Open access peer-reviewed chapter
Abstract
Understanding the acid coagulation of milk is the core of producing different fermented dairy products. The formation of the gelled structure includes the decreased stability of casein micelles, aggregation, and the gradual development of the bonding between proteins during acidification and cold storage. The coagulation behavior of casein micelles and the physical properties of the final gels can be modified by processing techniques. Exopolysaccharides (EPS) produced by starter culture during fermentation also contribute significantly to the microstructure and texture of acid gels. This chapter discusses the mechanisms of acid-induced gelation of milk based on the modified nanocluster model of casein micelles. The recent findings of heating, high-pressure treatment, ultrasonication, and enzymatic modification on the acid gelation behavior of milk are described. The influence of different ingredients such as polysaccharides (endogenous and exogenous) and phenolic compounds on the physical properties of acid gels are also summarized.
Keywords
- casein micelle
- acid gelation
- yogurt
- Exopolysaccharride
- whey proteins
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, 3, 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 (
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.
2. Acidification of milk
2.1 Method of acidification
There are different methods to acidify milk, including the direct addition of acids, such as lactic acid [15], citric acid [16], citric acid, and sulfuric acid [17], and indirect fermentation by bacterial culture or glucono-delta-lactone (GDL) which hydrolyses into gluconic acid in solution [3]. The addition of inorganic acid can decrease the pH of the milk rapidly, while indirect fermentation decreases the pH slowly. The hydrolysis of GDL is temperature-dependent, where the pH reduction is more rapid at a higher temperature. GDL decreases the pH much faster initially than culture but then stabilizes. By contrast, the pH of milk added with starter bacteria continues to decrease slowly with time. The final pH of GDL-induced gel is determined by the amount of GDL added to the milk, while the pH of culture-induced gel can reach a very low pH (e.g., 4.1) until the bacterial activity is inhibited [5]. The differences in the acidification rate lead to different changes in the physicochemical properties of casein micelles and the aggregation behavior of casein particles, which further influence the rheological and physical properties of the final acid gels.
2.2 Structure of casein micelle and its changes during acidification
The formation of a gel structure of milk is mainly from the changes in milk proteins, particularly for the caseins, which constitute approximately 80% of total milk protein. There are four main caseins: αS1, αS2, β, and κ-caseins with a ratio of 4:1:3.5:1.5 [18]. Caseins cannot form a globular structure due to the presence of a high amount of proline [3]. Alternatively, caseins can combine with calcium and assemble into a particular spherical micellar structure, named casein micelles, which have a diameter range of 50–500 nm (average 150 nm), containing 94% protein and 6% minerals (calcium, phosphate, magnesium, and citrate) [19]. The structure of casein micelle has been developed over the past decades. Among all the proposed models, the nanocluster model proposed by Holt et al. [3] can best characterize all phenomena that occurred to milk during processing. This nanocluster model was improved by Dalgleish and Corredig [18], considering the location of a large amount of water in the micellar interior, as shown in Figure 1.
Figure 1.
The structure of casein micelle: Black spheres represent the calcium phosphate nanoclusters that solubilize during the acidification process. Blue coils represent αS and β-caseins; red lines on the outermost part of the surface represent κ-caseins. (Source: Wang and Zhao [
In the nanocluster model, αS- and β-caseins (blue coils), which are rich in phosphoserine in their structure, are considered to interact with and surround the colloidal calcium phosphate nanoclusters (black spheres), forming the internal structure of casein micelles through hydrophobic interaction and hydrogen bond [4]. κ-caseins lack phosphate centers and are present on the surface providing strong steric repulsion to maintain their colloidal stability [20, 21]. Casein micelles have an isoelectric point of 4.6.
Acidification influences both the surface and internal structure of casein micelles. The colloidal calcium phosphate gradually dissociates from casein micelles [22], the surface charge of casein micelles decreases, and caseins are released into the serum phase [23]. The dissociation of caseins is temperature-dependent. At low temperatures (4°C), around 40% of caseins dissociated from micelles at pH 5.5, while no virtual dissociation of caseins occurred at 30°C [24, 25]. On the contrary, mineral solubilization is independent of acidification temperature. The extent of mineral solubilization increases markedly below pH 5.6 and is almost complete at around pH 5.0 [3, 26]. No changes in the hydrodynamic diameter of casein micelles occur during the acidification to pH 5.0 [22], although the charge of κ-casein decreases with acidification, resulting in the collapse of the κ-casein layer and the reduction in the stability of the micelles, as the intra- and inter-chain interactions are insufficient to keep the protein fraction extended in solution [27]. Further decrease to pH around 4.8 results in the aggregation of caseins and the formation of gel structure in unheated milk [2].
3. Characterization of acid milk gels
3.1 Mechanism of gel formation
Acid milk gels are typical particle gels where aggregated protein particle forms continuous network structures throughout the entire volume. The mechanism of the gel structure formation is still controversial. Different theoretical models have been proposed to characterize the gel-forming process, including the adhesive hard-sphere model, fractal model, and percolation model [5]. The adhesive hard-sphere model focuses on the surface κ-casein layer of casein micelles. In this model, the highly charged glycomacropeptide (GMP) part of the κ-casein sterically stabilizes the casein micelles [27]. The strong steric repulsion provided by GMP prevented the aggregation of casein micelles against pH and ionic modifications [22, 23, 28]. The rheological behavior of casein micelles fits well with the hard-sphere model [22]. The radius of casein micelles also remains stable at different concentrations. But once the pH approaches the pKa of the charged GMP brush, the surface of casein micelles collapses and aggregates. This model well defined the aggregation behavior of casein micelles. However, it fails to explain the development of gel structure and its mechanical properties [29].
The fractal aggregation model describes that the spherical particles of casein micelles can encounter each other through Brownian motion and form aggregates. The aggregates can then also aggregate with each other. Once no further changes happen among the particles in the aggregate and they are incorporated, this cluster-cluster aggregation process directs to the aggregates that obey the following scaling Eq. (1):
where Np is the number of particles in an aggregate of radius R, N0 is the total number of initial particles that could form the floc, D is the fractal dimensionality constant (D < 3), and αeff is the radius of the effective building blocks forming the fractal clusters. One limitation of this model is that it assumes all the aggregates have the same size, which is not the case in reality [5]. It also fails to explain the aggregates’ rearrangement (before, during, and after gelation), which can influence the D value.
