Controlling viscosity

Dressings, sauces and gravies may have varied uses, but have one thing in common: They all are viscous liquids. This viscosity not only is an important organoleptic feature, it affects processing and product shelf life.

"It's not just the building of viscosity that's important, part of the key is preventing or controlling the breakdown of viscosity," says Allen Freed, president, Gum Technology Corporation, Tucson, AZ. "The trick is to stabilize a product so that it maintains its apparent natural viscosity without looking unnatural."

With proper planning and formulation, product designers can perform this trick of properly controlling viscosity.

The role of viscosity

Be it a sauce, dressing or gravy, the adjectives, "thick" and "rich," often are used to describe organoleptic appeal. This refers not just to the texture and mouthfeel of the product, but the visual appeal as well. A sauce that runs and "disappears" on a plate of pasta is unappetizing, as is a salad dressing that runs to the bottom of the bowl, rather than clinging to the lettuce.

Viscosity also will affect how the product's flavor is released. This may be further influenced by any viscosity building ingredients used. An improperly selected starch may either contribute a flavor of its own, or mask other desirable flavors.

Less obvious than the organoleptic contributions, controlling viscosity often is critical to the successful processing of a product. Product thickness, for example, may have a tremendous influence on packaging.

"It's easier to fill something that is viscous, rather than something that's water thin," says James Jindra, applications specialist -- dressings and sauces, Systems Bio-Industries, Waukesha, WI. "If the product has particulates -- such as vegetables, or pickle relish -- they may settle out, or float to the top of the batch, causing some containers to have too many vegetables while others don't have enough. A greater viscosity will assist in providing a uniform amount of vegetables in each."

Although "controlled viscosity" often refers to maintaining consistent finished product viscosity, processing considerations often require the viscosity of a product to change in a predictable manner. For example, the viscosity needed for efficient filling may be higher than needed for the finished product. In such cases, a designer might select ingredients that build a high initial viscosity, but lose it somehow after packaging; during the heat of retorting, for example.

"A 'thick-to-thin' application might be a retorted pasta dish," says Jindra. "For depositing, a manufacturer will want the sauce viscous, but also want it to break down somewhat during retorting."

Although heat-sensitive viscosifiers often are looked upon unfavorably, they seem tailor-made in this application. On the other hand, an ingredient that increases in viscosity with time is required when a "thin-to-thick" change during retorting is desired.

"The reason behind this is that, with retorting, you want the fastest rate of heat transfer to the center of the container," says Jindra. With a high upfront viscosity, you aren't going to get a rapid heat transfer."

With regard to shelf life, avoiding viscosity breakdown during storage and distribution is going to be the primary role viscosifiers play. Maintaining viscosity while a sauce or gravy is held in a foodservice steam table also will be important. But viscosity and viscosifers also contribute to shelf stability in other ways.

One way is the elimination of syneresis. Controlling excess water also contributes freeze/thaw stability to a sauce that is used on frozen products, such as entrees. Stabilizing the emulsion of dressings is yet another contribution to shelf life.

"Some viscosifiers provide the necessary viscosity to assist in the creation of the emulsion," says Jindra. "Some, like xanthan gum and propylene glycol alginate, are very useful."

Structure of a sauce

Although viscosity building ingredients, such as gums and starches, are important, other factors also must be considered when controlling viscosity. These include the basic structure of the product, the other formula ingredients and the processing conditions. Variations of any of these can affect viscosity as much as changing the hydrocolloid. These factors also may directly affect the functionality of any hydrocolloids in the formula.

"You may have two identical formulas, but if one has shear and one doesn't, you will get very different results," says Freed. "You also may have only three ingredients, but as you change the ratio between them, you may have to change the stabilizer system."

Viscous liquid products in the same category can be made in many ways. However, there will be some common structural features within the category.

Sauces comprise the least definable category. They can be high-acid, or low-acid; be dairy-based, or tomato-based; and they may or may not be based on an emulsion.

"Sauces go all over the place, but more often than not they're not emulsions," says Freed. "Sometimes they're thickened water, sometimes they have a gel structure. What they have in common is they are products that have been thickened through the use of some form of stabilizer."

Gravy usually has some fat added to it, but not enough to make it a true emulsion. Although it may not be a true emulsion, thickness in a gravy is important in order to suspend the fat. Gravies most often contain flour, which contributes viscosity, opacity and is part of a gravy's expected flavor profile. Other viscosifiers, however, are a must because flour isn't always stable enough under processing conditions.

"It all depends on how much flour and salt is combined in the formula," says Jindra. "Above a certain flour level, freeze-thaw stability decreases dramatically. Also, the amount of salt in the product could affect freeze-thaw stability by affecting how the viscosifier hydrates. The pH of the gravy will further influence stability by affecting the hydration of the viscosifier during processing."

