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Body and viscosity, smooth mouthfeel, prevents syneresis by stabilizing protein gel Provides protein gel, body and viscosity, smooth mouthfeel Color and flavor Sweetness, increases total solids Firmness and consistency of protein gel, water binding (improves body and prevents syneresis) Firmness and consistency of protein gel, water binding (improves body and prevents syneresis) Consistency and viscosity of protein gel, water binding (prevents syneresis), maintains gel structure after cooling Consistency and viscosity of protein gel, water binding (prevents syneresis), maintains gel structure after cooling Produce acid to form protein gel and acetaldehyde to provide yogurt flavor

Acetaldehyde Yogurt Fermentation
Cold storage (4c,Cl Figure 5 Manufacture of yogurt-based products.

DVS culture is added, this percentage is decreased. Inoculum level and incubation temperature will depend on the culture manufacturer's recommendations and final product characteristics. The cultures required for yogurt production are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in nearly a 1:1 ratio at inoculation. Yogurt fermentation typically proceeds optimally between 40 and 43 °C. Temperatures of 35-40 °C favor S. thermophilus (acid production) whereas 43-45°C favors L. bulgaricus (acetaldehyde production). The growth of cocci and rods, and their ratio, change in yogurt during incubation. At first, the streptococci grow faster due to the formation of growth factors by the rods (25). As the acid level in yogurt increases, the cocci are slowed down and the rods grow faster because of the growth factors produced by the cocci (25).

Live yogurt cultures are present throughout processing and consumption. Although most strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus do not survive in the human intestinal tract, other microorganisms not only survive but also adhere to and proliferate in the human intestinal tract (27). Cultures typically utilized in yogurt because of their probiotic effects include Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. lactis, Lactobacillus casei, and Bifidobacterium subsp. Probiotics is a term used to describe microorganisms that, if consumed, exert beneficial effects upon health. Yogurt is an ideal food system for delivering probiotics to humans because yogurt buffers the stomach and raises its pH; yogurt's protein content and semisolid consistency give probiotic bacteria a chance to survive the stomach and adhere to the intestine (27).

Probiotic microorganisms must (5,27):

1. be nonpathogenic and nontoxigenic

2. adhere to the host's gastrointestinal tract and colonize it

3. be able to utilize nutrients and substrates available in the host's typical diet

4. survive passage through the host's digestive system

5. have good viability in the form delivered to the host

6. exert beneficial effects on the host by preventing infections or diseases, improving health, or improving nutrition

For the ''Live and Active Cultures'' logo from the National Yogurt Association to appear on a yogurt label, 108 cfu/g of lactic acid bacteria must be present at the time of manufacture (27). The lactic acid bacteria population may be 108 cfu/g at the time of production, but viability declines during storage. The therapeutic dose of probiotic bacteria per day is suggested to be 108 to 109 cells, so it is essential that products sold contain a minimum of 106 cfu/mL at the expiration date on the package (27). The viability of probiotic bacteria in yogurt depends on many factors, including (a) the strains used, (b) interaction among species present, (c) chemical/nutrient composition of the yogurt base, (d) final acidity, (e) presence of inhibitors, (f) dissolved oxygen, (g) inoculation level, (h) incubation temperature, (i) fermentation time, and (j) storage temperature (25). For instance, Bifido-bacterium subsp. are susceptible to acids and oxygen, so their counts, in particular, may decrease significantly during storage (25). However, because S. thermophilus acts as an oxygen scavenger and creates an anaerobic environment, its presence can enhance the survival and growth of Bifidobacterium subsp. (27).

The inoculated warm mixture may be incubated in a vat or pumped into consumer containers, standard milk cans, or stainless steel vats as large as 1500 gal. The mix is then held at the optimum temperature for 3-7 hr, until a pH of 4.4 or a titratable acidity (TA) of 0.9-1.2%, as lactic acid, is attained. Optimum TA prior to chilling is 0.9 and optimum pH is 4.5. At this point, yogurt is either cooled or ''broken'' rapidly in order to halt culture activity. Optionally, appropriate flavorings may be added according to the desired end product.

Cup-set yogurt is more prone to wheying off than vat-set yogurt, so particular care must be taken to minimize handling of the warm cups of yogurt (28). Positive displacement pumps are least destructive to yogurt (22). Once yogurt containers are packed, they are transferred to a blast cooler until the temperature is lowered to about 5-7 °C. The product can be held in a regular cooler until sale. Boxes of yogurt are palletized in circulating air chillrooms to maximize cooling rate (40).

