# Change of flavour in strong interactions?

Sorry if this question has been asked already but after researching I have found that quark flavour is not changed in the strong interaction. This confused me because how can a down and anti down quark both change flavour to a strange and antistrange quark in diagram below?

The process $$d + \bar{d} \to g$$ and $$g \to s + \bar{s}$$ does not change the net number of down quarks or the net number of strange quarks, where the net number is the number of quarks minus the number of antiquarks.