The standard applies to the determination of the density at 20 °C of buttermilk, fermented skimmed milk, fermented milk, milk yoghurt and skimmed milk yoghurt. NEN 958 applies to milk: "Determination of the density at 20 °C of milk with a lactodensimeter."
NEN 1358-1971 history
1971NEN 1358-1971 Determination of density at 20 °C of buttermilk and yoghurt with a density hydrometer (lactometer)