Fouling in thermal processing systems, particularly in dairy production, presents significant challenges in maintaining efficiency, product quality, and sustainability. It results in reduced heat transfer efficiency, increased energy consumption, and higher operational costs. Fouling can involve various components such as proteins, minerals, fats, and microorganisms, leading to substantial impacts on the dairy industry. Biofilm formation in heat exchangers, evaporators, and pasteurizers further complicates the issue.

This article explores the causes of fouling in dairy thermal processing, including pasteurization, ultrafiltration, and evaporation. It also investigates the impact of fouling on product quality and system performance, as well as strategies for mitigating fouling. Through understanding of the mechanisms behind fouling, this article offers insights into effective solutions that reduce fouling, enhance operational performance, and ensure food safety in dairy processing systems.

Introduction

Fouling is a significant issue in the dairy industry, particularly in the thermal processing of dairy products. Dairy products such as milk, cheese, dairy powders, yogurt, and cream are commonly subjected to heat treatment processes like pasteurization, ultra-high temperature (UHT) processing, evaporation, and concentration. These processes ensure microbiological safety, extend shelf life, and reduce microbial load. However, fouling during these processes remains an unavoidable challenge.

Fouling is characterized by the accumulation of unwanted deposits—proteins, minerals, fats, and microorganisms—on heat transfer surfaces such as evaporators, pasteurizers, and heat exchangers. As dairy processing systems become more complex and demand for energy efficiency and sustainability rises, fouling presents substantial challenges. Not only does fouling reduce heat transfer efficiency, but it can also impact product safety and quality, creating risks to both consumers and the production process itself.

This article examines the different types of fouling encountered in dairy thermal processing, the factors contributing to fouling, and the practical mitigation strategies that can be adopted to reduce its impact.

Types of Fouling in Dairy Products Thermal Processing

Fouling in dairy product thermal processing can be categorized into several types based on the nature of the deposits. Each type poses distinct challenges, depending on the specific dairy product and processing conditions.

• Protein fouling: Proteins, especially casein and whey proteins, are prone to denaturation when exposed to heat. Denaturation causes proteins to aggregate, forming a sticky layer that adheres to heat transfer surfaces. This protein layer acts as an insulator, reducing heat transfer efficiency.
• Mineral fouling (scale formation): Calcium and magnesium ions present in dairy products can form scale deposits on heat exchange surfaces. Mineral salts such as calcium phosphate and calcium carbonate tend to precipitate at higher temperatures, particularly in pasteurizers, evaporators, and heat exchangers. This scaling process impedes heat transfer and increases pressure drops in the system.
• Fat fouling: Fat globules, particularly in high-fat dairy products such as cream and butter, can stick to surfaces during thermal processing. These fats can undergo oxidation, which may lead to the formation of stubborn, greasy deposits that further degrade heat transfer performance and may affect product taste and texture.
• Biological fouling (biofouling): Microbial contamination is a significant concern in dairy processing. Bacteria, molds, and yeasts can colonize heat transfer surfaces and form biofilms. Biofilms are clusters of microorganisms embedded in a matrix of extracellular polymeric substances (EPS), which protect the microbes from cleaning agents and antimicrobial treatments. Biofilms pose serious food safety risks by potentially harboring pathogenic microorganisms.

“Raw milk contains microorganisms, which can form biofilms on heat transfer surfaces. These microorganisms can grow and proliferate, leading to further fouling and potentially affecting the safety and quality of the final product.”

Causes of Fouling in Dairy Thermal Processing

Fouling is influenced by several factors, ranging from the inherent composition of dairy products to the operating conditions in processing systems. Understanding these causes is crucial for developing effective fouling control strategies.

