Food Biochemistry and Food Processing (2 edition)

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23 Dairy Products 437

water-in-oil emulsion is formed as the small water droplets are

dispersed into the fat matrix.

The churning process is probably most critical to the textural

quality of the butter. As air is incorporated into the cream, the

fat globules surround the air bubbles and coalesce with other

fat-coated air bubbles to form clumps. These clumps continue

to coalesce during the churning process and the volume of air

bubbles decreases. The proportion of solid fat, which is influ-

enced by temperature, is critical to the aggregation of fat clumps

to form butter. If the rate of churning is too fast, the fat globules

are less stable and less likely to coalesce, resulting in a greater

loss of fat into the buttermilk.

Buttermilk

Natural buttermilk is a by-product of butter manufacture (Fig-

ure 23.2) and is categorized as sweet (nonfermented) or acidic

(fermented). Sweet and acid buttermilk are obtained from the

manufacture of butter from nonfermented, sweet cream and sour

or cultured cream, respectively. In general, the composition of

buttermilk is similar to that of skim milk, consisting of milk

proteins, lactose, minerals, and some lipids. The fat content of

natural buttermilk is approximately 0.4% and consists primar-

ily of the membrane components of the fat globules. Churning

disrupts the fat globules and the MFGM associates with the but-

termilk fraction. The MFGM is characterized as having a higher

content of phospholipids and unsaturated fatty acids than found

in whole milk. The high phospholipid content provides emulsi-

fication ability and expands the functionality of buttermilk as a

dairy ingredient. However, because of the high content of unsat-

urated fatty acids associated with the membrane lipids, natural

buttermilk has a characteristic flavor and is also more susceptible

to the development of oxidized off-flavors (Sodini et al. 2006).

Cultured buttermilk and cultured skim milk are more fre-

quently produced as alternatives to the traditional buttermilk

(Figure 23.3). The solids-not-fat and fat contents of cultured

buttermilk range from 7.4% to 11.4% and 0.25% to 1.9%, respec-

tively. The homogenized and pasteurized milk with the desired

fat content is inoculated with a 1% starter culture ofLc. lactisssp.

lactisandLc. lactisbiovar.diacetylactisandLn. mesenteroides

ssp.cremoris. These bacteria metabolize citrate to diacetyl to

contribute to the development of “buttery” flavor. Acid is pro-

duced by these bacteria at a relatively slow rate.

Only a minor percentage of the buttermilk produced is con-

sumed directly. The commercial use of buttermilk includes ap-

plications in the baking and dairy industries (Sodini et al. 2006).

For many of these applications, the buttermilk is concentrated by

evaporation and spray dried to form a powder. Spray drying has

been shown to result in a significant decrease in the phospholipid

content, which could have important implications with respect to

the functionality of buttermilk powders as an ingredient (Morin

et al. 2007).

Sour Cream

Sour cream is produced from pasteurized, homogenized cream

(20–30% fat) and has a pleasant acidic taste and buttery aroma

Figure 23.3.Processing scheme for cultured buttermilk.

(Fig. 23.4). Low-fat sour cream has a fat content ranging from

10% to 12%. Following standardization of the milk and cream

to the desired fat content, the mixture is warmed and homoge-

nized to improve the consistency of the final product. The starter

cultures includeLc. lactisssp.lactisandLc. lactisbiovar.di-

acetylactisandLn. mesenteroidesssp.cremoris. As in butter-

milk, these cultures contribute to the acid and buttery flavor

of the sour cream. The sour cream may be packaged prior to

or following fermentation at 20◦C until the pH is reduced to

4.5. Packaging prior to fermentation results in a thicker product

because the gel is not disturbed. As the pH decreases, the fat

clusters aggregate to form a viscous cream. Rennet or thicken-

ing agents, such as starch, guar gum, carrageenan, and locust

bean gum, are sometimes added to increase the firmness of the

sour cream and stabilize the gel to minimize syneresis (Hunt and

Maynes 1997).

Yogurt

The tremendous increase in the popularity of yogurt in recent

decades has been attributed to its health food image and the

wide diversity of flavors, compositions, and viscosities available

to consumers. The manufacturing methods, raw materials, and