The percolation model combines the concept of fractal aggregate formation and the hard-sphere model. It assumes that percolation clusters form random bonds between adjacent micelles in a lattice, which are random and their sizes increase with the increasing number of bonds. A larger cluster appears above a certain threshold which extends throughout the lattice. Analogies between percolation and gelation can be drawn with particles establishing an increasing number of links as they aggregate until, at a certain threshold, a cluster is created and spans the container/system [2]. Only a small portion of bonds are joined, and not all individual fractions are incorporated into the system-spanning cluster. This model successfully explained the continuous increase of elastic modulus (G′) after gelation [29]. However, it is hard to use this theory to model the mechanical properties of acid gels.
In addition to modeling the acid gelation process, the physical–chemical changes of milk during acidification have been well studied. When the pH of milk is decreased from 6.7 to 6.0, there is a decrease in the net negative charge and reduced electrostatic repulsions. But the solubilization of CCP is minimal, and the micelle integrity is preserved [22]. Decrease the pH from 6.0 to 5.0 leads to further neutralization of the surface charge and shrinkage/collapse of the hairy layer. CCP is fully dissolved, while the internal structure is more homogeneous [3]. At pH lower than 5.0, the destabilized caseins come closer to each other and form the gel structure [4]. Subsequent cooling/refrigeration causes the gels to swell, increasing the contact area of particles and the gel firmness/strength. As the hydrophobic interaction are lower at low temperatures, the increased gel firmness during cooling storage indicates that other forces, such as electrostatic and van der Waals’ interaction, also contribute to the gel integrity [1].
3.2 Rheological properties
Acid gels are viscoelastic materials. In the dairy industry, the rheometer is the most widely used technique to characterize acid gels. There are two main test methods: small-amplitude oscillatory rheology and large-amplitude oscillatory shear. Large deformation studies can provide information on properties related to the consistency during shearing (a step in the production of stirred-style yogurt) and consumption.
Small-amplitude oscillatory rheology (dynamic testing) is a nondestructive method, involving an applied oscillatory strain or stress that provides very useful information about the gelation process [2, 30]. The main parameters determined during this test include the elastic or storage modulus G′, which indicates the energy stored per oscillation cycle, the viscous or loss modulus G″, which indicates the energy dissipated per cycle, and the loss tangent (tan δ), which is the ratio between the viscous modulus and elastic modulus. The definition of these parameters is shown in the following equations:
where τ0 is the shear stress, γ0 is the shear strain, and δ is the phase angle.
In reality, the majority of preceding rheological measurements of milk gelation were performed under low strains (<1%) and oscillating strain rates (<0.1 Hz) to avoid gel destruction [7]. The gelation point is where the elastic and viscous modulus cross over (Tan δ = 1) [28]. The rheological properties of acid gel made from unheated milk at 30°C have been well studied, as summarized in previous reviews [1, 2]. After passing the gelation point, the G′ increased rapidly and plateaued during the aging of the gel. Loss tangent (Tan δ) decreased to <0.4 quickly after gelation and then to around 0.25 during aging. Heat treatment significantly increased the G′, and the gelation pH increased from 4.8 to 5.2. [3]. Renan et al. [31] compared the gelation profiles of acid gels produced with culture fermentation and GDL. As shown in Figure 2, the elastic modulus of acid gels fermented by culture increased much faster than the GDL. The resulted gels were firmer with a more heterogeneous structure. Both methods produced gels with a similar final loss tangent value of about 0.22. Moreover, acidification methods also influence the rheological properties of acid-induced gels. The presence of EPS, which are produced by starter culture during fermentation, enhances the protein distribution and viscoelastic properties of acid gels [8, 32].
Figure 2.
Rheological properties of heat-treated milk during acidification with glucono-delta-lactone at 20°C (black line) or a bacterial culture at 38°C (gray line) in coaxial cylinders versus time. Zero time for bacterial acidification was taken at the time when the temperature reached 38°C. three repetitions for each procedure. (Source: Renan et al. [
3.3 Microstructure of acid milk gels
The microstructure of the gels is directly correlated with their texture, appearance, and organoleptic properties. In the dairy industry, scanning electron microscopy (SEM) and confocal laser scanning microscopy (CSLM) are the most commonly used techniques to observe the microstructure of acid gels. Accordingly, the acid gels consist of a coarse particulate network of casein particles linked together in clusters, chains, and strands [5]. Gastaldi et al. [33] monitored the pH-induced changes of casein micelles during the acidification process. As shown in Figure 3a, casein micelles started to aggregate forming clusters when the pH was decreased from 6.7 to 5.8. The initial shape was still discernible. At pH 5.5 to 5.3, most casein particles lost their original structure and were deformed, stretched, and extensively coalesced, forming a pseudo-network with an open structure (Figure 3b and c). After decreasing the pH to between 4.8 and 4.7, the protein network appeared denser, and the pore size between casein aggregate particles became smaller. At this stage, the formation of acidified milk gels is completed, where the casein particles are aggregated into a three-dimensional network (Figure 3e and f). Much more related research using SEM to investigate the changes in the microstructure of acid gel has been done recently [34, 35]. One shortcoming of SEM is that many preparation steps are required, including dehydration, fixation, embedding, sectioning, and staining, which may disrupt the native structure of gel products and result in the formation of artifacts.
Figure 3.
SEM micrographs of acidified milk critical-point dried samples at different pH: pH 5.8 (a), pH 5.5 (b), pH 5.3 (c), pH 5.0 (d), pH 4.8 (e), pH 4.7 (f). The scale bar represents 1 μm. (Source: Gastaldi et al. [
Compared to SEM, CLSM is a relatively new technique. It allows observing the overall microstructure of milk gels with minimal preparation steps due to its unique optical sectioning abilities and high spatial resolution [2]. Figure 4 shows the CLSM images of acid gels produced by GDL or yogurt culture, GDL-produced gel exhibited a denser and more homogeneous structure compared to the gel fermented by culture [31]. Another advantage of CLSM is that it can identify different components in the gel by using specific fluorescence labels. The protein network has been stained with Congo red (0.01% in water) and fluorescein isothiocyanate (FITC, 0.025% in dimethyl sulfoxide) [36]. In another study, the microstructure of low-fat yogurt was observed with CLSM using fast green FCF fluorescent stain to label protein and lectin wheat germ agglutinin Alexafluor 55 conjugate to label EPS produced by starter culture [37]. In reality, the combination of SEM and CLSM can provide more thorough information about the overall and detailed microstructure.
