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I've heard of adding salt to coffee to reduce its bitterness. I've also heard that using freshly roasted and ground coffee would reduce the bitterness experienced when drinking coffee. Though it seems making freshly roasted coffee could actually make things worse, according to this question.
Is there a documented process (not additive) to reduce the bitterness of coffee and any research in why that's the case?
There are a lot of factors that contribute to the bitterness of coffee. Some of these have to do with the concentration of various compounds in the coffee and the roasting process, but the way you brew the coffee can make a big difference as well.
Assuming that you are not going to be harvesting and roasting your own coffee, we can focus on the brewing process.
Buying Beans: Robusta vs Arabica
To decrease perceived bitterness in coffee, the best choice is Arabica. Arabica beans are often sweeter with notes of sugars and fruits and a somewhat wine-like acidity. Robusta on the other hand tends to be harsher with more bitterness. This difference in quality is reflected in cost however, as Arabica tends to be more expensive than Robusta.
In terms of roast level, medium roasts generally contain fewer soluble solids, more acidity and a stronger aroma than dark roasts. All of these should contribute to making medium roasts on average less bitter than dark roasts.
Water: Temperature and Chemistry
You have to find the sweet spot when it comes to temperature, a little trial and error can be used here depending on the blend and brewing method. For example, the people at AeroPress recommend a much lower temperature to reduce bitterness that what you would use in a french press.
In general research has shown that coffee brewed in hot water is perceived to be less bitter than when cold water is used. Additionally, bitterness is reduced when hard or soft water is used when compared to distilled water.
Brewing [Extraction]
Bitterness is strongly correlated with the amount of dissolved solids in the brewed coffee. To ensure that the amount of dissolved solids and the overall extraction is correct (and minimally bitter), make sure to use the correct grind coarseness, water temperature, and brew time for your chosen method. Over extraction can be a major cause of bitterness.
Drip and pour-over brewing methods tend to have less bitterness than a french press or other immersion methods. This can be attributed to less dissolved solids in the coffee.
SCIENCE
Much of this section will be outside the control of most, but it is interesting nonetheless.
Research has found that the introduction of polyphenols reduces the bitterness of coffee. The metallic bitterness found in some coffee has been attributed to dicaffeoylquinic acids. Trigonelline is perceived as bitter at concentrations of 0.25% and its degredation is proportional to roast level. Roasting Trigonelline produces a byproduct called pyridines and helps create the roasty aroma found in some coffee.
Chlorogenic acid is also found in coffee, higher concentrations in Robusta than Arabica, and that contributes to the bitterness as well. Additionally, Quinic acid is a product formed from the degradation of Chlorogenic acid and is present in coffee at twenty times its taste threshold, contributing to the bitterness.
Furfuryl alcohol is also known to increase the bitterness of coffee.
More Information
Bitterness comes from over-extracting coffee. Over-extraction is mainly caused by:
1) brewing for too long: In the beginning of the brewing stage acidic flavors are first extracted, followed by sweetness. And towards the end of the brew, only bitter flavors are extracted. The trick is to stop the brewing just after all the sweetness is extracted.
From Wikipedia: Coffee extraction
Yields of under 18% are "under-extracted", specifically "under-developed" – desirable components have not been sufficiently extracted – and "unbalanced", specifically sour, because acids are extracted early, while balancing sugars (sweetness) and bitter components are extracted later.
Yields of over 22% are "over-extracted", specifically bitter, as bitter components continue to be extracted after acids and sugars have largely completed extraction.
2) Grind size being to small: small grind size means larger total surface area of the particles. Therefore, flavors get extracted more quickly and can lead to over-extraction.
The optimal brew time and grind size vary depending on the method (french press, manual pour, espresso etc..), and you'll need to experiment to get it just right. The type of coffee bean also matters, but probably to a lesser degree than the above two factors.
The way I reduce bitterness is to rinse the grinds before brewing.
I use an electric drip brewer that has a built-in blade grinder with a reusable basket filter. After grinding, I turn off the coffee maker, and transfer the basket to the sink where I spray the grinds with water until the water runs clear (about 5-10 seconds). Then I put the filter back into the coffee maker and start brewing.
I'm not clear on the science behind why this works, but I think it has to do with preventing over-extraction. Basically, the fine powdery bean dust gets rinsed into the sink instead of going into the carafe (where it would continue to brew). This pre-rinse eliminates the oily surface and bitter smell that the coffee has. Plus, I no longer get an upset stomach that unrinsed coffee usually gives me.
If you try this, be aware that it needs to be a mesh filter. Attempting to rinse through a paper filter just creates a big mess.
To reduce the bitterness it is important to know the brewing method. Each brewing method needs its own grinding of the beans. Espresso machines use a finer grinding than slow (filter) coffee for example.
Lets assume an espresso machine is being used by the OP. A finer grinding can result in 'channeling'. Channeling means the water creates a single channel trough the coffee puck instead of evenly distributed water flow. The single channel will lose its good taste because of the quantity of water it processes. So the result product aquires all of its taste of that single channel. In a matter of seconds you will be left with bitter coffee. The other effect of finer grinding is a longer exposure to the water, because the water takes longer to reach the bottom of the puck because of the fine grind.
Other factors resulting in bitter taste: water quality, bean type, bean freshness and roasting process, tamping (for espresso machines), cleaning of your brewing device, cleaning of your grinder, extraction time, water temperature, and so on. So a lot is involved and a single answer to this broad question cannot be given. Entire books have been written on this topic.