Causes of fouling in thermal processing of dairy products include:

• Composition of milk and dairy products: Raw milk is rich in proteins, fats, minerals, and water—all of which contribute to fouling. Casein proteins, which are sensitive to heat, can aggregate and form deposits on surfaces. Additionally, the high-fat content of dairy products can exacerbate fouling, especially in cream or whole milk. The mineral content, particularly calcium, can also lead to scale formation, especially under high-temperature conditions.
• Temperature: Temperature plays a critical role in fouling. High-temperature processes, such as pasteurization and UHT treatment, cause proteins to denature and minerals to precipitate. Excessive temperatures can also cause fats to melt and adhere to surfaces. Consequently, the temperature must be carefully controlled to balance effective pasteurization while minimizing fouling.
• Flow velocity and residence time: The flowrate and residence time in heat exchangers and evaporators can impact fouling. Low flowrates or inadequate residence time allow particles to settle on surfaces, increasing the likelihood of fouling. In contrast, high residence times may cause heat-sensitive proteins to denature further and form fouling layers.
• pH and ionic strength: The pH of dairy products significantly influences protein aggregation and mineral precipitation. Proteins are more likely to denature at pH levels outside their isoelectric point (approximately pH 4.6), which leads to the formation of aggregates. The ionic strength, which is influenced by the concentration of salts and minerals, also affects the tendency of calcium and magnesium salts to precipitate, leading to scale formation.
• Microbial contamination: Microbial contamination is a primary cause of biological fouling. Raw milk contains microorganisms, which can form biofilms on heat transfer surfaces. These microorganisms can grow and proliferate, leading to further fouling and potentially affecting the safety and quality of the final product.

Consequences of Fouling in Dairy Thermal Processing

Fouling in dairy thermal processing leads to several significant consequences, both in terms of operational performance and product quality. Understanding these impacts helps underscore the importance of fouling control.

• Reduced heat transfer efficiency: Fouling layers act as insulators, hindering the transfer of heat between fluids in the system. As a result, additional energy is required to maintain the desired temperature, leading to increased energy consumption. This inefficiency can significantly affect the overall performance and sustainability of the dairy plant.
• Increased operational costs: Fouling leads to higher maintenance and cleaning costs. Frequent cleaning cycles, as well as the potential need for equipment replacement, increase operational expenditures. Additionally, fouling may cause unscheduled downtime, further disrupting production.
• Decreased product quality: Fouling may result in uneven heating during pasteurization or UHT treatment, leading to under-pasteurization or inconsistency in product quality. Protein denaturation, oxidation of fats, and mineral scaling can negatively impact the texture, taste, and shelf life of dairy products. In the worst cases, these issues can lead to spoilage or off-flavors, which compromises consumer satisfaction.
• Safety concerns: Fouling, especially biological fouling, presents a significant risk to food safety. Biofilms can harbor harmful microorganisms, which may contaminate dairy products during thermal processing. If biofilms are not adequately cleaned, then pathogenic bacteria such as Salmonella, Listeria, and Escherichia coli can survive and contaminate the product, posing a public health risk.

“Clean-in-place (CIP) systems are widely used in dairy processing plants, employing both chemical and mechanical methods to remove fouling deposits.”

Mitigation Strategies for Fouling in Dairy Thermal Processing

To minimize the impact of fouling in dairy thermal processing, several strategies can be employed. These approaches range from optimizing processing conditions to implementing advanced cleaning technologies.

• Optimizing process conditions: Adjusting parameters such as temperature, flow velocity, and residence time can help reduce fouling. Lower temperatures can minimize protein denaturation and fat deposition, while optimizing flowrates can reduce the likelihood of particle settlement. Proper temperature control also prevents the supersaturation of calcium salts, reducing the risk of mineral scaling.
• Use of fouling inhibitors: Chemical additives such as phosphates can prevent protein aggregation and inhibit the formation of mineral scale. Antioxidants and emulsifiers can help reduce fat fouling by stabilizing fat globules during processing. These additives can be particularly effective in preventing fouling in high-fat products like cream.
• Regular cleaning and maintenance: Routine cleaning is essential for preventing the buildup of fouling layers. Clean-in-place (CIP) systems are widely used in dairy processing plants, employing both chemical and mechanical methods to remove fouling deposits. Using acidic and alkaline solutions, as well as high-pressure water systems, ensures that both protein and mineral deposits are effectively removed.
• Surface modification: The application of anti-fouling coatings to heat transfer surfaces can significantly reduce fouling. Coatings that reduce surface roughness or provide hydrophobic properties can prevent fat and protein deposition. Materials like stainless steel or specialized coatings can also reduce microbial attachment and biofilm formation.
• Microbial control: In systems prone to biofouling, it is crucial to implement antimicrobial measures to reduce microbial growth. Antimicrobial coatings, UV light, and biocides can be applied to prevent biofilm formation and microbial colonization on heat exchange surfaces.
• Membrane technology: In processes like ultrafiltration or reverse osmosis, fouling of the membranes can be mitigated by regular cleaning, chemical pretreatment of fluids, and the use of anti-fouling membranes. These measures help ensure high recovery rates and maintain process efficiency in membrane-based systems.

