Cool to 2 — 4°C and package (fruits are added after cooling)

Figure 2 Steps for the manufacture of yogurt. (From Kosikowski and Mistry, 1997.)

tries and codex regulations define yogurt as the product obtained by fermenting milk with a culture that includes Lb. delbrueckii subsp. bulgaricus and S. ther-mophilus. Some countries permit additional lactic acid bacteria, whereas others, such as Australia, require only S. thermophilus and a lactobacillus of choice. The United Kingdom requires Lb. delbrueckii subsp. bulgaricus to which other lactic acid bacteria can be added.

S. thermophilus (coccus) and Lb. delbrueckii subsp. bulgaricus (rod) are thermophilic organisms (Fig. 3) and grow best at approximately 45°C but not above 50°C (Chandan, 1992). They are typically added in a 1:1 ratio. Bulk cultures may be prepared separately from pure strains or frozen concentrates may be added directly to the mix. The latter eliminates the need to maintain culture transfer facilities (Kosikowski and Mistry, 1997). Rods and cocci function symbi-otically to produce typical yogurt characteristics. Either culture independently is unable to produce the ideal balance of acid and flavor. S. thermophilus initiates lactic acid production and lowers the oxygen level, which stimulates growth of Lb. delbrueckii subsp. bulgaricus (Vedamuthu, 1992). The pH is lowered to approximately 5 by the cocci and then to less than 4 by the rods. The rods in turn promote growth of S. thermophilus via production of peptides and amino acids.

Figure 3 Electronmicrograph of yogurt showing rods (solid white arrow) and cocci (dotted white arrow) embedded within the product.

S. thermophilus is more sensitive to acid than is Lb. delbrueckii subsp. bulgaricus; hence during extended storage of yogurt, the former (cocci) are likely to be injured by the acid and gradually die off. Therefore, although the initial ratio of rods to cocci may be 1:1, this ratio may change in favor of lactobacilli during storage of the yogurt. As the rate of acid and flavor production is strain dependent, the rod and coccal strains should be selected so there is a balance of acid and acetaldehyde production (Vedamuthu, 1992). Rate of acid production alone should not be the criterion for strain selection. Acetaldehyde is produced by both S. thermophilus and Lb. delbrueckii subsp. bulgaricus (Wilkins et al., 1986). Both organisms produce threonine aldolase which helps convert threonine to acetaldehyde but lactose is also a source.

It is now common in the yogurt industry, particularly in Europe, to enhance the body of yogurt by using cultures that produce exopolysaccharide (Hassan et al., 1996; Lavezzari et al., 1998). Some strains of lactic acid bacteria, including the thermophilic yogurt bacteria, can produce exopolysaccharides that act as stabilizers and thicken the body of yogurt. The polysaccharides can be extracellular or in encapsulated form (Hassan et al., 1996). Some strains of cultures produce polysaccharides that can lead to a ropy texture, whereas others provide a thickening effect without ropiness (Lavezzari et al., 1998). This may be important, because criteria for sensory evaluation of yogurt generally view ropiness as a defect (Bodyfelt et al., 1988).

In recent years bifidobacteria-containing yogurt has become popular in Japan, Canada, France, and Germany. Such yogurt is manufactured either with bifidobacteria singly or as mixed cultures with Lb. acidophilus and S. thermophilus and provide therapeutic properties to yogurt (Rasic and Kurmann, 1983). Bifidobacteria of human origin are preferred and include Bifidobacterium breve, Bi. longum, Bi. infantis, and Bi. bifidum. An inoculum rate of >10% has to be used, because bifidobacteria are slow acid producers. Incubation is at 36-42° C for 6-8 h to enable curd formation and provide viable counts of up to 100 million per gram in the final product. An advantage in using bifidobacteria is that over-acidification does not occur in the yogurt during production and storage. Bifido-bacteria yogurt therefore has a milder (less acidic) taste. To ensure viability during storage of yogurt, proper strains of bifidobacteria must be selected (Martin and Chou, 1992).

2. Defects

Yogurt by nature is a high-acid (low pH) product and is therefore inherently protected against defects caused by most contaminating organisms. Furthermore, the high pasteurization temperature used in processing the mix eliminates most contaminating bacteria. Nevertheless, certain defects, some microbially induced, may occur. Perhaps the most common defect is high acid and consequently high acetaldehyde flavor (Vedamuthu, 1992). This may develop under improper manufacturing and storage conditions. If the rods and cocci are maintained as a mixed culture, after repeated transfers at high temperature rods will dominate the culture. They then become the primary acid producers when used to make yogurt and produce excessive amounts of acid (over 2%). This can be prevented by maintaining the two cultures separately and adding them in a 1:1 ratio at the time of inoculation of the mix during manufacture of yogurt (Kosikowski and Mistry, 1997). Another critical factor is the rapid cooling of yogurt after incubation to prevent continued growth of lactobacilli. Many manufacturers use blast tunnels for cooling to 10°C within 50 min. Excessive acid production may also lead to body and texture defects such as shrinkage of curd and wheying-off. Other texture defects may also occur in yogurt, such as weak or excessively heavy body, which are generally related to improper use of stabilizers. Proper selection and use of ingredients, especially stabilizers in the mix, can address these defects. Yogurt manufacturers often add 2-4% nonfat dry milk to increase the total solids content to over 15%. This helps to develop a firm body (Kosikowski and Mistry, 1997; Tamime and Robinson, 1985), and is especially useful in low-fat and nonfat yogurts. A disadvantage is that the resulting yogurt will have a high lactose content (approximately 6%) that will allow the lactic fermentation to continue. Acidity of such yogurts is therefore high. An alternative is to concentrate milk by ultrafiltration to raise the protein content and lower the lactose level (Mistry and Hassan, 1992; Rasic et al., 1992). The protein concentration that can be used with such procedures is <5.6%, since excessive fortification leads to an undesirably firm body (Mistry and Hassan, 1992).

Another microbially induced defect is bitterness. This occurs if the milk supply contains spore-forming organisms such as Bacillus subtilis or B. cereus. Spores of these organisms are able to survive high heat treatment. Yeasts and molds are acid tolerant. Therefore, contamination by yeasts and molds can be a problem, particularly in fruit-flavored yogurts if poor-quality contaminated fruit preserves are used.

B. Cultured Buttermilk

Cultured buttermilk is a lightly salted fermented milk product that is manufactured from nonfat or low-fat milk using mesophilic cultures and flavor-producing organisms. Unlike yogurt, the flavor of buttermilk includes lactic acid, diacetyl, and acetic acid. Diacetyl is obtained from citric acid fermentation during manufacture of buttermilk. Cultured buttermilk should have a smooth thick body, with the correct balance of acid and diacetyl flavor (Vedamuthu, 1985). Steps in the manufacture of buttermilk are summarized in Fig. 4.

Preparation of milk: Standardize milk to desired fat content via separation of fat. Add 0.15% citric acid, if needed, and 0.1% salt

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