Thermophile Survival in Milk Fouling and on Stainless Steel During Cleaning

Abstract

Surface attached thermophiles in milk powder plants that survive cleaning after a manufacturing run can recontaminate the plant, thus reducing the subsequent run time. This study compares thermophile survival in a milk foulant with the survival on stainless steel during an incomplete cleaning process. Multilayered milk fouling deposits were produced on stainless steel surfaces to simulate samples obtained from poorly cleaned areas of industrial milk powder plants. The process involved alternate cycles of surface inoculation and fouling in a pilot plant heat exchanger. Un-fouled stainless steel surfaces were subject to the same bacterial inoculation procedure as the fouled surfaces but without fouling development. Surfaces were cleaned in hot caustic (2%, 65°C) providing no agitation for durations up to 20 min. The cleaning conditions simulated the very low flow velocities found in hard-to-reach areas of milk powder plants. Impedance microbiology was used to assess microbial activity and confocal laser scanning microscopy was used to determine whether thermophiles remained at the surface. After 15 min of cleaning thermophile activity was detected on the fouled surfaces at a level of 105cfu cm−2 but no activity was detected on the un-fouled stainless steel after this time. Microscopy showed that biofilm still remained on the un-fouled stainless steel surfaces after 15 min of cleaning indicating that the cells were killed rather than removed. This suggests that milk fouling layers can provide thermophiles with greater protection from cleaning than a biofilm provides. Therefore it is important to target hard-to-clean areas in milk powder plants and ensure all fouling is removed to reduce the chance of recontamination in subsequent runs.