1. General

Ultra-high-temperature treatment is a continuous inflow 

process. It is based on the rapid heating of the product to the sterilisation temperature and short holding at that temperature, followed by rapid cooling. The purpose of UHT treatment is to achieve commercial sterility of the product. Full sterilisation efficiency requires rapid heat transfer which is only possible in liquid systems. If powders are used in the formulation of a product to be UHT-treated, special attention has to be paid to proper soaking: all powder particles must be completely wetted through.

1.1 Low-Acid, Liquid-Food Products

Low-acid food products are characterised by a pH value of > 4.5 or > 4.6, depending on local food legislation. These products require careful treatment because:

a) microorganisms can grow and multiply. Bacterial spores are also able to germinate and cause product spoilage;

b) practically all pathogenic (disease-causing) microorganisms can develop.

Consumption of products thus spoiled may lead to food poisoning and/or food-borne infections.

Typical processing temperatures for low-acid foods are 130-150°C with holding times of a few seconds (60), usually 4 seconds.

1.2 High-Acid Liquid-Food Products

High-acid food products have a pH value equal to 4.5 or 4.6 or less. These are mainly fruit juices, fruit juice drinks and “belly washers”. High-acid food products are safer than low-acid foods because:

a) they are not prone to pathogenic bacteria and are therefore regarded as safe from the point of view of public health;

b) bacterial spores cannot germinate under high-acid conditions and, consequently, cannot cause food spoilage;

c) the sterilising efficiency of any heat treatment increases with decreasing pH. Thus, lower temperatures can be applied in order to achieve commercial sterility;

d) in addition, some organic acids common in fruits specifically decrease the temperature resistance of possible spoilage organisms;

e) the main spoilage microorganisms are yeast, mould, and a few bacteria (Lactobacillus, Streptococcus, and some others). Processing temperatures for high-acid foods are rather low. With few exceptions temperatures of 85 -95°C with holding times of 30 to 15 seconds, sometimes a few minutes, are sufficient (60). However, there are some exceptions, particularly tomato products which may require considerably higher temperatures, often above 100°C. The pH values of a number of fruits and fruit juices (131) are given in table 1.

1.3 Acidified Liquid-Food Products

Acidified foods are low-acid products, the pH of which has been lowered into the high-acid range by pre-processing. Such products have the same microbiological advantages as high-acid food products. The acidification process is crucial. It is necessary to achieve an even and low pH throughout the product. Acidification can be carried out biologically by the addition and growth of a starter culture, usually Lactobacillus and/or Streptococcus followed by a ripening period at a suitable temperature. Another possibility is an artificial or chemical procedure in which an acid (usually citric or lactic acid) is added to the product which has to be mixed thoroughly afterwards (Figure 1). To achieve commercial sterility, the acidified product has to be processed after the acidification process, usually by heat treatment, and then packaged.

2. Indirect UHT Systems

A heat exchange surface separates the product from the heating or cooling media in indirect UHT systems. The heating medium may be either steam or superheated water (Figure 2). A time-temperature diagram for indirect systems is shown in 

figure 3. Typically, the product enters the steriliser via a balance inlet tank and a centrifugal feeding pump at about 4°C. Subsequently, it is heated to 70 -75°C, at which temperature the product is homogenised. For milk homogenisation, pressures of 200 to 250 kg/cm2 (150 to 200 at the first stage, and about 50 kg/cm2 at the second) are often used. The omogeniser is a positive piston pump which pushes the product through the remaining equipment directly to the aseptic filling machine or into an aseptic tank for subsequent filling. Sterilisation temperatures are usually between 135°C and 140°C. Holding times between 2 and 6 seconds at the sterilisation temperature are common. Though more expensive, downstream (aseptic) homogenisation results in better flavour characteristics for some products. In order to lower the oxygen content of the product, deaeration can be introduced prior to upstream homogenisation. Milk entering the steriliser is normally saturated with oxygen. Since indirect working sterilisers are closed systems, the incoming and outgoing oxygen content of the product is the same. Depending upon temperature, milk may contain 6 to 9 ppm of oxygen (ca 7ppm), (168). Passing a vacuum chamber (deaerator) at high temperature prior to homogenisation (~ 70°C), the oxygen 

content can be reduced to about < 1 to 3 ppm (0.3 - 0.9 ppm), (168). The degree of deaeration depends upon the temperature and the underpressure applied in the process. During heat processing, deposits form on heat exchange surfaces, particularly in the temperature range > 80°C. In order to reduce this deposit formation and thus prolong the running time, a holding cell can be introduced: the product (milk) is held for a few minutes at a temperature of about 90°C (Figure 4). Indirect systems offer good possibilities for regenerative energy recovery: the incoming productis heated by the outgoing.

Direct UHT Systems

    Direct UHT plants feature direct contact between the heating medium and the product. The heating medium is usually steam but electrical heating (“Elecster”, “Ohmic”) has been introduced to a very limited extent(figure 7).

