The effect of milk fat globule membrane damage in the absence of air on fouling in heat exchangers : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology of Massey University
The fouling by whole milk of processing plant surfaces, especially in heat exchangers, is a serious problem, but is incompletely understood despite extensive past investigations. While milk fat has generally been thought to play a minor role in fouling, the results of some previous work suggest that this is not always the case. The state and form of the fat, as well as processing conditions, may have effects on milk fouling behaviour. Careless mechanical handling of whole milk is known to cause fat damage. The present study set out to investigate the effect on fouling of damage to the milk fat globule membrane (MFGM) by mechanical stresses in the absence of air. Pasteurised non-homogenised whole milk was deliberately stressed to differing degrees by passing it through a cavitating pump a variable number of times. The extent of damage was measured using four different techniques: a free fat (FF) test (a modified extraction method), a lipolysable free fat (LFF) test (free fatty acid determination after incubation of the sample with pig pancreatic lipase, a technique developed during this work), particle size distribution (PSD) measurement by laser light scattering, and confocal laser scanning microscopy (CLSM). The fouling behaviour of both damaged and undamaged milk was investigated by heating the milk from about 4°C to about 94°C in a custom built double pipe heat exchanger, which could be disassembled easily to access the fouling layer. Milk flowed in the annulus, with a Reynolds number range of about 220-310. The fouling rate was measured and expressed as the rate of increase of the overall resistance to heat transfer, normalized using the overall heat transfer coefficient determined at the start of a run. The fouling rate exhibited by damaged milk (normalized by the rate for undamaged milk, to account for batch-to-batch variation) was found to increase significantly with the extent of cavitation treatment. There was also a clear positive relationship between both the FF and LFF contents of milk and the extent of cavitation treatment, suggesting strongly that the observed increases in fouling rate were the result of increased MFGM damage. PSD measurement and CLSM both showed that cavitation caused the appearance in the milk of some large, irregularly shaped fat globules, presumably the result of coalescence. The FF results, and observation by CLSM, indicated that only a small proportion (> 6°C) of the total milk fat had to be measurably damaged to cause extensive fouling. The fat contents of the fouling layers were found to be very high (>45% on a dry weight basis). Although some of the experimental conditions, especially the low Reynolds numbers, may have contributed to this result, other fouling investigations made in New Zealand have produced similar results. It is hypothesised that large globules formed by the coalescence of native globules whose membrane have been damaged could migrate more easily to the stainless steel heating surface. There, they could act as anchor points for the build-up of a fouling layer with a continuous protein phase. This hypothesis is supported by CLS micrographs of the fouling layer. Further investigation is warranted. Recommendations are made for improving the methods used to measure MFGM damage, fouling and fouling rate, and the structure of the fouling layer.