microbial research reusable cup hygiene

Hi all,

I am currently doing research on the hygiene of a reusable cup.
In this research, I am contaminating the cup with coffee+milk+sugar and chocolate milk.
Then the cups will be tested under different variables like different storage times, 1 time used versus multiple time used cups and also comparisons between conventional alternatives will be made.
after contamination, the cups will be washed in a professional dishwasher and examined organoleptically. 
Then the cups will be swabbed and the swabs sent to an external certified microbiological lab. They will test the cups for :
-CFU/cm2
-enumeration of Enterobacteriaceae

My questions are :
- how much cups should I test per variable to get a statistically reliable result?  (i have spoken to labs and they say n=5 should be sufficient but this is normally done for process controls so periodical testing, and not for self-contained researches like this). 
-would you suggest testing for CFU/cm2 and Enterobacteriaceae is sufficient to draw conclusions about the hygiene?
- Could chocolate milk be considered a worst case scenario? ( considering its neutral PH high nutritional value and stickyness?)

Thanks in advance!
 

Edited April 5, 20196 yr by LCA ZWC
forgot to mention something