STRUCTURAL CHARACTERISTICS OF STEAM-FLAKED SORGHUM

Sorghum undergoes structural changes during tempering, steam cooking, and flaking at various tempering moisture levels. Physical properties of flakes, digestibility, birefringence, scanning electron microscopy (SEM), and environmental SEM (ESEM) were used to evaluate the quality of steam-flaked sorghum from grain containing 11-23% water. As moistur e levels increased, the flakes became stronger (57-69% whole flakes) a nd less dusty (9-4% fines). The diameter of the flakes varied among mo isture levels, but preflake and final flake moisture contents increase d as the temper level increased. Starch birefringence remained unchang ed after tempering, and decreased only slightly after steam cooking; g elatinization occurred primarily during the flaking process. The steam ing process prepared the grain for flaking by heating and softening th e kernels. Tempering allowed extra water to penetrate inside the kerne l endosperm. More starch granules had the opportunity to reach the gla ss transition temperature (T-g) during flaking. Based on subjective ev aluation, birefringence, and SEM, poor quality flakes were opaque, cha lky in appearance, and prone to high levels of breakage. The starch gr anules were more intact and less tightly packed into the flake. Good q ualify flakes were translucent, thin, and strong, with little chalkine ss, and low levels of dust and fines. The dehydrated gelatinized starc h continuous phase surrounding the granules reduced the amount of air spaces within the flake, increasing the translucency. ESEM inspection revealed that starch was more extensively gelatinized in the tempered samples. The starch granules were much larger in diameter, and the rel ative level of gelatinization, evident by the presence of starch granu les with collapsed centers, was much higher.