This underlines the need to take on new resposibilities on a three month trial basis! Sorry to be wise after the event. What you need to do now is talk to your line manager.
You need to point out how much of your time is now taken up with this new duty - keep a diary for a few weeks & log the increase of volume here too. Plus how this affects your other duties.
Find out what happens when you are not available for this duty. Maybe there could be a time limit on the hours during which you can be expected to perform these duties.
Suggest an investment in labour saving devices as the volume has increased so markedly. For example an 'instant' kettle that dispenses hot water in 6 seconds, or a water boiler for large volumes. A large filter coffee machine that you just replenish a few times a day. etc.
Ask how much more volume would you be expected to handle before some other help or arrangement would be considered. Is there another member of staff that could share these duties - one in the morning, then you in the afternoon?
And don't forget to include what you expected your job to entail when you first started there. Plus what your expectation of your 'prospects for advancement' of your 'career' was then & ask what they may be now & how they have changed? Don't be frightened to express your disappointment at how the last change has turned out for you.
Do not use accusing language (making them wrong will just put their back up & label you as a moaner). Ask for His/her help, stick to your perceptions, reasonable expectations & feelings. Check out what you are going to say with someone outside your work place first, preferably more experienced than you. & the ultimate sanction, check out some other jobs/places to work.