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August 1, 1995
Beyond Cultural Tradition
By: Scott Hegenbart
Cultured dairy products such as sour cream and yogurt have a long historical tradition. Although they faced many challenges in their day, the dairymen of the old-fashioned creameries probably could never fathom contemporary quality challenges presented by widespread product distribution. Compounding this are the diverse and ever-changing consumer demands regarding product texture -- particularly with respect to the creation of reduced-fat products that are indistinguishable from full-fat. Fortunately, stabilizing ingredients offers modern food product designers the ability to create cultured products with the right quality features that will last through the product's entire shelf life.
Playing the role
Stabilizers perform two basic functions in cultured products. The first is building texture. Sour cream has a very rigid gel-like structure," says Mary Ann Meschi, senior food technologist, American Maize-Products Co., Hammond, IN. "The stabilizers used will influence the texture of the final product."
But texture is not always universal. Different consumers have different requirements depending on what they're accustomed to. Using stabilizers easily provides product flexibility.
"Depending on the geographic area, sour cream texture will vary," says Mark Mandell, technical sales manager, Bunge Foods, Atlanta. "The northeast prefers more a more firm texture than, say, the southeast. In some cases, you need more texture than can be contributed by the fat content. Here you'll often add a gelatin to build the body."
But texture doesn't necessarily refer exclusively to overall body. Some products require very subtle enhancements to creaminess and mouthfeel. Again, stabilizers can fit the bill. This is particularly true of low- and no-fat products where the mouthfeel of the fat will be missing entirely.
Keep in mind that the stabilizer not only has to build texture, it also must help maintain it consistently through the product's shelf life.
Speaking of shelf life, no one likes yogurt or sour cream with a watery layer on top. Because whole milk is about 87% water and skim milk 91%, controlling syneresis throughout product distribution is the other critical quality function of stabilizers.
The necessary ingredients
Before adding other ingredients, product designers must first understand that a certain degree of stabilization in cultured products comes from the milk proteins themselves during fermentation.
Two mechanisms account for this. First, the acid formed during culturing lowers the overall pH to 4,6 -- the isoelectric point of the milk protein. Once the protein's charges are reversed, they will exhibit a mild affinity for one another and form a soft gel.
The lowered pH also will tend to denature the proteins. During the denaturing process, the normally folded protein molecules will tend to unravel. The structure of the unraveled protein molecules allows them to bind moisture.
In addition to the right pH, a significant amount of time is required to denature enough protein to have a significant effect on product stability. Unfortunately, this will slow throughput in a modern dairy plant. This is one of the primary reasons that additional stabilizers are needed in cultured products. The rigors of product distribution and consumer desires for smooth texture round out the need for these ingredients.
"In a traditional full-fat sour cream, you have a protein gel with fat trapped in it," says Anne Tieleman, Ph.D., senior food scientist, Hercules, Inc., Wilmington, DE. "Although you have a gel, it will tend to undergo syneresis and won't be smooth."
Because stability in cultured products requires building texture and binding moisture, the natural tools to reach for are starches and gums. Which starches and gums to use, however, is not always such an easy decision. Different starch sources, for example, offer different functionality. These also can be modified to further expand the functional variety. The same is true with the many different gums.
"A number of different gums are used in cultured products," says Mandell. "They predominantly include locust bean gum, xanthan gum, some carrageenan and some guar gum. You also may see some gelatin and agar. It depends on the system and what functionality you're trying to gain."
In yogurt, for example, a typical stabilizer system may include starch, gelatin and, possibly, pectin.
"Gelatin is effective for controlling the moisture yet melts cleanly in the mouth," says C. Douglas Vargo, cultured products technical manager, Germantown International, Broomall, PA. "At a 0.4% use level, though the product texture may resemble that of a gelatin dessert. Using that same level of gelatin, adding 0.6% of a modified food starch will create a creamier texture."
This combination also helps make processing more efficient. Because gelatin doesn't set until 55°F or lower, yogurt stabilized with just gelatin may be too fluid at the filler. Adding a starch can build enough viscosity so the machinery can be run faster. This viscosity can further help the visual appeal of a yogurt by suspending fruit pieces in a blended yogurt so they don't settle out before the gelatin can set. The right stabilizer also can improve the visual appeal of fruit-on-the-bottom, or "Swiss Style" yogurts.
