Foam cleaning in closed circuits: effect of surfactant type on foam structure and Bacillus subtilis spore removal from stainless steel surfaces

Foam cleaning represents an environmentally and economically attractive alternative to conventional cleaning-in-place (CIP) processes for removing microbial contamination in food and bioprocessing industries. This study systematically compared three surfactants, sodium dodecyl sulfate (SDS), Ammonyx® LO, and Capstone® FS-30, to understand how surfactant type influences foam structure and cleaning performance. Stainless steel coupons contaminated with Bacillus subtilis 98/7 spores were cleaned under identical flow conditions using foam generated at a nominal air fraction of 0.5 and a mean velocity of 1.8 cm s -1 . SDS achieved the highest spore removal (1.9 log 10 reduction after 20 min), with superior kinetic detachment (K 1 = 114.75 s -1 ; f = 98.2%), while Ammonyx® LO (0.83 log 10 ) and Capstone® FS-30 (0.55 log 10 ) performed significantly worse. These differences were attributed to foam structural properties: SDS produced fine, stable bubbles persisting for 24 h, while Ammonyx® LO and Capstone® FS-30 collapsed after 8 and 3 h, respectively. Image analysis revealed local air fractions of 0.88 (SDS), 0.79 (Ammonyx), and 0.96 (Capstone) in the test section, confirming dry foam behavior. Theoretical analysis using Bretherton’s model and microscopic observations showed that SDS’s low capillary number promotes strong Marangoni stabilization and thin lamellae, generating sustained wall shear stress fluctuations essential for spore detachment. This study demonstrates that foam cleaning efficiency is primarily determined by surfactant-controlled variations in bubble size, foam stability, and interfacial properties. SDS provides the optimal balance for achieving efficient and sustainable foam-based cleaning.