For a start a little chemistry. I promise extremely little, but we need it to be able to have fun on the next level.
Green coffee beans contain a lot of cellulose. Cellulose is a polysaccharide, basically it's wood, the solid parts in the stems. Cellulose is the most abundant polymer in the world. If we want to brew coffee from such beans, it will not succeed because
- We'll destroy the grinder. It's hard (like wood - literally)
- There won't be much or how to extract
When we start roasting coffee, the cellulose polysaccharide starts to "cut" into glucose monosaccharide. And that's a whole different animal.
Glucose is really sweet. Surprisingly, it is the most abundant monosaccharide on the planet.
One would start to think that coffee is sweet for that. Well, it's not.
Because when coffee is roasted, this glucose reacts with the proteins in the coffee to form melanoidins. The brown stuff.
The roasting process consumes all the glucose we cut from the cellulose before, leaving nothing sweet in the coffee. Also almost all sucrose degrades during roasting
So when we make coffee (in any way) how can they say, "This is sweet!" and about other they can thing "This one isn't so sweet!" ?
It's because they feel something sweet. For example, honey. Or fruit that tends to be very sweet. Or vanilla, which we often associate with sweetness. We simply smell the coffee and feel the sweetness. Even though there is no sugar in this coffee, no glucose, no honey.
Feeling the sweetness in your coffee is definitely a pleasant experience.