What causes a tomato sauce to have a bitterness and getting rid of it?

A few things can cause tomato sauces to become bitter:

• Overcooked spices. Both basil and oregano can become bitter with long simmers. Add them near the end of the process.
• Under-ripe tomatos. Store bought tomatoes are often picked green and ripened in the store. These tomatoes make less sweet sauces (which may be contributing).
• Cooking in an aluminium pan. Aluminium reacts with the acid in the tomatoes and adds an off-putting flavour.
• Seeds/skin in the sauce. Both seeds and skins can be bitter.

You can improve a bitter sauce by adding a small amount of baking soda (or salt), and something sweet (but not too much).

In the same vein as Bruce's answer, try using plain diced tomatoes and adding the spices yourself instead of using the "italian" variety. Also, If you use garlic in your sauce too that might sweeten it a little and counteract the bitterness without adding sugar.

Skin the tomatoes but keep them whole in the sauce 'til cooked. They will break down when they're ready. Seeds are bitter. Also I add a couple of sweet bay leaves.

I've found the following to make tomato sauces bitter:

• Tomato seeds
• Underripe tomatoes
• Burnt garlic

Usually, adding sweetness helps somewhat, although letting the garlic get too brown, let alone burn it, can hardly be corrected for. Some things I've found to help:

• Extra carrot
• Brown sugar

You can add half a carrot to the sauce and remove at the end. The carrot absorbs the acidity :)

I find the seeds of fresh tomatoes to be the cause of the bitterness. Try to remove as many seeds as possible by squeezing tomatoes after blanching and peeling. Its hard to get them all but that is okay. Then a bit of cane sugar. This will balance the flavors a bit but some bitterness is good as it is a flavor of fresh tomatoes.

I can’t answer the first part of the question: why tomatoes are bitter (and it looks like a few people already have anyway).

I use three methods for sweetening tomato sauce. The first two you can use with anything: red current jelly and port (port is my favourite cooking ingredient because you can use it in place of red wine in most cases, too).

The third I came across online (can’t find the source), but basically, put your tinned tomatoes (or whatever) on the hob and then cut a brown onion in half and drop that in the sauce. Then cook it for a long time (maybe two hours). At the end, just take the onion out and chuck it.

For being more precise:

Tomatos are acid! This is the characteristic of tomatos. And this is also the reason of tomato success. Thanks to this quality tomato makes edible all other acidless food. Acidity produces a strong salivation that makes easiest the first approach with food you eat, clean palate and give the wish to eat again at once. But also acidity makes bite fairly bound in mouth up to bottom.

In fact before tomato, acidity was taken from citrus and fruits.

Of course acidity makes food bitter. This is the only reason of bitterness of tomato sauce.

There are two main ways we use in Italy to remove that bitterness:

• a pinch of sugar in the sauce while cooking it. This is the main and traditional one!
• a pinch of bicarbonate of soda.

The garlic and also triturated carrot are other ways but alter the sauce. I don't advice those for genuine tomato sauce.

I hope you have a better understanding of bitterness and I could help you validating what Bruce said about how to remove it.

The Italian cooker ;)