Culturally Speaking: Making Yogurt Drinkable
A According to Federal Standards of Identity, to be called a "yogurt drink," the white mass (yogurt portion) of the product must meet the requirements for yogurt. That is, it must contain a minimum of 8.25% milk solids not fat (MSNF) and 3.25% milkfat prior to the addition of other ingredients. It must also be fermented with Streptococcus thermophilius and Lactobacillus delbrueckii ssp. bulgaricus. After the addition of bulky flavors (e.g., fruit), the complete product does not need to meet these compositional standards. Of course, other fat descriptors such as reduced-fat and nonfat are allowed. Fancifully named similar products such as smoothies are not standardized and therefore are not required to contain yogurt. However, if the descriptor for such products states "a blend of yogurt and juice," then yogurt should be used in product preparation. If, on the other hand, the descriptor indicates a blend of juice and milk, then a directly acidified milk product may be employed.
Q What is in yogurt drinks?
A The average finished yogurt drink has a pH of 4.0 to 4.5, contains 8.0% to 9.5% MSNF, is generally low in fat (less than 2%) and contains a substantial amount of sugar (8% to 12%). Reduced-calorie products usually use intense sweeteners rather than sugar. The level of stabilizer varies depending upon the choice of hydrocolloid, but usually ranges from 0.01% to 0.5%. The amount of flavoring varies. When using natural fruit flavors, usage levels range from 8% to 15%. Some products, though, may contain as much as 30% fruit.
Often, yogurt drinks are formulated as "synbiotics," that is, they contain synergistic combinations of prebiotic materials and probiotic microorganisms (e.g. Lactobacillus casei and various species of Bifidobacterium), which are designed to promote intestinal health.
Q How does the stabilizer function?
A Selection of the appropriate stabilizer system is perhaps the most critical decision when formulating a yogurt beverage. The role of the stabilizing agent in yogurt drinks is two-fold. First, the stabilizer needs to prevent protein from aggregating during storage in order to minimize or eliminate phase separation, which is the formation of a layer of clear liquid on top of the container. Second, the stabilizer needs to provide the appropriate drinking viscosity. While a blend of stabilizer hydrocolloids is usually required to achieve both stability and appropriate viscosity, a particularly functional stabilizer for yogurt drinks is high-methoxy pectin. This negatively charged hydrocolloid interacts with the positively charged casein particles to prevent protein aggregation. Good dispersion of the pectin and an initially small particle size of the casein are critical for the success of the product.
Q How does the manufacturing procedure for yogurt drinks differ from yogurt?
A The manufacturing process for yogurt beverages closely resembles that used for stirred-style yogurt. After combining ingredients for the yogurt base, the mix is pasteurized, homogenized and subjected to a high-heat treatment of 185°F for 30 minutes to denature the whey proteins. It is then cooled to a fermentation temperature of 102°F to 109°F, inoculated with a yogurt starter culture and incubated until the desired pH (4.0-4.5) is reached. Finally, the product is cooled to 64°F to 68°F. It is at this point the processing of yogurt drinks varies from that of regular stirred-style yogurt.
Unlike typical packaged yogurt where the coagulum should be treated as gently as possible following fermentation, after initial cooling but before cooling to storage temperature, the yogurt drink is usually passed through a homogenizer to reduce the particle size. This assures complete hydrocolloid distribution and stabilizes the protein suspension. Flavor may be added immediately prior to homogenization or the white mass may be homogenized and then flavored. Though this post-fermentation homogenization step presents the most significant challenge to most manufacturers, it is crucial to obtain the appropriate finished product viscosity and to prevent syneresis during storage. The pressure required for homogenization is quite low. Applied pressures up to 500 psi using a single-stage homogenizer are typically sufficient.