Excess phlegm only after dinner. Any tips?

PreferNotToSay, what is the "paste" made from? Could be by supper time, it is just too much of that type of paste in her system. It may cause GERD which can produce a cough and phlegm. For myself, I found if I take an antihistamine, it helps. You would need to check with your Mom's doctor.

It could be acid reflux (GERD)...mine acts up at night. It can produce lots of phlegm.

I get this nightly and I'm not even 60. This does not happen after breakfast/lunch only after dinner. I have described this to my PCP and they are stumped. When I get this it's helpful (to me at least) to cough up as much as I can then go have something to drink. I usually drink something fizzy (probiotic tea) or something warm (decaf Chai) and I swish whatever I drink around a lot. Sometimes it's gross but it helps get the phlegm up / down. I never drink milk/dairy etc after dinner.

Are you noticing any swallow difficulties at dinnertime. Often elders lose their swallow, and things are going down into airways. This can happen more often at the end of the day because they are more tired. If there is a swallow deficit there is the danger of aspiration pneumonia. Be certain they sit up after dinner. Your Mom should have this reported to the MD and there may be a check for GERD or for swallow deficits.
Wishing you the best and hope you'll update.

Digestive Enzymes are really important as we get older. We produce less as we age, but the need is still there. Applesauce is great for getting capsules down, but they can be opened and the enzymes mixed in something soft, like applesauce or puddings. Undigested food will ferment in the stomach, and create phlegm and gas that comes back up. Probiotics are important for intestinal health, but stomach acid will kill most of them. There are many good ones that are coated to protect them, but best taken on an empty stomach with a little of something soft and smooth. Preferably jello. These things seem so simple, but they will solve a lot of digestive problems.
small meals more often than large ones work best for elderly. Peristalsis slows down with age and it takes longer for food to digest. Acid reflux happens when undigested food ferments and can’t leave the stomach. It backs up in the esophagus. Digestive enzymes will correct this problem.
i hope that this helps someone.

I wonder if there is something that affects her ability to swallow at the end of the day; as has already been pointed out a medication or that she is overtired.
Or maybe she can't handle so much for dinner because her digestion is very slow and by dinner time her stomach is overfilled and she is regurgitating, maybe a much smaller portion or a different meal altogether - sedentary elders need surprisingly few calories.
My mom periodically had so much phlegm near the end of her life. In hindsight it is obvious her inability to swallow her own saliva was the cause and it had been slowly pooling in her airway - at night it wouldn't be a problem as she would be more apt to drool.

What is her night time medication?

PreferNotToSay: Try to change out her dietary intake and see if it helps.

Have you tried changing her diet to see if that helps?

She could have a slight allergy that is triggered by to much of whatever is causing it and that's why it happens after the 3rd meal.

I know this is a real thing because I can eat a half an avocado and I'm fine, if I eat a whole one, migraine headache for 3 days. Just to much for my system.

PreferNotToSay, I'm very interested to see if anyone posts some suggestions because my 103-yr old Aunt has been having the same problem for the past 2 or more years. Same thing: never after breakfast, seldom after lunch but often right after her first few mouthfuls of dinner. We've been dealing with it by having her take Mucinex (for congestion). She needs a pretty high dose and the pills are giant so I found it in liquid form. If she takes it in the early afternoon, it mostly works. Otherwise if she forgot to take the Mucinex then when the problem starts she sips some very hot water as she feels this thins the phlegm (or whatever it is...saliva? mucus?) and also relaxes her throat, but this is just a theory. Sometimes this helps, sometimes it doesn't. She's been to an ENT (Ear, Nose, Throat doctor) and he's the one who suggested the Mucinex. This isn't a swallowing issue. It seems like eating triggers excessive saliva production (assuming it is saliva). But I can't figure out why it doesn't happen at breakfast or lunch. I tried having her cut out gluten and wheat just to see if it was a sudden allergy she developed because she has toast every single morning and often pasta or a sandwich for lunch and maybe by dinner time her body just over reacts. It didn't really improve anything much. I've done internet searches for this issue but haven't found any description of this problem. Whatever secretion it is, it's a lot and we can't tell if it's coming from her chest, nose/sinuses or mouth. I think it's saliva because it's so clear, but it is rather thick. Hoping you'll get some helpful info here.