Table 4.

Foaming assay scale^a

Rating	Description
0	As for pure water; no foam
1	1.0–3.0 cm of foam with fragile ill-formed bubbles; insufficient stability to form films; immediate collapse upon cessation of aeration
1ab	Flotation of clumped bacterial cells to the surface of the air-water interface; clumped cells remain afloat upon cessation of aeration, producing a scum layer
2	Intermitted films sufficiently stable; usually generated from a fragile foam structure of limited ht; films unstable upon cessation of aeration
3	Substantial foaming (i.e., bubbles about 10 cm in diam) to ht of 3–8 cm; infrequent or regular film formation, with both film and foam semistable upon cessation of aeration
4	Initially 8–15 cm of foam (about 1-cm-diam bubbles), with stable films being formed at regular intervals; body of the foam and films stable for 3–5 min once aeration ceases
5	Stable foam 5–10 cm in ht in 2 min, after which it collapses to ht of 3–5 cm; foam is stable when aeration ceases; no films
6	Stable foam 15–30 cm in ht with no films; bubble size is about 0.5 cm during aeration and increases to 2.0–3.0 cm in diam in 3–5 min from the time when aeration ceases

^aAdapted from reference 28.

^bThis key has been added to represent the “scummers.”