From a legal perspective technically an offence of theft was committed 'Theft by consumption' albeit to a very minor level which i personally wouldn't expect warranted such a response from store or management. I currently work within a well known high street chemist store as a retail security officer and on a weekly basis probably see 5-10 people 'test' stock. majority i will just ignore however some have to be stopped and advised as ultimately breaking through a seal or use of the product renders product no longer sellable such as in this instance as the item has been opened and is a food item. (normally baby food jars and cosmetics) that being said i think the store have gone massively OTT. as for DPA you can ask for their name although can refuse to give full name and just say first name and position or surname and position.