Technology of drinking milk: pasteurized milk, sterilized and UHT


To become fluid milk, raw milk must undergo the physical treatment, such as clarification, standardization, homogenization and heat treatment course. Only the latter which differentiate pasteurized milk, sterilized milk and UHT milk, and other treatments are almost identical for these products.

Before addressing the technical preparation of milk consumption, we will present first time, the steps are common to all these products. Then we will introduce the techniques specific to each product: Pasteurized milk, sterilized milk and UHT milk

Common Steps


The step of receiving the milk must be done without damage to fat globules or incorporation of air into the conduct of milk while maintaining the necessary quality controls, these are detailed in the chapter on quality control of milk and milk products .


The clarification is the process by which milk is subjected to a centrifugal force in order to extract more dense particles, such as cellular debris, leucocytes and foreign material. Without this treatment, these particles sediment in homogenized milk, to become visible in transparent containers.

The principle of operation of a clarifier is identical to that of a centrifugal separator (Figure 1). The characteristic difference between a clarifier and a centrifugal separator is the design of the stack of discs: holes without distribution in the clarifier and the number of outlets (one on the clarifier, two on the splitter).

The choice of location clarifier may depend on certain factors, including the ability of the device and the method of standardization. The clarification is accomplished by either of the following steps:

  • At the reception, prior to storage of raw milk;
  • Between storage and standardization;
  • Between standardization and the entrance to the pasteurizer;
  • Between the regeneration and the heating plate pasteurizer, in which case the device is a combination separator-clarifier.


In many large dairies, it is not possible to pasteurize and process the milk immediately after receipt. Part of the milk must be stored in tanks for several hours or several days. In these conditions, even refrigeration is not enough thrust to avoid serious deterioration of quality.

Many dairies thus preheat the milk to a temperature below the temperature of pasteurization, to temporarily inhibit the growth of bacteria, including pathogens. This process is called thermisation. The milk is heated to 63-65 ° C for about 15 seconds, a combination of temperature and time n'inactive not that the enzyme phosphatase.

To avoid the proliferation of aerobic spore forming bacteria after thermisation, milk must be cooled rapidly to 4 ° C or less and must not be mixed with untreated milk. Many experts believe that thermisation has a positive effect on some spore forming bacteria. Heat treatment returns many spores in a vegetative state, and they are destroyed during pasteurization of milk later.
Thermisation is only used when the dairy is unable to process all deliveries. Otherwise, this step is ignored.

Separation of fat (Skimming)

Although the lipid and aqueous phases of milk do not mix, decanting and spontaneous coalescence of fat globules on the surface of milk is slow, so they are accelerated by means of centrifugal separators which discharge continuously cream one hand, and skim milk on the other. Figure 1 illustrates schematically the operation of these devices.

Principle of operation of a centrifugal separator

The amount of fat that we can separate the milk depends on the design of the separator, the flow of milk through it and the size distribution of fat globules. In general, removing 100 l of milk, 10 liters of cream 35-40% fat. Skim milk contains no more that about 0.1% fat.


By offering its customers a choice of milk at different fat content, the company should adhere precisely to the standards established for each of these levels. In practice, this means accurate measurements during standardization, both compliance with regulations for reasons of efficiency and economy.

Standardization can be done in batch or continuous. In the first case it is to mix in a container of whole milk, skim milk or the cream in proportions calculated to arrive at the percentage of fat in the desired mixture.

As for the continuous process, it may be more or less automatic. This can to a controlled injection of skimmed milk in whole milk pasteurization direction (Figure 2). This system can be easily controlled, since the mixing is done with two products whose fat content is known. But increasingly rely on one type of fully automated device: The separator-clarifier-normalizer is indeed programmed remixes for skimmed milk and cream, separated in a first step in a partially skimmed or total milk. To obtain the desired fat content, the mixture is carried out in controlled proportions according to the results of a system for continuous analysis. A special release is to evacuate the aircraft surplus cream. In the manufacturing, this method of standardization is out of the regeneration of raw milk pasteurizer.