"When there is fruit on the bottom, the color may bleed through the white yogurt and is seen on the surface as a defect," says Jamie Senkeleski, development chemist, National Starch & Chemical, Bridgewater, NJ. "Selecting the right starch can minimize this."
Working with the process
Stabilizing ingredients in cultured products are not only selected for the properties they contribute, their tolerance for processing conditions must be considered. All cultured products undergo homogenization and pasteurization at some point. While the resulting shear, pressure and heat aren't much of a concern for gums, they can affect the performance of the starch.
Starch is composed of granules that will swell when cooked. These swollen granules may rupture while passing through the homogenizer. Although modified starch granules will still hold water, their ability to do so will be diminished.
This situation can be avoided through ingredient selection and process modification. First, starches can be cross-linked to make them more resistant to shear. Next, when heating the product to cook the starch prior to homogenization, adjust the time and temperature so the starch is slightly undercooked when it passes through the homogenizer.
"The starch must adapt to the process," says Meschi. "It'd be best if the granules were only slightly gelatinized. If not gelatinized at all, they won't be friable and could still be damaged by shear."
Optimizing the homogenizer settings also helps. Use just enough heat and pressure to get the job done.
"The critical point in the process is homogenization," says Senkeleski. "The lower the pressure and temperature through the homogenizer, the better the starch functionality."
Not only can the process affect stabilizer performance, the stabilizer can affect the process, too. If too much viscosity is built too early in the process, the equipment may not be able to handle the product mix.
"You want the body, but you don't want it all early in the process," says Tieleman. "If you have a really thick sour cream mix before adding the culture, it will be hard to disperse it evenly."
Selecting a gum that is activated at a lower pH is one method to delay viscosity building until after the culture is introduced.
A matter of culture
In most cases, culturing means the introduction of a microorganism. These convert lactose to lactic acid. In some products, such as sour cream, lactic acid itself may be added (direct acidification).
"Culturing provides finished products with flavor and texture," says John Breeden, product manager, functional blends, Danisco Ingredients, New Century, KS. "Direct acidification does not produce the same flavor profile as culturing a product does."
During culturing, acidity increases during the growth phase when the microorganisms are multiplying on a logarithmic scale. The product will go from a pH of 6 or 7 down to around 4. As the pH approaches the isoelectric point of the milk protein (4.6), coagulation begins to occur.
"As the product becomes more acidic, the acid will begin to slow the growth of the microorganisms," says Breeden. "Care should be taken to avoid breaking the product around pH 4.6 since it can produce a grainy texture."
In addition to coagulating the protein, converting lactose to lactic acid develops the product's characteristic flavor. But it doesn't do so alone. In yogurt, for example, the culture also produces acetic acid, acetaldehyde and diacetyl -- all of which contribute flavor.
Of these reactions, the increase in acidity is the only one that significantly affects the stabilizer -- specifically, the starch. When exposed to acidic conditions, a starch may hydrolyze into smaller polysaccharides. These will be less adept at building texture and holding water.
Because starches can be made acid-tolerant, however, any problems are avoided by simply selecting a starch that is stable in the 4.4 to 4.6 pH range. Keep in mind, though, that the presence of acid may encourage the granules of even an acid-tolerant starch to swell slightly. If the granules are at the point of rupture from previous process stress, the increase in acidity still may rupture the granules and cause a slight decrease in starch functionality.
Although the acid is more of a concern, in some cases, the microorganisms can affect stabilizer performance as well.
"Some microorganisms will produce enzymes," says Mandell. "Some of these are capable of breaking down certain hydrocolloids."
Although enzymes won't completely destroy the stabilizers, they can, like lactic acid, diminish the functionality. Certain hydrocolloids -- such as agar and xanthan gum -- are more immune to enzymatic degradation and can be used, if desired. For the most part, though, modern starter cultures are specifically designed to minimize concerns over these enzymes.
Not only can the culturing process affect the stabilizer system. The stabilizer system can influence how culturing proceeds.
"Sometimes the starches or stabilizers you add may change the fermentation conditions of the starter culture," says Khalid Shammet, Ph.D., dairy scientist, A.E. Staley Manufacturing Co., Decatur, IL. "Monitoring the pH or the titratable acidity during culturing can easily determine if such changes are taking place."
Shammet adds, however, that most stabilizers are used at very low levels --0.1% to 0.4% for gums and as high as 2% for starch. Any inhibition of the culture is likely to be insignificant.