Figure 4.
A comparison of the microstructure of acid milk gels produced by yogurt culture (a and c) and GDL (b and d). (Source: Renan et al. [
3.4 Syneresis/whey separation
Syneresis is defined as the spontaneous contraction of a gel, leading to the expulsion of liquid from the pores. In acid milk gel, syneresis is also called whey separation, which refers to the occurrence of whey on the surface of a milk gel. Syneresis relates to the instability of the protein network, which causes a loss of the capacity to entrap the whey in the network [38]. Rapid fermentation, proteolysis, and high incubation temperatures are the main factors that lead to the whey separation of acid gels [39]. Proteolysis during fermentation causes the reduction of interconnections within the protein network and the rearrangement of the intra-network. On the other hand, the acid curds are more prone to syneresis at increased temperatures due to higher rearrangements causing contractions in the gel network, which creates pressure for the whey to move [40].
Whey separation can be simply quantified by determining the quantity of whey expelled from yogurt after centrifugation or drainage through a screen [38, 41, 42]. Both methods are not related to the spontaneous separation of whey from set-style yogurt. The centrifugation method determines the water-holding capacity of the gels under different forces. The drainage of whey from a disrupted gel distributed over a screen measures the whey separation over a large surface area, which is more relevant to the products such as cottage or casein than to set yogurt [2]. Lucey et al. [43] proposed a new method that produces the gels directly in a container and determines the quantity of expelled whey on the surface. During the manufacture of acid-induced gel products, heat treatment is used to sterilize the milk, and gelation is done at a high temperature, which increases whey separation in acid gels. Dairy scientists have used different ways to increase the gel properties, such as the use of high-EPS yield culture [44], enzymatic treatment to strengthen the protein network [3], and increase the protein concentration or adding different exogenous polysaccharides [42, 44, 45].
3.5 Texture properties
The textural properties of acid milk gels can be measured by different instrumental methods, such as dynamic-amplitude oscillation, large-amplitude oscillatory, texture analyzer (penetration), and rotational viscometry [2]. The main challenge for the acid milk gels is the “lumpiness” or “granular” body texture, which is against consumers’ expectation of a smooth, fine-bodied product. This textural defect is due to forming large protein aggregates that often range in size from 1 to 5 mm [46]. Many factors contribute to the formation of dense protein clusters, including incubation at a high temperature, rennet, and adding excessive starters [2, 47]. A recent publication indicated that the vibration during fermentation resulted in the formation of bigger aggregates, which caused the graininess of set-style yogurt [48]. In addition, other factors such as a very high amount of total solid and adding excess whey protein concentrate to the milk also increased the “lumpy” or granular defect [2, 49]. Stabilizers, both exogenous and endogenous, have been proved to provide smooth body texture to the acid gel products [44, 50]. Optimizing parameters such as heat treatment, total solid level, amount of additives, amount/variety of starter added, and incubation temperature are necessary to produce acid milk gels with desired texture.
4. Strategies to improve the acid-induced milk gels
4.1 Heating
Heat treatment is a standard procedure before further processing of the dairy product. In the production of fermented dairy products such as yogurt, the heat treatment is usually performed at high temperatures (such as 90°C, 5 min). Heating can destroy the raw milk flora and decrease the dissolved oxygen level which can prevent the growth of the starter cultures. More importantly, heat treatment can denature the whey proteins in milk, increasing the firmness and texture of acid milk gels [3, 51].
The structure of casein micelle is relatively heat-stable as they lack tertiary structure. On the contrary, the main globular whey proteins such as α-lactoalbumin (α-la) and β-lactoglobulin (β-LG) undergo irreversible denaturation at temperatures higher than 70°C [52]. The denatured whey proteins can aggregate with themselves or free caseins (mainly dissociated κ-casein) through disulfide bonds and hydrophobic interaction [53]. Denatured β-LG can also attach to the surface of casein micelles by interacting with the surface κ-casein layer. The coating of denatured whey proteins is pH-dependent. At neutral pH 6.7, heating resulted in around 30% of denatured whey proteins associated with the surface of casein micelles, and this number increased to 75% when heating at pH 6.3 [54]. Whey protein denaturation significantly altered the acid gelation behavior of milk, particularly at a higher denaturation degree (>40%). The gelation pH increased from 4.9 to values between 5.1 and 5.3, and the elastic increased drastically [55].
The distribution of denatured whey proteins between serum and the surface of casein micelles has a significant influence on the gelation process. At lower pH 6.3, most denatured whey proteins are present on the surface of casein micelles, and they gel first entrapping the casein micelles and triggering gelation at pH 5.3. In contrast, at pH 7.0, most denatured whey proteins are present in the serum phase as soluble aggregates, which contribute to the formation of stiff gels by associating with casein micelles during acidification [51]. Both heating at lower pH (<6.7) and higher pH (>6.7) resulted in slightly weaker acid gels than at neutral pH (6.7) [56, 57, 58].
There are still some divergent opinions regarding the role of soluble whey protein complexes and micelle bind complexes in the acid milk gels. Some researchers think that the small number of denatured whey proteins associated with casein micelles during heating is responsible for the increased gel properties [43, 58]. In contrast, other researchers think that the soluble denatured whey proteins play a more crucial role than the denatured whey proteins associated with casein micelle [56, 59]. In recent research, glutaraldehyde was added to milk to reduce micellar kappa-casein dissociation, which decreased the formation of soluble protein complexes. This reduction in soluble complexes resulted in the forming of weaker gels [60]. In addition to the gelation process, high heat treatment increases the brittleness of acid gels prepared by microbial fermentation, while it decreases the brittleness of gels prepared via GDL [51, 61]. The differences are due to different acidification rates between these two methods.
4.2 High-pressure treatment
High-pressure treatment can reduce milk fat globule size, disintegrate/re-associate casein micelles, and denature whey proteins [4]. The size of casein micelles is stable under pressures lower than 200 MPa [62]. Increasing pressure to 250 MPa increased micelle size by 25%, whereas a further increase of pressure (300–800 MPa) decreased casein micelle radius by about 50% [63]. The soluble caseins and soluble calcium increased after high-pressure treatment [64].
