Why is my mom holding on to food in her mouth?

Yes, it’s called pocketing. It’s very common in later stages of dementia and Alzheimer’s.
At this stage a speech therapist is usually sent to evaluate and a modified barium swallow test to see if food is quite literally going down the right pipe or not is ordered. This is to ensure she’s not aspirating good into the lungs and recommend the proper food and fluid textures.
A change in diet texture to mechanical soft such as eggs and soft toast, to purée which is where food is given after puréed in a blender and even thickened liquids are given to help her swallow them efficiently.
As a long term care nurse for over 20 years, I can also recommend that a urinalysis is given and possibly a chest X-ray and lab work too. So if you haven’t called the Dr. yet do that. If she is ill especially with a urinary tract infection, this could be a very treatable indicator that an infection is taking place and will slowly resolve as it is treated. Even if no other symptom is present.
Sometimes even in the presence of an infection the set back has already continued to stage them further along and they do not regain the ability that was lost. The longer the set back goes on the more true this appears to be.
I would start there first with tests then get the speech therapist next or simultaneously.
The sweet taste is helpful to trigger the swallowing reflex as is the massaging as the first poster said. Definitely do these tips in the meantime.
A decision as to place a feeding tube or not is pretty common with end stages of dementia and Alzheimer’s as well. It is generally not recommended however, for many reasons, that I won’t go into here. But I will say this a huge decision not to be taken lightly or thought of as obvious. An educated decision and family feedback is warranted. Aspiration is generally just as big of a risk if not more with a tube feeding. I only mention this now as once the pocketing of food starts this decision is often not far behind.
Lets hope it’s a treatable infection and she recovers from this fully. All my best!

My mom with end-stage Alzheimer's does this, and even textured food including thick it will not stop it. My mom just spits out the food right in front of me. I do only give her soft food, and ironically she love popcorn. I give her a piece of pop corn she will swallow whatever is in her mouth without problems. It takes over an hour to feed her, and something as simple as feeding her produces a considerable amount of stress for me. If I do not do this right she will spit the food out in front of me and make a terrible mess. Alzheimer's is a very cruel disease not just for the victim but the caregiver.

Alzheimer's does not kill the person, but the complications of it will. Most die of either a fall or pneumonia. Feeding tubes are not a good idea with someone with end-stage Alzheimer's because they will tend to yank them out. They can still vomit and aspirate with tube feeding, and the tube insertion area can get infected.

One study showed 40% seemed bothered by the feeding tube to the point the patient had to be tied down or sedated to prevent them from pulling it out
see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3254052/

Another complication of tube feeding appears to be diarrhea..more likely from "dumping syndrome" and this can set off urinary tract infections.
https://www.managedhealthcareconnect.com/article/8614

Maybe when it comes down to letting them die vs. trying a tube as a last resort it still requires decision making by the family. I suppose in a nursing home you all will not have to see your loved one tied down or sedated because they are trying to pull it out, but it is a risk you have to take.

This is common enough to have a name - pocketing. It usually has to do with having difficulty chewing, manipulating the food in the mouth and swallowing; she should be evaluated by a speech and language pathologist who is trained to assess what is going on and to make recommendations to help her eat safely.

Yes. My father did the same as well, and would keep it there for nearly 15 minutes, as well as toothpaste, and pills. My father had late stage dementia...my father was as well on Hospice, but was home first, when it began. This is one of the signs of patients near death, that they will not want to eat anymore...my father went downhill fast.

My Mother does this occasionally and to help prevent it, I have her rinse her mouth with mouthwash after meals.  I tell her to "swish" it around and spit it out.  If you can do this with your mother, it really helps.  Also, give her smaller bites and when she's finished, have her drink something. I'm just sharing what works for us - you may want to seek advice from your physician.  Bless you for caring.

Dysphagia:
Difficulty swallowing foods or liquids, arising from the throat or esophagus, ranging from mild difficulty to complete and painful blockage.
Mom pockets her food too. They did an evaluation of and found she also has Dysphagia. Many have commented on "pocketing" so I need not go into that phase.
I've been caring for Mom for over 6 years now, this started about 2 years ago and progressively got worse. However, with patience, and small pieces of cookies on between bites, she eats her food. The sad part is that the care facilities do not have someone that can "sit for an hour" to feed one resident. In addition, if her mouth has food in it and they lay her down she may possibly aspirate. I suggest you google Dysphagia, Aspiration, and aspiration Pneumonia. "Patience for Patients IS KEY."

In late terms ALZ, patients often "forget" how to swallow. A speech therapist once told me there were 26 or some number like that different muscles we all use to coordinate our swallowing. ALZ patients lack the brain capacity to direct this.

What I would do with my Husband is I would encourage him to take a sip of juice, water any fluid after a few bites. I would also rub the outside of his cheeks to loosen up any food that had drifted into the area between his gums and cheeks.
I also used a swab after he ate to remove any debris that was there.
I did this for 2 reasons.
1. So the food would not stay there and become a problem as it broke down.
2. So he would not aspirate food particles.

When my mother reached end stage dementia, she fell asleep while I was feeding her, just after she would take a bite she fell asleep and did not swallow. I had to wake her up. After we put her in a nursing home, every morning when I arrived I would find her mouth still had food in it. So I washed it out and brushed her teeth which they had not done, I stayed the day and fed her lunch and then supper or dinner what ever you call the evening meal and each time I cleaned her mouth. I know that every one can't take the time that I did but if I didn't she would have had quite a bit of food in her mouth by bed time. She only lived 2 months in that home and while she was there she got gangrene in her foot because they did not take enough time with her. At home I washed her feet daily and made sure they were dry. She was diabetic. Nursing homes do the best they can but in some instances it just is not good enough.

Her brain may be so broken that she has forgotten how to swallow. Seek out the help of her neurologist.