How do you achieve body in your end product... Do you roast or source for it?

Tough question: Depends on where you are starting. If you are trying to get more body out of what you have then it's roasting. If you are looking to blend then it'll be sourcing and roasting to maximize body

You want a longer middle phase...

Yes.

Each coffee presents a unique mouthfeel. If roasting for espresso, I think it's often about drawing out sugar chains, building a base of sweetness and layering textures and flavors through a combination of roasting and blending. You can think about body in generalities, but each coffee presents a unique opportunity to learn something.

One such generality may be that you will have more body from a dry processed Ethiopian than you will from a washed Costa Rican, but your outcome can be altered by grind and overall density. So that which is not equal can be made equal by applying a little bit of logic, and a lot of tasting and adjusting.

To not factor in this or that is an invitation for something bad to happen. Every step is important.

I don't know how to get more body - but I know how to get less - underdeveloped beans. Proper development considerably helps solubility which helps body. So roasting is important.

But solubility is also a function of the bean as well as roast: For an extreme example consider stale & dry beans - they make for a bad roasting experience as well as bad pulls that result in a watery body and muted flavors.

I would think body is a function of TDS - the higher the TDS the fuller the mouthfeel. If my assumption is correct solubility (as a function of roast and beans) and a short pull would be key.

To me, body is more of a "mouth feel" and I associate it with oils.

So that said, a bean that starts to exude oils after the roast (day or so) will seem to have more "body" than one roasted lighter.

Understand you may have a trade off between "taste" and "body". You may lose some of the subtitle flavors to get more mouth feel. This is where blending post roast comes into play.

Your brew method can effect "body". Go away from filters for more oils in the cup.

Good question.

As with any aspect of flavor, it's best to first source for the desired attribute, then roast to bring out or enhance that attribute.

Modulating the Flavor Profile of Coffee by Rob Hoos is a good source for answering questions like yours. In the case of body, the general consensus, as JK stated above, is that a longer middle phase (aka ramp or milliard) results in increased body. This is apparently because the molecules that are created during this phase are relatively large, and therefore contribute to greater mouthfeel.

In my experience, both the dry phase and ramp influence body... actually the entire roast, but I'll just talk about the dry phase (charge to yellow) right now. As Rao insists in his book A Roasters Companion this phase is crucial to even development throughout the seed. And even development throughout the seed is prerequisite to a pleasant mouthfeel. For example, if the inside of your coffee is underdeveloped, it will not matter how long you stretched your middle phase, the cup will not have a full body. Brothy is a descriptor that comes to mind.

So how do you achieve even development through careful roasting during dry? Well... there are different approaches based on density and processing, but generally speaking, the best way to "modulate" the dry phase to achieve an even development is what I call the "Roaster Joe Method"--5-6 min to yellow with very low airflow. As Joe explains in a number of the Mill City videos, the idea behind this method is to preserve the moisture in the seed and use it to conduct heat into the center of the seed. Water, after all, is a much better conductor of heat than dry cellulose.

I took a roasting class taught by Joe a while back. I was roasting on a Probat and I made the mistake of turning on the airflow at a bean temp of 260F. Joe grabbed the airflow lever and closed it, politely reminding me never to give a Probat any air until at least 270F bean temp.

In other words, middle phase will develop large molecules which contribute to mouthfeel, and a "Roaster Joe" approach to drying will ensure that those large molecules develop in the center of the seed as well as the outer layer.

Every coffee will have a different amount of body, mouthfeel. Heavy Sumatras, light Mexicos....etc

I agree with what has been said about roasting.

Another thing to think about is brew method. I find that clarity is many times the trade off for body. Ristretto = Heavy Body, Low Clarity. Lungo = Light Body, High Clarity. French Press = Heavy Body, Low Clarity. Chemex = Light Body, High Clarity

Finding a brew method that may suit your taste may be a factor as well