Biofilm Formation and Food Safety Risks

Biofilms formed by microbial growth during dairy thermal processing represent one of the most challenging forms of fouling. These biofilms are resistant to conventional cleaning methods and can harbor pathogens, leading to contamination of dairy products.

• Biofilm formation mechanisms: Biofilm formation begins when microorganisms adhere to heat exchange surfaces and produce extracellular polymeric substances (EPS). Factors such as surface roughness, temperature, and nutrient availability in dairy products influence microbial attachment. Once adhered, microorganisms proliferate and produce EPS, creating a protective biofilm matrix.
• Impact on food safety: Biofilms increase the risk of microbial contamination because they protect microorganisms from heat, chemicals, and mechanical cleaning forces. This protection can allow harmful pathogens such as Salmonella, Listeria, and E. coli to survive pasteurization or UHT treatment, potentially leading to foodborne illness. Biofilms may also harbor spoilage microorganisms, affecting the quality of dairy products.
• Mitigation of biofouling: Enhanced cleaning protocols, including stronger chemical cleaners and enzymatic detergents, can help break down the biofilm matrix. Surface modifications and antimicrobial coatings can prevent microbial attachment, while biocides such as chlorine dioxide and ozone are effective against biofilms.

Takeaway

Fouling in dairy thermal processing is a multifaceted problem with significant implications for product quality, safety, and system efficiency. Protein, mineral, fat, and biological fouling all contribute to operational inefficiencies and increased costs. The formation of biofilms, in particular, presents a considerable food safety risk by harboring pathogenic microorganisms that resist standard cleaning methods.

By understanding the mechanisms of fouling and employing strategies like process optimization, advanced cleaning technologies, and surface modifications, the dairy industry can reduce fouling-related issues. Ongoing research into improved fouling control techniques and the development of more effective cleaning systems will continue to enhance dairy processing efficiency and food safety, ensuring the production of high-quality and safe dairy products.

References Consulted and Further Reading

• Huppertz, T. and H. Nieuwenhuijse. "Constituent Fouling During Heat Treatment of Milk: A Review." International Dairy Journal 126 (March 2022): 105236. https://www.sciencedirect.com/science/article/pii/S0958694621002648.
• Flint, S., P. Bremer, J. Brooks, et al. "Bacterial Fouling in Dairy Processing." International Dairy Journal 101 (February 2020): 104593. https://www.sciencedirect.com/science/article/abs/pii/S0958694619302304.
• Sharama, A. and S. Macchietto. "Fouling and Cleaning of Plate Heat Exchangers: Dairy Application." Food and Bioproducts Processing 126 (March 2021): 32-41. https://www.sciencedirect.com/science/article/abs/pii/S0960308520305708.
• Daufin, G. and J.P. Labbé. "Equipment Fouling in the Dairy Application: Problem and Pretreatment." In Calcium Phosphates in Biological and Industrial Systems. Springer, 1998. https://link.springer.com/chapter/10.1007/978-1-4615-5517-9_19.
• Murphy, T.R., E.W. Finnegan, J. Tarapata, T.F. O'Callaghan, and J.A. O'Mahony. "The Impact of pH on Fouling and Related Physicochemical Properties of Skim Milk Concentrate During Heat Treatment Using a Laboratory-Scale Fouling Rig." Foods 13, no. 19 (September 2024): 3100. https://pubmed.ncbi.nlm.nih.gov/39410137/.