Starches

Starch is the main carbohydrate found in grains (wheat, corn, rice) and tubers (potatoes and tapioca). In photosynthesis, glucose is produced from the chlorophyll in the leaves and stalks of all green plants. In the kernel or tuber of the plant, the excess glucose is transformed into starch, to be stored as the source of energy for the new plant. Since scientists haven’t discovered how to duplicate this process, we obtain our starch by breaking down these kernels to release the starch inside.

Starch thickens dairy products and desserts. It contributes viscosity to soups, sauces and gravies. It imparts the gel structure to wine gums and puddings. Starch is the binder for processed meats and extruded snacks. It keeps baked goods moist longer. It adheres the coating to frozen fish, meat schnitzel and vegetables.

Most of the properties that make starch so useful to food manufacturers only develop when the starch is cooked (gelatinized). Native starches are used in powder form to absorb moisture and as fillers and bulking agents. When cooked, they serve as thickeners and texturizers, stabilizing, adding viscosity, and binding the ingredients in processed foods.

Modifying starch enhances or represses its inherent properties to make it appropriate for specific applications: to provide thickening, improve binding, increase stability, to improve mouth feel and sheen, to gel, disperse or cloud.

The ingredients present during the cooking cycle have very definite effects on the swelling characteristics of the starch and the finished viscosity of the paste. Acids disrupt hydrogen bonding to bring about more rapid swelling of the granule. Soluble solids such as sugars interfere by tying up the water necessary for hydration. Fats and proteins tend to coat starch which delays granule hydration and lowers the rate of viscosity development.

The pH of the medium is very important in determining the proper starch selection. An unmodified starch will peak earlier and break down faster at a pH of 2.5 than it will at a pH of 4.0. We cross-link to control texture and to provide heat, acid and shear tolerance. As a result, we have better control and improved flexibility in dealing with formulation, processing and product shelf-life. Cooked pastes are more viscous and heavy bodied and are less likely to breakdown with extended cooking times, increased acid or severe agitation.
Another important starch modification is that of stabilization. This modification prevents gelling and weeping while maintaining textural appearance. The linear fraction of cooked corn starch will re-associate through hydrogen bonding causing gelling, opacity and weeping. To prevent this condition, anionic groups are scattered throughout the granule to block molecular association through ionic repulsion as well as steric hindrance. The result of this treatment is a stabilized starch which produces pastes that withstand several freeze-thaw cycles before syneresis (weeping) occurs. Freeze-thaw stabilized starches are essential to the frozen food industry and for cold temperature storage of other processed foods, such as canned sauce, gravies and dairy desserts.
The impact of processing equipment on the starch granule cannot be overemphasized. Shearing forces exerted by high speed mixing, milling, and homogenization or pumping can damage the starch granule. Cross-linking the starch can build in tolerance to shear as well as to temperature and acid. This is an absolute requirement for salad dressing starches which are cooked at low pH, at high temperatures and are also subjected to colloid milling. Pudding starches subjected to flash cooling are another example of the need for shear tolerance.