Boric acid–induced suppression of culturable microorganisms in water-miscible metalworking fluids under industrial conditions

• Boric acid suppresses culturable microorganisms in industrially aged MWFs. • Complete CFU suppression is achieved at boric acid concentrations ≥ 3 wt.%. • Identical inhibition thresholds are observed in filtered and unfiltered samples. • Microbial suppression is chemically driven rather than mechanically induced. • Boric acid enables rapid short-term microbial control under industrial conditions. Microbial contamination of water-miscible process fluids represents a long-term technological, hygienic and operational problem. Although the microbial degradation of these fluids and the antimicrobial effect of boric acid have been repeatedly described in the professional literature, systematic studies conducted on real industrial fluids aged in operation are still limited. Moreover, previous work usually does not sufficiently separate the chemical inhibitory effect of boric acid from the mechanical effect of filtration, and the cultivation results are often interpreted without explicitly taking into account the fact that they capture only the culturable part of the microbial population. Therefore, the presented study evaluates the effect of boric acid concentration on the microbial contamination of a real operationally loaded process fluid taken from an industrial machining center after long-term use, with a parallel comparison of filtered and unfiltered samples. The novelty of the work lies in the methodological separation of chemical inhibition from the mechanical effect of filtration and in the careful interpretation of the cultivation results as a manifestation of the suppression of culturable microorganisms under defined experimental conditions. Microorganisms were determined by standardized cultivation procedures after exposure to different concentrations of boric acid, which allowed assessing changes in microbial load depending on chemical treatment and sample condition. The results thus provide a methodologically more precisely defined and industrially relevant evaluation of the short-term use of boric acid for suppressing microbial activity in contaminated process fluids