Maintaining Textural Integrity - April 2007
Dehydration from moisture loss is certainly a major concern for product developers, but there are other factors that negatively impact texture over the course of a product’s shelflife, depending on the product in question. Sugar crystallization can adversely affect the texture of dairy foods and confectionery-based products. Foods formulated as emulsions can separate, coated items such as nuts and cereal can lose their crispness and items such as puddings, sauces, soups and gravies can exhibit weeping, or syneresis.
The list of potential textural defects that traverses all product categories is endless. As such, product developers must be aware of all of the complex matrices (i.e., chemical composition, ingredient interactions, atmosphere during storage, processing parameters) of a food’s system, so that they can ensure the delivery of high-quality products with extended shelflife that consumers expect.
Ice Cream—Hold the IceSmoothness is one of the key parameters used in evaluating the texture of ice cream. This attribute is a “hallmark of quality” often judged by the absence of noticeable ice in the product, notes Bruce W. Tharp, Ph.D., Tharp’s Food Technology and adjunct professor of food science at Penn State University. According to Tharp, descriptive terms applied to ice cream’s texture include icy/coarse, sandy and greasy. Sandiness, which may be perceived when lactose crystallizes, is rarely an issue today because ingredients such as stabilizers and sugar alcohols prevent lactose nucleation from occurring. Lactose crystals cannot form if nuclei are not present. A greasy mouthfeel can occur when large agglomerates of fat are present—a result of the mixing and freezing process. Iciness, however, is the most common textural defect associated with frozen desserts such as ice cream.
“Fluctuation in storage and distribution temperatures, also known as ‘heat shock,’ damages the quality of ice cream,” explains Linda Dunning, technical application director for a global ingredients company. “As temperatures increase, the small ice crystals that manufacturers work so hard to create during processing melt. Then, as the temperature decreases, that melted water refreezes, but not as small ice crystals. Instead, the water refreezes onto existing larger ice crystals. As a result, the small, desirable ice crystals are replaced by larger ice crystals that can be detected in the mouth. Initially, the ice cream containing discernable ice crystals will be described as ‘coarse.’ But as the ice crystals continue to grow, the ice cream will become known as ‘icy.’”
A stabilizer system comprised of gums, emulsifiers or often a proprietary mix of both types of ingredients can help achieve a desirable ice cream texture. For instance, locust bean and guar gums work synergistically with one another and are frequently used as viscofiers in ice cream blends. Hydrocolloids can form gel-like structures and control water mobility during freeze/thaw cycling. “Most hydrocolloids and blends of hydrocolloids are used at levels below 0.20% in the finished product,” says Dunning. “While this contributes some viscosity, the majority of viscosity contribution and water immobilization occurs during freeze concentration. Water is removed from the system as ice forms, resulting in a concentration of ingredients into the portion of water that has yet to freeze. As the hydrocolloids concentrate, they have a much greater impact in increasing the viscosity of the remaining solution."
Jennifer Lindsey, dairy industry manager of a global supplier of stabilizers and emulsifiers, provides an example of how one of her company’s patent-pending stabilizer blends delivers both direct and indirect benefits to an ice cream’s finished texture. “This stabilizer blend actually controls the growth of ice crystals over time, which in turn prevents the quality deterioration of ice cream throughout its shelflife. As an added benefit, both internal tests and customer feedback have shown that butterfat may be reduced by 1% to 2% in the overall ice cream formulation while actually maintaining the perception of the original fat level and, in some cases, improving the quality perception of the finished product.”
Non-fat and low-fat frozen desserts are common in today’s health-conscious market, as are no-sugar-added blends. Polydextrose adds creaminess and provides heat-shock stability in frozen desserts such as these. “It is the goal of the developer to find a balance between the ingredients that highlight the desirable characteristics of the finished product, such as heat-shock protection or fat replacement, while minimizing the undesirable characteristics that these formulations might impact, such as off flavors or gumminess,” says Dunning.
Bread for the AgesWhen a customer expressed interest in a shelflife of 18 months for a bakery product, Celeste Sullivan, senior applications specialist for a manufacturer of specialty starches, could hardly conceal her surprise. Baked goods include a wide range of products including breads, bars, muffins and cookies, all of which can be par-baked, fresh or frozen. Temperature cycling causes moisture migration and loss, which in turn decreases shelflife, notes Casey Lopez, associate scientist and colleague of Sullivan. Product development of bakery items has become increasingly challenging, as customers demand longer shelflives for their products.
Instant starches are commonly used for their solubility (i.e., hydration) properties and moisture binding capability, but not all types of instant starch perform equally. In a cake with icing, moisture can migrate to the surface, causing the icing to fall off, notes Tonya Armstrong, senior applications scientist and fellow colleague of Sullivan and Lopez. When the proper starch is used, the cake stays moist and the icing does not crack and fall off.
