21 CFR §112.145

1. (a)You must establish and implement a written environmental monitoring plan that is designed to identify L. monocytogenes if it is present in the growing, harvesting, packing, or holding environment.
2. (b)Your written environmental monitoring plan must be directed to sampling and testing for either Listeria species or L. monocytogenes.
3. (c)Your written environmental monitoring plan must include a sampling plan that specifies:
   1. (1)What you will test collected samples for (i.e., Listeria species or L. monocytogenes);
   2. (2)How often you will collect environmental samples, which must be no less than monthly, and at what point during production you will collect the samples; and
   3. (3)Sample collection sites; the number and location of sampling sites must be sufficient to determine whether measures are effective and must include appropriate food contact surfaces and non-food-contact surfaces of equipment, and other surfaces within the growing, harvesting, packing, and holding environment.
4. (d)You must aseptically collect environmental samples and test them for Listeria species or L. monocytogenes using a method as set forth in § 112.152.
5. (e)Your written environmental monitoring plan must include a corrective action plan that, at a minimum, requires you to take the actions in § 112.146, and details when and how you will accomplish those actions, if the growing, harvesting, packing, or holding environment tests positive for Listeria species or L. monocytogenes.