To prevent (bio)chemical changes to happen in any sample the lower the temperature the better, as chemical reactions slow down with decreasing temperatures.

So it depends on which analyses you are planning to do. - 180C will probably not be necessary. Most common storage temperatures for biological samples are -80 and -20 C. It all depends on how stable the substances that you want to analyse are. 

Thank you very much for your help. Peter

For our research milk samples were frozen in plastic containers and stored at - 20C until analysis. However, it depends on what type of compounds you want to analyse.