Zein, a protein extracted from maize, can be employed to easily produce nanoscale particles suitable for use as carrier systems. This review investigates the main methods for obtaining zein nanoparticles, as well as the problems and options available in the development of stable colloidal suspensions. Considerable gaps were identified in the literature concerning this topic, with studies being unclear about the factors that affect the stability of zein particles. In the vast majority of cases, no data are presented in relation to the stability of the formulations over time. It could be concluded that in order to produce a high quality system, detailed evaluation is required, considering factors including the zein concentration, pH, ionic strength, thermal treatment of the protein prior to preparation of the nanoparticles, strategies employing other materials as coatings, and the storage conditions. It is extremely important that these aspects should be considered during product development, prior to commercial-scale manufacture.

Introduction
Zein is the main protein present in maize, accounting for around 50% of the total protein content. It belongs to the prolamin class and is composed of lipophilic amino acid residues. The α-zein form accounts for over 70% of the total zein protein and is the type that is commercially available. It is not used for direct human consumption, due to its negative nitrogen balance and low solubility in water. However, it can be easily converted to spherical colloidal nanoparticles. Due to its high coating capacity, biodegradability, and biocompatibility, zein has been used in modified release systems for the delivery of enzymes, drugs, and essential oils, among other substances. The purpose of the present work is to provide an overview of the main methods of preparation of zein nanoparticles, as well as the main problems related to the temporal stability of these systems. Possible options for increasing the colloidal stability of zein nanoparticles are presented, together with future perspectives for the development of these carrier systems.

Colloidal Stability of Zein Nanoparticles
Despite the effectiveness of the methods used to prepare zein nanoparticles, considerable challenges remain concerning the temporal chemical stability of these systems under different storage conditions. Chen and Zhong studied dispersions of zein nanoparticles and concluded that they presented poor colloidal stability, readily forming aggregates and precipitates in the formulations, hence losing their functionality. At higher pH, formulations of zein nanoparticles have been found to exhibit aggregation and precipitation, due to the fact that in solutions with pH above 5, zein is close to its isoelectric point (pH 6.2).
In the case of ionic strength and pH, these nanoparticles have been shown to be highly liable to aggregation at a low concentration of sodium chloride (NaCl), and to be unstable at pH above 5. The salt added to formulations increases the ionic strength, with consequent increases in van der Waals interactions and hydrophobic effects among the protein chains, favoring aggregation and precipitation of the proteins. Similar findings have been reported in other studies.