**1. Introduction**

Acid-induced milk gel products are one of the most traditional and widely consumed foods, which have a variety of health claims and curative benefits [1]. The acid coagulation of milk proteins is an irreversible and complicated process accompanied by demineralization, reduction of electrostatic interactions between protein molecules, and aggregation of caseins through hydrophobic interaction and calcium bridging [2–4]. The formation, structure, and physicochemical properties of acid gels have been reviewed recently [5]. Much-related research has been done to understand the gelation mechanism and the role of different components in the final gel texture. The influence mechanism of exopolysaccharides (EPS), produced by starter culture during the acidification process, on gel formation is under debate and has become a hot research topic in recent years.

In general, there are two types of acid-gel dairy products: fresh acid-coagulated cheese products (cream cheese, cottage cheese, quarg, tvorog, and frais) and yogurt products. In fresh acid cheeses, acid and heat are usually combined to coagulate the milk, cream, or whey. The acidification can be achieved by adding acids (such as HCl) or fermentation through culture. Utilization of EPS producing culture can improve the functionality (such as stiffness, serum retention, and creaminess) of fresh acid cheese [6]. In real production, a small amount of rennet, which specifically works on the surface κ-casein layer and thus decreases the steric repulsion between casein micelles, is usually combined with acidification to increase the gel properties, such as decreased coagulation time, increased gel strength, and decreased syneresis [3, 7]. Unlike fresh acid cheese products, yogurt is acidified by the thermophilic starter bacteria (*Streptococcus thermophilus* and *Lactobacillus bulgaricus*), which ferment the lactose to lactic acid and produce EPS during acidification [8]. There are two types of yogurt products: set-style yogurt that is fermented (undisturbed) in the retail pot, and stirred-type yogurt produced by breaking the set gel before mixing with fruit or other ingredients and filling it into containers [9]. The effect of different starter cultures and processing conditions on the physicochemical properties of yogurt and fermented milk has been reviewed [10, 11].

The production of yogurts and fresh acid cheeses usually involves the pretreatment of milk like heat treatment and homogenization. Those processing show significant influences on the structure of milk proteins and their gelation behavior [3]. For instance, the heat-induced whey protein denaturation and the attachment of denatured whey protein aggregates on the surface of casein micelles are known to increase the gelation pH (from 4.6 to 5.3) and the gel strength [12]. Homogenization increases the protein hydration and the density of network strands, resulting in increased gel rigidity and resistance to syneresis [13]. In addition, other techniques such as ultrasonication and enzymatic treatment (rennet and transglutaminase (TG)), and the addition of prebiotic or bioactive compounds, can also improve the gel texture and the gelation properties of milk [4, 11, 14]. This chapter summarizes the formation mechanism of acid-induced gels, methods to characterize the physicochemical properties of gels, and updates recent progress in using different strategies to improve the texture and microstructural properties of acid gels.