High-pressure treatments significantly improve the acid coagulation behavior of milk. The rigidity, strength, and resistance to syneresis of acid gels were improved [13, 65, 66], which are a result of the increases in protein hydration and density of network strands, resulting from the incorporation of denatured whey proteins in the acid gel [13, 67]. The elastic modulus and yield stress of acid milk gels increased with decreasing fat globule size as the adsorption of proteins onto the newly created surface of fat globules after high-pressure treatment, resulting in the formation of a more porous protein network with thick strands [66]. Homogenization performed prior to heating resulted in higher adsorption of proteins to the fat globules than homogenization after heating, which further led to the formation of acid gels with higher elastic modulus and yield stress [68].
4.3 Ultrasonication
Ultrasound refers to sound waves with a frequency higher than 20 kHz, which modifies the structure functionality of protein molecules through the cavitation effect, based on the implosion of bubbles that produce shock waves surrounding the probe and jets of high velocity. It is a relatively new technique used in dairy processing to improve the acid gelation properties of milk. The influence of ultrasound on the lactic fermentation, growth and cell viability of lactic acid bacteria, lactose metabolism, texture, and sensory attributes of fermented dairy products has been reviewed recently [69, 70].
Pretreatment of caseins with ultrasound postponed the gelation point to lower pH, decreased the syneresis, and enhanced the elasticity of acid gels, which have a more interconnected structure [71, 72]. The increased acid gelation properties are related to the increased surface hydrophobicity [73]. Whey protein denaturation and increased association of casein with the milk fat globule membrane during ultrasonication also contribute to the increased gel strength [74]. In contrast, ultrasound treatment during the lag phase of lactic acid bacteria reduced the fermentation time, promoted the speed of lactose hydrolysis, and increased the storage modulus of the final gels [75]. For the yogurt products with high protein concentration, ultrasonication during fermentation (for instance from pH 5.8 to 5.1) decreased the firmness and provided a smooth texture for yogurt products, which solved the difficulty of further processing issues [76]. The influence of ultrasonication on acid gelation properties is temperature-dependent. Ultrasonication at temperatures lower than 60°C produced acid gels with higher firmness than those produced at temperatures higher than 60°C [77].
4.4 Enzymatic treatment
Rennet is a complex of enzymes with the active enzyme chymosin, which works on the Phe105-Met106 bond of surface κ-caseins. It cuts κ-casein into hydrophobic para-κ-casein, which remains on the surface of casein micelle, and hydrophilic GMP, which is cleaved from casein micelle [3]. Partial hydrolysis of κ-caseins reduces the negative charge of casein micelles, promoting aggregation during acidification [78]. The gelation pH and elastic modulus increased with increasing hydrolysis degree [79, 80]. Inactivation of enzymes at a lower temperature (60°C) resulted in firmer acid gels than at a higher temperature (85°C). However, gels produced after partial κ-casein hydrolysis exhibited higher syneresis [80].
Transglutaminase (TG) is another enzyme used to improve the acid gelation behavior of milk. It crosslinks peptides and proteins through an acyl transfer mechanism between glutamine and lysine residues [81]. At neutral pH, TG predominately works on the κ-casein surface layer of casein micelles, which prevents the dissociation of κ-caseins, and increases the colloidal stability of casein micelles [82]. TG treatment positively influences the physical properties and microstructure of the yogurt gels [83]. It prevented the release of the caseins into the serum phase which further decreased the formation of soluble complexes during heating [84, 85]. The rearrangements within the protein network were also limited by TG during the gelation, which produced acid gels with a more homogeneous network consisting of smaller aggregates and better WHC [83].
4.5 Endogenous and exogenous polysaccharides
Exopolysaccharide (EPS)-producing starter cultures are preferred in the manufacture of fermented products. The production of EPS in situ has been shown to improve the texture and rheological properties of the yogurt [46, 86]. EPS can improve the structure of milk gels by attaching to the protein network and the bacteria and forming a web-like structure [87]. The influence of EPS on the physical properties of acid gels are affected by EPS location, its structure (molecular mass, side chains, stiffness, and charge), and the interactions of EPS with other components (proteins and minerals) [88]. Depending on the location, EPS can be divided into ropy-EPS (free EPS in the medium) and capsular EPS (located on the surface of the bacterial cells) [89]. Ropy-EPS can produce a stringy and slimy appearance and affect the rheological properties and microstructure of milk gels. Most EPS-producing strains can increase the firmness and WHC of acid gels compared with non-EPS strains. However, slightly weaker gels produced from EPS-producing strains have been reported [90]. Charged EPS can interact with milk proteins through electrostatic attractions during fermentation which increases the gel texture, whereas uncharged EPS can induce depletion flocculation in casein systems [91].
A wide variety of endogenous polysaccharides have been used as additives in the production of acid milk gels in recent years. They can combine with water in the gel and interact with milk proteins during fermentation and storage, resulting in the formation of gels with improved texture and sensory properties. When selecting polysaccharide additives, many factors need to be considered, such as structure, charge properties, and adding amount. Pang et al. [92] found that anionic polysaccharides enhanced the acid gelation properties of yogurt. In contrast, neutral polysaccharides inhibited milk gelation from the beginning [92]. Apple pomace (pectin and soluble fibers) improved the firmness and cohesiveness of set yogurt. At the highest adding amount of 1%, gelation happened at a much higher point (pH 5.9) [93]. Many other polysaccharides, such as okra polysaccharide, dietary fiber, salecan, and oat β-glucan, have been used to improve the structure and rheological properties of acid milk gels recently [44, 94, 95, 96].
4.6 Functional bioactive compounds
Phenolic compounds can combine with milk proteins through hydrophobic interactions [4, 97]. Polyphenol addition does not influence the fermentation process or the lactic acid bacteria viability during the storage of yogurt [98]. Because of the capacity of polyphenols to interact with milk proteins, the acid gels incorporated with phenolic compounds had a higher firmness value and a stronger water-binding capacity within the gel matrix [99, 100, 101]. The influence of phenolic compounds on gelation and the physical properties of acid milk gels depends on the source and addition timing. Phenolic compounds extracted from different herbs (thistle, hawthorn, marjoram, and sage) prevented the syneresis and improved the water-holding capacity of yogurt [102]. Polyphenols extracted from the honeysuckle berries did not influence the rheological properties of yogurt but decreased the viscosity during storage [103]. In contrast, EI-Said et al. noticed that adding pomegranate peel extracts to milk decreased the viscosity of stirred yogurt [104]. The addition of gallic acid before heat treatment resulted in a longer gelation time and decreased final storage modulus (G′) and fracture stress. On the other hand, no influence was found when adding gallic acid after heat treatment [105].
