Nobody has mentioned tannin levels in a discussion about fruit?
I think they certainly help add some balance, especially in a sour. But have you noticed a correlation between tannins and flavor in beer?
Just now we all know how to pronounce the name of He-Who-Must-Not-Be-Named (–hint: it’s a silent -t–), now JayKay has to start giving us pointers on how to spell He-Who-Must-Not-Be-Spelled’s name.
In an attempt to re-rail this trainwreck: assuming the original comment about fruit flavours “binding” to sugars actually meant “something about flavours, sugar and acid” renders it more meaningful.
Regarding raspberries vs strawberries: even if their acid and sugar contents are comparable (*), their flavour profile obviously isn’t. Raspberries have a more pronounced flavour than strawberries, which is why you can eat a bowl of the latter, but seldom eat more than a handful of the former. Also: what reverseapachemaster said.
*) “acid content” vs “acid composition”. Which acids are we talking about, and how does strawbery’s acid composition differ from rasp’s?
A potentially interesting tangent is how artificial fruit flavours try to mimic the natural flavours. Strawberry flavour tends to not taste like strawberries at all. Rasp’s even less so, possibly because the acidity is all gone from the flavour.
The Guacamolambic challenge has been accepted. Expect more news in a year or two 
@klickitatjim. See what you have done? For the next two years there will be this sickly green-yellow-brownish sour concoction with oily mouthfeel popping up in my worst nightmares. And then I won’t even have tasted the vileness.
You are still saying not much more than that raspberries are more flavorful than strawberries.
I think tannins certainly contribute flavor and structure, especially with berries. Maybe tannins affect the quality of the fruit flavor? The few fruit beers is have done, I greatly preferred those on whole fruit over juice concentrates. I know in my meads, usually a little tannin helps greatly to enhance the overall quality.
I don’t think so. Both have primarily citric acid, which does tend to get reduced during fermentation. Cherries and apples have mostly malic acid, which is only reduced by a small amount. Tartaric acid (grapes) is reduced even less by the fermentation chemistry, but grapes experience a drastic flavor change when fermented.
I don’t know the answer to your question, but my experience tells me that while both acid and sugar can be important factors in flavor enhancement, they are not at the key you are looking for. Some flavors (like raspberry and cherry) are just more stable.
This is interesting because we occasionally add a little tea to the must of some of our melomels. I always thought it was to balance sweetness without adding more acidity but maybe it does more.
Tea definitely has a lot of tannin. I add wine tannin to hard cider to give it more complexity.
http://nzic.org.nz/ChemProcesses/food/6B.pdf
That link is completely about wine, not beer, but a great comparative list of compounds in grape juice before and after fermentation. This is where I think there is more than a simple sweet/acid vector in adding fruit to beer. There can be significant transfer of organic acids (more or less directly) as well as phenols. In particular, anthocyanins are bound to a sugar, but I am not sure of which sugar molecule or if the bond is cleaved in fermentation and the sugar used as a carbon source.
As to the flavor transfers, I suspect that raspberries water/sugar ratio is far less than strawberries, meaning far more strawberries needed per 5 gallons and probably some additional sugars to prevent dilution. I would also speculate that strawberry flavors and aromas are more volatile. I have had some strawberry melo’s that were horribly astringent and unpleasant from the bulk of the seeds. All speculation of course.
I also think ripeness is a huge factor. When you think about it raspberries are always picked and used very ripe, they’re either ripe or not. Peaches and strawberries often get picked underipe. The time between perfectly ripe and moldy is very short, sometimes 1 day. I get great peach flavor in mead using my own fresh picked ripe peaches (usually picked ripe and immediately frozen before using, actually). I know there is a chemical component to ripeness. Maybe its partly because most don’t use very ripe fruit that the flavor doesn’t come through.
Pete
The pdf is very interesting. The components of wine are (a.o.) carbohydrates, organic acids, phenolics, and flavor components. Since there are different types of organic acids they may play a role. I don’t think the phenolics are at work here, because these are mostly tannins. The flavor components are interesting: mostly volatile monotherpenes (in wine well-known critters such as geraniol, citronell, and linalool.