Variation in nutrient composition and structure of high-moisture maize dried at different temperatures

Abstract

Maize cobs with grains were harvested at a relatively high moisture content (23%) from the field in northern New South Wales, Australia. The cobs were divided into four categories and dried in the sun or artificially in a forced draft oven at 80, 90 or 100°C for 24 h. The samples were subjected to proximate and detailed nutrient analyses. 'In vitro' nutrient digestibility and ultra structure of samples were also assessed. Proximate analysis of maize revealed that dry matter (DM, 980 g/kg) and ash (1.32 g/kg) content were highest in the 100°C samples, but crude protein (98.4 g/kg), ether extract (45.0 g/kg) and phytate-P (1.8 g/kg) content were the highest in the sundried samples. Gross energy was little affected by heat treatment but the metabolizable energy value increased with rising temperature. The concentrations of most of the amino acids but not lysine were increased in samples dried at 80, 90 and 100°C in comparison to sun drying. Total starch, resistant starch (RS) and amylose content were slightly increased by artificial drying while amylopectin was reduced under the same conditions. Starch content (691 g/kg) was highest at 80°C while RS (363 g/kg) and amylose (304 g/k) were lowest in the same batch. The mineral concentration of samples decreased with increasing temperature except copper, which was slightly increased. The soluble non-starch polysaccharide (NSP) content was increased while the insoluble NSP decreased with increasing temperature. The morphological structure of maize observed under a scanning electron microscope showed some shrinkage of starch granules as a consequence of artificial drying temperature. 'In vitro' digestibility of DM was improved as a result of artificial drying of high moisture maize but starch digestibility was reduced. It may be inferred that the nutritive value of maize grain varies with drying temperature and drying process. These differences may explain changes in nutritive value of the grain when fed to chickens.