Kafirin and zein as coatings for the controlled release of amino acid supplements

Abstract

This experimental work investigated the development and testing of a controlled release system for methionine. Methionine is one of the limiting amino acids for the milk production in dairy cows. The quantities of methionine which reach the small intestine are affected by the bacteria in the rumen which utilize methionine. A controlled release system which will offer a protective barrier for methionine may ensure that the methionine reaches the small intestine in sufficient quantities. The work involved the development of a coating around methionine crystals, which would act as a barrier, protecting it from the rumen conditions. Zein and kafirin proteins from maize and sorghum, respectively, were used as the principal coating components for the controlled release system. Two different approaches were used in the development of the controlled release system. First, the zein and kafirin proteins were tested for their ability to act as barriers for the controlled release of methionine, and second, zein and kafirin microparticles were used as the controlled release agents. Relatively successful, laboratory-scale methods were developed for coating the methionine with the proteins and the microparticles. Protein coatings were made by addition of methionine crystals to acid-dissolved proteins which led to the formation of a protein/methionine matrix. For coating the methionine with microparticles, glacial acetic acid was used to fuse microparticles around the methionine crystals. Dissolution assays were performed to test the release of methionine from the coatings under simulated rumen conditions. Both the zein and kafirin and microparticle coatings exhibited a barrier effect for methionine. The barrier effects of these coatings were influenced by several factors. Increasing the proportion of the coating agents led to improved barrier properties. However, this only occurred until a certain proportion of coating agent was present (50%), after which the barrier properties no longer increased. Heat treatment of the coatings also increased the barrier properties of the coatings. This may be due to the formation of disulphide cross-links being formed during the application of heat. When a simple extrusion method was used to form the coatings, the barrier properties also improved in comparison to those coatings which were not formed using extrusion. When producing the microparticles, it was found that only the laboratory extracted kafirin preparation with 85% (db) protein formed microparticles. It was hypothesized that microparticle formation might be related to the purity of the protein preparations. Scanning electron microscopy of the coatings after the dissolution tests and pepsin digestion revealed pores on the surface of the coating. These were probably where the methionine leached from the coating into the dissolution medium. The protein coatings did act as partial barriers, extending the release of methionine. From the release curves of methionine from the coatings, it could be seen that a sustained release of methionine occurred over a period of time, rather than a controlled release of methionine at a certain time. The aim of the application was thus only partially achieved as a complete protective barrier for methionine was not obtained from the protein coatings. No significant difference between the barrier properties of the coatings prepared from the proteins themselves and the microparticles were found. However, when based on equal protein purity the kafirin protein coatings showed the most effective barrier properties. Further research regarding kafirin coatings as a controlled release agent is recommended based on the results of the above named calculation. This research would entail investigating various coating technologies and methods.