“Traditional drum-dried instant starches contain fragments of starch, but an instant starch developed with new technology does a better job of binding moisture, because its granules are intact,” explains Sullivan. “Intact-granule instant starches are physically and chemically modified in such a way as to get granule integrity,” adds Lopez. “These starches become fully hydrated during mixing, hold on to water during baking, stay intact during the hold time and withstand the harshness of the baking process. In addition, intact-granule instant starch works well with mold inhibitors and doesn’t overstabilize the system.”
Preventing overstabilization is every bit as important as stabilization is to a product’s overall textural quality, as the former can produce undesirable effects as well. For instance, too high of a level of gums and starches can overstabilize a system, resulting in gummy texture, large cell structure, tunneling and shrinkage, particularly in a chemically leavened product, notes Armstrong.
Fibers, sweeteners and enzymes are also commonly used to preserve the freshness of baked goods, as are wheat protein isolates (WPIs), which can be used to protect a product during freezing. “WPIs are lower molecular weight, film-forming proteins, which allows them to form a protective barrier around a product’s components to prevent ice crystal damage,” says Brook Carson, technical product manager for a manufacturer of a diverse range of food ingredients. The isolates that are derived from wheat protein have thermal properties that differ from other types of wheat protein, which makes them more effective in protecting baked goods during frozen storage.
Enzyme preparations also are being developed to counteract the compounds that interfere with dough formation and elasticity, while other types interact with compounds that cause staling. Traditional enzymes, particularly those derived from bacterial amylase, produced breads that developed gummy textures over time. Newer enzymes are not only helping to extend shelflife, but improving quality in bread volume, appearance and softness as well as dough machinability.
Battling Complex ForcesEmulsions are formed when one immiscible phase is dispersed in another through mechanical action such as homogenization. When an emulsion breaks, the two phases separate. Destabilization of an emulsion can occur in various ways. Creaming is a gravitational separation of phases (i.e., oil phase floats to the surface). During flocculation, clumping occurs without a disruption to the interfacial film, whereas with coalescence, the interfacial film is disrupted as droplets collide and form a separate phase. Flocculation is reversible, while coalescence is not.
The complexity of emulsion stability is compounded by the many factors that influence it. Aside from interfacial tension, emulsion stability is affected by the viscosity of the continuous phase and density differences between phases (e.g., ester gum added to flavor oil in beverages to “weigh it” and prevent ringing). In addition, droplet size in the internal phase (the smaller the size, the more stable the emulsion), temperature extremes and the presence of solids (finely divided solids “wetted” equally well by both phases at the interface will stabilize the emulsion, whereas a higher percentage of solids dispersed in the internal phase will destabilize the emulsion) also affect viscosity.
The reduction of interfacial surface tension is just one possible function of an emulsifier. Other functions associated with this class of ingredients are varied and application specific. A product developer’s choice of an emulsifier or combination of such is often based on factors such as the complexity of the food matrix and other processing-related issues. For instance, “Surfactants stabilize oil-in-water emulsions to give them stability to phase separation during temperature fluctuation over time,” says Tyre Lanier, Ph.D., professor of food science, North Carolina State University.
Starch retrogradation that leads to crystallization is another factor that can cause undesirable textural characteristics in the form of either staling or syneresis. Use of unmodified starches or those that have the incorrect type of chemical modification often will result in dramatic textural changes in frozen/thawed puddings, sauces and even gelled products such as imitation crabmeat. “Proper modification of the starch can add side chains that create steric hindrance to tight packing of starch molecules (i.e., retrogradation) that would lead to the release of water and textural collapse,” explains Lanier.
There are many other examples of foods that are subject to textural degradation and just as many solutions that can potentially mitigate the damage. What the product developer must keep in mind is that processed foods are complex systems composed of ingredients that not only interact with one another, but with the environmental conditions that a product is subject to throughout its shelflife. Processing conditions are also critical to the quality of a product’s textural integrity and must be monitored closely to ensure that optimal conditions are met.
“The texture of a food is usually the result of a somewhat delicate balance of forces between macromolecules such as starch and proteins as they interact with water and as they are modified by salts, sugars and other low molecular weight compounds,” explains Lanier. Liquid foods with noticeable viscosity and solid, or viscoelastic foods (such as gels or baked goods), are particularly sensitive to this balance of forces. Understanding the underlying basis for how the macromolecules of a particular mixture subjected to smaller molecules in a particular process impart texture in the finished product is crucial to product control and shelflife.
Choosing the right balance of functional ingredients may be the key to producing that creamy, high-quality ice cream versus one that is icy; that moist bread versus one that is dry and crumbly; that sauce that does not separate; or that fondant that is not coarse and gritty.