Strangeness is only conserved in strong and electromagnetic interactions

Quarks, Leptons & Conservation Laws — AQA A-Level Physics

Strange particles (kaons, lambda particles, sigma particles) are produced in pairs via the strong interaction — this is called associated production.
In strong interactions, strangeness is always conserved.
Strange particles decay slowly via the weak interaction. In weak decays, strangeness can change by $\pm 1$.
If strangeness changes by more than 1, the interaction is forbidden.

Strangeness values of common particles

$K^{+}$ ($u\bar{s}$): $S = +1$.
$K^{-}$ ($\bar{u}s$): $S = -1$.
$K^{0}$ ($d\bar{s}$): $S = +1$.
$\Lambda^{0}$ ($uds$): $S = -1$.
Protons, neutrons, pions, electrons, neutrinos: $S = 0$.

Common Mistake MEDIUM

Students often: Don't say strangeness must always be conserved.
Instead: Strangeness is conserved in strong and electromagnetic interactions only. In weak interactions, it can change by 0, +1 or −1.

Related:Particle Interactions