The food in the script/shooting script is referred to as "foam rubber food" or "multi-colored foam rubber." There are other references but that's how it's described, without really telling what the food is. I think the audience is expected to assume it's some sort of dehydrated supplement or something—you know, future food, where edible rubber is the highlight. I imagine this is some kind of commentary on food quality in the 60s and 70s, where food was really becoming processed and manufactured.
Here is a interesting scene that describes its qualities:
16. MAIN ROOM, THX'S QUARTERS
THX is laughing hysterically. As soon as he starts to
gain control of himself, he breaks out laughing again.
LUH has taken a spherical piece of food from her tray and
is bouncing it on the table. She is laughing, herself,
but still trying to keep control of the ball.
THX
(laughing very hard)
Violation...violation...You're
bouncing in...violation..
This makes them laugh even harder and LUH loses the ball.
They both dive for it and wind up face to face under the
table.
Eventually, they stop laughing.
THX
I've never been under the table
before.
LUH
(pointing at the
underside of the
table)
Look,dirt!
THX
(mocking)
That's not dirt...Can't be dirt..
Dirt is a violation.
LUH
Looks like dirt.
THX picks up the red food ball and holds it up at LUH.
THX
Look, food!
LUH
That's not food. Can't be food.
THX
Looks like food.
THX takes a bite out of it and grimaces.
THX
Tastes like food.
He scrambles out from underneath the table and tosses it
in the product consumer. It disappears instantly.
THX
Produced to be consumed!
LUH goes over to the table and scoops up the rest of the
food.
She begins tossing it into the product consumer.
THX
Hey! Wait a minute. Not all of
it. I'm hungry.
She begins to break out laughing, throwing the food into
the consumer as fast as she can. THX starts for her and
she begins throwing the food at him.
THX catches her by the foot and pulls her to the floor.
They roll around playfully, laughing hysterically all the
time.
Slowly and subtly, their play becomes more violent and
sadistic, although they keep on laughing.
LUH
(laughing)
The pain, no...the pain, I can't
stand it.
LUH turns on THX, pinning him to the floor. She sits
astride him, holding his arms.
They are both out of breath and panting heavily, confused
by a slowly increasing awareness of each others sexuality.
LUH slowly releases THX's arms and sits up. She gives a
last, embarrassed giggle and then falls silent. They stare
at each other.
What is interesting is the removal of this scene, particularly because it builds tension and explains their sudden romance with each other. While it is a good segway, removing it doesn't hurt the scene or the transition from roommates to lovers. And the humor itself feels ackward and out of place with the rest of the theme; however, I do like it and would have like to seen it as an outtake/deleted scene.
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Just search for the keyword: food