Fermentation

As with all fermented dairy products, the choice of starter culture is crucial for the production of high-quality sour cream (7). Mixed strains of mesophilic lactic acid bacteria are used for sour cream. In general, both acid and aroma producers are utilized. Acid producers include Lactococcus (Lc) lactis subsp lactic and Lc lactis subsp cremoris. Lc lactis subsp lactis biovar diacetylactis (or Cit Lactococci) and Leuconostoc mesenteroides subsp cremoris are commonly used aroma producers.

The acid producers convert lactose into l-lactate through a homofermentative pathway. They can produce up to 0.8% lactic acid in milk (8) and are responsible for lowering pH in the fermented product.

In contrast, aroma producers are heterofermentative and can convert lactose into d-lactate, ethanol, acetate, and CO2. In addition, these strains convert citrate into diacetyl, which is one of the major flavor compounds responsible for typical sour cream flavor.

Diacetyl is subsequently partially converted into acetoin, which is a flavorless compound (9). Extensive research at starter culture companies has led to the development of Leu-conostoc strains that show less of a tendency to convert diacetyl into acetoin, thus retaining high levels of diacetyl (D. Winters, personal communication, 2002). Use of such strains can extend the shelf life of sour cream because it takes longer for the product to turn stale. Leuconostocs also reduces acetaldehyde to ethanol (10,11). In fact, acetaldehyde has been shown to promote the growth of Leuconostoc mesenteroides subsp cremoris (12,13). Acetaldehyde is typically assocaited with yogurt flavor (green apple) but is considered an off-flavor in sour cream.

The choice of starter cultures will affect product texture as well. Strains of acid producers have been developed that increase viscosity through the production of exopo-lysaccharides (14). These polysaccharide chains contain galactose, glucose, fructose, man-nose, and other sugars. Quantity and type depend on the bacteria strain and growth conditions (15,16). The exopolysaccharides interact with the protein matrix creating a firmer network and increasing water-binding capacity. The importance of this behavior was confirmed by Adapa and Schmidt (17), who found that low-fat sour cream, fermented by exopolysaccharide-producing lactic acid bacteria, was less susceptible to syneresis and had a higher viscosity.

Production of high-quality sour cream requires a fine balance of acid-, viscosity-, and flavor-producing bacteria. This balance varies among commercially available strains, but a typical combination would be 60% acid producers, 25% acid and viscosity producers, and 15% flavor producers (D. Winters, personal communication, 2002).

IV. GEL FORMATION

Fermentation leads to a significant increase in viscosity. Two physicochemical changes cause this behavior (18,19). The casein submicelles disaggregate because of solubilization of colloidal calcium phosphate. In addition, the negative surface charge on the casein micelles decreases as pH approaches the isoelectric point. This creates the opportunity for casein micelles to enter into a more ordered system. Besides the protein network, cream gains viscosity from the formation of homogenization clusters (20). Following single-stage homo-genization at room temperature, milk fat globules will cluster, and these clusters may contain up to about 105 globules (21). Casein molecules adsorb onto newly formed fat globule membranes and, in the case of high fat content, form bridges between fat globules. Clustering increases viscosity because (a) serum is entrapped between the globules and (b) irregularly shaped cluster are formed.

V. STABILIZERS

The gel structure may not be sufficiently firm to withstand abuse during transportation, handling, and storage. This could result in a weak-bodied sour cream and whey syneresis in the container. These defects are especially noticeable for low-fat products. To ensure consistent firm texture, dairy processors often choose to add nondairy stabilizers (22). Stabilizers commonly found in sour cream include polysaccharides and gelatin.

Stabilizer must be food grade and approved. The type and quantity used vary widely dependent on fat content, starter culture, and required sensory characteristics of the final product. Types and quantities of potential stabilizer mixtures used in sour cream are

Table 1 Examples of Stabilizer and Other Ingredients Used in Sour Cream

Product

Ingredients

Usage level

Sour cream

Modified food starch, grade A whey, sodium phosphate, guar gum, sodium citrate, calcium sulfate, carrageenan, locust bean gum

Low-fat sour cream Nonfat sour cream

Modified food starch, microcrystalline cellulose, propylene glycol monoester, gum arabic, artificial color, cellulose gum

Same as above

Source: From Ref. 27.

outlined in Table 1. Especially, the nonfat formulation contains other ingredients such as emulsifiers, color, and protein.

Polysaccharides bind water and increase viscosity. Commonly used plant polysaccharides include carrageenans, guar gums, and cellulose derivatives. Modified starches are frequently utilized as well. It is necessary to fully hydrate these polysaccharides to optimize their functionality. Depending on the ingredient, this may require efficient blending systems for incorporation of the ingredient into the cream, though care should be taken to avoid churning the cream. Complete hydration can sometimes only be accomplished following heating and cooling steps, which conveniently are done by the pasteurization process. Time may also be a factor for hydration to occur. Besides binding with water molecules, polysaccharides may also interact with milk proteins and form a network, which limits the movement of water and increases viscosity. A short description of the stabilizers is provided below:

Carrageenans. Extract of seaweed. Three types of carrageenans are commercially available, lambda, iota, and kappa, which differ based on the amount of sulfate. They have low viscosity at high temperature but viscosity increases during cooling. Lambda has the highest sulfate content, is soluble in cold milk, and forms weak gels. Iota is soluble in hot milk (55 °C) and prevents syneresis. Kappa dissolves only in hot milk (>70°C) and forms brittle gels (23).

Guar gum. Endosperm of seed from Cyanopsis tetragonolobus plant. Different types of guar gum are available to fit processing conditions. Maximum viscosity develops over time. All are soluble in cold milk. The main component is mannose with attached galactose units.

Methylcellulose. A cellulose that improves freeze-thaw stability and prevents melt upon heating (22).

Gelatin. In contrast to the polysaccharides described above, gelatin consists primarily of protein (84-86%) and is derived from animal sources such as skin and bones (24). Gelatin is an excellent gelling agent but some off-flavors are perceived when used at excessive concentrations.

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