Homogenization

Homogenization is a critical step in the process of making high-quality yogurt or sour cream. The milk fat globules in raw milk may be as large as 2.0-20.0 Am in diameter (17). At that size, and in their raw state, fat globules tend to rise to the surface and clump or coalesce. Homogenization is used to stabilize milk fat against gravity separation. The process of forcing fat globules through tiny holes under high pressure breaks the large globules into smaller ones, about 0.1-3 Am in diameter, rendering them less likely to coalesce and rise to the surface (18). Homogenization is most efficient when fat globules are in the liquid state; that is, the milk has been preheated. Thus, milk is processed through a heat exchanger in the regeneration section of the HTST (Fig. 3), where the temperature is raised to at least 60 °C prior to homogenization (18).

There are two types of homogenizers: single-stage and two-stage. Both are composed of the same stainless steel parts, but a two-stage homogenizer has two sets of components. Homogenization takes place in the first stage of the homogenizer (10-25 MPa/100-250 bar/90-225 atm/1320-3300 psi). The second stage (48 bar/47 atm/696 psi) makes homogenization more efficient, by supplying controlled back-pressure. It also breaks apart any fat clumps that may have formed after the first stage. When liquid passes through the narrow gap of the homogenizer (Fig. 4), the flow velocity increases until the pressure is so low that the liquid starts to boil (1). When the liquid leaves the gap, the speed decreases and the pressure increases again. The liquid stops boiling and the steam bubbles implode. Theories suggest that fat globules may be broken down by either (a) numerous small

Milt Tanks Implode

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Figure 2 Product flow under partial stream homogenization (skim milk is not homogenized): 1. Heat exchanger; 2. centrifugal separator; 3. Automatic fat standardization device; 4. Homogenizer. (Courtesy of Tetra Pak.)

■—• Cw it, 40& fat mmmmmmmm Stom mtk 0 0S% /at ■ CM®, res^r

Figure 2 Product flow under partial stream homogenization (skim milk is not homogenized): 1. Heat exchanger; 2. centrifugal separator; 3. Automatic fat standardization device; 4. Homogenizer. (Courtesy of Tetra Pak.)

Tetra Pak Pasteurizer

Figure 3 The complete pasteurizer consists of 1. balance tank; 2. feed pump; 3. flow controller; 4. regenerative preheating sections; 5. centrifugal clarifier/separator; 6. heating section; 7. holding tube; 8. booster pump; 9. hot water heating system; 10. regenerative cooling sections; 11. cooling sections; 12. flow diversion valve; 13. control panel. (Courtesy of Tetra Pak.)

Figure 3 The complete pasteurizer consists of 1. balance tank; 2. feed pump; 3. flow controller; 4. regenerative preheating sections; 5. centrifugal clarifier/separator; 6. heating section; 7. holding tube; 8. booster pump; 9. hot water heating system; 10. regenerative cooling sections; 11. cooling sections; 12. flow diversion valve; 13. control panel. (Courtesy of Tetra Pak.)

eddies (or whirls) present in high-velocity flow systems or (b) cavitation, shock waves created when steam bubbles break and disrupt the fat globule (1).

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