Sedimentation in UHT Milk

Long-life milk is processed via sterilization or ultra-high temperature (UHT) and packed via an aseptic packaging system. Milk becomes shelf stable, resulting in milk with an extended shelf life. However, milk undergoes various changes during storage, which can reduce its shelf life.

Sedimentation is a process that occurs in milk during storage in which a layer of material, primarily protein, accumulates at the bottom of the pack. On storage, almost all UHT milk samples will form a sediment layer, however, this layer is usually thin. Furthermore, when stored in an undisturbed container for several months, casein micelles will gradually settle to the bottom; however, this settling does not normally create a densely packed and durable sediment and can usually be reversed by light inversion or shaking.

When sedimentation becomes excessive, generating a thick layer relatively quickly after manufacture, it is considered a defect. The mechanisms underlying the development of sediment in UHT milk are not well known, however, various factors have been examined and shown to be associated with sedimentation.

Sedimentation in packed UHT milk is caused by the formation of protein at the bottom of the pack. Sediment from ultra-high temperature (UHT) processed milk was composed of κ-casein depleted casein micelles, with low levels of denatured whey protein.

The quality of the raw milk, the type and intensity of heat treatment, pH change, Ca ion concentration, storage temperature, and duration all influence sedimentation.

• Raw milk Quality: Protein is thought to coagulate on heat treatment. Protein sediment formation can occur with an unbalanced protein composition or excessive protein breakdown. High microbial loads in raw milk can promote enzymatic activity and bacterial development, resulting in milk component breakdown and sedimentation. Because the UHT process does not eliminate naturally occurring lipase and proteases, they play an essential role in UHT milk stability.
• Protein Aggregation: Casein, a protein found in milk, can aggregate and precipitate under specific conditions. Heat treatment during UHT processing can cause protein denaturation or structural alterations, resulting in protein aggregation and sediment formation. These aggregated proteins may eventually settle to the bottom of the milk container.
• Calcium & Minerals:?The heating during the UHT process might increase the precipitation of natural minerals, particularly calcium, resulting in their deposition as sediment in the milk. The calcium level of UHT milk can also influence sedimentation. Milk with a high calcium concentration will sediment more than milk with a low calcium content.
• Homogenization:?The homogenization process can also affect the likelihood of sedimentation. Milk that is not homogenized is more likely to sediment than homogenized milk.
• Storage conditions:?Milk that is stored at high temperatures is more likely to sediment than milk that is stored at lower temperatures. Physicochemical changes leading to accelerated sedimentation of UHT milk at high storage temperature primarily involve changes in the secondary structure of proteins due to increased proteolysis and protein crosslinking through Maillard-type reactions.

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Numerous tactics and formulation elements can be utilized in an industrial UHT facility to reduce sedimentation in packed UHT milk. Raw milk quality is critical, and it is critical to guarantee that it is of high quality before processing.

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• Heat Treatment Optimization: The UHT process has an impact on sedimentation. Using a milder heat treatment will help to preserve the stability of the casein micelles and reduce the likelihood of aggregation. It is crucial to optimize the heat load applied. Similarly, the direct UHT process tends to produce more sedimentation than indirect UHT systems.
• Particle Size Control: Controlling the particle size of casein micelles is crucial for minimizing sedimentation. Proper homogenization helps reduce the size. Emulsification is ensured during homogenization and drag force on heavy particles is reduced. Downstream homogenization is employed to reduce sedimentation. In-container sterilization of milk may increase sedimentation if the heating profile is not optimized.
• Adjust the pH:?Adjusting the pH of UHT milk to a higher level will also help to reduce the likelihood of aggregation. This can be achieved by adding a buffering agent. Milk with a pH below 6.6 has a high likelihood of sedimentation.
• Reducing the calcium content:?Reducing the calcium content of UHT milk will also help to reduce the likelihood of aggregation. This can be achieved by using milk that is low in calcium or adding a chelating agent.
• Stabilizers and Emulsifiers: Adding stabilizers and emulsifiers to the milk formulation can improve its stability and prevent sedimentation. These ingredients can help maintain a homogeneous distribution of fat and protein, reducing the risk of sediment formation. Carrageenan is widely applied in this regard.
• Packaging: Selecting appropriate packaging materials and ensuring their compatibility with UHT milk is essential. Packaging should provide effective barriers against light, oxygen, and other factors that can contribute to sedimentation.
• Storage Conditions: After UHT processing, storage temperature should be controlled and maintained below 25C. Good rotation of stock to ensure FIFO (First-In, First-Out) can also prevent the accumulation of older milk that may be more prone to sedimentation.

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An industrial UHT facility can reduce sedimentation in packs and produce UHT milk with increased stability and quality by following these steps and taking formulation considerations into account. To assure the effectiveness of these tactics and make required adjustments, it is critical to undertake regular testing and monitoring.