Abstract
In the cleaning operations of heat exchange surfaces in dairy processing plants, the effect of heating/run time (HT) and aging on the fouling/cleaning of heat exchangers is not well understood. Longer heating time of the same deposit is expected to result in the formation of stronger and perhaps more cleaning resistant deposit. In this study, the phenomenon of heating and aging on the formation and cleaning of dairy fouling is investigated using the heat induced whey protein gels (HIWPG) produced in laboratory. The processes were also investigated with the whey protein deposits formed in pilot-scale plant trails. The HIWPGs were produced in tubular capsules for various heating (or run) times (60, 120, 240, 1440 and 2880 min respectively) and then dissolved in aqueous sodium hydroxide (0.5 wt %). The heating process here would have been of ‘pure’ aging. The dissolution rate was calculated based on the previously established UV Spectrophotometer analysis. The structure and texture of the gels were analysed using Scanning Electron Microscope (SEM) and texture analyser. In the pilot-scale plant study, whey protein fouling layers were generated by recirculating whey protein solution (6 wt %) for various heating periods (30, 60, and 90 min respectively). The deposit layers were then removed by recirculating aqueous sodium hydroxide (0.5 wt %) and the cleaning efficiency was monitored in the form of the recovery of heat transfer coefficient while both fluid electric conductivity and turbidity were recorded as indications of cleaning completion. It was found that increasing the HIWPG heating time significantly increased the gel hardness and dissolution time, implicating the difficulty in cleaning. Similarly to these results on gels, increasing the pilot-scale plant heating/run time increased the extent of fouling. The fouling layer formed next to the metal surface experienced the longest period of aging and the slope of the heat transfer coefficient increase seen at the final cleaning stage is related to this aging effect. The rate of cleaning for deposits formed initially on the metal surface is lower indicating a ‘pure’ aging effect of the deposit near surface.
Highlights
It is known that heating of dairy fluids is a normal operation for pasteurisation or varying viscosity purposes
More recent studies have shown that aging facilitates longer reaction time between the fouling layer matrixes (Liu et al, 2002) and affects the fouling process resulting in a strong and firm fouling layer that is difficult to clean when the fouling layer is aged for a long period (Wilson, 2005)
The scanning electron microscope (SEM) images of the heat induced whey protein gels (HIWPG) deposits microstructure show that the HIWPG deposit formed at shorter heating time (30 min) contains small and perhaps undeveloped aggregates, while the HIWPG deposit formed at longer heating time (1440 min) contained more and larger aggregates
Summary
It is known that heating of dairy fluids is a normal operation for pasteurisation or varying viscosity purposes. Longer HT means that the fouling layers on the heating surface will experience aging. Though often mentioned as being an important process, the aging process of the fouled dairy deposits is poorly understood in a quantitative manner. It was early documented by Epstein in 1983 as one of the principal mechanistic stages in the fouling process. More recent studies have shown that aging facilitates longer reaction time between the fouling layer matrixes (Liu et al, 2002) and affects the fouling process resulting in a strong and firm fouling layer that is difficult to clean when the fouling layer is aged for a long period (Wilson, 2005)
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