Ok,
1) the edited file MUST go in a mod directory for crusade e.g.
C:\Users\User\Documents\My Games\GC3Crusade\Mods\'name'\Game\MasterTechDefs.xml
2) edit the MasterTechDefs.xml file and save it as above
find the MasterPlanetarySpecialization1 section and make this change
<Stats>
<EffectType>Food</EffectType>
<Scope>Global</Scope>
<Target>
<TargetType>Faction</TargetType>
</Target>
<BonusType>Flat</BonusType>
EDIT: OOPS i may have spoken too soon, thought it was working but need to do more testing, sorry