The role of surface tension of re-used caustic soda on the cleaning efficiency in dairy plants

During the cleaning in place with recycled cleaning solutions, an increase of suspended solids (SS) and a decrease of the surface tension (γ) of caustic soda (NaOH) solutions were observed for various types of dairy equipment. The decrease of γ was shown to result from the chemical reactions of the milk protein and fat with the cleaning solutions. In this work, the respective roles of SS and γ on the cleaning efficiency, cleanliness and cleaning rate were determined. The cleaning test consisted of an ultrafiltration membrane fouled with skimmed milk and cleaned using four NaOH-based solutions: newly prepared NaOH (high γ), re-used NaOH (low γ) with and without pre-clarification and a newly prepared commercial detergent solution. In the range of γ (28.7–74.0 mJ m^?2) and SS (0.0–1.7 g kg^?1) studied, the cleaning efficiency and the membrane hydraulic cleanliness were similar (0.91–0.94 and 0.69–0.71, respectively). However, it was found that solutions with a low γ resulted in a much faster cleaning rate. The presence of SS was mainly detrimental to the cleaning rate. For an efficient cleaning rate, high pH and low γ were important. A re-used NaOH solution with both low γ and SS was as fast as the commercial detergent solution at the same temperature (50 °C).