温度和pH值对乳清蛋白热稳定性的影响。
　　1、分别从牦牛和荷斯坦奶牛初乳以及常乳中分离乳清，并采用酶联免疫吸附技术(ELISA)测定了乳清总蛋白、免疫球蛋白G(TgG)、乳铁蛋白(LF）、β-乳球蛋白(β-Lg)、α-乳白蛋白(α-Lac)和牛血清白蛋白(BSA)的含量。结果表明，牦牛和荷斯坦奶牛初乳中乳清总蛋白、IgG和LF的含量显著高于常乳。其中牦牛初乳和常乳中乳清总蛋白、IgG的含量与荷斯坦奶牛无显著差异，然而以高品质初乳IgG含量大于等于50 gL-1为参照标准，牦牛高品质初乳产出率100％，而荷斯坦奶牛仅为73.33％。LF、α-Lac和BSA的含量在牦牛初乳中显著低于荷斯坦奶牛，而在牦牛常乳中与荷斯坦奶牛相近。β-Lg是牛乳中主要的过敏原之一，其含量在牦牛初乳和常乳中均低于荷斯坦奶牛。这些结果表明，与荷斯坦奶牛的初乳和常乳相比，牦牛初乳和常乳是较好的功能性食品原料。
　　2、将牦牛乳清蛋白在60℃、63℃、72℃、90℃的温度下，分别加热15sec和30min，采用ELISA和UV分光光度技术通过检测牦牛乳清蛋白的浊度和自由巯基来评价其热稳定性。结果表明，在pH=3.4-4.6范围内，加热温度较低时，乳清蛋白的热稳定性较高，随着温度和pH值的上升，乳清蛋白的稳定性开始降低，其浊度和自由巯基逐渐增加，在pH=5.5、温度高于63℃的加热条件下，乳清蛋白的变性最明显，热稳定性最差。
　　3、在牦牛乳清蛋白中分别加入三种保护剂（氯化钙、SDS和甘油），对其在pH值为3.5，4.6和5.5时的稳定性以及在72℃和90℃时的稳定性进行测定。研究结果表明:加入保护剂后对乳清蛋白的热稳定性有显著的提高，当pH为3.5时，乳清蛋白中沉淀最少，且浊度变化最小，热稳定性最高。随着加热温度的升高，牦牛初乳和常乳中IgG、LF、α-Lac、β-Lg和BSA开始发生变性，含量逐渐降低。对三种保护剂的保护效果比较发现，甘油对乳清蛋白热稳定性的提高效果最佳，其次为SDS和氯化钙。

目录

声明

Abstract

摘要

TABLE OF CONTENT

List of Abbreviation

CHAPTER Ⅰ Introduction

1．1 Background information

1．2 Nutritional importance of yak milk

1．3 Factors influencing milk composition

1．4 Functional and nutraceutical foods

1．5 Milk protein components

1．5．1 Casein proteins

1．5．2 Whey proteins

1．6 Colostrum composition

1．6．1 Factors affecting Immunoglobulins concentration in colostrum and milk

1．6．2 Colostrum safety

1．7 Whey proteins processing

1．7．1 Whey proteins separation techniques

1．7．2 Whey proteins denaturation

1．7．3 Improvement of heat stability on whey proteins

1．8 Research objectives

CHAPTER 2 Effect of heat treatment on yak and sows’ milk／colostral liquid whey proteins at various pH levels

2．1 Introduction

2．2 Material and methods

2．2．1 Collection of samples

2．2．2 Liquid whey extraction from milk／colostrum

2．2．3 Total whey proteins assay

2．2．4 Whey protein components quantification by ELISA method

2．2．5 Heat treatment of liquid whey

2．2．6 Turbidity

2．2．7 Free sulfhydryl concentration

2．3 Statistical analysis

2．4 Results and discussion

2．4．1 Composition of proteins in liquid whey

2．4．2 Effect of heat on liquid whey turbidity at various pH levels

2．4．3 Effect of heat on free sulfhydryl concentration in liquid whey at various pH levels

2．4．4 Effect of heat in liquid whey IgG concentration at various pH levels

2．4．5 Effect of heat in liquid whey LF concentration at various pH levels

2．4．6 Effect of heat and pH on major whey proteins concentration

2．5 Conclusion

CHAPTER 3 Heat stability improvement of yak milk and colostral liquid whey proteins at various acidic conditions

3．1 Introduction

3．2 Materials and methods

3．2．1 Sample collection

3．2．2 Liquid whey extraction from milk／colostrum

3．2．3 Heat treatments

3．2．4 Total whey proteins assay

3．2．5 Determination of whey protein components concentration

3．3 Statistical analysis

3．4 Results

3．4．2 Influence of protectants on heat denaturation of liquid whey proteins at various pH levels

3．5 Discussion

3．5．2 Influence of protectants on heat stability of IgG and LF in liquid whey at various pH levels

3．5．3 Influenee of protectants on heat stability of major whey proteins at various pH levels

3．6 Conclusion

GENERAL CONCLUSIONS AND PERSPECTIVES

REFERENCES

ACKNOWLEDGEMENTS

CURRICULUM VITAE