Excipients serve a dual purpose, to both provide bulk to a lyophilized sample and to stabilize biomolecules, particularly proteins. 

The ideal solution will have a component that helps to form a solid "cake" which gives body to the samples once freeze dried.  A second component of a good medium is a lyoprotectant which will help to preserve the structure of biomolecules throughout the lyophilization process.

Excipient Comparison

Disaccharide, inert compound. Traditionally, used to stabilize proteins and other biomolecules.	Sugar alcohol Commonly used pharmaceutical applications to preserve nucleic acids and proteins.	Disaccharide, inert compound. Commonly used in preserving both microorganisms and biomolecules.
When used at concentrations as low as 2% it can effectively protect proteins and other biomolecules. Trehalose fills the void left by exiting water and prevents this denaturation.	When used at concentrations up to 10% w/v, mannitol forms an amorphous (non-crystalline) matrix which supports proteins and other biomolecules for freeze drying.	Sucrose is the traditional lyoprotectant used in many formulations and is less expensive than the alternative, trehalose