The Secret to Success
April 1, 2006
by Donna Berry
Starch is an important food constituent, which for classification purposes falls under the category of carbohydrate. So, it is paramount that formulators and bakers understand more about it.
For starters, starch is the major component of wheat flour and accounts for about 70% of its weight. Even though starch is the majority component of what is usually the primary ingredient in grain-based foods, many manufacturers will add one or more starch ingredients to a formula for extra functionality. Working closely with flour and starch suppliers ensures the highest-quality, most economically made baked foods.
MORE DIFFERENCES THAN SIMILARITIES
Indeed, starches and flours can be very different. Starches are primarily derived from corn, wheat, tapioca and potato plants, but other sources including rice and arrowroot are also available in food-grade form. Starch is produced in these plants through the photosynthesis of sugar. Plant source, along with any physical disruption of the starch granule during milling and any further modification of the starch, generates a variety of properties that contribute to functionality in specific applications.
For example, native starches will thicken and gel when heated, thus building system viscosity and binding water. Modified starches, on the other hand, can be designed as “thinned” starches with functionalities involving setting, solubility, adhesion, gel strengthening and film forming. They also have low viscosity when heated.
About the only thing that starches have in common before being processed into ingredients is their general chemistry. All starches consist of large molecules composed of chains of glucose units linked together to form one of two polymers. Amylose is the mostly straight-chain polymer with long chains of glucose units joined by alpha 1,4 linkages. Amylopectin, the branchedchain molecule, consists of shorter chains of glucose monomers linked by some alpha 1,4 linkages and many alpha 1,6 branch points. The proportion of these two polymers in any given starch granule depends on the plant of origin, which also influences the number of glucose units.
Interestingly, while they exist together in starch granules, amylose and amylopectin are inherently incompatible molecules. Amylose has a lower molecular weight with a relatively extended shape, and amylopectin is much larger but also more compact. Amylopectin typically consists of branched chains of 20 to 30 glucose units; each molecule can contain as many as 2 million glucose units. On the other hand, amylose chains vary in length, starting with about 200 glucose units to more than 20,000.
Amylose and amylopectin molecules are packed tightly into the solid starch granules where they strongly associate via the many possible hydrogen bonds along the glucose units. In some regions of the granule, the molecules are so regularly ordered that they form a strong, crystalline structure. However, in other regions, the molecules are more random and have an amorphous, readily disrupted structure.
The strongly bound crystalline areas of ordinary starch granules render them insoluble in cold water. They will form an opaque suspension, which upon heating slowly becomes translucent as the granules swell and the suspension transforms into a viscous solution. This process is referred to as gelatinization. In general, the straight amylose molecules collide with each other and lock firmly in place. They then connect into a rigid network throughout the liquid, and the starch paste settles into a gel. The branched amylopectin molecules have little tendency to lock together and, thus, do not produce a gel.
There’s an exception to every generalization including starch-gel forming. For example, ordinary potato starch is made up of very long amylose chains. However, these amylose chains do not readily lock and, thus, do not form a gel. Instead they make a viscous starch paste that remains a paste when cooled. Because starch gelatinization is critical in bread making (swollen starch granules gel between gluten strands and provide bread with structure), ordinary potato starch should be avoided in such applications.
Interestingly, as important as starch gelatinization is to making quality bread, it is precisely the gel that causes the bread to deteriorate, to go stale. Upon cooling, the gelatinized amylose molecules begin to recrystalize and network to form an ordered structure. They shrink as some of the liquid separates from the gel. This change is referred to as retrogradation and is the complete opposite of gelatinization. Over time, retrogradation results in crumb firmness, which is also described as staling. Retrogradation occurs very slowly, if at all, in waxy starches because the amylose and amylose-like fractions are minute and very dispersed.
Besides tapioca, which can have lower amounts, most native starches are about 20 to 30% amylose, with the rest amylopectin. Plant-breeding techniques have allowed development of starches with varying ratios of amylose to amylopectin. For example, the term “waxy” describes starch that is almost completely amylopectin. This takes advantage of amylopectin’s unique functionality because waxy starches form thick, clear pastes but gel only at very high concentrations, such as 30%. On the other hand, standard cornstarch, at 25% amylose, forms a gel at a level of 4 to 5%.
High-amylose starches, which contain 50 to 70% amylose, have a variety of unique properties. They are able to form films, bind ingredients, act as a barrier against oxidation and form quick-setting, stable gels. High-amylose starches impart a crispy crunch to baked snacks and cereals; however, so does standard cornstarch. Cornstarch is a very economical ingredient for snack products because it can withstand high levels of heat and stress and it has strong adhesive properties, which keep topical seasonings on the snack piece.
When it comes to softer baked foods such as breads and rolls, high-amylose starches are typically avoided.
