Sorbitol in Yeast Transformations

[Peter PAWELEK]

I'm trying to optimize a lithium acetate-mediated yeast transformation 
protocol in order to obtain a higher efficiency. I've dug up about four
different variations of the protocol in the literature. Some include 
1M sorbitol in the LiAc/TE buffer while others (including one claiming
an efficiency of 10^5 transformants/ug) do not.

Therefore, I have the following questions:

(a)  What is the purpose of including 1M sorbitol in the LiAc/TE buffer?
(b)  Does the inclusion of sorbitol at this stage improve transformation
     efficiency or can it be omitted?

Thanks in advance.

Peter Pawelek