Article: Formulating for Texture and Viscosity -- September 2008
September 1, 2008
Throughout the evolution of humans, the texture of foods has played an immense role in the taste preferences and acceptance of comestibles. Ingredients that contribute to taste and texture via viscosity, mouthfeel and bite, among other attributes, have been a driving force as well. Texture is also a priority in food processing and preservation, due to the ever-increasing need for shelflife, managing moisture, reducing fat or calories, adding healthier fats, optimized processing and developing specific textural properties. The convenience and foodservice boom that started in the early 1970s demands more food properties, such as freeze/thaw stability, particle suspension, creaminess and enhanced melt characteristics. Product developers have a number of ingredients at their disposal that can help manage texture.
At some point in the formulating process, the issue of texture and viscosity comes up. According to Webster’s dictionary, texture is defined as “characteristics of the solid and rheological state of a product,” and viscosity is “the resistance of liquid to shear forces or flow.” Virtually every processed food, from soup to nut bread, exhibits some level of texture and viscosity during processing to final consumption. In some foods, such as baked goods and candies, texture is most prominent. Viscosity, on the other hand, is more clearly observed in soups, puddings and beverages. However, both groups exhibit texture and viscosity to a greater or lesser degree. While the low moisture content of baked goods and candies significantly reduces the importance of viscosity in the finished product, it is present during processing and consumption. The texture of soups, puddings and beverages can range anywhere from a solid, gel-like flan to a full-bodied sports drink.
Food FormulationThe purpose of most formulating projects is to create a new, different or improved food product. This product will generally fall into one of the following four broad categories; each requires a different solution to meet the desired textural expectations.
1. Traditional foods. These are staple products, such as white bread, yogurt, soup or fruit filling. These could be an improvement on a current product line or an entirely new venture.
2. Copy (or match) food. In this case, it is the reproduction of a popular food item, such as Oreos, Miracle Whip or Cheerios.
3. Alternative food. This category covers most functional foods. A basic food ingredient is either removed (e.g., fat, sugar, gluten or eggs) or added (e.g., protein, fiber, calcium) to a traditional food. Maintaining desired texture and viscosity is probably most challenging and often crucial in this category. More recently, the digestive health introductions that contain prebiotics and probiotics are growing at a very high rate.
4. Innovative food. This small, but growing, category involves the creation of a totally new food that is not currently on the market. Examples may include restructured foods and vegetarian analogs.
All of these categories require an expected or assumed texture and viscosity. In traditional foods, consumers will expect a bread to look like bread, smell like a bread and taste like a bread. When reproducing a popular food item, certain functional and textural characteristics are taken for granted. The cookies of an Oreo look-alike should snap as the original; the Miracle Whip-type product should spread like its counterpart; the generic Cheerios-type product is expected to crunch like its famous parent cereal. The biggest draw to functional foods is producing a food item that is as close as possible to the original, but with healthier features. This is often easier said than done. Although the innovative food often does not have a traditional expectation, it requires more ingenuity and creativity.
With the technological advances in recent years, a whole array of ingredients—from hydrocolloids to proteins—is available to produce every conceivable texture and viscosity. However, before the formulating process begins, formulators must define and set specific parameters in order to select the proper ingredient(s) for the specific application. Firstly, list the desired textures and viscosities of the product at various stages of production (including consumer consumption). Some areas to consider are: particle suspension during pumping, viscosity at deposit fill, final mouthfeel and flavor release. Secondly, define processing conditions such as pH, time and temperature of cook, shear rates and overall plant conditions, including storage temperature. Thirdly, list packaging conditions, expected shelflife and end-use. Lastly, define the cost limitations.
Modified Starch and Food TexturesStarches are, by far, the most common hydrocolloid used in the food industry. The wide range of starch types, physical and chemical modification, versatility and low cost make them an attractive ingredient option. They are composed of long-chained straight (amylose) or branched (amylopectin) glucose molecules in varying ratios. Starch types consist of corn, tapioca, wheat, potato and rice. Each type has unique and distinct characteristics and is available in a variety of chemical and physical modifications. Cornstarch is the most prevalent and well-known of the starch types. Common cornstarch provides a firm, gel-like structure. However, left unmodified, it is prone to syneresis (water separation). This can be corrected by modification or the addition of another hydrocolloid to the formulation. Its modified waxy counterpart with 100% amylopectin, on the other hand, produces a smooth and creamy texture. Unmodified, however, results in an undesirable, stringy texture.