Dressings are traditionally oil-in-water emulsions. Thanks to the low-fat trend, however, dressings can be as varied in structure as sauces. Dressings don't have to be emulsions because they don't necessarily contain oil.

"Salad dressings are a unique category," says Dorothy Peterson, a food technologist with Hammond, IN-based American Maize-Products Co. They may contain anywhere from 30% to 50% oil."

While starches and hydrocolloids can help stabilize dressing products and help build texture and body, much of a dressing's viscosity is obtained through the formation of the emulsion itself.

Ingredient selection

Following the basic product structure, ingredient selection will be the next element of effective viscosity control. For many products, particularly those based on emulsions, the oil itself exhibits a huge influence.

According to Don Banks, president of Dallas-based Edible Oil Technology, the viscosity contributed by an oil is not the relevant issue because most domestic oils have very similar viscosities. However, the ratio of the oil to the aqueous portion of the emulsion will directly affect the final product viscosity. In these days of fat sparing, the reduction or elimination of the oils may greatly alter product viscosity.

In reduced-fat products, viscosifiers must be added to account for the missing oil's viscosity contributions and to deal with the extra water that now must be added to the system. Typically, the job is handled by some combination of hydrocolloids, such as xanthan gum, propylene glycol alginate and starch. The exact combination and ratios of individual hydrocolloids will depend on the nature of the product.

"If you're making a ranch dressing, propylene glycol alginate can be used for mouthfeel properties," says Jindra. "A French no-fat dressing may also contain starch for body and mouthfeel. Both typically will contain xanthan gum. The type and amount of starch and hydrocolloid will vary depending on whether you want to produce a pourable dressing or a spoonable one."

But, dressings don't have to be low-fat to use hydrocolloids as effective viscosity controllers. The oil may form the fundamental viscosity in the emulsion, but a hydrocolloid, such as xantham gum, helps create and maintain this emulsion through the process as well as enhancing product texture in salad dressings.

As previously noted for retorted products, viscosity building ingredients often must be selected so they change in a desirable way. This includes both during processing and when used by the consumer.

"Pourable salad dressing can be very viscous at rest; then, if formulated correctly, thins when poured and can flow easily out of the bottle," says Jindra. "Once poured onto the salad, the conditions of shear are removed and the dressing regains its initial viscosity."

In this case, xanthan gum -- a shear-thinning hydrocolloid that regains viscosity once the shear is removed -- may be the solution.

When selecting a gum or a starch, product designers must take care not to over-stabilize a product. Over-stabilization not only can make a product seem overly thick and unnatural, it can actually reduce the functionality of the system or mask flavors. Over-stabilization may even affect the interfacial activity of the emulsion, causing it to break and the oil to separate out.

"People often use a hydrocolloid at too high a level and think that it doesn't work," says Freed. "It's hard to convince them that they have too much. They think if 1% doesn't work, 0.5% certainly won't."

In addition to gums and starches, other formula ingredients will help build viscosity. Sugars, corn syrup solids, tomato paste -- anything that contributes total solids -- will enhance viscosity. At the same time, these other ingredients also may affect the functionality of added viscosifiers.

"Salt, protein and, in some cases, a higher sugar content will all compete with starch for the available water. This competition can actually change the gelatinization properties," says Peterson. "An added acid may disrupt the hydrogen bonds and cleave the starch molecule, so you have an increase in gelatinization and breakdown. Fats and proteins can coat the surface of the starch granule and delay gelatinization. It's really important to look at the other ingredients in the system."

If acidity will have an effect on stabilizer performance, keep in mind that a product's pH may change with time. A fresh salsa in the refrigerated section will change pH gradually over time. The stabilizer must be selected to function within this range, rather than at a specific pH. Remember, too, that acidic particulates in the product -- such as sun-dried tomatoes -- can generate "hot spots" of lower pH around them. Again, the stabilizer must be selected to be functional within the potential pH range.

Processing effects

As formulas for sauces, gravies and dressings vary, so do the methods for processing them. Some products are heated, some aren't; some are homogenized, some won't be. At the same time, how ingredients react to process conditions also varies. Certain ingredients lose viscosity with heat and shear, while other stabilizers need heat and/or shear to become activated and build viscosity. This makes it difficult to describe a single processing guideline for controlling viscosity. Still, designers can take process conditions into consideradon by evaluating the amount of heat and shear to which the product will be exposed.

While determining heat exposure usually is pretty obvious, shear may occur anywhere throughout the process. Naturally, shear will occur during mixing. The type of mixers used, however, will provide different degrees of shear. An open tank with a paddle mixer will definitely provide less shear than a high-vortex mixer.

When heated, a product also will experience shear and, like mixing, the type of equipment will determine the degree of shear.

"For instance, an open kettle will be a relatively low-shear system," says Peterson. "A swept-surface heat exchanger will be moderate, while a plate heat exchanger is high in shear and pretty abusive."

