Figure 4-3. Formation of acetaldehyde by yogurt bacteria from pyruvate and threonine. Adapted from Chaves et al. 2002.
consumers as flavor. Coagulated milk is essentially a protein gel that imparts viscosity, mouth feel, and body to the finished product. Formation and maintenance of the gel structure, therefore, is important for yogurt quality. The gel properties are affected by the ingredients in the yogurt mix, how the yogurt mix is processed and produced, culture activity, and post-fermentation factors. For example, proper heat treatment of the milk has a profound effect on gel strength and, specifically, the water holding capacity. If the gel is poorly formed or disrupted syneresis will occur, leading to the defect known as "wheying off" (see below).To control syneresis and maintain a suitable gel structure, yogurt manufacturers in the United States commonly add stabilizers to the mix. Typically, stabilizers are hydrophilic polysaccharides whose purpose is to bind water. The most popular stabilizers are naturally derived gums and starches, including carrageenan, locust bean, and guar gums; corn starch; tapioca;
and pectin. Gelatin is also frequently added as a stabilizer.
In some European countries (e.g., France), stabilizers are not allowed in yogurt.Therefore, the ability of some strains of S. thermophilus and L. delbrueckii subsp. bulgaricus to produce and secrete natural polysaccharide material directly in the yogurt during the fermentation is an especially important trait. This is because many of these extracellular polysac-charides (exopolysaccharides or EPS) have excellent stabilizing and rheological properties. The producer strains are now widely used and included in yogurt starter cultures to provide the desired body characteristics. A variety of EPS produced by these bacteria have been identified (Box 4-3). Some EPS contain only a single sugar (homopolysaccharides). In contrast, other EPS consist of a heterogenous mixture of different sugar monomers (heteropoly-saccharides) in varying ratios. The latter contain isomers of galactose and glucose, along with rhamnose and other sugars.
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