(Unmodified) tapioca is known for its clean flavor and creamy texture and is commonly used in dairy products, while unmodified wheat starch may be found in soups, breads and gravies and exhibits a complementary flavor in these applications. Unmodified potato starch provides the highest viscosity of all the starch types. It produces a thick and somewhat pulpy paste and is often found in meats, tomato-based soups, gravies and fruit fillings. On the other hand, rice, with its fine particle size, creates a very smooth texture with clean flavor. Also available in a waxy variety, it is commonly used in soups, dairy and meats.
Most applications, however, will require a modified starch to achieve the required texture and viscosity. While modification is often necessary to withstand adverse processing conditions, it also allows the development of different textures. Starches are modified to withstand low pH (less than 4.5), high and/or long cook or storage temperatures, high shear or pumping rates, freezing/thawing, refrigerator storage and consumer use. The most important chemical modification of starches across the board is the cross-linking by mono-functional reagents, such as phosphorus oxyclorides (POCl3) and STMP (sodium tri-meta phosphate and epichlohydrin); the POCl3 dominates this modification. Only a very minute amount of POCl3 (0.1-0.01%) is needed to make low, medium to high cross-linked (more stable) starches that will withstand low-pH, high-temp and high-shear processing conditions. Higher levels of STMP recently have been utilized for producing resistant starches. (Note: The maximum residual level of STMP allowed by the FDA is 0.40%. See CFR Title 21, Part 172, Sec. 172.892 Food starch-modified.)
To maintain space constraints, this article will focus on the cross-linking modification of starches and the processing effects of different levels of cross-linking. The charts “Phosphate Cross-linking,” “Cross-linking: Objective and Principle” and “Effect of Cross-linking Level on Waxy Starch” demonstrate the chemistry and the changes to starch molecules. The charts “Phosphate Cross-linking” and “Cross-linking: Objective and Principle” depict chemical reactions that produce stabilized, modified starch by cross-linking and the starch granule. The chart “Cross-linking: Objective and Principle” shows the effect that cross-linking has, in that it is like “spot welding” of adjacent chains of molecules that provides stability to processing conditions. Cross-linking inhibits granule swelling, the primary reason for its stability to processing conditions. In “Effect of Cross-linking Level on Waxy Starch,” results charted from a Brabender amylograph show the effect of different levels of cross-linking agents on viscosity.
In modified starch categories, cross-linking is generally supplemented by a substitution with reagents, such as acetic anhydride or propylene oxide, which enhance viscosity, water holding, etc. Foods are processed to produce the highest level of sterilization (safe for consumption) by heat and acid. Shear equipment, such as high-speed mixers, homogenizers, etc., help enhance the sterile state, as well as produce the appropriate texture.
During the production of a cherry pie filling, for example, the low-pH (3.5) product will be heated in a tank to a minimum of 180°F (82°C), in order to develop enough viscosity to suspend the cherries, while the product is pumped to the depositor. The filling may be hot-filled or retorted, subjecting the starch to another or continued heat treatment. The starch must develop the correct viscosity and texture without breaking down. Prior to final use, the filling is further heated, and possibly frozen or refrigerated, prior to consumption. During each stage of the process, the starch must maintain the desired texture, up to and including mouthfeel during consumption.
Other modifications include: 1) encapsulating starches used in beverage emulsions to suspend flavor components; 2) fat replacers mimicking the creaminess, mouth-coating and flavor release of fat; and 3) thin-boiling starches for a chewy and sticky texture in gumdrops.
Instant starches are available in pre-gel or cold water-soluble versions. The pre-gels come in a variety of granulation sizes, producing textures that vary from very smooth to pulpy. To prevent clumping, pre-gels must be blended with other dry ingredients prior to mixing. They provide texture, mouthfeel and viscosity to puddings, icings, baked goods and mixes. Cold water-soluble starches, on the other hand, provide the texture and viscosity of cook-up starches in a cold process, without pre-blending.