5. Conclusion
Acid-induced coagulation of milk is a complicated process. It involves the demineralization of casein micelles, dissociation of caseins, the collapse of the surface κ-casein layer, casein aggregation, and the development of the protein network. The acid-induced coagulation behavior of milk can be influenced by different processing techniques such as heating, high-pressure treatment, ultrasonication, and enzymatic treatment. Utilizing appropriate processing parameters and polysaccharide additives can improve the rheological properties and microstructure of acid gels. Although much work aims to improve the texture and physical properties of acid gel products has been performed, many “unknowns” need to be resolved. The role of the distribution of heat-induced whey protein/κ-casein complexes and their influence on the acid coagulation behavior of milk needs to be clarified. The effect of released caseins during acidification on the gel structure development during storage requires further investigation. The secretion of EPS by starter culture, the interaction between EPS with milk proteins, and its improvement mechanism of the final gel structure is undoubtedly one of the primary scientific challenges to be solved.
References
- 1.
Phadungath C. The mechanism and properties of acid-coagulated milk gels. Songklanakarin Journal of Science and Technology. 2005; 27 :433-448 - 2.
Lucey JA, Singh H. Acid coagulation of milk. In: Fox PF, PLH MS, editors. Advanced Dairy Chemistry. 2nd ed. Vol. 2: proteins. New York: Aspen Publishers; 2003. pp. 997-1021 - 3.
Li Q, Zhao Z. Acid and rennet-induced coagulation behavior of casein micelles with modified structure. Journal of Food Chemistry. 2019; 291 :231-238 - 4.
Wang X, Zhao Z. Improved encapsulation capacity of casein micelles with modified structure. Journal of Food Engineering. 2022; 333 :111138 - 5.
Lucey JA. Formation, structural properties, and rheology of acid coagulated Milk gels. In: McSweeney PLH, Fox PF, Cotter PD, Everett DW, editors. Cheese. Fourth ed. London: Academic Press; 2017. pp. 179-197 - 6.
Surber G, Schaper C, Wefers D, Rohm H, Jaros D. Exopolysaccharides from Lactococcus lactis affect manufacture, texture and sensory properties of concentrated acid milk gel suspensions (fresh cheese). International Dairy Journal. 2021; 112 :104854 - 7.
Zhao Z, Corredig M. Effects of pH-modification on the rennet coagulation of concentrated casein micelles. Food Chemistry. 2020; 316 :126199 - 8.
Arab M, Yousefi M, Khanniri E, Azari M, Ghasemzadeh-Mohammadi VG, Mollakhalili-Meybodi N. A comprehensive review on yogurt syneresis: Effect of procecssing conditions and added additives. Journal of Food Science and Technology. 2022. DOI: 10.1007/s13197-022-05403-6 - 9.
Li S, Ye A, Singh H. Effects of seasonal variations on the quality of set yogurt, stirred yogurt, and Greek-style yogurt. Journal of Dairy Science. 2021; 104 :1424-1432 - 10.
Farag MA, Saleh HA, Ahmady SE, Elmassry MM. Dissecting yogurt: The impact of milk types, probiotics, and selected additives on yogurt quality. Food Reviews International. 2021. DOI: 10.1080/87559129.2021.1877301 - 11.
Das K, Choudhary R, Thompson-witrick KA. Effects of new technology on the current manufacturing process of yogurt-to increase the overall marketability of yogurt. LWT-Food Science and Technology. 2019; 108 :69-80 - 12.
Ichimura T, Osada T, Yonekura K, Horiuchi H. A new method for producing superior set yogurt, focusing on heat treatment and homogenization. Journal of Dairy Science. 2022; 105 :2978-2987 - 13.
Anema SG. Heat and/or high-pressure treatment of skim milk: Changes to the casein micelle size, whey proteins and the acid gelation properties of the milk. International Journal of Dairy Technology. 2008; 61 :245-252 - 14.
Cadavid AM, Bohigas L, Toldra M, Carretero C, Pares D, Saguer E. Improving quark-type cheese yield and quality by treating semi-skimmed cow milk with microbial transglutaminase. LWT-Food Science and Technology. 2020; 131 :109756 - 15.
Thybo CD, Lillevang SK, Skibsted LH, Ahrne L. Calcium balance during direct acidification of milk for mozzarella cheese production. LWT-Food Science and Technology. 2020; 131 :109677 - 16.
Choi I, Zhong Q. Gluconic acid as a chelator to improve clarity of skim milk powder dispersions at pH 3.0. Food Chemistry. 2021; 344 :128639 - 17.
Robles-Rodriguez CE, Szymanska E, Huppertz T, Ozkan L. Dynamic modeling of milk acidification: An empirical approach. Food and Bioproducts Processing. 2021; 128 :41-51 - 18.
Dalgleish DG, Corredig M. The structure of the casein micelle of milk and its changes during processing. Annual Review of Food Science and Technology. 2012; 3 :449-467 - 19.
Zhao Z, Corredig M. Changes in the physico-chemical properties of casein micelles in the presence of sodium chloride in untreated and concentrated milk protein. Dairy Science and Technology. 2015; 95 :87-99 - 20.
Sadiq U, Gill H, Chandrapalam J. Casein micelles as an emerging delivery system for bioactive food components. Food. 2021; 10 :1965 - 21.
Holt C, Carver JA. Quantitative multivalent binding model of the structure, size distribution and composition of the casein micelles of cow milk. International Dairy Journal. 2022; 126 :105292 - 22.
Zhao Z, Corredig M. Colloidal properties of casein micelles suspensions as a function of pH during concentration by osmotic stressing. Food Hydrocolloids. 2016; 60 :445-452 - 23.
