Contribution to the study of the effect of drying on the nutritive value of corn grain (Zea mays L.) for poultry and pigs feeding

Corn grain (Zea mays L.) is widely produced all over the world and is intensively used in poultry and pigs feeding. Drying with hot air is usually utilized to preserve corn after harvest by decreasing the water content. During high temperature drying, major components (starch and proteins) of corn grain undergo some structural changes with possible nutritional implication. This thesis aimed at increasing knowledge regarding the effect of the drying process on the feeding value of the corn grain for broilers and pigs feeding.
After the general introduction and a review which attempts to summarize the major findings of previous studies carried out on the effects of drying on the nutritive value of corn for broilers feeding, the first in vitro experiment performed aimed to assess the effect of fluidized-bed drying on the digestibility of dry matter, starch and protein using a sequential pepsin-pancreatin protocol. It appeared that when corn grains harvested at similar moisture content are dried at different temperatures, the digestibility of starch and that of crude proteins are not affected in a similar way. Higher drying temperatures increased the final digestibility of starch while the digestibility of crude proteins decreased mainly during the pepsin step of digestion. Differences in the kinetics of starch digestion according to the corn grain temperature were attributed to the structural modifications induced within starch granules and proteins in response to the drying conditions.
As the digestion of starch involves the breakdown of glycosidic bonds and results in the production of various oligosaccharides, a second experiment was performed in order to get more insight into the nature of saccharides released after the in vitro digestion of corn grains. Three methods were thus compared in their ability to provide relevant information about sugar saccharides released during an in vitro digestion procedure that mimics the pig digestive model. High Performance Anion Exchange Chromatography coupled with a Pulsed Amperometric Detector was revealed to give the most effective qualitative and quantitative information regarding the digestion of starch. Five major saccharides were found as the result of in vitro pepsin-pancreatin digestion of corn grain: glucose, isomaltose, maltose, maltotriose and glucosyl-maltotriose. The relative concentration of these saccharides varied according to the temperature applied during the corn drying, suggesting that structural changes occurring within starch granules (partial gelatinization) would lead to different digestion profiles in the small intestine and may therefore affect the nutritional physiology in pig intestinal tract.
Protein is also affected by the high drying temperature. Among corn protein subgroups, salt-soluble proteins are the most sensitive to heat and their solubility decreases when the drying temperature increases. In a third experiment, the denaturation of salt-soluble proteins during corn drying and hydrothermal treatment was monitored and modelled. The impairment of corn protein solubility in saline buffer was revealed to depend mainly on temperature, time of processing and moisture content of materials treated. The evolution of extractible salt-soluble proteins content of corn kernels during drying at high temperature was more correctly described using the second order kinetic reaction than with the first order kinetic reaction when classical kinetic models were compared.
In a forth study the relationship between the in vitro digestibility of dry matter and the salt-soluble protein index was assessed. It was found that the use of salt-soluble protein index as an indicator of the may lead to doubtful conclusion due to confounding factors (temperature and moisture content).
Although this study has achieved some progress in understanding the possible link between corn drying parameters and their effects on the performance of poultry and pigs feeding using corn-based diets, as well as on methodologies that may be used to assess the nutritive value of dried corn grains, complementary studies aiming to determine the effectiveness of these effects on livestock and pigs have to be done.