Any ideas on how to close off the kitchen from mother-in-law (81 with Alzheimer's) and father-in-law (82 with dementia)?

Wow this is a very challenging problem. It is hard to know what to suggest about "blocking off the area" without knowing if this also blocks the exit from the house or other important rooms, like bathroom.

If you do add a door: I remember the child-proof doorknob cases from when my kids were young...they spin without actually turning the knob unless one has the smarts and hand-strength to squeeze it tight enough to make it turn the knob.

Is there a way to keep your FIL more segregated in his own area of the home? What would happen if that area was locked from the outside so he couldn't wander into your area until you were ready for him?

Does he eat all night long as well? This is most likely a phase, as my aunt went through it as well (in terms of just eating constantly, although not a fridge raider or kitchen creeper). Is anyone in your home able to distract him or take him out for a ride while you do the meal prep?

Let us know if you get a winning suggestion and tell us how it goes. Wishing you success in keeping out your kitchen creeper!

If you can use a gate why not a Dutch door. From what I see here, you could have locks put on.

https://www.nicksbuilding.com/dutch_doors.php?gclid=CjwKCAiAmrOBBhA0EiwArn3mfIHLtTXBuBHKg2_CT_lTMErTfNzt0BuPWKdh56DSpKYq70KKuwp6HhoCOXEQAvD_BwE

This is a new one!

Do you folks live 'with you' meaning they have free reign to the whole house or do they have an apartment within your home?

IF they are somewhat separated, just buy a small fridge for their space and fill it with the stuff dad likes to snack on. Stock the family fridge with things right up front that he WON'T snack on (probably raw veggies & fruit, raw meat, whatever he won't just grab'n'go with.

This may take some time for dad to retrain the thought process of raiding your fridge...but if he equates new fridge with the snacks he likes, hopefully, he'll catch on pretty quick.

The baby gate concerns me--I am a shorty and I can't QUITE get over one of those, so having them up when we had 'groups' of toddlers and had to corral them--was fraught with MY falling. Which happened a few times, to the utter delight of the toddlers!

You can install a actual LOCK on a fridge. Dad did that to our 2nd home fridge--it's a pain to have to unlock it all day long, but it will quickly solve your problem.

A screen door with latches up high on both sides of the door may help to make a sort of visual block--again, everybody has to be tall enough to unlatch--and dad may well figure that out pretty quickly.

I'd say the baby-proof locks that go on doorknobs would be great, but anyone with arthritis (me!) cannot open those. I get the toddler I'm tending to open it for me (SMH over THAT)--and if you cannot quickly pop those open, they can pose a hazard if the person you need to get to is on the other side.

I don't mean to make fun of what is clearly something that's driving you nuts--but this reminds me of when my kids were teens. We had the home closest to the Jr High AND the HS. My house was literally FILLED with tweens and teens every single day. Gallons of milk, a batch of cookies were consumed EVERY.SINGLE.DAY. I had to make a grocery run every day. I also baked double batches of treats almost every day.

Small price to pay for being the KoolAid mom and knowing where everyone was at all times, esp my own kids.

Now I have DH who is WFH and raiding the fridge all day long. I put all the stuff I need for meals in the downstairs fridge and put the 'healthy snacks' in the upstairs fridge. It's helped, he doesn't snack on the stuff I need for meals.

Use baby gates maybe willl help

A first concern I would have is safety for all in the event of an emergency.    Is anyone in the house able to get out through another method w/o going through the kitchen?   And is there an exit in the kitchen for someone who might need to get out quickly?

I was thinking of a numerical lock; the combinations could be infinite and it would be difficult for FIL to guess the right one.   But it does slow down access in and out for others.

There are some other good suggestions already posted.   I've never experienced this situation, so I'm just guessing and don't have any good insight into real life  experiences in this kind of situation.

What about a keypad lock? I think it could work, if the door is sturdy enough. See Amazon for pricing. The fancy ones are even bluetooth enabled for remote control. Plus if he guesses the combination for some reason you can always reset it.

If you had a small freezer you could store your PBJ sandwiches and anything else frozen in it. Those types of freezers have a lock and a key. I had to use the childproof locks on my kitchen cabinets and refrigerator when I cared for my Mom. She would try so hard to get those locks off. I guess I really didn't help you out very much because I can't think of anything else to keep your FIL out of your kitchen. If I can figure out something that might work for you I will let you know. Good Luck.

Don't worry about the locks. I had to get fridge and freezer lock as well. There's nothing like waking up in the morning to find everything all the food in the freezer had been taken out, completely opened and left on the counter.

Baby gates can pose a problem if someone tries stepping over it a fall could result. This is anyone not just mom or dad.
A screen door would work and you could put a hook and eye latch up high where it would be out of view of dad. (people generally do not look up for a lock or latch)
One of the screens that they advertise for sliding doors might work. The screen can be pushed through, the 2 parts are magnetized and would be easy to get though in an emergency or if you just had to get out of the kitchen BUT it is a visual and somewhat of a physical barrier that might slow dad down a bit. (Put bells on it so that it would alert you if he starts to go through it.)

I almost hate to say it but this might be a case where dad would be safer in a facility where he would have supervision 24/7.

I went through a lot of this with my Husband. for a time I had to run a cable around all my dining room furniture because he would take the chairs to the deck and try to climb over a gate that I had installed that I had a combination lock on. (I had to drill out all the screw heads because he tried taking all the screws out with a screwdriver. (almost got them all out of the latch. ) I had to change the "up/down" opener on the inside of my garage with a keypad so he could not get out through the garage. I had to lock my keys in a safe or wear them so he could not get the keys and drive off. Had to lock the car so he could not get the garage door opener....

Contact Alzheimer's association as they will have suggestions for you. They have an 800 number and are very helpful.