Zhao Z, Corredig M. Short communication: Serum composition of milk subjected to re-equilibration by dialysis at different temperatures, after pH adjustments. Journal of Dairy Science. 2016; 99 :2588-2593 - 24.
Dalgleish DG, Law AJR. pH-induced dissociation of bovine casein micelles. I. Analysis of liberated caseins. Journal of Dairy Research. 1988; 55 :529-538 - 25.
Dalgleish DG, Law AJR. pH-induced dissociation of bovine casein micelles. II. Mineral solubilization and its relation to casein release. Journal of Dairy Research. 1989; 56 :727-735 - 26.
Le Graet Y, Gaucheron F. pH-induced solubilization of minerals from caseinmicelles: Influence of casein concentration and ionic strength. Journal of Dairy Research. 1999; 66 :215-224 - 27.
De Kruif CG. Casein micelle interactions. International Dairy Journal. 1999; 9 :183-188 - 28.
Zhao Z, Corredig M. Influence of sodium chloride on the colloidal and rennet coagulation properties of concentrated casein micelle suspensions. Journal of Dairy Science. 2016; 99 :6036-6045 - 29.
Horne DS. Formation and structure of acidified milk gels. International Dairy Journal. 1999; 9 :261-268 - 30.
Li Q, Zhao Z. Interaction between lactoferrin and whey proteins and its influence on the heat-induced gelation of whey proteins. Food Chemistry. 2018; 252 :92-98 - 31.
Renan M, Arnoult-Delest V, Paquet D, Brule G, Farmelat M. Changes in the rheological properties of stirred acid milk gels as induced by the acidifification procedure. Dairy Science & Technology. 2008; 88 :341-353 - 32.
Tiwari S, Kavitake D, Devi PB, Shetty PH. Bacterial exopolysaccharrides for improvement of technological, functional and rheological properties. International Journal of Biological Macromolecules. 2021; 183 :1585-1595 - 33.
Gastaldi E, Lagaude A, De La Fuente BT. Micellar transition state in casein between pH 5.5 and 5.0. Journal of Food Science. 1996; 61 :59-64 - 34.
Fang T, Shen X, Hou J, Guo M. Effects of polymerized whey protein prepared directly from cheese whey as fat replacer on physiochemical, texture, microstructure and sensory properties of low-fat set yogurt. LWT - Food Science and Technology. 2019; 115 :108268 - 35.
Hashim MA, Nadtochii LA, Muradova MB, Proskura AV, Alsaleem KA, Hammam ARA. Non-fat yogurt fortified with whey protein isolate: Physicochemical, rheological, and microstructural properties. Food. 2021; 10 :1762 - 36.
McCann TH, Fabre F, Day L. Microstructure, rheology and storage stability of low-fat yogurt structured by carrot cell wall particles. Food Research International. 2011; 44 :884-892 - 37.
London LEE, Chaurin V, Auty MAE, Fenelon MA, Fitzgerald GF, Ross RP, et al. Use of lactobacillus mucosae DPC 6426, an exopolysaccharride-production strain, positively influences the techno-functional properties of yogurt. International Dairy Journal. 2015; 40 :33-38 - 38.
Tarapata J, Lobacz A, Zulewska J. Physicochemical properties of skim milk gels obtained by combined bacterial fermentation and renneting: Effect of incubation temperature at constant inoculum level. International Dairy Journal. 2021; 123 :105167 - 39.
Castillo M, Lucey JA, Wang T, Payne FA. Effect of temperature and inoculum concentration on gel microstructure, permeability and syneresis kinetics. Cottage cheese-type gels. International Dairy Journal. 2006; 16 :153-163 - 40.
Panthi RR, Kelly AL, Sheehan JJ, Bulbul K, Vollmer AH, McMahon DJ. Influence of protein concentration and coagulation temperature on rennet-induced gelation characteristics and curd microstructure. Journal of Dairy Science. 2019; 102 :177-189 - 41.
Harwalkar VR, Kalab M. Susceptibility of yogurt to syneresis. Comparison of centrifugation and drainage methods. Milchwissenschaft. 1983; 38 :517-522 - 42.
Gilbert A, Rioux LV, St-Gelais D, Turgeon SL. Characterization of syneresis phenomena in stirred acid milk gel using low frequency nuclear magnetic resonance on hydrogen and image analysis. Food Hydrocolloids. 2020; 106 :105907 - 43.
Lucey JA, Munro PA, Singh H. Whey separation in in acid skim milk gels made with glucono-δ-lactone: Effects of heat treatment and gelation temperature. Journal of Texture Study. 1998; 29 :413-426 - 44.
Xu K, Guo M, Du J, Zhang Z. Okra polysaccharride: Effect on the texture and microstructure of set yogurt as a new natural stabilizer. International Journal of Biological Macromolecules. 2019; 133 :117-126 - 45.
Yang T, Wu K, Wang F, Liang X, Liu Q, Li G, et al. Effect of exopolysaccharides from lactic acid bacteria on the texture and microstructure of buffalo yogurt. International Dairy Journal. 2014; 34 :252-256 - 46.
Xu Z, Guo Q, Zhang H, Xiong Z, Zhang X, Ai L. Structural characterisation of EPS of Streptococcus thermophilus S-3 and its application in milk fermentation. International Journal of Biological Macromolecules. 2021; 178 :263-269 - 47.
Lucey JA, Singh H. Formation and physical properties of acid milk gels: A review. Food Research International. 1997; 30 :529-542 - 48.
Temme P, Korzendorfer A, Hinrichs J, Schlucker E. Vibration as a cause of texture defects during yogurt manufacturing-formation of vibrations and their propagation in dairy production lines. Journal of Food Engineering. 2021; 98 :17-24 - 49.
Lesme H, Rannou C, Farmelart M, Bouhallab S, Prost C. Yogurts enriched with milk proteins: Texture properties, aroma release and sensory perception. Trends in Food Science & Technology. 2020; 98 :140-149 - 50.
Han X, Yang Z, Jing X, Yu P, Zhang Y, Yi H, et al. Improvement of the texture of yogurt by use of exopolysaccharide producing lactic acid bacteria. Biomed Research International. 2016; 2016 :7945675 - 51.
Lucey JA, Wilbanks DJ, Horne DS. Impact of heat treatment of milk on acid gelation. International Dairy Journal. 2022; 125 :105222 - 52.
