Slicing tri-tip is a bit more confusing than most other cuts of beef. While the straightforward answer is to cut against the grain, the trouble with tri-tip is that it has two apposing grain directions.
My best advice is still to cut against the grain but to make note of the grain direction prior to cooking the meat. Even if you forget, just remember the grain transitions at the tri-tips boomerang inflection point.
The tri-tip is a singular muscle called the tensor fasciae latae muscle.
It is a sub-primal cut sourced from the bottom of the sirloin of the beef hindquarter. It's commonly called the "triangle roast" due to it's triangular shape.
This muscle has two different, defined grain directions and should be sliced accordingly. To help explain this, we can look at both portions of the muscle.
To start, here's the entire tri-tip muscle taken right out of the vacuum seal packaging:
As I hope is readily apparent, there is a flat, short end (to the left) and a long, cylindrical end to the right. Not only can these parts cook differently (strategically) but they also have muscle fibers that run in different directions.
Here's the flat end - the grain direction is indicated by the dotted white lines:
Then you have the long, cylindrical end. This end will tense up and become more horizontal when it cooks which is demonstrated below.
However, grain direction can be somewhat hard to illustrate, especially with the muscle laying flat.
Here's a front view of this end:
Here's what happens when this muscle is reverse seared:
The long, cylindrical end (to the right) has tightened up; Which helps to articulate how these fibers are actually running.
We know the separation point where the grain transitions is roughly where my dotted white line is, pictured below:
With the above out of the way, we now have a general idea of how the muscle fibers or grain run on the meat; Our goal is to slice against these grains.
Note: It's significantly easier to identify grain direction in raw meat as apposed to cooked meat; Rather when meat is cooked it will brown, char, etc.
A helpful tip is to take a picture of the meat prior to cooking it and then use it as a guide when you go to slice.
1. The first recommendation I always make with Tri-tip is to separate these two portions after cooking; I even do the same thing when slicing brisket.
Meaning, you'd slice where the grain transitions:
This makes it much easier to slice the individual portions as you won't have to turn your knife or the meat.
2. Once the halves are separated, reference your raw meat image and look at the direction the fibers are running.
In our example, the grain on the left side looks like this:
Meaning, you'd slice the cooked meat, like this:
The right side grain direction looks like this when cooked:
Meaning, you'd slice the cooked meat like this:
That's all there is to it - slice the meat against the grain structure of the muscle.
Slicing cuts of meat like tri-tip is super simple, when you know what to look for. Simply identify the direction the meat fibers run and slice against it with your knife.
You also don't need to be super anal when slicing.
All I mean by that is, you don't need to take out a ruler or protractor to find the exact, perpendicular slicing angle.
It's also not super necessary to slice on a bias - angling the knife as apposed to slicing up and down.
You'll notice the most significant difference in eating experience when simply slicing against the grain as apposed to with it.
