I need to start putting lock on food places like refrigerator and pantry; what should I say?

There are locks that are magnetic that you can use for the cabinets. You do not see them so she would not even know they were there you can just say the cabinet is stuck and you have called someone to fix it but it will be a while.
For the fridge there are locks that are an elongated "U" shape and if you are lucky enough to have a white fridge she might not even notice it is there. Again say the door is stuck.
If she is furious about the locks that will pass as it becomes part of her every day life.
Leave baskets of "healthy" items out apples, oranges, bananas things like that maybe 1 or 2 granola bars, individual packs crackers. This will limit what she can take at any given time.
Is there a possibility of putting a small dorm type fridge that can be hers while the other fridge is getting the door fixed? Keep water maybe a juice, a pudding cup and a yogurt in the little one she can take those to satisfy her snacking. My Husband was a snacker also and I left out fruit and 2 packages of one of his favorite cookies and he was happy when he "found" his snacks.

Is she eating nonfood substances?  Otherwise, what is your specific concern?

Could you do the dementia thing about saying that someone is stealing food, and you have been advised to put locks in place to stop the theft?

If you need to do this than do it. You can explain but she probably won't remember why.

Yes, your feelings are normal. I too didn't like what being a caregiver made me feel I didn't like me. I had to harden myself a little so I could be the parent instead of the child. My patience was nill. I really didn't think I would be dealing with the hands on as long as I did. I was 65. My kids had been out of the house for a while. It was just me and hubby. Doing what we wanted when we wanted. I retired at 62 a year later babysitting infant Grandson. He was approaching 2 so ready for Daycare. Then Mom needed 24/7 care, In a couple of months I would have gotten my life back. Now I had another 2 yr old. Mom needed everything done for her and I was it. Brothers not nearby. Eventually, I was able to place her into an AL. Later LTC. Such a lift off my shoulders.

Do you? Really?

Of course I don't know if something like this would be practical; but just off the top of my head, as an example, one alternative might be to limit what you keep in the existing places, and set up another fridge + storage unit that she doesn't know about in the garage.

The problem is your mother won't only be furious because she's being locked out of her own storage. She will also be daily frustrated and agitated because she won't remember about the locks or what they're for. I honestly would avoid placing this obstacle in her living environment unless you absolutely have to.

You can try telling her the fridge and pantry are "broken".. let me try to get them fixed...

71? That's young to have such a level of dementia. My FIL had early onset in his mid 60s so I do know it's not unheard of.

How are you doing with being the caregiver? It surely can be challenging.

If you need to keep your mom from accessing areas of the home for her safety, just do it. Make up a theraputic lie - "I don't know why it's locked, mom". I can't open it either. Weird. Etc. It's sad when it comes to this, but here you are.