Raikos V. Effect of heat treatment on milk protein functionality at emulsioninterfaces: A review. Food Hydrocolloids. 2010; 24 :259-265 - 53.
Loveday SM. β-Lactoglobulin heat denaturation: A critical assessment of kinetic modelling. International Dairy Journal. 2016; 52 :92-100 - 54.
Anema SG, Li Y. Effect of pH on the association of denatured whey proteins with casein micelles in heated reconstituted skim milk. Journal of Agricultural and Food Chemistry. 2003; 51 :1640-1646 - 55.
Lucey JA, Teo CT, Munro PA, Singh H. Rheological properties at small (dynamic) and large (yield) deformations of acid gels made from heated skim milk. Journal of Dairy Research. 1997; 64 :591-600 - 56.
Anema SG, Lee SK, Lowe EK, Klostermeyer H. Rheological properties of acid gels prepared from heated pH-adjusted skim milk. Journal of Agricultural and Food Chemistry. 2004; 52 :337-343 - 57.
Lakemond CMM, van Vliet T. Acid skim milk gels: The gelation process as affected by preheating pH. International Dairy Journal. 2008; 18 :574-584 - 58.
Ozcan T, Horne DS, Lucey JA. Yogurt made from milk heated at different pH values. Journal of Dairy Science. 2015; 98 :6749-6758 - 59.
Anema SG. Effect of whey protein addition and pH on the acid gelation of heated skim milk. International Dairy Journal. 2018; 79 :5-14 - 60.
Mahomud MS, Katsuno N, Nishizu T. Effect of micellar kappa-casein dissociation on the formation of soluble protein complexes and acid gel properties. LWT-Food Science and Technology. 2018; 89 :255-261 - 61.
van Vliet T, Keetels CJAM. Effect of preheating of milk on the structure of acidifified milk gels. Netherlands Milk and Dairy Journal. 1995; 49 :27-35 - 62.
Gaucheron F, Famelart MH, Mariette F, Raulot K, Michel F, Graeta YL. Combined effects of temperature and high-pressure treatments on physicochemical characteristics of skim milk. Food Chemistry. 1997; 59 :439-447 - 63.
Huppertz T, Fox PF, Kelly AL. Dissociation of caseins in high pressure treated bovine milk. International Dairy Journal. 2004; 14 :675-680 - 64.
Cadesky L, Walkling-Ribeiro M, Kriner KT, Karwe MV, Moraru CI. Structural changes induced by high-pressure processing in micellar casein and milk protein concentrates. Journal of Dairy Science. 2017; 100 :7055-7070 - 65.
Johnston DE, Austin BA, Murphy RJ. Properties of acid-set gels prepared from high pressure treated milk. Milchwissenschaft. 1993; 48 :206-209 - 66.
Ji Y, Lee SK, Anema SG. Effect of heat treatments and homogenization pressure on the acid gelation properties of recombined whole milk. Food Chemistry. 2011; 129 :463-471 - 67.
Anema SG. Effect of pH at pressure treatment on the acid gelation of skim milk. Innovative Food Science & Emerging Technologies. 2010; 11 :265-273 - 68.
Anantawat V, Loveday SM, Singh H, Anema SG. Acid gelation of heat treated recombined milk: Fat globule membrane composition and gelation functionality. International Dairy Journal. 2022; 125 :105243 - 69.
Abesinghe AMNL, Islam N, Vidanarachchi JK, Prakash S, Silva KFST, Karim MA. Effects of ultrasound on the fermentation profile of fermented milk products incorporated with lactic acid bacteria. International Dairy Journal. 2019; 90 :1-14 - 70.
Carrilo-Lopez LM, Garcia-Galicia IA, Tirado-Gallegos JM, Sanchez-Vega R, Huerta-Jimenez M, Ashokkumar M, et al. Recent advanced in the application of ultrasound in dairy products: Effect on functional, physical, chemical, microbiological and sensory properties. Ultrasonics Sonochemistry. 2021; 73 :105467 - 71.
Chandrapala J, Bui D, Kentish S, Ashokkumar M. Heat stability and acid gelation properties of calcium-enriched reconstituted skim milk affected by ultrasonication. Journal of Dairy Research. 2014; 81 :238-244 - 72.
Madalou A, Emam-Djomeh Z, Mousavi ME, Mohamadifar M. Acid-induced gelation behaviour of sonicated casein solutions. Ultrasonics Sonochemistry. 2010; 17 :153-158 - 73.
Chandrapala J, Zisu B, Kentish S, Ashokkumar M. Influence of ultrasound on chemically induced gelation of micellar casein systems. Journal of Dairy Research. 2013; 80 :138-143 - 74.
Gregersen SB, Wiking L, Hammershoj M. Acceleration of acid gel formation by high intensity ultrasound is linked to whey protein denaturation and formation of functional milk fat globule-protein complexes. Journal of Food Engineering. 2019; 254 :17-24 - 75.
Abesinghe AMNL, Vidanarachchi JK, Islam N, Karim M. Effects of ultrasound on the fermentation profile and metabolic activity of lactic acid bacteria in buffalo’s (Bubalus bubalis) milk. Innovative Food Science & Emerging Technologies. 2022; 79 :103048 - 76.
Korzendorfer A, Schafer J, Hinrichs J, Nobel S. Power ultrasound as a tool to improve the processability of protein-enriched fermented milk gels for Greek yogurt manufacture. Journal of Dairy Science. 2019; 102 :7826-7837 - 77.
Nguyen NHA, Anema SG. Ultrasonication of reconstituted whole milk and its effect on acid gelation. Food Chemistry. 2017; 217 :593-601 - 78.
Nouri M, Ezzatpanah H, Abbasi S, Aminafshar M, Behmadi H. Effect of partially hydrolyzed kappa-casein on physicochemical and sensory properties of heated milk. Journal of Dispersion Science and Technology. 2012; 33 :1204-1209 - 79.
Gastaldi E, Trial N, Guillaume C, Bourret E, Gontard N, Cuq JL. Effect of controlled κ-casein hydrolysis on rheological properties of acid milk gels. Journal of Dairy Science. 2003; 86 :704-711 - 80.
Sinaga H, Bansal N, Bhandari B. Gelation properties of partially renneted milk. International Journal of Food Properties. 2017; 20 :1700-1714 - 81.
