Surface conditioning of stainless steel coupons with skim milk solutions at different pH values and its effect on bacterial adherence

Abstract

In the dairy industry, processing surfaces contacting milk or milk residues at different pH values during washing and fermentation could change bacterial attachment behavior. In order to simulate and elucidate this process, stainless steel coupons were conditioned with skim milk at different pH values, and the influence of surface conditioning on bacterial adherence was investigated. When milk pH was decreased (by the addition of hydrochloric acid), milk proteins coagulated, and precipitates were made on stainless steel coupons. When milk pH was increased (by the addition of sodium hydroxide), little visible deposit was observed on the surfaces after conditioning. Obtained surfaces were then used to verify bacterial adherence by being exposed to a suspension of Lactobacillus paracasei subsp. paracasei NBRC 15889. The enumeration of adherent cells on these surfaces was carried out under a fluorescence microscope. After short contact with bacterial cells (30 min), surfaces which were previously conditioned with acidic milk, especially in pH from 3.8 to 5.5, had lower adherence in comparison with the control (untreated surface) and with basic milk samples. Conditioning at pH of 7.03 had the highest adherence in the tested pH range. Results also indicated that attachment increased with the exposure time. Very few bacterial cells appeared inside milk precipitates after 30 min of exposure. In contrast, many cells were observed to be present inside these precipitates after 12 h. To decrease bacterial adherence, surface conditioning with milk at low pH is recommended when bacterial contact is brief. Long periods of bacterial exposure are not advisable for any conditioning case.