The most important substance to the Strogg is Stroyent. It is a viscous green slime that when fully processed becomes a vibrant red or neon orange chemical which contains large amounts of protein and other nutrients that the Strogg require. It's created from the recycled corpses of their victims and former allies. In addition to being their one and only food source, Stroyent is used by the Strogg as a kind of renewable bio-fuel instead of oil. It's used in everything from bio-tech vehicles, sentry devices, weapons, ammunition and recovery stations such as "Stroyent health stations" which are what appears to be a large bag of flesh attached to a device that dispenses Stroyent to Strogg requiring medical assistance.
Stroyent is produced in specialized factories, such as the one seen in Quake II, where human prisoners, who have been tortured into insanity and rendered harmless, are transported along conveyor belts and then gruesomely dismembered by various automated machinery. Their body parts and blood are collected in huge vats, where the substance is treated with chemicals and refined into Stroyent.
Quake 4 expands upon this system, showing Stroyent being further refined using various bio-engineered systems, including something called a "Stroyent Processing Creature", as well as a giant organ much like a massive heart that pumps the Stroyent through a series of tunnels and pipelines running across Stroggos. This is part of a biological factory the Strogg maintain for creating Stroyent. It is likely this is the central system for Stroyent's production and dispersal. However, even during the Earth Invasion the Strogg were equally capable of producing Stroyent on Earth and could even convert existing human oil refineries into Stroyent factories. For this reason it is likely that the Strogg can create Stroyent with or without the use of the Stroyent processing creature and the heart, though these systems inevitably made the process a lot faster.
Unlike oil, Stroyent is a non-combustible fuel and is likely only able to fuel Strogg machinery by being fed into various interconnected biological components. Regardless of this, the Strogg still use combustible fuels also, likely Stroyent-derived, since there are explosive red barrels in many Strogg facilities, probably containing gasoline or some similar fuel.
Trivia
- Stroyent could be a reference to Soylent Green, a solution to a fictional food crisis in a movie of the same name, which, unbeknownst to the general populace, is made from processed human corpses.
- The Stroyent is not used by any enemy in game; Matthew Kane also cannot use this until he is stroggified. This can be treated as an "aid station" if he needs to use it.