Speaking of abuse, salad dressings often are processed through homogenizers and colloid mills. These subject the product to, quite possibly, the most extreme levels of shear. Still, every time the product is pumped, the shear exposure increases. This occurs not only in the pump itself, but in the piping as well. Any time product moves from a larger vessel into a smaller tube, it will receive shear. Shear additionally occurs at every elbow in the piping path. Here, the entire product pathway and the type of pumps used must be added to the final determination of shear exposure.

With such seemingly insignificant process stages affecting the product, it's no wonder that some manufacturers have problems transferring a formula from one plant to another. In fact, even if the overall process line is identical the product may be different because of how it is packaged. A gravity filler, for example, will expose the finished product to less shear than a piston filler. With this being the case, product scale-up will obviously be a challenge.

"Often times, a starch is selected for optimal performance in the lab or pilot plant, then doesn't work in production," says Peterson. "The process must be in mind when developing the sauces. A process involving high-shear will not be easy to duplicate in the lab."

Putting it all together

At this point, viscosity control of dressings, sauces and gravies has reached a paradox: The viscosity directly affects process efficiency in many ways, yet the process itself affects the viscosity of the product. For the most part, the paradox is solved because product designers most often are requested to formulate products to run on existing lines. Even if the corporate powers-that-be are planning to invest in a new line, it's best to solve the paradox by having at least a rough idea of what the desired end process will be and use that as a guide for selecting viscosity building ingredients.

"The First thing you have to consider is the process," says Jindra. "Is it going to be a hot process/hot fill, or is it going to be something that's cold processed and filled. Right away, this will answer the question, 'Do I want something that is cold-water-soluble, or something that is going to require heat to hydrate?' "

Once the level of heat is determined, potential viscosifiers become more obvious. Some gums will need the heat to activate, others are broken down by it. Understanding the process first easily sorts out which one to use. At the same time, however, the product's acidity may complicate the issue.

"Some gums and starches are fine at a certain pH level until you start heating them," says Freed. "As you reduce in pH and increase heat, you further degrade the stabilizer."

Keep in mind, also, that while viscous liquids may be processed in many ways, all processes will not be suitable for all types of products. The desired end-product viscosity may make the use of existing lines impossible.

"In reduced-fat, mayonnaise-type dressings, the viscosity can be so high as to make cold processing unfeasible," says Jindra. "You're trying to hydrate ingredients in water without any additional heat, so you can only achieve a certain viscosity before you have difficulty in dispensing the remaining dry ingredients. In a case like that, a hot process is preferred."

In addition to heat, examine the process line and determine its potential for shear. This includes mixers, homogenizers, pumps and all of the transfer piping, and the packaging line. The total shear potential must be used to select an ingredient with the proper shear stability.

With the process in mind, evaluate the viscosity goals for the product. Determine what type of body the product should have. If it's a salad dressing, for instance, it may be pourable or spoonable. In either case, the dressing must cling to lettuce. If the product requires vinegar or some other acid, look for viscosifiers that are appropriate for the pH conditions that will be encountered.

"You just have to look and see what the conditions of the formulation and processing are going to be and use that as your starting point," says Freed. "As time goes on, you start learning the more esoteric textural effects they will contribute. Locust bean gum will have a very different texture from guar gum. Different starches will have different eating textures."

Another feature of the product that must be considered is how well it stands up to storage and distribution. A jarred gravy may experience extremes of heat and cold that can test the water-holding properties of the hydrocolloids. The cheese sauce on a frozen side dish may experience a certain number of freeze-thaw cycles.

Finally, proper viscosity control requires understanding how the final product will be used. A gravy designed for a buffet-style foodservice operation must maintain viscosity at inconsistent heat levels for long periods of time. Similar qualities are even necessary for a home-use simmer sauce because some consumer will invariably over- or under-cook the product, yet will expect consistent results.

In fact, any in-home preparation by the consumer must be scrutinized. The sauce may be required to perform identically after either microwaving or oven preparation. A dry sauce mix must produce a consistent product in spite of the greater potential for variation with preparation in the home kitchen. In many cases, the sauce must maintain quality after being stored and reheated as leftovers.

Storage presents an even greater challenge for salsas and dressings. These products often are opened and held for extended periods of time in the refrigerator. When consumers get a craving for a salad or nachos, they don't care if acid conditions are hard on viscosifiers, they want the product to have the same qualities it had when it was first opened.

Make no mistake about it, controlling viscosity in dressings, sauces and gravies is no easy task. Any one of a number of variables along the way from formulation, to processing, to the consumer's culinary skill (or lack thereof) has the potential to make or break a product. Although it may seem daunting, the effects of ingredients and processing can be identified and compensated for to keep the finished product in the thick of it.