"Still, you want to be sure your stabilizer and the starter culture work in harmony at the same temperature and pH," says Shammet. "These two are critical in the manufacture of any fermented product."
The stabilizer can affect the culturing process in other ways, too. One of the byproducts of fermentation is carbon dioxide gas. This must be allowed to escape during the culturing process.
"If the coagulum is too thick, this gas may become trapped," says Vargo. "When that happens, you get a gassy, airy product that will form a dome on top. A viscosity of 600 centipoise is still OK, but you run into trouble up around 1,000 centipoise."
Reduced-fat products, however, require higher levels of stabilizers and the extra viscosity during fermentation may be unavoidable. Here, the starter culture itself may be modified to reduce the amount of carbon dioxide formed.
For instance, sour cream contains two cultures -- Streptococcus lactis produces lactic acid while Lucinostoc citrovorum ferments the citrates and produces diacetyl and carbon dioxide. A normal ratio between the two in sour cream is 60-40. By changing this to either 80-20, or even 90-10, fewer gas-producing microorganisms will be introduced. Keep in mind that the reduction of diacetyl will affect the flavor profile. Still, most reduced-fat products require a re-balanced flavor system anyway.
Finding the right mixture
Knowing what challenges a cultured product presents, product designers may now more easily identify and implement the correct stabilizer system. This process starts with an overview of the products performance requirements.
"You first have to appraise what you're looking for. Is syneresis the problem, or do you want to build body?" says Mandell. "Once you've established the functionality, then you have many options from which to choose; all have pros and cons."
Because different ingredients offer different advantages, most cultured products contain a system of different stabilizer ingredients. A gum, for example, can be used to form a firm, gel-like structure. It may not, however, control syneresis to the degree required. As previously mentioned, it also may make the texture too much like a gelatin dessert. (Although consumers in some regions prefer sour cream and yogurt with this sort of texture.) Combining it with a starch can improve syneresis control and make the texture more creamy.
"Gums and starches complement each other," says Meschi. "In one system I worked on, the gums contributed a more rigid gel, while the starch held onto the water."
When selecting the stabilizers, though, remember that they are not the only factors controlling texture. Certain cultures will produce polysaccharides during fermentation. This can give a ropy texture to yogurt. While an extremely stringy texture would be undesirable, a certain degree of ropiness can make a fruit-on-the-bottom yogurt smoother when stirred together by the consumer.
In addition to texture, flavor must be considered when selecting stabilizers. This isn't typically a problem of flavor contribution, but rather one of flavor masking.
"The more starch you use, the more creamy and soft the texture will be when used with gelatin," says Vargo. "But there's a point where you must limit starch use because it has a tendency to mask subtle flavors."
In sour cream and buttermilk the diacetyl may be affected. In yogurt, it may be the acetaldehyde. Keeping the starch use level below 1% to 1.5% will help reduce the risk of masking these characteristic flavor components.
Next, compatibility with the system must be addressed. The stabilizers and cultures must work together without inhibiting one another. Different stabilizers also will have varying pH and temperature requirements that must be considered.
"When using gums, sometimes you need a high temperature to activate them. The pH also has to be right," says Shammet. "At the same time you want it to be the same pH for the starter culture to grow."
Once the stabilizer system has been assembled, it must be thoroughly tested in process in order to evaluate performance. Newer small-scale equipment will allow some of this to take place in the lab. Most testing, however, will be limited to at least the pilot plant scale.
"The best way to formulate and test stabilizers is to subject them to every imaginable processing situation," says Breeden. "This can be accomplished by varying pasteurization temperatures, holding times, homogenizer pressures and pumping distances."
Also, if the product requires screening to smooth it, use various homogenizer valves and screens to determine the shearing effects on the product. Also, vary the cooling temperatures and the titratable acidity or pH of breaking. Typically, the higher the titratable acidity, the more viscous the product. The cooling rate of the product and the time the product is allowed to set also must be considered.
The dairymen of yesteryear certainly had their share of challenges when working with cultured products. Although food science has solved many of them, the use of ingredients such as stabilizers generates new challenges for product designers. Fortunately, a thorough look at stabilizer functionality and a logical formulation approach help designers make products that are the cream of the crop.
© 1995 by Weeks Publishing Company
3400 Dundee Rd. Suite #100
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