Paste rheology (body)
Relative shear stability
Tendency to gel
"Under ordinary cooking conditions (100°c).
"Under ordinary cooking conditions (100°c).
Solution stability Gels
Solubility Complex formation
Soft, reversible, flowable gels become irreversibly stifFer with time; undergo syneresis
Difficult soluble; high-temperature solubility only
Complexes with iodine, lipids, and various polar organic molecules
Solutions do not gel Soluble
Does not complex
to make strong, tough films. Their solutions and gels undergo rétrogradation rapidly. Waxy maize starches, even when unmodified, gelatinize easily and yield viscous, almost transparent solutions that do not gel.
In general, the properties of a starch paste or gel are a function of the amounts of amylose and amylopectin dispersed or solubilized, the amounts of insoluble swollen granules and granule fragments, and interactions between components. Amylose increases gel strength; amylopectin decreases gel strength and viscosity.
GRANULAR NATURE, COOKING CHARACTERISTICS, AND SOLUTION AND GEL PROPERTIES
All green plants package and store carbohydrate (d-glucose) in the form of discrete particles of starch called granules (1,5). In granule form, starch is insoluble in cold water and only slightly hydrated. The sizes and shapes of granules are specific for the plant of origin and can be identified microscopically (1,6). Diameters (in micrometers) of some commercial starches are in the following ranges: rice, 3 to 8; corn, 5 to 25; tapioca, 5 to 35; potato, 15 to 100.
Occurrence in granule form makes starch unique. Starch granules can be dispersed in water, producing low-viscosity slurries that can be easily mixed and pumped. Starch granules can be reacted while slurried. They can be isolated/recovered, either with or without reaction, by filtration or centrifugation and resuspended for cooking. The thickening power of granular starch is realized only when the slurry is cooked.
Starch granules reversibly absorb water and swell slightly but remain as granules until an aqueous suspension (slurry) is heated. When dry corn starch (normal moisture content 10-12%) is placed in cold water, the moisture content of its granules increases to approximately 30%, and the average granule size of normal yellow dent corn starch increases by ca 9% and of waxy maize starch approximately 23%.
When heated in water, starch granules gelatinize. Ge-latinization is the collapse (disruption) of molecular orders within granules resulting in irreversible changes in properties such as granule swelling, native crystallite melting, loss of birefringence, and leaching of soluble components (primarily amylose) (7). (Some amylose leaching can occur at temperatures below the gelatinization temperature.) The temperature of initial gelatinization and the range over which gelatinization occurs depends on the method used to determine it and is governed by the starch type and concentration and heterogeneities within the granule population under observation (Table 1).
Pasting is the phenomenon following gelatinization when a starch slurry containing excess water is heated (7). It involves further granule swelling, additional leaching of soluble components, and eventually, especially with the application of shear, total disruption of granules, resulting in molecules and aggregates of molecules in dispersion or solution. In most if not all cases, some granule remnants remain.
The cooking behavior of starches and the viscosity of the resulting pastes is most often followed with an instrument with which a starch suspension is heated at a designated rate to a designated temperature while continuously measuring and recording the viscosity. The resulting paste is held at the designated temperature (usually 95°C) for a designated time and then cooled. The resulting curve reveals the pasting temperature, rate of viscosity development, peak viscosity, rate and extent of viscosity breakdown, and rate and extent of viscosity development during paste cooling (Fig. 2).
The property of forming thick pastes or gels is the basis of most starch uses. The extent of starch gelatinization and pasting is the principal factor controlling texture, other product properties such as storageability, and digestibility. In some baked goods, many starch granules remain un-gelatinized (as much as 90% in pie crust and some cookies that are high in fat and low in water content). In the processing and preparation of most other food products containing starch, which usually occur in the presence of excess water, starch granules swell beyond the reversible point. On heating a starch slurry (usually 2-5% starch), granules absorb water until almost all water is within highly swollen granules, forcing the granules to press against one another, filling the container with a highly viscous paste or gel. These highly swollen granules are fragile. Stirring then effects granule rupture and disintegration and a decrease in viscosity.
Starches from different sources have different cooking characteristics (Table 1). Tuber and root (potato and tapioca) starches gelatinize more easily than do cereal starches and produce more viscous pastes that easily and rapidly lose viscosity on application of shear. These pastes are rather clear and slow to gel and have a bland flavor. Ordinary corn starch produces an opaque, cohesive gel that undergoes syneresis and has a slight cereal flavor. The properties of waxy maize starch pastes are in between those of potato starch and corn starch. Rice and wheat starches also produce opaque pastes. All starches can be overcooked to give stringy pastes; they can also be undercooked.
The viscosity obtained by cooking a suspension of starch is determined by starch type, type and amount of modification, solids concentration, pH, amount of agitation during heating, rate of heating, maximum temperature reached, time held at that temperature, agitation during holding, and the presence of other ingredients. High sugar concentrations decrease the rate of gelatinization and lower the peak viscosity and gel strength. Emulsifiers such as monoacylglycerols and lipids that complex with amylose inhibit granule swelling, increase both the gelatinization temperature and the temperature needed to produce maximum viscosity, and decrease both the temperature needed for gel formation and gel strength.
Starches can be cooked and the paste dried in a way that destroys granular structure and allows the product to be redispersed (dissolved) in water at temperatures below the gelatinization temperature. Various types of these pregelatinized (instant) starches that do not require cooking to achieve thickening are produced. Some will produce smooth dispersions; others produce pulpy or grainy dispersions and find use in fhiit drinks and tomato products. Pregelatinized starches are often used in dry mixes because they disperse readily, even when mixed with other ingredients such as sugar.
By heating common corn starch in media containing a limited amount of water, granular products that swell extensively in cold water can be made. Cold water-swellable starches can be used in instant products of various kinds. When they are dispersed in sugar syrups with rapid stirring and the mixture is poured into molds, a rigid gel that can be sliced is formed.
Starch molecules in an unordered state (in solution, in a dispersion, or in gelatinized granules) will undergo a process called rétrogradation. Rétrogradation (setback) is the reassociation of starch polymer molecules. It occurs when molecules that have become disordered during cooking begin to reassociate in an ordered structure (8). In the initial phases of rétrogradation, linear segments of two or more starch chains may form a simple juncture point that may develop into more extensively ordered regions. Ultimately, under favorable conditions, a crystalline order appears. The result is gelation or precipitation. Generally, extensive rétrogradation is undesirable. Various approaches are used to retard rétrogradation; one is the complexation described earlier (see the section "Molecular Components").
Derivatization to produce stabilized starch is also used to reduce the tendency for rétrogradation (see the section "Stabilized Starch").
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