Abstract: The invention relates to a gel body of a delimited physical form preferably containing water and characterized in that it a) contains an encapsulated apathogenic micro-organism having a known and reproducible resistance; b) has a porous structure with the size of the pores defined, said structure preventing the encapsulated micro-organism from diffusing out but allows a nutrient solution adjusted to the micro-organism to diffuse in and metabolites produced to diffuse; c) is thermally stable in various sterilization, pasteurization or cooking processes; d) is mechanically stable when transported through the process equipment for sterilization, pasteurization or cooking; e) is surface sterile; and f) is transparent in order to allow observation of possible growth of the micro-organism when incubated in a nutrient solution.