Why is my icecream bitter? I am new to making homemade ice cream. I just started to feel confident with plain vanilla, so it was time for the next step - adding fruit. 

The custard recipe says it can be turned into fruit ice cream by adding 30g pureed fruit per yolk. I looked into the basket for a fruit likely to make a good puree, and settled for a honey dew melon. I made the custard base (yolks, cream and sugar with a pinch of salt), and, because I wanted more fruit taste, made 50g of melon puree per yolk and thickened it with a bit of guar gum, because I feared crystals from the rather liquid melon. I added the melon to the prepared custard, it wasn't exposed to heat. 

After chilling and transferring to the freezing plate, I indulged in licking the thermometer, spoon and bowl used for the making of the ice cream. I was unpleasantly surprised at the lack of melon aroma and strong bitter taste. It was as if I was chewing the seeds of the melon, although there were none in the ice cream, I am sure I'd have seen them if they had fallen into the puree. Combined with the overbearing sweetness of unfrozen ice cream, I thought of throwing the batch out. But I froze it anyway. Frozen, there is much less bitterness (and of course much less sweetness), it tastes as if I am eating a raw gherkin. In fact, if I didn't know that there is melon in there, I probably wouldn't have recognized. It is eatable, but not especially good. 

The aroma problem was solved when I ate a piece of the melon: this exemplar must have grown in a dark greenhouse. It was watery, only a hint of sugar content, only a hint of aroma. But it wasn't bitter, just tasteless. 

But I am still wondering what went wrong. Does melon react with guar to create a bitter compound? Does melon react with cream to create a bitter compound (I know kiwies do, but melons?) Was there so much heat in the custard base as to cook the melon, and does cooked melon taste bitter? Would a better melon have made a difference, or would it have been bitter, sweet and aromatic at the same time? Is melon generally unsuitable for making ice cream?
It's no surprise to me that you describe the final taste as being similar to a raw gherkin, because both melons and gherkins are closely related to the cucumber.

Melons actually do have bitter notes; according to McGee, they contain "green, grassy compounds" as well as sulfur compounds, the latter being the same sort of thing you'd find in garlic.

Since melons only get sweeter on the vine, I would assume that the general blandness was caused by the melons being harvested too early and possibly sitting on the grocery store shelf for a long time.

If there was noticeable bitterness beyond what you tasted in the raw fruit, it was probably just because you got a concentrated dose of it in puree form.  Overcooking it would also increase the bitterness, but you say it wasn't exposed to heat (could the custard still have been very hot when you incorporated the melon?).

There is no way it was a reaction with guar gum - it's a very common additive in melon-based drinks and desserts.  It's also extremely unlikely to have been a normal reaction with the cream; I've made melon mousse and other dairy items with melon and never noticed any kind of unusual bitterness.

The only other possibility I can think of is that there may have been a problem with the cream itself.  This varies quite a bit from brand to brand and region to region, but most of the time, the "cream" you buy does not just have cream in it; for example, I'm looking at a typical supermarket carton that contains cream, milk, skim milk powder, dextrose, and a bunch of E numbers, and even some sodium citrate.  These creams can easily take on bitterness before they go sour.  So if you were using the typical supermarket brand of heavy cream, and it was sitting in your fridge for a while, that may have been a contributing factor.

All in all, I think it was probably a very random and unfortunate combination of factors that would have been due to the specific ingredients used rather than the types of ingredients.  I doubt you'd get the same results if you tried again with fresh melons, cream, and eggs.