In facilities where, for reasons of efficiency and energy conservation, only the cream passing through the homogenizer, the fat must be reduced to 10-12% in order to obtain satisfactory homogenization. This is done by injecting the cream at the entrance to the homogenizer, part skim milk from the separator.

Principle of direct standardization


Pasteurization is a heat treatment sufficient to moderate and the destruction of pathogenic microorganisms and a large number of microorganisms of alteration. This treatment first, to ensure the safety of the product and secondly, to improve its keeping quality. This step is used to produce multiple products such as pasteurized milk and pasteurized butter.

The product undergoes scales of temperature and time (63 ° C for 30 minutes or 73 ° C for 16 seconds). However, in order to obtain prolonged storage of pasteurized milk, it generally applies a more severe temperature and / or holding time, while avoiding areas exceed limits beyond which the milk would taste of cooked or suffer a reduction of excessive nutritional value.

Pasteurization tank at low temperature is still used, in particular to prepare different types of products in small quantities. But it is the continuous process at high temperature (HTST: High Temperature, Short Time), by systems in plaques (Figure 3), which is most in use. It is an economical method, accurate and reliable. The equipment can operate at great speeds and be fully automated.

Operating principle of a plate heat exchanger

To improve the flavor of milk, can be integrated in pasteuriser a vacuum chamber which would remove certain volatile compounds. However, equipment for this treatment requires more monitoring and increases the losses when changing products.

Pasteurization can also destroy the lipase, before homogenization, or immediately after, which prevents the lipolysis. In the same vein, it is important to avoid mixing of raw milk, even small amounts, pasteurized homogenized milk because raw milk lipase cause rancidity in homogenized milk.

It remains to note that the use of a plate system for pasteurization of many different products, especially for small volumes, reduces the efficiency of operation, increases the risk of quality deterioration by addition of water and cause the loss of fat on the walls if the product-water mixtures are not fully recovered. Figure 4 shows the time needed to change in percentage of fat at the start and stop of a plate pasteurizer.

Changing the fat in a heat exchanger


Homogenization is a process that serves to prevent the fat globules rising to the surface of milk by reducing their diameter. It is obtained by passing milk under high pressure through orifices or valves very close (Figure 5).

Principle of operation of a homogenizer

Because it has the advantage of stabilizing the emulsion of the fat evenly dispersed throughout the liquid, the homogenization of milk consumption was widespread. On the other hand, this treatment gives the milk flavor and texture softer, more smooth for the same fat content.
One consequence of physico-chemical homogenization is that it slightly affects the stability of proteins, in that the milk is homogenized clot more easily, under the influence of heat, for example, that the same milk not homogenized. Homogenized milk curd end will give a soft, more porous and permeable.

The efficiency of homogenization depends mainly on three factors: temperature, pressure and type of valve. The mechanical condition of the homogenizer, the incorporation of air into the circuit and the nature of the treated products can also modify the effects of treatment.

It goes without saying that the homogenization should take place at temperatures that keep all the fat in the liquid state, otherwise there would be of churning: to ensure effective treatment requires temperatures above 54 ° C .

The choice of the pressure depends on the type and number of homogenising valves. Most often the homogenization is done in two stages by means of two valves that are broken in the second, aggregates of fat globules formed after the passage through the first. In this case, a pressure of 14,000 to 17,000 kPa first stage and 3000 (Kpa) in the second generally gives good results. However with certain types of valve, such as multiple stages, the homogenizer can give a good performance while operating at significantly lower pressures.

Homogenization must be sufficiently effective to prevent crémage. The result can be verified immediately by determining the index of homogeneity (see Chapter 5: Quality control). Another method can also be used to measure at the microscopic examination of a sample of milk, the size of fat globules and their distribution according to their diameter.


All microorganisms were not eliminated by pasteurization, the heat treatment must be followed by an abrupt cooling. Thus, after pasteurization, milk is cooled to a temperature near freezing to slow the development of germs still present.

