Evaluation of UHT milk processed by direct steam injection and steam infusion technology

Abstract: UHT direct steam injection and steam infusion are widely used; however there is no comparison of their impact on milk components. This study evaluates the structural changes at different steps during the UHT processing of whole milk in a full-scale UHT plant by transmission electron microscopy (TEM) and particle size measurements, and follows the enzymatic activity.

The results of structural analyses by TEM show the formation of agglomerates during the final heating step using both technologies; however in samples from steam injection, the agglomerates were larger and more frequent. Regardless of the heating system, big agglomerates were still present after the vacuum cooling, but very rarely in the final product. The creation of big protein agglomerates in the milk from steam injection could be a result of the accumulation of proteins on the surface of steam bubbles formed during the introduction of steam into the product and may influence its stability at the early stage of storage (Paper II.).

Sedimentation was the only parameter significantly dependent on the final heating technology and preheating temperature. The amount of sediment was significantly higher in milk processed by steam injection and when preheated to 75ºC compared to 80ºC. Sediment in all samples contained large protein particles, some of them containing fat globule agglomerates covered by a thick protein layer (Paper III.).

The casein micelles in milk stored at 5ºC, 22ºC and 30ºC had an irregular surface that reduced in size on storage while tendrils became extended. These structural changes led to gelation within four or five months? storage at 22ºC and 30ºC. At 40ºC the tendrils dissociated and the samples never gelled. The absence of gel formation could be a combined effect of plasmin activity and lactosylation (Paper III.).

The volume-weighted mean diameter of casein micelles in samples produced by steam infusion was larger than in samples from steam injection. This could be explained by the b-lactoglobulin/?-casein complex being better anchored to the casein micelle surface than in samples from steam injection, where the complex was released to a greater extent or mechanical damage of casein micelles during steam injection leading to micelle disintegration and disaggregation (Paper III.).

The structural changes and localization of the individual proteins in UHT milk kept at 22ºC or 40ºC for 6 months were followed using transmission electron microscopy combined with immunogold labelling on total caseins, ?-casein and ?-lactoglobulins A and B. In the freshly produced UHT milk, the labelling of total casein concentrated mainly on the micelles, while ?-lactoglobulin and ?-casein were localized in the serum phase and on the micelle surface. In the stored samples, tendrils protruding from the micelles showed positive labelling against all the analyzed proteins. The heavy particles that sedimented after 6 months storage showed positive labelling for total casein but not for ?-casein suggesting that the sediment composition based on caseins was most probably the hydrophobic parts of the casein micelles or protein fragments from proteolyses by plasmin (Paper IV).

The enzymatic activity was the same in milk produced in both systems; however the sedimentation rate was greater in samples treated with steam injection. Based on that, we can say that analyses of results of proteolytic action measured by capillary electrophoresis of supernatant cannot alone be used for predicting the shelf life or stability of UHT milk (Paper I.).

Plasmin activity was negligible at 5ºC, increased with temperature at 22ºC and 30ºC, but decreased at 40ºC. Gel formation occurred in samples stored at 22ºC and 30ºC, but not at 5ºC and 40ºC. Lactosylation started at 22ºC and increased with increasing storage temperature. After 6 months storage at 40ºC, all ?-lactoglobulin molecules contained at least 2 lactose residues, and the proteose-peptones were also lactosylated. The extensive lactosylation at 40ºC may play an important role in the absence of gelation. Destabilization of casein micelles and restructuring of proteins have a huge impact on both sediment and gel formation (Paper I.).