Ercili-Cura D, Partanen R, Husband F, Ridout M, Macierzanka A, Lille M, et al. Enzymatic cross-linking of β-lactoglobulin in solution and at air–water interface: Structural constraints. Food Hydrocolloids. 2012; 28 :1-9 - 82.
Huppertz T, De Kruif CG. Structural and stability of nanogel particles prepared by internal cross-linking of casein micelles. International Dairy Journal. 2008; 18 :556-565 - 83.
Silva NFN, Casanova F, Gaucheron F, de Carvalho Teixeira AVN, da Silva GM, Minim LA, et al. Combined effect of transglutaminase and sodium citrate on the microstructure and rheological properties of acid milk gel. Food Hydrocolloids. 2018; 82 :304-311 - 84.
Chen L, Li Y, Han J, Yuan D, Lu Z, Zhang L. Influence of transglutaminase-induced modification of milk protein concentrate (MPC) on yogurt texture. International Dairy Journal. 2018; 78 :65-72 - 85.
Lam E, Otter D, Huppertz T, Zhou P, Hemar Y. Effect of transglutaminase and acidification temperature on the gelation of reconstituted skim milk. International Dairy Journal. 2019; 92 :59-68 - 86.
Ale EC, Perezlindo MJ, Pavon Y, Petralta GH, Costa S, Sabbage N, et al. Technological rheological and sensory characterizations of yogurt containing a exopoplysaccharide extract from lactobacillus fermentum Lf2, a new food additive. Food Research International. 2016; 90 :259-267 - 87.
Yilmaz MT, Dertli E, Toker OS, Tatlisu NB, Sagdic O, Arici M. Effect of in situ exopolysaccharide production on microstructural, rheological, sensory, and microstructural properties of the yogurt drink ayran: An optimization study based on fermentation kinetics. Journal of Dairy Science. 2015; 98 :1604-1624 - 88.
Mende S, Rohm H, Jaros D. Influence of exopolysaccharides on the structure, texture, stability and sensory properties of yogurt and related products. International Dairy Journal. 2016; 52 :57-71 - 89.
Pachekrepapol U, Lucey JA, Gong Y, Naran R, Azadi P. Characterization of the chemical structures and physical properties of exopolysaccharides produced by various Streptococcus thermophilus strains. Journal of Dairy Science. 2017; 100 :3424-3435 - 90.
Khanal SN, Lucey JA. Evaluation of the yield, molar mass of exopolysaccharides, and rheological properties of gels formed during fermentation of milk by Streptococcus thermophilus strains St-143 and ST-10255y. Journal of Dairy Science. 2017; 100 :6906-6917 - 91.
Khanal SN, Lucey JA. Effect of fermentation temperature on the properties of exopolysaccharides and the acid gelation behaviour for milk fermented by Streptoccus thermophilus strains DGCC7785 and St-143. Journal of Dairy Science. 2018; 101 :3799-3811 - 92.
Pang Z, Deeth H, Bansal N. Effect of polysaccharides with different ionic charge on the rheological, microstructural and textural properties of acid milk gels. Food Research International. 2015; 72 :62-73 - 93.
Wang X, Kristo E, LaPointe G. The effect of apple pomace on the texture, rheoloogy and microstructure of set type yogurt. Food Hydrocolloids. 2019; 91 :83-91 - 94.
Ramirez-Santiago C, Ramos-Solis L, Lobato-Calleros C, Pena-Valdivia C, Vernon-Carter EJ, Alvarez-Ramirez JA. Effect of stirred yogurt with soluble dietary fiber from Pachyrhizus erosus L. urban: Effect on syneresis, microstructure and rheological properties. Journal of Food Engineering. 2010; 101 :229-235 - 95.
Fu R, Li J, Zhang T, Zhu T, Cheng R, Wang S, et al. Salecan stabilizes the microstructure and improves the rheological performance of yogurt. Food Hydrocolloids. 2018; 81 :474-480 - 96.
Lazaridou A, Serafeimidou A, Biliaderis CG, Moschakis T, Tzanetakis N. Structure development and acidification kinetics in fermented milk containing oat β-glucan, a yogurt culture and a probiotic strain. Food Hydrocolloids. 2014; 39 :204-214 - 97.
Villalva M, Jaime L, Arranz E, Zhao Z, Corredig M, Reglero G, et al. Nanoemulsions and acidified milk gels as a strategy for improving stability and antioxidant activity of yarrow phenolic compounds after gatrointestinal digestion. Food Research International. 2020; 130 :108922 - 98.
Romero MLM, von Staszewksi M, Martinez MJ. The effect of green tea polyphenols addition on the physicochemical, microbiological and bioactive characteristics of yogurt. British Food Journal. 2021; 123 :2380-2397 - 99.
Harbourne N, Christophe J, Jacquier JC, O’Riordan D. Effects of addition of phenolic compounds on the acid gelation of milk. International Dairy Journal. 2011; 21 :185-191 - 100.
Tosif MM, Najda A, Bains A, Krishna TC, Chawla P, Dyduch-Sieminska M, et al. A comprehensive review on the interaction of milk protein concentrates with plant-based polyphenols. International Journal of Molecular Sciences. 2021; 22 :13548 - 101.
Chen H, Zheng H, Brennan MA, Chen W, Guo X, Brennan CS. Effect of black tea infusion on physicochemical properties, antioxidant capacity and microstructure of acidified dairy gel during cold storage. Food. 2020; 9 :893 - 102.
Dabija A, Codină GG, Ropciuc S, Gâtlan AM, Rusu L. Assessment of the antioxidant activity and quality attributes of yogurt enhanced with wild herbs extracts. Journal of Food Quality. 2018; 5329386 :1-12 - 103.
Szoltysik M, Kucharska AZ, Dabrowska A, Zieba T, Bobak L, Chrzanowska. Effects of two combined functional additives on yogurt properties. Food. 2021; 10 :1159 - 104.
MM EI-S, Haggag HF, HM FEI-D, Gad AS, Farahat AM. Antioxidant activities and physical properties of stirred yogurt fortified with pomegranate peel extract. Annals of Agricultural Sciences. 2014; 59 :207-212 - 105.
Bayraktar M. Incorporation of polyphenols into dairy matrices [Doctor thesis]. Central Archive at the University of Reading; 2018