Data/hora: 28/05/2024 02:10:55

Biblioteca(s): | Embrapa Agroindústria de Alimentos.
Data corrente: | 05/11/2002
Data da última atualização: | 05/11/2002
Autoria: | CARVALHO, C. W. P.
Afiliação: | Embrapa Agroindustria de Alimentos.
Título: | Effect of sugar on the extrusion of maize and wheat.
Ano de publicação: | 2001
Fonte/Imprenta: | Loughborough: University of Nottingham, 2001.
Páginas: | 175 p.
Idioma: | Inglês
Notas: | Tese (Doutorado) - University of Nottingham.
Conteúdo: | Maize and wheat varied particle size distribution were mixed with sucrose to give sucrose levels of 0, 5, 10, 15 and 20% on dry cereal weight basis, were extruded using a twin screw extruder Clextral BC-21 (Clextral Ltd., Firminy, France). Wheat and maize starch mixtures with gluten (12% on dry starch weight) were studied in order to investigate the effect of gluten on extrudate expansion. Puffed and non puffed extrudates were produced by changing processing conditions (temperature, die type, feed rate and water content). Non-puffed flat extrudates allowed the glass transition temperature (Tg) to be measured by dynamical mechanical thermal analyser. The degree of starch conversion was determined from viscosity of the starch solubilized in alkali, X-ray diffraction , differential scanning calorimetric and Rapid Viscosity Analysis. The expansion of the puffed and non puffed extrudates were quantified by using three expansion indices (sectional, longitudinal and volumetric) and also the ratio of shrinkage and specific mechanical energy (SME) were determined. Replacement of maize by sucrose at constant water content reduced the SME and probably as a consequence, reduced the cross-sectional expansion and the degree of starch conversion. In contrast, replacing wheat flour by sucrose, even at levels as high as 20% of the flour weight, had little effect. Possible causes were less efficient plasticisation of wheat flour by sucrose compared with maize grits at low water contents, a specific role for gluten and the larger particle size of maize grits compared with wheat flour. In non puffed extrudates starch conversion was greater in the fine material compared with the coarse material and greater for wheat compared with maize. Sugar addition decreased starch conversion in all cases, but the effect was greater for maize compared with wheat and for the coarse material compared with the fine material. As expected, the mechanically determined Tg was reduced by sugar addition. Water plasticised the coarse wheat particles less than maize grits, maize flour and wheat flour because the extruded coarse wheat was entirely amorphous, whereas X-ray analysis showed some crystallinity in the other three materials. Die swell was much less for maize grits possibly because elasticity decreased with decreasing starch conversion. The implications for the role of both water and sugar on the behaviour of directly expanded extrudates are discussed. The rheological properties of maize and wheat with similar particle size mixed with sucrose (0, 10 and 20% db) were investigated using a twin bore capillary rheometer RH7 Flowmaster (Bohlin Instruments Limited, Cirencester, England) at varied temperatures 90, 100, 110 and 120 oC and at wall shear rates from 10 to 500 s-1. The extrudates from the capillary die were collected at two distinct wall shear rates, 10 and 500 s-1. Die swelling and starch conversin measured by various techniques (X-ray, DSC and RVA) of these capillary extrudates were determined. The apparent viscosity of the wheat melt was lower than maize. Increasing temperature decreased the apparent viscosity of both cereals. Sugar reduced the apparent viscosity of maize but did not clearly affect the viscosity of the wheat system.
Thesagro: | Açúcar; Extrusão; Milho; Trigo.
Categoria do assunto: | --
Registro original: | Embrapa Agroindústria de Alimentos (CTAA)
Exemplares: | CTAA - (5544-1) - ADD - TS - -- - 0562 -  - 2002.00008 - DIS