THE WAXY APPROACH
Researchers at the U.S. Department of Agriculture’s Agricultural Research Service (ARS), Fargo, ND, bred a new kind of durum wheat — a waxy durum wheat (WDW). The researchers found that WDW flour allows a baker to cut shortening out of the recipe without losing the desired properties shortening confers to bread. This fat-replacing function is credited to WDW being nearly 100% amylopectin.
The research suggests that WDW flour works best as a shortening substitute when it comprises 20% of a dough formulation. In trials, loaves of the experimental bread had the same softness, texture and volume as those containing 100% bread wheat flour and 3.25 g of shortening. And in tests for freshness, the WDW bread stayed much softer than the non-waxy wheat bread after five days. Results from this study suggest that blending waxy flour with bread flour in a bread formulation can make low-fat bread with excellent softness and improved shelflife. A single bread loaf might have two tablespoons of shortening, so replacing that with WDW flour saves about 26 g of fat, or 234 Cal.
In the baking industry, waxy starches offer some functional advantages to specific applications. For example, waxy maize starch has good expansion properties. It is well suited for puffed cereals, baked chips and cakes. In filling applications where boil-out and heat stability are issues, waxy maize starch is recommended because it is thicker than other starches, which helps reduce boil-out. Using a modified waxy maize starch in snacks yields a crunchy texture and eliminates the risk of starch breakdown during processing. And waxy rice starch performs very well in baked foods that are frozen or face risk of starch retrogradation.
All starch ingredients start out as native or unmodified. Such starches are not chemically altered and can be labeled, for example, as simply cornstarch, rice starch and wheat starch. These starches are typically used in baked products that require little shear, low baking temperatures or short bake times.
In order to withstand modern processing and storage conditions, native starches can be chemically modified to alter their properties. Modification can stabilize a starch, which prepares it for temperature or shearing extremes. Modification is usually done to develop characteristics for specialized purposes by mild degradation, cross-linking of chains, derivatizing with phosphate or other esters, or pre-gelatinization of the starch.
High-moisture baked products can benefit from modified starches because moisture migration can become a serious quality and shelflife problem. Modified starches are used to provide sheen to fillings, bind moisture and deliver a short paste texture.
If ease of dispersement is desired, suppliers can instantize both native and modified starches. Instantizing makes it easier to disperse the ingredient in bakery mixes including cake, cookie, muffin and quick bread. Instant starches also work well in commercial production when the formula combines most ingredients upfront rather than going through a preliminary dough development stage. Because the instant starch goes directly into the complete system, shear and breakdown are avoided.
Not to be confused with instant starch, pre-gelatinized starch, also sometimes called pre-gelled or cold-water swelling, is processed to swell in cold water; instant starch typically requires heat. Pregelatinized starches are made by creating a starch paste that is then heated to its gelatinization temperature. Once gelled, the mixture is dried on a drum dryer and ground into a powder. When dispersed in cold water, pre-gelatinized starches gel quickly, but the solution tends to be less viscous than gels from non pre-gelatinized starch.
Like instant starches, pre-gels work very well in 1- or 2-step bakery mix products. For example, a pre-gelatinized modified wheat starch quickly thickens cake and muffin batter. It increases viscosity and keeps inclusions such as chocolate chips and raisins in suspension during baking. After baking, the starch contributes moistness and tenderness to the finished product.
Another pre-gel that combines native and modified wheat starch can provide instant thickening with exceptional mouthfeel and texture. It improves moisture retention and provides freeze-thaw stability to bakery products through refrigerated and frozen storage conditions.
With such high-tech starches available, it’s a wonder that anyone still uses old-fashioned, cook-up starches; however, many applications still work best using such native starches. Cookup starches require heat to reach their full functional temperature. They are often used in products that will reach 180°F for at least 10 minutes. In fresh and frozen applications, cook-up starches add viscosity ; are resistant to acid, heat and shear; increase overall stability; provide clarity and remain stable in cold storage. It is this durability, and often economics, that makes native cook-up starches attractive.
For example, a native cook-up wheat starch that possesses a high water-holding capacity also happens to be an excellent fat emulsion stabilizer. With its brilliant whiteness and bland flavor profile, the starch provides functionality in a wide array of applications including cakes, icings, glazes, cookies and pie crusts.
There’s one last general category of starch that must be addressed. These starches are typically included in a product formulation more for their behavior as fiber than as a thickener. Called resistant starches because they resist digestion in the small intestine and get fermented in the large intestine, these starches do not have the typical off flavor or negative effect on baking as more traditional sources of fiber including bran, psyllium and whole grains. In fact, resistant starches have been described as invisible fiber when added to a product as simple as white bread.
With so many flour and starch options available to bakers, it’s imperative to work with suppliers to choose the ingredients with the right ratio of amylose to amylopectin and the proper physical and chemical processing. The right starch is the secret to your product’s success.