Yogurt should be a smooth, viscous gel with a characteristic sour, green-apple flavor. Flavor compounds produced during fermentation by yogurt bacteria include lactic acid (sour), acetaldehyde (green apple), acetic acid (vinegar), diacetyl (buttery), and volatile fatty acids (butyric, caproic, capryllic, capric). Incubation temperature, inoculum level, incubation period, choice or source of the culture (strains used), heat treatment of yogurt milk base, and pH of the finished product all influence the body, texture, and flavor of the final product. Other factors that affect yogurt quality include choice of milk, milk standardization, additives, deaeration, homogenization, culture preparation, incubation temperature, and plant design (1).

B. Plain, Extract- or Concentrate-Flavored, Sundae Style

Simply put, set yogurt is incubated and cooled in the retail package. Set yogurt may be plain or may include color and/or flavoring and/or fruit. Plain or ''natural style'' yogurt has no added flavoring material or fruit and is seldom, if ever, sweetened. Plain yogurt has a tart or sour, somewhat gree apple-like flavor, due to the high levels of lactic acid (0.9-1.2%) and acetaldehyde (5-40 ppm) present, respectively (3). Sweeteners, fruits, and/or flavorings tend to mask some of the naturally occurring acid and acetaldehyde flavor compounds in flavored yogurts.

If fruit is incorporated into the bottom of the cup, the yogurt is called sundae style or fruit-on-the-bottom. Fruit is rarely added to the top of the yogurt (fruit-on-top) in the United States. Ingredients used in Western-type sundae-style yogurt include coloring agent(s), flavor extract(s) or concentrate(s), and/or sweeteners, which are added to the yogurt base (milk base is colored according to the given flavor). Eastern-type sundae-style yogurt contains no coloring, flavoring, or sweetener in the yogurt base (milk base is white). Fruit-on-top style yogurt is made by filling yogurt cups with milk base first, followed by the addition of flavoring material on top.

The amount of fruit flavoring, fruit preserves, puree, or jam varies from 1.25 to 2.0 oz (37-59 g) per 8 oz (230 g) cup. In addition to the lactic acid and acetaldehyde flavors being somewhat masked by sweeteners and flavorings, production of these flavor compounds is limited because the metabolism of these end products by L. bulgaricus and S. thermophilus is inhibited by high sugar concentrations (3).

C. Swiss-Style Yogurt

The most popular type of yogurt in the United States is stirred. Stirred yogurt may also be referred to as Swiss-style, French, prestirred, or preblended yogurt. Although the yogurt base used to manufacture plain, Swiss, and sundae-style yogurts may be essentially the same, the manufacturing processes differ. Stirred yogurt is typically incubated in tanks and cooled before flavor addition and packaging. However, some facilities blend the fruit and flavors prior to incubation and allow the yogurt to set in retail containers.

Due to the differences in processing methods, set and stirred yogurts have markedly different textures (25). Sundae-style yogurt is allowed to set in the cup without being disturbed, but stirred yogurt structure is broken upon mixing with fruit. Thus, it is important to formulate Swiss-style yogurt appropriately to obtain the desired texture after blending. The texture of yogurt can be manipulated by formulation and processing techniques. Some of the factors that influence firmness in yogurt include casein or fat content of the milk, homogenization, heat treatment, yogurt cultures, pH, and incubation temperature (25). Casein enhances curd firmness, but as fat level increases, gel firmness decreases because the fat globules interrupt the protein network (25). Homogenization and heat treatment of the milk enhance firmness (25). Because yogurt structure is broken during the stirring process, exopolysaccharide-producing strains of L. bulgaricus or S. thermophilus are particularly useful for Swiss-style yogurt because they provide viscosity (19,20). Yo gurt firmness increases with decreasing pH, but yogurt pH should not be less than 4.1 (25). At low incubation temperatures, it will take longer for a certain pH to be attained, but the finished product is much firmer (25).

D. Drinkable Yogurt

Although one way to manufacture drinkable yogurt is shown in the process diagram, Fig. 5, several options exist. Yogurt beverages are similar to stirred-style yogurt, but the coagulum is ''broken'' into a liquid form before being either cooled, repasteurized, or UHT treated, depending on the shelf life desired, and homogenized prior to packaging. If repasteurized or UHT-treated, bacteria are killed, so ''contains live culture'' will not appear on the label. At Dannon Co. (West Jordan, UT), Danimals Drinkables bottles are sent to an unscrambler and blown by air to conveyer belts connected to fillers (29). At the filler, the bottles are mechanically turned upside-down, rinsed, sterilized, and sent through to the filler. Bottles are sent to a 30-unit rotary wheel to be filled, then are capped with an aluminum foil seal. Every 15 mins, a random sample is tested for viscosity, pH, and color. Bottles are code-dated, then shrink-wraped in groups of four. Case packers group four-packs into sets of six, then the groups are corrugated, the boxes are code-dated and sent to the cooling room (29).