Stage post-pasteurization and during the packaging, it is also important to avoid contamination, especially by psychrotrophic bacteria, which are the main responsible for the subsequent deterioration of pasteurized products. Jezeski and Greene (1954) have indeed shown that pasteurized milk inoculated Pseudomonas fluorescens deteriorated after four days at 10 ° C, 16 days at 5 ° C and 36 days at 0 ° C.


Intended to convey the fluid milk products in networks of production and distribution, the container must have certain qualities:

  • be attractive in its form and presentation;
  • provide effective protection to the product against physical shock, light and heat;
  • preserve the contents of odors or flavors;
  • Facilitate handling of the product;
  • being adapted to modern requirements of production.

Types of containers

Cardboard container, despite its low resistance, is appreciated by consumers for its appearance, its form and convenience. It also affords good protection of the product. Made of cardboard coated with polyethylene, it is generally pre-assembled at the factory. Sometimes more sophisticated manufacturing to respond to particular uses, it can be covered with a thin layer of aluminum on its inner surface, forming an effective barrier against the ingress of oxygen and better preserve the product.

The equipment used for this type of packaging are numerous and offer great flexibility and speed. The entire operation, including the mounting of the container, filling, sealing and crating, is increasingly integrated to meet the new requirements of plants.

The plastic container, as rigid or flexible, is widely used in the dairy industry. The advantages of the flexible form are: lower cost, the ability to manufacture or assemble the containers directly on the dosing, reducing the need for storage space, the lower price of metering required. .. Among the disadvantages of flexible packaging, handling difficult point for the consumer and a product of inadequate protection against the rays of light.

The glass container, although virtually disappeared from many markets, still exists in some areas of Canada and especially the United States. Its rigidity provides some protection against physical shocks and makes the product more attractive.

Conditioning devices

Facilities vary depending on the type of packaging used and the desired pace, they are 2 types: those which operate by gravity and those so-called volumetric generally manufactured in stainless steel for parts that come into contact with the food and provided a cleaning system.

It is advantageous to have an elevated tank to supply the filling by gravity. Operating without pumping, this arrangement avoids the incorporation of air into the product, facilitates the precision of filling, draining and cleaning by circulation.

Pasteurized milk

The design of processing lines pasteurized milk trade varies greatly from one country to another, and even a dairy to another, according to the law and local regulations. For example, the possible standardization of the fat may occur before, during or after pasteurization. Homogenization may be total or partial etc.

The process "as simple" is to simply pasteurise whole milk. The processing line comprises a pasteurizer, a tank and a filling machine. The process becomes more complex if it must make several types of dairy products trade, ie whole milk, skim milk and standardized milk at different fat content and the cream at different levels fat. Figure 6 represents an example of diagram of manufacture of pasteurized milk: Raw milk is clarified, preheated, skimmed, standardized, homogenized, pasteurized, cooled and packaged aseptically.

Pasteurized milk must be kept cold (4 to 6 ° C). Its shelf life is approximately 7 days. However, a retention period shorter less may be imposed by the regulations of certain countries.

On the safety, pasteurized milk is supposed to be free of all pathogens. It is therefore no need to bring to the boil before consumption.
On the nutrient, pasteurized milk is richer compared to the sterilized milk.

Diagram pasteurized milk

Sterilized milk

Sterilization is to destroy, by a high heat, all of the microbial flora of milk. The milk then presents a full guarantee of hygiene and conservation.

The milk is pre-sterilized at 130 ° C for 3 or 4 seconds, and then bottled "hot" at 80 ° C, and sterilized in its packaging at 115 ° C for 15 to 20 minutes. Another method is to sterilize milk at 115 ° C for 20 seconds, then store and aseptic packaging.

The main faults to avoid are the color of milk and made the taste of cooked (Maillard reactions and caramelization).

UHT milk

Definition and History

The UHT milk is heated in continuous flow at a temperature of at least 132 ° C for a few seconds to cool to room temperature and aseptically packed in the order:

  • to ensure its stability and nutritional value long enough to meet the requirements;
  • to release of all pathogenic microorganisms and toxins that may affect the health of the consumer;
  • to destroy any microorganism that can grow during storage.

