Simple, just respect their taste. I had friends who grew up with instant coffee and they hated even the best brewed coffet I could make..
I once bought my Dad some nice expensive freshly roasted coffee to try and help him experience something better. I was shocked a couple weeks later when he told me it “didn’t have enough kick” so he was adding cheap espresso grounds to it when he would make it. I gave up on that right there because no amount of logic was going to get through that one.
Why do you feel you need to convince them of anything? I totally get having your eyes opened to the broader spectrum of coffees available but end of the day it's a preference based on subjective senses. Let 'em do them, what's the harm?
Taste buds change over years in the same person. Flavor preference holds no logic because people are individuals.
Just because you like something doesn't mean other people have to.
I think you may be SOL.
If they like coffee this way, that may just be that. And it's okay. By all means try making them a proper cup of a nice dark roast, but at the end of the day, if they've come to expect bitterness outta their morning cup, you ain't gonna satisfy the habit with something that ain't terribly bitter.
Let people enjoy things, man.
I just tell them it's too bitter for me and that I don't want any. They are the ones not letting me enjoy tea instead when I'm there. Lol
how do you convince someone that bitter coffee is not good coffee?
That's the neat part; you don't.
Bitter coffee is also cheaper for a reason, as you can use worse beans and burn them. These beans need to be used as well, so it's a good thing people like it bitter.
When it gets weird is when people pay for expensive bitter coffee.
I love bitter coffee. And chocolate.
Just because it's not your cup of tea (or coffee) doesn't make it bad or wrong.
They like espresso,you like Americano. Get a grip. It's just coffee.
I like espresso, just not from a drip coffee machine.
People have different preferences. That's why light and dark roasts exist. Learn to enjoy more than just your favorite, and encourage others to do the same. Don't take it personally when someone doesn't like what you like, and don't be afraid to state your own preferences. If their coffee is too bitter for you to drink, add a tablespoon or two of water
But yes, clean the grinder.
Are they on the older side? I've noticed that my in laws, who are almost 80, like very strongly flavored things, and I suspect it's because they just don't taste things as well as when they were younger.
Imagine my disappointment when the story didn't involve butter.
If I'm reading this 
correctly, bitter can be caused by over extraction. The solution would be to extract less, which can be done by using more coffee.
There are lots of variables. The coffee itself, the grind (lots of fines?), the brewing method, etc.
I agree with others, though. If they like their coffee, let them enjoy it.
Thought for sure this would be about bulletproof coffee.
Isn't taste also affected by age? Like eyesight and hearing, it diminishes with age.
Their coffee might taste to them the way your coffee tastes to you.
Make them better coffee and then don't comment on it until they ask.
Most people seem to connect bitter and strong and it's a hard habit to break out of. People are used to crappy coffee.
Just getting them to a stage where they buy quality beans and grind them fresh is a big leap, but once they reach that baseline you can introduce roast levels.
Catering for years I found that the older the guests were, they really didnt care about the taste as long as it was piping hot! Live and let live.
Without judging, some people confuse bitter for strong. You should try brewing their coffee different ways that will make it intense, but not bitter to see if they like it better. I'm not an expert, but maybe Chemex? Aeropress? Pour-over?
That's all you can do if they end up preferring ~~butter~~ bitter coffee!
Well they are about easy when it comes to making coffee. I'm going to use the coffee compass as a guide to see what happens next time I visit.
Have you tried adding a few granules of sea salt to your cup? To counteract the bitterness a little. One or two usually does the trick for me when I have to drink the coffee at work.
Not sure if you’re in the US but that seems likely with this sort of question so this is probably untrue for most other countries besides maybe the UK
Until somewhat recently, late 80s I’d reckon, while there obviously were places you could get ‘good’ coffee for enthusiasts, for the vast majority of people coffee was just something you drank to get through the day and stay awake. The folks that grew up and lived in that world have always mocked the ‘fancy’ stuff even when it started growing in popularity. How they define what coffee tastes like is likely harsh and bitter.
Same thing happened to beer, used to be you were a Budweiser, Miller, or Coors person, maybe there’s something similar in your region like Lonestar or Ranier but it’s nothing like today. And there are plenty of older folks that hate everything that isn’t a light American lager. It’s just happens.
Sorry that doesn’t really help you, just adding my thoughts
Helping with maintenance tasks like cleaning the grinder is a good idea, but I wouldn't count on changing your parents' preferences. If you don't enjoy their preferred roast/ratio, just bring some of your own beans and make a separate pot for yourself when you visit. I had a similar situation with my in-laws and that is our solution. I bring my coffee when I visit them and they bring theirs when they visit us and we're all happy.
If they tried your way and didn't like it, then just let them be?
Simple. Give them a well-extracted shot of straight up espresso. Pick a natural process, fruity East African kinda thing.
Welcome to boomer psychology 101: you scream at them very hard and they will scream back and keep their position, but after you have left they will change their mind
Welcome to asshole logic 101- you must scream at people for absolutely inane reasons, such as their coffee being "too bitter".
I agree anybody is entitled to have preferences and the bitterness of coffee is trivial. This was mainly a joke and a recollection of how it worked in my family
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