Understanding the Effects of UHT Processing and Storage on Milk Proteins

According to scientific research, ultra-high-temperature (UHT) processing and the subsequent ambient storage can cause significant biochemical changes to milk proteins. These alterations, which affect the whey and casein fractions, are critical to understanding the effects of UHT processing and storage on milk proteins and the product's overall quality and shelf-life.

Quick Summary

This article details the complex alterations to milk proteins during UHT processing and extended storage, covering thermal denaturation of whey proteins, casein micelle aggregation, non-enzymatic browning via the Maillard reaction, and enzymatic proteolysis leading to age gelation and sedimentation.

Key Points

• Whey Protein Denaturation: UHT heat causes whey proteins to unfold, exposing reactive groups that lead to aggregation and interaction with casein micelles.

• Casein Micelle Interactions: Denatured whey proteins covalently bind to κ-casein on the surface of casein micelles, slightly increasing micelle size and affecting stability.

• Storage Proteolysis: Heat-resistant enzymes like plasmin and bacterial proteases can survive UHT and cause slow protein degradation during storage, leading to age gelation and bitterness over time.

• Maillard Reaction Progression: The non-enzymatic browning reaction between milk proteins and lactose begins during UHT and continues during storage, causing changes in flavor, color, and reducing lysine's bioavailability.

• Age Gelation and Sedimentation: Long-term storage can lead to physicochemical instability issues such as age gelation, where a protein network forms, and sedimentation, where aggregates settle at the bottom.

• Processing Method Impacts: Direct UHT processing, with its rapid heating and cooling, can cause fewer chemical changes to proteins than the more severe thermal load of indirect methods.

• Nutritional Changes: While major nutrients like protein and calcium are generally stable, the Maillard reaction can cause some loss of the essential amino acid lysine, potentially reducing nutritional quality over time.

How UHT Processing Impacts Milk Proteins

Ultra-high-temperature (UHT) processing, involving heating milk to 135–150°C for seconds, ensures commercial sterility but also triggers chemical and physical protein changes, primarily denaturing whey proteins.

Whey Protein Denaturation and Aggregation

Heat-sensitive whey proteins unfold, exposing reactive sulfhydryl groups that form new disulfide bonds. This causes whey proteins to aggregate with each other and interact with κ-casein on the surface of casein micelles. These interactions increase protein particle size and are influenced by factors like pH and heating method.

Effects on Casein Micelles

Caseins, while more heat-stable, are affected by UHT as κ-casein dissociates from the micelle surface, allowing denatured whey proteins to bind. Some dephosphorylation can occur, though the core micelle structure remains largely intact during heating.

Long-Term Storage Effects on UHT Milk Proteins

Storage at ambient temperatures continues to alter UHT milk proteins through proteolysis and the Maillard reaction.

Proteolysis and Age Gelation

Heat-resistant enzymes like native plasmin and bacterial proteases can survive UHT. These enzymes slowly hydrolyze caseins during storage, particularly β-casein. This breakdown weakens micelles and can lead to 'age gelation', a network formation causing the milk to thicken. Bacterial proteases can also cause off-flavors like bitterness.

The Maillard Reaction and Other Cross-links

The Maillard reaction between sugars (lactose) and protein amino groups, initiated by UHT heat, progresses during storage. This non-enzymatic browning can make lysine unavailable, affects flavor and color, and contributes to protein cross-linking and sedimentation.

Comparison of Protein Changes in UHT vs. Pasteurized Milk

A comparison of protein changes in UHT and pasteurized milk highlights key differences in heat intensity, whey protein denaturation, whey-casein interaction, enzyme activity, proteolysis during storage, and the Maillard reaction. UHT processing involves much higher temperatures, leading to extensive, irreversible whey protein denaturation and significant whey-casein interaction. While both processes inactivate many enzymes, heat-stable enzymes like plasmin can survive both, potentially causing storage proteolysis in UHT milk. The Maillard reaction is significantly more pronounced in UHT milk and progresses during ambient storage, unlike in refrigerated pasteurized milk. UHT milk is prone to long-term storage issues like age gelation, whereas pasteurized milk has high stability over its shorter refrigerated shelf-life.

Conclusion

UHT processing provides a long shelf-life by significantly altering protein structure. Initial heat causes whey denaturation and casein interaction, while storage continues changes through proteolysis and the Maillard reaction. These processes affect flavor, color, texture, and nutrition, potentially causing age gelation or sedimentation. Maintaining quality requires controlling raw milk, processing, and storage. For further reading, a review of heat-induced changes is available {Link: NIH https://pmc.ncbi.nlm.nih.gov/articles/PMC8468757/}.

References

• Influence of Ultra-Heat Treatment on Properties of Milk Proteins.

Frequently Asked Questions

Does UHT processing destroy all milk proteins?

No, UHT processing does not destroy all milk proteins. The main effect is the extensive denaturation (unfolding) of whey proteins and some surface changes to casein micelles, but the amino acid building blocks remain largely intact.

Why does UHT milk sometimes taste different?

UHT milk can have a slightly sweeter, cooked flavor due to the high-heat treatment causing the Maillard reaction. This reaction between milk proteins and lactose produces new flavor compounds and can also lead to a minor increase in the milk's viscosity.

What is age gelation in UHT milk?

Age gelation is the formation of a protein network during UHT milk storage that causes the product to lose its fluidity and become semi-solid. It is primarily caused by the slow, heat-resistant activity of enzymes like plasmin, which degrade the casein micelles over time.

Are heat-stable bacterial proteases a problem in UHT milk?

Yes, heat-stable bacterial proteases are a significant issue. If the raw milk had high levels of psychrotrophic bacteria, their enzymes can survive the UHT process and cause off-flavors, especially bitterness, and contribute to age gelation during storage.

Does UHT treatment change the nutritional value of milk proteins?

UHT processing has a minor impact on protein nutritional value. The Maillard reaction can render some lysine amino acids unavailable, but overall, the milk's protein content remains high. Most heat-sensitive nutrients are not the main components of milk.

What is the difference in protein changes between direct and indirect UHT processing?

Direct UHT, which involves steam injection, is a more rapid process with a lower thermal load than indirect methods. Consequently, direct UHT tends to cause fewer chemical changes to proteins, resulting in better flavor and color preservation initially.

Does UHT milk lose protein over time?

UHT milk does not lose total protein content over time. However, the protein undergoes structural changes like aggregation and partial degradation by enzymes. These changes can affect the protein's stability and how it functions, leading to textural issues.