The impacts of protein concentration, milk solids content and heat/run time "AGING" on the fouling and cleaning of a dairy heat exchange surface

The mechanisms of the fouling and cleaning processes of dairy heat exchangers are affected by various factors such as the bulk fluid protein concentration, the bulk fluid solids content and the heating/run time 'aging' during the processing of dairy fluids. Little has been published about the effect of the last two factors on the fouling and cleaning processes in dairy plants. In this study, laboratory produced heat induced whey protein gels (HIWPGs), and fouling layers from different dairy solutions (produced using a pilot-scale test rig) were used to investigate the effect of protein concentration, milk solids content and heating/run time on the fouling and cleaning of dairy heat exchange surfaces. HIWPGs were used, because they are considered to be a suitable model material for studying proteinaceous milk fouling. This is because HIWPGs have the same nature as type "A" milk fouling and they are easy to reproduce and to dissolve in the laboratory under well-controlled conditions. The results from the HIWPGs investigations provided a good foundation for the pilot-plant investigation. In the HIWPGs studies, the effect of protein concentration and the effect of the heating/run time on the formation and dissolution of HIWPGs were investigated using HIWPGs made from different protein concentrations (14, 20, and 26 wt %) and HIWPGs produced at various heating times (60, 120,240, 1440 and 2880 minutes). The HIWPGs were formed in tubular capsules and then dissolved in aqueous sodium hydroxide at an optimal condition of 0.5 wt % at 60°C. The dissolution rate calculation was based on a UV Spectrophotometer analysis. The microstructure and texture of the HIWPGs was analysed using scanning electron microscope (SEM) and texture analyser. It was found that increasing the HIWPGs protein concentration from 17 wt % to 26 wt % significantly increased the gel hardness and the penetration force from approximately 8N to 48N and decreased the dissolution rate from 0.34 g m-2 S-1 to 0.12 g m-2 S-I. It was also found that increasing the heating time from 60 min to 1440 mm significantly increased the gel hardness and penetration force from approximately 3N to 25N, and decreased the dissolution rate from 0.40 g m-2 S-1 to 0.24 g m-2 S-I. The HIWPGs contained larger aggregates and the structure became more rigid, implicating the difficulty in cleaning. In the pilot-scale plant studies, the effects of bulk protein concentration, bulk solids content and heating/run time on the fouling and cleaning of dairy heat exchange surfaces were investigated using fouling layers produced from different concentrations of whey protein fluid, different solids content of milk fluid and fouling layers produced at various heating/run times. The fouling layers were produced by re-circulating the dairy fluids in the pilot-scale test rig. The dairy fluids used include: reconstituted whey protein, fresh whole milk, fresh skim milk, reconstituted whole milk, and reconstituted skim milk. The cleaning of the fouling layers was achieved by re-circulating aqueous sodium hydroxide (0.5 wt %) at 60°C, and the cleaning efficiency was monitored in the form of the recovery of the overall heat transfer coefficient while both fluid electric conductivity and turbidity were recorded as an indication of the cleaning completion. The microstructure structure of the fouling deposits was analyzed usmg scanmng electron microscope. It was found that the pilot-scale plant study results mirror the results from the HIWPGs study, increases in the bulk protein concentration, bulk solids content and the heating/run time in the pilot plant study significantly increased both the rate of fouling and the time required to remove the fouling deposit. The slopes of the overall heat transfer coefficient versus time in the cleaning curves provide a good indication of the effect of bulk protein concentration, bulk solids content and aging, particularly for the fouling formed at the earliest stage (the inner fouling layer). The inner fouling layers were affected by the initial high bulk concentration and experienced the longest aging period during the heating time, and they were the hardest to clean. Contribution of this research This research has contributed reliable data for improving the understanding of the effect of protein concentration; solids content and dairy fluids heating/run time on the fouling and cleaning of dairy heat exchangers, and will provide a strong basis and guide the decision for further studies.