    Electrical heating features the passage of an electrical current through the product. Although it is also used for liquids, the system is mainly intended to be applied in the sterilisation of products containing particles. Commercial application is very limited although a number of plants have been installed in the research and development departments of some major food and animal-feed processing companies. One problem is the difference in electrical resistance which can exist between the liquid and solid phases. Other problems generally associated with the processing of products containing particles are those of separation during processing and aseptic transfer, and mechanical damage to particles softened by processing.

    Injection implies direct steam injection into the product. In infusion plants, the product is infused into a steam chamber.

    Injection and infusion systems must be operated with culinary steam.

    The minimum requirements outlined in figure 10 must be observed for the steam in such plants (not necessary for electrical heating). Indirect working plants should also use culinary steam especially if they are sterilised with steam.

    The US Department of Health and Human Services (17) lists the following requirements for culinary steam:

1. Boiler Feed Water: If boiler feed water is treated, the process must be under the supervision of trained personnel. Only compounds complying with Section 173.310 of 21 CFR (258) may be used to prevent corrosion and scale.

2. Boiler Operation: A supply of clean, dry saturated steam is necessary for proper equipment operation. It is recommended that periodic analysis be made of condensate samples.

3. Piping Assemblies: The steam supply line should be as shown in figure 11.

    A typical time-temperature diagram for direct heating is shown in figure 12.

    The product enters the steriliser via a balance inlet tank and a centrifugal feeding pump at a temperature of about 4°C. It is heated by plate or tubular heat exchangers to about 70°C. At this stage, steam is injected into the product or the product is infused into a steam chamber. Steam condensation increases the temperature almost instantaneously (~ 0.1 sec in injection and ~ 0.25 sec in infusion systems) to the sterilisation temperature which is typically between 145°C and 150°C. The average holding time at the sterilisation temperature is usually around 4 seconds. In both the injection and infusion processes, water condenses in the product and dilutes it. Depending on the temperature difference, an increase in volume of about 10% results. This water must be subsequently removed. The outlet of the holding cell connects to a vacuum chamber. To prevent boiling in the product holding cell, a sufficient overpressure by a suitable restriction device must be introduced. Being exposed to underpressure, the product starts boiling vigorously and steam is flushed off. Careful adjustment of the injection (infusion) temperature and the underpressure in the vacuum chamber guarantees the same dry matter content of the incoming and outgoing product. The resulting pressure drop requires the installation of an aseptic extraction pump for further product transportation. In order to avoid an accumulation of product in the expansion cooler or its emptying, the capacity of both the product feeding pump and the extraction pump at the outlet of the expansion cooler must be carefully

matched.

    Cavitation forces during steam condensation and the boiling in the expansioncooler destabilise milk protein and fat. To compensate this effect requires downstream homogenisation which has to be done under aseptic conditions. Homogenisation pressures for milk are usually 200 to 250 kg/cm2 (150 to 200

kg/cm2 at the first stage and about 50 kg/cm2 at the second). The homogeniser pushes the product through the final cooling section of the steriliser, either into a sterile tank or directly to the aseptic filling machine.

    In the expansion cooler, not only water is removed from the product but also all other volatile compounds. In addition, the vacuum chamber functions as a very effective deaerator that removes oxygen and other dissolved gases, mainly carbon dioxide (CO2). As a consequence, the freezing point increases. At the outlet of the expansion cooler, the oxygen content of milk is down to ~ 0.1 ppm.

    Advantages of injection and infusion heating are:

a) a lower total heat load, as a result of which fewer chemical changes are inflicted on the product;

b) less scaling, particularly in the temperature range of 70°C and above resulting in longer production runs (less frequent cleaning and sterilisation);

c) the low oxygen content in the product increases the stability of some vitamins and, during storage, reduces flavour changes caused by oxidation;

d) more suitable for viscous products.

    Recently, special UHT heat exchangers have been developed combining direct and indirect heating. In one assembly, tubular heating and steam injection have been combined. The product enters at ~4°C and is heated to 95°C by tubular heat exchangers. It then passes a holding cell at the same temperature to stabilise the protein. Steam injection raises the temperature almost instantly to 140-150°C. The product is held at this temperature for a few seconds before being cooled down. Pre-cooling is performed in a tubular heat exchanger where the heat energy is utilised for regenerative heating. The injected steam is flushed off as vapour in a vacuum cooler. The temperature drops to 80°C. Aseptic homogenisation is needed. Subsequently, the product is cooled down to ambient and filled aseptically.

    Prior to production, the equipment must be sterilised. This can be achieved either by superheated water or by steam. Because of the expansion and infusion vessels, direct working systems must be sterilised by steam. It is essential that the temperature controller/guarding sensor is placed at the most sensitive spot, usually somewhere in the return.

    If superheated water is used to sterilise the plant, the temperature, time, and flow rates are critical (142). Possible air pockets and the interface between the product steriliser and the aseptic tank circuit are significant. Air is effectively removed from the system if the flow rate is > 1.5 m/second.

    An additional sterilisation circuit is required in installations operating through an aseptic tank. The sterilisation medium is always steam. In steam sterilisation processes, a possible problem is condensate which must be removed from the system.

    Sterilisation of the aseptic filling equipment is always performed separately either by heat alone or by chemicals and heat. Attention should be paid to the interface between the product line and the filler.