Modified starches provide body and textural attributes to the final product, such as cheese products and sauces, through appropriate levels of gel formation. Starches are typically more economical than gums, can be used as bulking agents and usually have little or no flavor influence on the final products. Starches also can be cost-effective replacements for casein in products, such as imitation cheeses, providing sensory and performance characteristics.
It is important to consider all aspects of the requirements for the final product, plus processing and storage, when choosing a starch. Some of the most important attributes are: hydration and gelation during processing, final viscosity, storage and freeze/thaw stability, texture/mouthfeel and cost. Similar to dairy products, modified starches provide appropriate water holding, viscosity, etc.
New and innovative technologies in starch development have produced natural starches and functional flours without chemical modification. Consumers are becoming more and more sensitive to chemical ingredients in their foods. They are requesting clean ingredients and clean labels. Natural starches are intended for applications with abusive processing conditions requiring a clean label. These starches provide the same texture and viscosity as more traditional starches. In baked goods, functional flours have the same opacity, texture and flavor as traditional flours but are cold water-soluble, process-tolerant and freeze/thaw resistant.
Hydrocolloids and Food TexturesWhile starches are mostly known for their viscosifying capabilities, gums can provide strong gels and unique mouthfeel characteristics. Like starches, gums are affected by heat, shear and pH, as well as the presence and levels of other ingredients in the formulation. Acids and salts may inhibit hydration in some gums. While certain gums thrive in products with high levels of alcohol and Brix, others languish. Proteins and salts such as calcium, potassium and sodium may enhance gel strength in some gums and decrease it in others. Processing conditions, especially shear rates, will thicken gels with some gums, but reduce it with others. Different usage levels of the same gum will produce widely varying textures. Usage levels are typically quite low (<0.5%), but the higher cost of gums may influence the type and level of gum used. Due to this great variety, it is essential to determine the formulation, processing conditions and cost prior to gum selection.
Xanthan gum is the workhorse of gums. It is acid-, shear- and heat-resistant, cold water-soluble, and freeze/thaw and refrigerator stable. At proper levels, its light, clinging texture suspends particulates, pours or flows when disturbed and thickens upon setting. It is ideal for pourable salad dressings, fruit fillings and as a general, all-around thickener. Since it does not work well with milk protein and tends to have a somewhat slimy texture at higher levels, it often is used in combination with other gums and starches.
Other common gums include guar gum and locust bean gum. While guar gum is not low-pH-stable, its low cost, cold water-solubility and synergistic effect with other gums make it a desirable thickener. Locust bean gum, on the other hand, is hot water-soluble and, in conjunction with other gums, forms gels of varying strengths.
In contrast, carrageenans are thin when heated, or sheared and thicken upon set. This unique characteristic eases mixing and pumping during production. There are three types of carageenans: kappa, iota and lambda. Lambda is primarily used as a thickening agent, while kappa and iota form gels in the presence of proteins and certain salts. Due to structural differences in proteins (such as whether they are from milk or soy), the interaction with kappa and iota will vary. Kappa tends to form a brittle gel with milk proteins, while iota’s gel will be more elastic. Potassium salts will increase kappa’s gel strength, but sodium salts will decrease it. Iota’s gel strength, however, will increase in the presence of calcium salts. Both gels are sensitive to low-pH levels.
For gel formation in low-pH environments, high-solid or high-sugar systems, pectin, agar and gelatin are commonly used. Pectin requires high concentrations (>50%) of sugar and a low pH to gel. While low-methoxyl pectin will gel at lower sugar concentrations, the gels tend to be softer. Pectin may also restore mouthfeel in pulp-free juices. Agar is hot-soluble, and gels form upon cooling. Both pectin and agar produce the tender, short texture found in gum candies and are often found in yogurts. Gelatin develops into a thermoreversible gel that softens at higher temperatures and hardens upon cooling. Its low melting point (around 80°F), desirable mouthfeel and flavor release make it ideal for gelatin desserts, jellies and candies.
