There are many advantages for the producer, retailer and consumer if a product does not require refrigeration and can be stored for long periods without spoiling. The producer can, for example, reach geographically wider markets, simplify production planning by reducing product changes and losses, make deliveries easier by using fewer and cheaper distribution vehicles, and eliminate return of unsold products. Handling becomes easier for the retailer, as expensive refrigerated display space is not necessary and stock planning is simplified. This is also an environmentally friendly technology due to the reduced waste and energy consumption.
Finally, the consumer gains in convenience thanks to less frequent shopping, less congestion in the home refrigerator and emergency reserves on hand for unexpected guests. This includes expensive products such as cream, desserts and sauces.
Raw material quality
Milk exposed to high heat treatment must be of very good quality. It is particularly important that the proteins in the raw milk do not cause thermal instability. The heat stability of the proteins can be quickly determined by an alcohol test. When samples of the milk are mixed with equal volumes of an ethyl alcohol solution, the proteins may become unstable and the milk flocculates. The higher the concentration of ethyl alcohol solution that can be added without getting flocculation, the better the heat stability of the milk. Production and shelf life problems can be avoided to a great extent if the milk remains stable (does not precipitate) even after addition of alcohol solutions with a 75 % alcohol concentration.
The alcohol test is typically used to reject milk that is unsuitable for UHT treatment because it:
- Is sour, due to a high bacterial count of acid-producing microorganisms
- Has the wrong salt balance
- Contains a high level of serum proteins – typical of colostrum
Raw milk of bad quality has an adverse effect on both processability and final product quality. Milk with pH below 6.65 at 20 °C has reduced thermal stability and causes not only processing problems, e.g. burning-on on the heating surfaces resulting in short running times, but also difficulties with cleaning, as well as sedimentation of proteins at the bottom of the packages during storage.
Milk stored for a long time at low temperature may contain high numbers of psychrotrophic bacteria, which can produce heat-resistant enzymes that are difficult to completely inactivate by heat treatment. During storage the enzymes can cause organoleptic changes such as rancidity, bitterness or even gelation (age-thickening or sweet curdling).
The bacteriological quality of the milk must be high. This applies not only to the total bacterial count, but also, and more importantly, to the count of spore-forming bacteria that influence the rate of insterility.
- Has pH < 6.65
- Has alcohol stability < 75%
- Was stored for a long time at low temperatures
The expression “commercial sterility” is frequently used for UHT-treated products. A commercially sterile product is defined as one which is free from microorganisms that grow under the prevailing conditions. In low acid products – products with pH above 4.5 – the most heat-resistant microorganisms which can grow are spores. As their heat resistance is much higher then that of vegetative microorganisms, sterilization processes concentrate attention to the killing effect on spores only. The group of low-acid products comprise not only milk, but most milk-based products.
When microorganisms and/or bacterial spores are subjected to heat treatment or any other kind of sterilizing/disinfectant procedure, not all microorganisms are killed at once. Instead, a certain proportion is destroyed in a given period of time while the remainder survives. If the surviving microorganisms are once more subjected to the same treatment for the same length of time, an equal proportion of the remainders will be killed, and so on. In other words, a given exposure to sterilizing or disinfectant agents always kills the same proportion of microorganisms present.
N = number of micro-organisms (spores) originally present,
Nt = number of micro-organisms (spores) present after a given time of treatment (t), and
K = a constant
t = time of treatment
Logarithmic reduction of spores
The lethal effect of sterilization on microorganisms can thus be expressed mathematically as the logarithmic function to the left.
This formula results in a straight line when drawn as a semi-logarithmic graph with the duration of treatment plotted on the linear axis and the number of survivors on the logarithmic axis.
A logarithmic function can approach zero, but never reach it! To put it another way, sterility defined as the absence of living bacterial spores in an unlimited volume of product is impossible to achieve. In unlimited volumes we have always to count with some survivors, otherwise known as the “sterilizing effect” or “sterilizing efficiency” concept. These terms state the number of decimal reductions in counts of bacterial spores achieved by a sterilization process.
Each time a sterilization process is performed, it can be characterised by a certain sterilizing effect. In any heat sterilization process, the sterilizing effect is determined by the time/temperature condition applied. The higher the temperature and the longer the holding time, the more efficient the process, i.e. the greater the sterilizing effect.