E. Extra-Creamy Yogurt

Four-ounce Dannon La Creme yogurts boast ''a touch of cream'' that provides''naturally smooth dairy taste and a rich, creamy texture that goes beyond traditional yogurts'' (30). At approximately 4% fat, creamy yogurts contain 25% more fat than standard yogurts (approximately 3% fat) in the market. Modified food starch, corn starch, and Kosher gelatin help maintain the creamy texture. Since these products require only a formulation modification, processing need not be modified. However, because the creamy product may be packaged in smaller containers, the packaging line must be modified to accommodate the change in volume.

F. Portable Yogurt

Portable yogurt has met a consumer need for healthful portable foods. Go-GURT, Expresse, YoSqueeze, and Yo-Stix portable yogurts, in 2.0- to 2.5-ounce single-serve tubes, may be frozen, then packed in a lunch box to be eaten later in the day (13). It is likely that higher levels of stabilizers are used in these portable yogurts than standard yogurts in order to maintain desirable viscosity even after freezing. Although the manufacturing process is similar to that of standard yogurt, significant modifications must be made to the filling and packaging lines to accommodate filling and sealing of the convenient tubes.

G. Frozen Yogurt

Frozen yogurt, which can be soft or hard, resembles ice cream in physical structure more than it does yogurt. Higher levels of sugar (11-15%) and stabilizers (about 0.85%) than used in yogurt are required for frozen yogurt to maintain air bubble structure during freezing (1,22). Incubation time is extended due to the higher level of carbohydrates (1). No federal standards apply to frozen yogurt, but several states have regulations requiring a minimum titratable acidity. Yogurt for frozen yogurt is typically high in milk solids (25%) and incubated to high titratable acidities (1.50-2.00). A small amount of yogurt (10-15%)

is then blended with an ice cream mix and frozen like ice cream. An alternative method is to ferment a large amount of the base (30-100%) to a low titratable acidity (0.30-0.90) prior to blending and freezing (28). Ice cream and frozen yogurt are frozen in a scraped-surface heat exchanger. As sensible heat and latent heat are removed from the product in the freezer chamber, ice crystals form on the edges of the freezer barrel. Dashers scrape the frozen product from the freezer barrel and air cells simultaneously form within the serum, thus increasing product volume. The semi-frozen mass may be extracted from the freezer barrel at about —8°C and serve ''soft-serve'' or filled into retail cartons and hardened in a blast freezer at about —25°C (1).

H. Whipped Yogurt

Yoplait Whips! were the first whipped yogurt to enter the market. Light and fluffy in texture, Whips! contain 25% air compared to traditional yogurt: the same packages used for 6 oz of the standard product contain 4 oz of the whipped product. Air is whipped into the yogurt base using a proprietary process, much like the ice cream process but without freezing. Higher levels of sugar and stabilizers are used in whipped yogurt than used in standard yogurts to maintain air bubble structure during processing. Essential ingredients specific to whipped yogurts include gelatin, for foam structure; an emulsifier, for interphase stability; and pectin/starch, for viscosity and mouthfeel (31). During processing, the viscosity and temperature of the mix, as well as the flow rate, overrun, aging, and cooling kinetics are critical parameters that must be controlled for proper aeration of whipped yogurt (31).

I. Concentrated Yogurt

Concentrated yogurt, sometimes called strained yogurt or Labneh, is formulated much the same way as plain yogurt, described previously. The yogurt is incubated in tanks, concentrated by straining to about 24% total solids, and cooled before being packaged (22). Concentrated yogurt is used in recipes by the baking industry.

J. Other Yogurt Product Considerations

1. All Natural

''Natural'' is an arbitrary term in the food industry because the FDA has not established a formal definition for the term, but the word means a lot to many consumers (32). Yogurt can be made successfully with a minimum of ingredients, when made under controlled conditions of processing and handling. All natural yogurt products are easily attainable. Suppliers must be consulted to obtain natural ingredients. If products are to be handled roughly, gums and stabilizers are almost a necessity, due to the fragile nature of the yogurt body.