In addition to providing unique textures and gels, many thickeners are also emulsifiers. These include gum Arabic, the cellulose family and alginates. While gum Arabic gives chewing gum and gum candies their characteristic texture, its emulsification properties are commonly used to stabilize beverage emulsions. Although many cellulose ingredients may function as emulsifiers, their gel formation varies greatly. Methylcellulose (MC) and hydroxypropylmethyl cellulose (HPMC) form gels when heated and thin upon cooling. Carboxymethyl cellulose (CMC) produces a clear gel and is stable in high-alcohol systems. Microcrystalline cellulose (MCC), on the other hand, is opaque and thins upon shearing and sets upon rest. Alginates often serve as an emulsifier and suspension aid in salad dressings, yogurts and sour cream. Propylene glycol alginate works well as a thickening agent in low-pH and low-protein systems. Sodium alginate, however, will form a gel in the presence of calcium and often is used in restructured foods, such as olive pimento strips.
With all the variety offered by these individual starches and gums, the most stable and varied textures and viscosities are found when they are used in combination with each other. These combinations may be starch with starch, starch with gum or gum with gum. They may produce synergistic textures not found individually. One may compensate for the weakness or shortcomings of the other ingredient. For example, one ingredient will give viscosity and the other enhance mouthfeel or flavor release. Sometimes they may simply lower cost.
Starch blends have been used successfully for decades. One of the most popular blends is the common and waxy corn blend used in spoonable salad dressing. Common cornstarch sets to a firm, gel-like paste, while waxy tends to produce a smooth, creamy texture. Varying the ratios of common or waxy will create a firmer or creamier texture. Modified starch blends are often found in reduced-fat products, where they mimic fat characteristics. One starch will provide a high mouth-coating, while another will cling to the tongue, producing a low melt-away sensation. Combinations of starch types are typically found in many bakery goods, dry mixes, soups or sauces. A potato starch may be added for high viscosity and flavor, along with a modified tapioca starch for stability. Another combination common in baked goods may be wheat starch for flavor and texture, with cornstarch providing firmness and body. In restructured foods, such as vegetarian meats and cheeses, three or more starch types will be needed for different functions. Cost often dictates the use of starch combinations.
In most processed foods, however, a starch/gum blend is typically found. Gums are often combined with starches to compensate for starch breakdown, especially under abusive processing conditions or to reduce off-flavors. The most common combination is xanthan gum with a modified starch. The gum provides a clinging texture and stability to the starch’s viscosity and creaminess. Other blend combinations include guar gum, locust bean gum and modified starch in sour cream; or guar gum and pre-gel starches in cakes or bakery mixes. A carrageenan and modified starch blend also is found in dairy products, such as yogurts and puddings. Since carrageenans thin during the heating and shearing process, the overall thinner viscosity during production results in easier processing and less shear to the starch, thereby reducing starch breakdown. Since carrageenan greatly increases final viscosity, a small amount may lower the starch concentration by as much as 25-30%, resulting in less starch off-flavors. Microcrystalline cellulose also reduces starch concentration, as well as improves cling in both hot and cold processes. In reduced-fat applications, the addition of pectin to modified starches enhances mouthfeel.
Gum-on-gum interaction tends to produce synergistic textures. While there are many combinations to choose from, most include xanthan gum. Guar gum is often added to xanthan gum to reduce cost, increase viscosity and improve texture, while xanthan gum prevents syneresis. In low-pH systems, such as pie filling, locust bean and xanthan gum combinations are generally used. Together, they form a soft elastic gel, with xanthan gum acting as a thickener and suspending aid and locust bean gum enhancing mouthfeel. When combined with konjac flour, xanthan gum produces an extremely elastic gel suitable for meat and vegetarian analogs, chewy candy, pie fillings and salad dressings. At low levels in beverages, it provides a clean mouthfeel and flavor release. The addition of cellulose gum to this blend firms the gel and mimics fat mouthfeel. The water-binding capacity of this combination reduces staling in baked goods and controls crystallization in frozen products. When locust bean gum or konjac flour is added to kappa carrageenan, gel strength and moisture binding is increased. At various ratios and levels, texture is modified and improved.