Experimental cookery

viscid suspension is formed. If left standing, the starch granules settle to the bottom of the container and the supernatant liquid appears clear. If the starch suspension is heated a change takes place. It becomes thick, forming a paste which is opaque or transparent. The viscosity of the warm paste is far greater than that of the water suspension, but on cooling it loses some of its fluidity, becoming less plastic and more viscid, firm, or stiff. Alsberg has suggested that the reason for the loss of plasticity of the cool paste is that it regains some of its former rigidity at lower temperatures and that the surface of the granules may be more sticky at the lower tempera- ture, thus preventing one granule from moving or flowing so easily past another. It has also been suggested that some of the starch is less soluble at the lower temperature. When cornstarch is heated with water the resulting paste is clear or transparent. When flour is used the paste is opaque. It is for this reason that a clearer color is obtained when cornstarch is used with colored fruit juices than when flour is used. Samec ascribes the opaque color with wheat starch pastes to the combination of the wheat starch with phosphorus and protein. In another reference he says, "Pastes from wheat starches are white, cloudy, and a little slimy. The starches of cereals do not contain as large quantities of phosphorus, but contain fatty acids. Pastes from starch of such starch are 'pasty' and not 'pectinous.' " Elsewhere it has been stated that combination of starch with phosphorus increases, whereas fatty acids decrease, the hydration capacity, which may have some effect on the translucency of the paste. Granule injury. If starch granules are injured or ruptured in milling or in their separation from other material, some soluble starch is found dis- persed in cold solutions; and with increasing proportions of soluble starch the unheated solution becomes more viscid, the heated solution correspond- ingly less stiff or viscous. Long grinding appears to break some of the starch chains into shorter units. Gelatinization. The change upon heating a cold water starch sus- pension, so that a transparent or opaque, plastic, viscid product that forms a gel results, is known as gelatinization. Gelatinization depends upon the starch's being heated to a definite temperature, but the temperature at which gelatinization starts depends upon ( 1 ) the length of time the granule is wet before it is heated, (2) the kind of starch, (3) the size of the granule, and (4) possibly the salts in the water or combined with the phosphoric acid of the starch. During gelatinization some starches tend to disperse, that is the physical units become smaller or the granule may disintegrate. With other starches most of the granule may remain intact but may assume a typical hydrated shape. Thus the photomicrographs of Woodruff and of Sjostrom indicate that wheat starch when hydrated tends to become kidney-shaped. Gelatinization occurs and extends over a wide range of temperature. Swelling may start and the paste becomes more viscid at a fairly low tem- 408 FLOUR AND BREAD perature, but complete swelling and maximum stiffening ability may not occur until the starch is heated to a much higher temperature.