The principle of milk treated at ultra high temperature (UHT) is known for nearly a century: already in 1893, it was built devices that can process the milk to 125 ° C for 6 minutes. In 1909, there was a continuous flow tubular able to heat the milk to 130 ° -140 ° C, but it was not until 1951 that it was possible to aseptically packaged by the process "Martin Canning" . Ten years later (1961), the Company Tetra Pak, Sweden provided the first satisfactory commercial aseptic packing.

Physico-chemical aspects of UHT

The heat treatments used to eradicate the seeds of milk trigger chemical reactions or physico-chemical reactions generally. By raising the temperature, these reactions are accelerated, but the rate of destruction of microorganisms is considerably higher than the speed of chemical reactions. In other words, the rate of destruction of microorganisms is proportional to the temperature, while the speed of chemical reactions is mainly linked to the duration of treatment.

Minimum heat treatment to destroy microorganisms is sufficient to denature the β-lactoglobulin, which causes a release of sulfhydriles groups (-SH), which have a decisive effect on the taste of the product. They are indeed responsible for the taste of cooked fresh milk treated, their property of reducing groups will then be used to retard the oxidation of milk during storage.

The different phases of the evolution of taste are:

  • Primary phase:
    • a) taste of cooked very pronounced
    • b) taste of cooked
    • c) acceptable flavor with slight flavor of cooked
  • Secondary phase:
    • d) acceptable to neutral flavor
    • e) flavor neutral to slightly oxidized
    • f) oxidized flavor

In principle, the period c) to e) will be brought to market the product, which would be mainly affected by the quality and age of raw milk, heat treatment and storage temperature of the product finished.

To mitigate the effects of chemical changes must take into account the resistance of enzymes in the choice of the scale of heat treatment (Figure 7).

Curve denaturing enzymes and UHT curve

Manufacturing processes UHT milk

Preparation of UHT milk is done in two steps: the sterilization of milk and aseptic packaging.


This heat treatment applied in two modes: direct treatment, by injection of steam into the milk (upérisation) or dispersion of the milk in a room heated by steam: in one case as in the other, the diagram of the treatment will be the same, the indirect treatment by means of a plate, a tubular system or combination of both. Again, the diagram remains almost the same, using the same principle of regeneration that for pasteurized milk. Figure 8 represents a system of UHT sterilization of milk

Diagram UHT milk

Aseptic Packaging

Once sterilized, the milk must remain in a state of total asepsis: the packaging device can be connected directly to one or more packaged, or it can be coupled to an aseptic tank, depending on the choice of the process.

The aseptic tank (Figure 9) is used to store milk ready to be placed in containers. It is first sterilized and valves for milk are surrounded by vapor barriers that keep sterile. When the milk is directed to the aseptic packing, its volume is replaced with sterile filtered air.

Aseptic Tank

The packer must ensure aseptic sterilization of the container and maintaining a sterile environment for its filling. The container is usually sterilized using hydrogen peroxide at a concentration of 30-35%, which is then evaporated at a temperature above 100 ° C, which is essential for the sterilization of paper.

In most systems, the packaging material is composed of polyethylene, aluminum and laminated paper. Such material adequately protects the product from light and gases. Aseptic packaging is the most delicate operation of UHT, and in terms of control and preventive measures: It requires a vigilant staff and well trained both in terms of operations and at the interview.

Advantages of UHT milk

UHT treatment is considered a major revolution in dairy technology, this process offers, in particular, the double advantage of a long-life milk consumption without the need for refrigeration. The distribution becomes more economical because it can be extended on a weekly time limit for example, and is not subject to the limits of course. Thanks to its long shelf life, the rate of returns is very low.
Preparation of UHT milk lends itself well to automation. In this respect, it is important that equipment used in this process is equipped with an automated system of cleaning and sanitation CIP (Cleaning In Place).

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