Pasteurization is a “process of heating fluid milk products to render them safe for human consumption by destroying the disease-producing organisms (pathogens). The process inactivates approximately 95% of all microorganisms in milk” . The scientist Louis Pasteur is the eponym of the process, after having discovered that microbes could be killed in wine by raising its temperature below the boiling point. Pasteurization has one more advantage, aside from making milk safe for human consumption: it can improve shelf-life up to 16 days .
Two processes are used to reach the goals above: batch or continuous, each employing different combinations of temperature and time. Of the two, the continuous process is the one more widely used due to its economic advantages. In this method, the heat treatment is commonly accomplished with a plate heat exchanger heating the milk to temperatures above 60°C. When milk is heated above this temperature, milk fouling starts to form.
Fouling can be defined as the continuous accumulation of undesired material on the heat exchanger surfaces (See Fig. 1). For milk, this undesired material is mainly composed of calcium phosphate and whey protein . In the dairy industry, every end product requires heating in some part of the process. These deposits cause severe impact on the plant’s economy, as they hamper heat transfer, increase resistance to flow (and if dislodged, they may cause contamination), and enhance pressure drop.
As the equipment’s performance steadily declines, a point is reached where insufficient heat affects product quality during steps such as pasteurization and sterilization. Before this moment, the equipment must be taken offline for cleaning.
In an interesting parallel with oil & gas, the cost of cleaning milk related heat exchangers is relatively low when compared to the cost of halting production. And in contrast with oil & gas, fouling triggered shutdowns are more related to product quality than heat exchanger performance . According to , “milk fouling is so rapid that heat exchangers need to be cleaned every day to maintain production capability and efficiency and meet strict hygiene standards. And about 80% of the total production costs in the dairy industry can be attributed to fouling and cleaning of the process equipment”.
Types of heat exchangers
There are basically three types of heat exchanger used in the dairy industry: plate, tubular and scraped-surface. Plate heat exchanger (PHE) is the most common, due to its relatively higher convection coefficient, lower propensity of fouling (high turbulence), easy of cleaning and compactness (Fig. 2). “Tubular heat exchangers can be used when long running times are essential (Figure 3). Scraped-surface heat exchangers (Fig. 4) are used for viscous products” .
Dealing with Fouling
Two recent mitigation techniques have been selected as examples of how the dairy industry is dealing with fouling. One is based on treatments for the heat surface and other is based on the elimination of the heat surface altogether.
Surface properties like roughness, composition and surface energy are known to affect fouling behaviour .
For example, the strength at which the deposits adhere to the metal surface can be reduced “by either decreasing the surface energy of the metal or by coating the metal surface with low surface energy materials” . One coating process that has gained much favor in the food industry is electroless plating of nickel based alloys. Electroless plating “is an autocatalytic method in which the reduction of the metallic ions in the solution and the film deposition can be carried out through the oxidation of a chemical compound present in the solution itself, i.e., a reducing agent, which supplies an internal current. The process requires that a cation of the metal to be deposited is reduced by the receiving electrons, from the surface of a metal substrate or from the surface of the catalysts used to initiate the deposition. The reductant in turn delivers electrons to this surface and is thereby oxidized” . Electroless nickel plating is today regarded as the most efficient coating to avoid milk fouling. Particularly the combination Ni–P–PTFE (Nickel-Phosphor-Poly-Tetrafluorethylene), which is made by applying a polytetrafluoroethylene (PTFE) layer into a Ni-P alloy coating. These treatments not only reduce fouling rate but also the time required for cleaning . Current research is now looking into electroless nano-coatings as a promising technology. The main difference from traditional coatings reside in the thickness of the deposited film, which is measured in nanometers, or 1×10-9 meter. It is expected that such coatings have a superior anti-fouling characteristic, but, as of today, there are still issues related to a relative fragility in an industrial scenario .
You cannot foul a surface that does not exist. If there was a way of heating milk without a heat surface, then fouling would not be a problem. The so-called “Non-Surface” heat transfer methods do exactly that. One of the most common is direct steam injection, but for the food industry the most used method is Ohmic heating or direct resistance heating. This method heats milk by passing an electrical current through it, just like an electrical wire heats up when current flows through it. Although in ohmic heating there is no “heat surface”, the method does employ electrodes immersed in the fluid. These electrodes do get fouled with time, but as the fouling adds resistance, the effect is to increase the temperature in the fluid (in contrast to a conventional heat exchanger where fouling decreases temperature). As this causes a dramatic change in the temperature profile, the electrodes also require periodic cleaning . Fig. 5 presents a commercial ohmic heater.
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