Theft of milk and food probably ranks as the top students' complaint. In my ebook about social life in university, "Going Up: a Social Troubleshooter", I outline various strategies to deal with this. The main point is - DO NOT tamper with your edibles in order to catch the thief out, you could find yourself liable for any injuries, allergic reactions or ill-health caused. Here's a short exerpt:
"Go to your tutor and present them with a diary of thefts, which you should by this stage have started keeping. Tell your tutor this is important because of the financial situation students have to face, and explain the effect the stress is having on the quality of your studies and assignments.
"NB: on the web you will find a range of solutions to this conundrum that range from the surreal to the obscene. If you feel sufficiently stressed-out by the situation to choose one of them, ask yourself: how far are you willing to go? If you make your stuff appear unpalatable with an additive (eg food dye in milk), are you sure your kitchenmates don’t have a serious allergy to the contents? What would your personal and indeed legal position be if an allergic reaction were to require medical attention?"
Going Up - Starting university in England: a social troubleshooter by Kriss Day - currently only 99p on Kindle! ($0.99 equivalent plus VAT) A comprehensive troubleshooter for the social side at university in England. Also for sale at Smashwords, $1 equivalent plus VAT.