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Yogurt, a cultured dairy product, is milk fermented by lactic acid bacteria, mainly Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. Because of the existence of a large number of live bacteria, yogurt has therapeutic effects, such as digestion enhancement, enterogastric peristalsis boosting, appetite enhancement, anticarcinogenic activity, and reduction of serum cholesterol [1]. Yogurt also contains many bioavailable proteins, minerals, and vitamins [2]. Therefore, yogurt has become a popular favorite among consumers around the world. In China, yogurt production has risen dramatically with an annual growth rate of more than 10% in 2009–2013 [3].
Yogurt is a complex gel that is mainly composed of denatured protein and milk fat globule. The fat content of yogurt directly influences the final strength of the gel network structure [4]. When the fat content of yogurt decreases, a more fragile gel network structure of yogurt forms, and this also leads to less desirable rheological properties, texture characteristics, taste, and flavor [5].
In order to produce good-quality fat free, low fat, and reduced fat yogurt, it is a common practice to add stabilizers to yogurt, such as pectin, gelatin, and κ-carrageenan [6,7]. The addition of stabilizers in yogurt has a negative influence on consumers' acceptance, because more natural yogurt products are preferred [8]. Under the premise of not using stabilizers, it is a challenge to manufacture fat free, low fat, and reduced fat yogurt with the desired gel network structure, creamy aroma, mouthfeel, and little whey-off during storage [9].
Increasing the protein content of yogurt offers an alternative way to strengthen the gel network structure [10]. Denatured proteins act as fillers or binders within a casein matrix [5]. The main components of denatured protein are caseins that are mainly composed of four kinds of monomeric protein: αs1-casein, -αs2-casein, β-casein, and κ-casein [11]. The content of these four kinds of monomeric proteins directly influence the final strength of the gel network structure [8]. Ozlem and Nursel [12] assessed the gel network and water holding capacity of yogurt with the increasing protein content. In the research of Denin-Djurdjević et al [13] and Fetahagić et al [14], the viscosity of yogurt with added dry...