2. Organic

Natural is not legally defined, but organic is. Ingredients used in organic yogurt must be certified organic. This means the products were obtained from certified organic farmland, feeds for cows were grown on certified organic farmland, and/or cows were raised on certified organic farmland. Certified organic farmland must be free of synthetic pesticides, herbicides, and fungicides for at least 3 years prior to being used to grow organic crops intended as ingredients or dairy cow feed (33). As of October 21, 2002, producers and handlers must meet the National Organic Program's standards and be certified by a USDA-accredited certifying agent to sell, label or represent products as organic (34). Products labeled '' 100 percent organic'' are products exclusively produced using organic methods. To be labeled ''organic,'' at least 95% of the ingredients (by weight, excluding water and salt) must be organically produced (34). The remaining components can only be allowed natural or synthetic ingredients as stated in the Code of Federal Regulations (34). '' Made with organic'' products must contain 70-95% organic ingredients (34). Products with less than 70% organic ingredients can list the individual organic ingredients on the label (34).

3. Kosher

Several different symbols may appear on dairy products to indicate that the product is suitable for consumption by individuals observing dietary guidelines according to the Jewish faith. To receive Kosher certification, guidelines for handling of raw materials, processing, packaging, and distribution of foods and use of processing equipment must be followed (2). If a Kosher food is to be made after a non-Kosher food has been made, all equipment must be thoroughly cleaned, left idle for 24 hours, then washed with hot water prior to use for making Kosher foods (2). A rabbi or a Kosher Supervisory Agency must review and approve product processing procedures before Kosher symbols may be used. Products certified by the Orthodox Union are marked with the OU symbol (U enclosed by an O), placing a D beside the OU to signify the presence of dairy ingredients. Other Kosher symbols include CRC (Chicago Rabbinical Council), K, U, and parve or K-D (2). For sour cream or yogurt to receive Kosher certification, all ingredients used must be certified Kosher, including Kosher gelatin, colorings, and flavorings (2).

4. Multi-Packs

Four- and six-ounce size yogurts are options for consumers looking for yogurt snack options (33). These sizes are well-suited for multi-packs. Multi-packs of a single flavor or multiple flavors of cup or drinkable yogurt exist in the marketplace. Modifications must be made to the filling and packaging lines to accommodate the multi-pack feature.

5. Package Inclusions

Yogurt containers topped with separate enclosures of granola, candy, or other creative inclusions may also be found in the marketplace. Dannon Double Delights combine lowfat yogurt with dessert-like toppings in a dual-compartment container. Modifications must be made to the packaging line to accommodate the cup-topping or dual compartment features.

6. Yogurt with Vegetables

The natural sweetness ofsome vegetable purees (carrot, sweet potato, and Hubbard squash) lend themselves to addition in yogurt in place of fruits. In 2002, students from Washington State University demonstrated that the market may be ready for YoVe, an all-natural blend of yogurt and vegetables (12). Their product, a stirred yogurt with vegetable purees, was highlighted as a finalist at the 2002 Annual Meeting of the Institute of Food Technologists Student Association Product Development Competition, but it has yet to be launched in the marketplace. Minimal modifications to product formulation and processing line need to be made to produce yogurt with vegetables. Although the three flavors of YoVe were desirable to consumers upon tasting, the team determined that significant marketing and promotion would be necessary to overcome consumer perceptions that vegetables do not belong in yogurt and to guarantee the success of such a product.

Table 5 Troubleshooting Guide for Yogurt Production

Observation

Explanation

Solution

Bitter flavor

Cooked flavor

High acetaldehyde flavor

High acid flavor

Low acid flavor

Old ingredient flavor

Oxidized flavor

Rancid flavor

Gel-like body Grainy texture

Ropy body Too firm body

A bitter aftertaste may be due to culture contamination, excessive use of certain stabilizers, or poor quality ingredients.

A nutty or sulfur-like note may be apparent due to an excessively high heat treatment.

Excessive ''green apple'' flavor indicates L. bulgaricus, stimulated at higher incubation temperatures (112jF), dominated the fermentation.

Excessive lactic acid may cause extreme tartness (some sourness is appropriate) due to 1) extended incubation period, 2) elevated incubation temperature, 3) imbalance of bacteria, and 4) inadequate cooling to arrest culture activity.

Under-incubation of the yogurt, imbalanced yogurt cultures, or partial failure (due to inhibitors) are possible causes for the lack of typical refreshing flavor and moderate tartness associated with good yogurt.

When yogurt contains an old ingredient (NFDM, fruits, etc.), a ''state'' or bitter aftertaste persists after swallowing.

A cardboardy, metallic, or tallowy off-flavor may result if oxidized ingredients are used in the yogurt base.

