Large-scale fed-batch culture of mammalian cells, especially Chinese Hamster Ovary (CHO) cells, is widely used to produce proteins used in a variety of applications, such as diagnostic, therapeutic, and research uses.  Particular attention has focused on CHO cells because they have been extensively characterized and have been approved for use in clinical manufacturing by regulatory agencies.  Such cultures are typically maintained for days, or even weeks, while the cells produce the desired protein(s).  During this time the culture can be supplemented with a feed medium containing components, such as nutrients and amino acids, which are consumed during the course of the culture.  Such feeding has been shown to improve protein production by a mammalian cell culture.  See e.g., U.S. Pat.  No.  5,672,502.  Even incremental improvements in protein production can be valuable, given the expense and difficulty of building and obtaining regulatory approval for large-scale, commercial culture facilities. 
Concentrated feed media are often used in fed batch culture processes to improve protein titer, cell growth, and/or cell viability.  Some components present at high concentration in feed media may precipitate during storage, especially when the pH of the medium is near neutrality.  Precipitation of medium components during storage prior to use of a medium is very undesirable because it adds an element of uncertainty.  When medium components precipitate, the concentration of medium components in solution, versus in the precipitate, will be unknown.  Since concentrations of various medium components can affect the quantity and quality of a protein produced by a culture, this is an element of uncertainty that is highly undesirable in a commercial culture process, in which culture conditions are, optimally, carefully controlled.  Moreover, commercial processes may be subjected to stringent regulatory review.  Thus, feed media with high concentrations of amino acids that can be stored for a period of time without precipitating would provide significant advantages.