The set yogurt gel structure results from an acid-casein interaction in which casein micelles at or near their isoelectric point flocculate and the colloidal calcium phosphate partially solubilizes as acidity increases. During the fermentation of milk, the pH gradually declines to around 4.5 and destabilized micelles aggregate into a three-dimensional network in which whey is entrapped (43,44). Appearance of whey on the surface of yogurt gel is due to syneresis, separation of serum from curd. In yogurt this defect is called wheying off (45).

In stirred-style yogurt, the three-dimensional network is disturbed when fruit and flavors are mixed into the plain yogurt. The texture and physical properties of the yogurt depend on the fruit, the stabilizer, and the rate of cooling (45).

Stabilizers are added to prevent surface appearance of whey and to improve and maintain body, texture, viscosity, and mouthfeel. Yogurt with lower milk solids have a greater tendency to synerese. Numerous stabilizers are available on the market. Generally a combination of several stabilizers is included in the formulation to avoid defects that may result from the use of a single stabilizer. A partial list of stabilizers used in yogurt include the following:

1. Gelatin is a protein of animal origin. It is derived from the hydrolysis of collagen. Only high-bloom gelatin should be used in yogurt making due to improved gelatin/ casein interactions, its higher melting point, and its higher stabilizing ability (5). The term bloom refers to the gel strength. It disperses in cold but requires heat for activation. It is used at a 0.3-0.5% level. Microstructure of yogurt made with 0.5% gelatin under scanning electron microscopy did not show gelatin, and the structure did not differ from that of a plain unfortified yogurt. This yogurt was rated smooth in a sensory evaluation (46).

2. Whey protein concentrates are used at 1-2% of protein addition. In a study with skim milk yogurt fortified with dairy-based proteins, the yogurts made with casein-based products were coarser and inferior compared to those made with WPC at 1-1.5%. It was recommended that WPC should be used along with other stabilizers (47).

3. Gums are water soluble or dispersible polysaccharides and their derivatives. In general, they thicken or gel aqueous systems when used at low concentration. Gums are used to stabilize emulsion and prevent wheying off. Food gums are tasteless, odorless, colorless, and nontoxic (48). All are essentially noncaloric and are classified as soluble fiber. These are used at 0.2-1.5% depending on the application.

Locust beam gum is a seed gum. It has low cold water solubility. It is generally used where delayed viscosity development is needed. Dispersion of this gum when heated to about 185°F, and then allowed to cool, is high in viscosity. It works synergistically with carrageenan in some applications (48).

Guar gum is very similar to locust bean gum but is more soluble in cold water. It hydrates readily at pH 6-9. Its solubility is not affected by pH in 4.8 to 5.0 range. It does not cross-link well with carrageenan (48).

Carrageenan is derived from red sea weed. It is a mixture of various types, kappa, iota, and lambda. It may contain 60% of kappa form and 40% lambda. The kappa type forms a gel, whereas lambda does not. The polymer is stable at pH above 7.0 and has a tendency to degrade slightly at pH 5-7; it degrades rapidly below pH 5.0. The potassium salt of this gum is the best gel former, but the gels are brittle and prone to syneresis. This defect is prevented by the addition of a small amount of locust bean gum. It interacts with casein in milk and promotes stabilization of the yogurt gel (48).

Xanthan is produced by microbial fermentation. It is readily soluble in cold and hot water. It is not affected by pH changes. A synergistic increase in viscosity results from interaction of xanthan with n-carrageenan and locust bean gum. These gels are prone to shear thinning. It also gives sheen to products, which may not be desirable in yogurt (48).

Protein, starch, modified starch, and tapioca-based starches can be used without affecting the flavor of yogurt. The stabilizer used in yogurt is generally a blend of stabilizers incorporated at 0.5 to 0.7% or less. The amount used also depends on milk solids level (5). A recent consumer survey of marketplace yogurts has indicated that many products are gummy, perhaps overstabilized (12).

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