If lipase is not inactivated prior to ingredient blending, the enzyme may act on the milk fat.

Excessive product firmness, slickness, or rigidity due to overstabilization.

Product lacks smoothness and uniformity due to undissolved solids, resulting from 1) unstable casein, 2) too-high homogenization temperature, 3) too-rapid acid development, 4) too-high incubation temperature, 5) excessive amount of culture, 6) incorrect stabilization system, 7) improper blending of yogurt base with fruit.

Yogurt strings out when spooned due to certain bacterial polysaccharides or stabilizers.

Product resists mastication and gives a pudding-like sensation in the mouth due to overstabilization.

Careful selection of ingredients and maintenance of culture quality is essential for high-quality yogurt.

Careful control of the pasteurization process and appropriate balance of flavor systems can minimize or mask moderate cooked flavors.

Maintain incubation temperature at or below 105° F to limit L. bulgaricus (S. thermophilus dominates below 105°F).

Incubate yogurt between 41 and 43°C (105-110°F) for 4-7 hr or until the yogurt pH is between 4.45 and 4.65 and TA reaches 0.9%.

The pH should be <4.5 for appropriate acid flavor. Use appropriate heat treatment conditions to maximize culture activity. Reduce sweeteners in yogurt base (<12%).

Careful selection and monitoring of ingredients is essential for high quality yogurt.

Careful selection and monitoring of ingredients is essential for high quality yogurt.

Careful monitoring of pasteurization and homogeniza-tion temperatures is essential.

Use appropriate levels of stabilizer.

Allow complete hydration of milk powder and stabilizer(s), homogenize and incubate at appropriate temperatures, select stabilizer(s) carefully, blend yogurt base and fruit flavoring adequately.

A certain level of ropiness may be desirable. Balance culture and stabilizer(s). Use appropriate levels of stabilizer.

Table 5 Continued

Observation

Explanation

Solution

Weak body Weak yogurt does not stand up in the spoon,

Careful yogurt base formulation and strict control of pasteurization temperature.

and/or may appear watery due to 1) under-stabilization, 2) low casein or low total milk solids, 3) under incubation, or 4) low pasteurization temperature.

Color leaching Pigment trails or leaching of color from

Use acid-stable colors within the typical pH range of yogurt (3.8-4.3). Properly blend yogurt base with flavoring material before filling.

fruit into yogurt, caused by low pH (<3.8) or incomplete blending of yogurt base with fruit flavoring material.

Free whey

Whey release from curd may be due to (1) low milk solids, (2) excess acid development, (3) agitation during incubation, (4) insufficient milk pasteurization temperature or holding time, (5) poor product formulation,

(6) inadequate stabilizer(s),

(7) temperature fluctuations.

MSNF must be at least 8.25%. Practice good overall process control, (1) accurately monitor pH, (2) halt acid production between pH 4.5 and 4.0,

(3) carefully select stabilizers,

(4) minimize handling of warm yogurt.

Shrunken

Curd pulls away from carton due to tightening of curd structure; often associated with free whey.

Practice process control measures used to prevent free whey defect.

K. Troubleshooting

Although the importance of quality ingredients, aseptic culturing techniques, and strict time-temperature controls has been highlighted, sometimes product quality suffers. Some of the defects that may be found in yogurt products, along with how to overcome the defects, are outlined in Table 5.

IV. CULTURED OR SOUR CREAM PROCESSING A. Processing

1. Sour cream Ingredients

According to the CFR (Title 21, Section 131.160), sour cream must contain at least 18% milk fat and have a titratable acidity of at least 0.5%, expressed as lactic acid (36,37). Optional ingredients may be used to improve texture, prevent syneresis, improve flavor, or extend shelf life, but sour cream may not contain more than 0.1% sodium citrate, salt, rennet, nutritive sweeteners, flavoring, or coloring (36,37).

Production of high-quality sour cream (Fig. 6) must begin with high-quality milk and/or cream. If high viscosity is desired, or if mechanical agitation will be employed, the body may be improved by including stabilizers (hydrophilic colloids bind water), WPC, NFDM, or by using cultures that naturally excrete polysaccharides. Sour cream made with 18% fat may not need stabilizer (38). Stabilizers may be used at about 0.5%, and 0.2% citric acid may be used for flavor and enhanced culture activity. Occasionally, particularly outside the United States, chymosin, which is used in cheesemaking to augment coagu-

J ligh-qiiLililv milk/crciim i

Standardization (10-30% fill)

Pasteurization (90"C, > 3 min)

Homogcnization (single or dual)

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