The sterilizing effect is expressed by the number of decimal reductions achieved in the process. For example, a sterilizing effect of 9 indicates that out of 109 bacterial spores fed into the process, only 1 (100) will survive.
The efficiency of the sterilization process is mainly determined by two factors:
- The temperature and the length of time it is applied
- The heat resistance of the microorganisms
Other factors such as product composition, viscosity, uniformity and pH will also affect sterilizing efficiency. Equipment for in-flow sterilization – UltraHighTemperature (UHT) treatment – usually has a sterilizing effect of around 9 – 10 on bacterial spores growing at ambient temperature.
Spores of Bacillus subtilis or Bacillus stearothermophilus are generally used as test organisms to determine the sterilizing effect of UHT equipment, since these strains – especially B. stearothermophilus – form fairly heat-resistant spores. Clostridium botulinum has been traditionally used for calculation of the effect of in-container sterilization (see F0 value calculation) .
The sterilization process must be designed in such a way that there is only a negligible risk that a product will be spoiled before the consumer uses it, or that it contains surviving and growing pathogenic microorganisms. Clostridium botulinum has always been considered as the most significant microorganism in public health terms. Sterilization processes were designed with destruction of this microorganism’s spores in mind. However, in heat-treated milk and milk products, the probability of survival and growth of Clostridium botulinum spores is very low indeed.
The lethal effect on bacterial spores starts at a temperature around 115 °C and increases very rapidly with rising temperature.
Bacteria can be divided into two groups:
- Those existing as vegetative cells only (easy to kill by heat or other means),
- Those existing in a vegetative state and as spores as well, i.e. spore-forming bacteria. While these bacteria are easily killed as long as they are in the vegetative state, their spores are difficult to eliminate.
Products to be sterilized usually contain a mixed flora of both vegetative cells and bacterial spores, as shown in Figure 9.1. Unfortunately, the correlation between the two is not very good. High spore counts may be found in products with low total counts, and vice versa, so total count determination cannot serve as a reliable basis for enumeration of spores in food products.
As mentioned above, the sterilizing effect of a heat sterilization process increases rapidly with increasing temperature. This, of course, also applies to chemical reactions occurring as a consequence of heat treatment. The Q10 value has been introduced as an expression of this increase in speed of a reaction. It states how many times the speed of a reaction increases if the temperature of the system is raised by 10 °C.
The Q10 value for flavour changes – and for most chemical reactions – is around 2 to 3, i.e. if the temperature of a process is raised by 10 °C, the speed of chemical reactions doubles or triples. Q10 values can also be determined for the killing of bacterial spores. The values found range between 8 and 30. The variation is so wide because different kinds of bacterial spores react differently to temperature increases. The changes in chemical properties and spore destruction by the influence of increased temperature are shown in Figure 9.2.
From this graph we can also see that in the range of UHT temperatures, the bacteriological killing effect increases considerably with temperature, whereas the chemical changes remain mild. This clearly illustrates the advantages of UHT treatment against in-container sterilization operating at low temperature for a long time. Using ultra high temperatures with short holding times can provide a high sterilization effect while causing only minimal chemical changes in the treated product. In-container sterilization operating at low temperature for a long time leads to more extensive changes in product quality. See Figures 9.5 and 9.6.
The combined effect of time and temperature on the sterilization of Clostridium botulinum spores is expressed in terms of the F0 value in units of time, most frequently minutes. It is calculated according to the following mathematical formula:
t = sterilization time most often expressed in minutes, e.g. the holding time in seconds divided by 60
T = sterilization temperature in °C
There is a direct correlation between F0 value and the logarithmic reduction of Cl. botulinum spores:
N = is the number of spores of Clostridium botulinum at time 0
Nt = is the number of spores in the final product at time t
D121.1 = is time necessary for decimal reduction (1 logarithm = 1 log) of spores of Clostridium botulinum (is about 0.25 minutes)
Based on that F0 value 1 minute gives 4 log reduction of Clostridium botulinum spores.
To obtain commercially sterile milk from good quality raw milk in practice, UHT plants are designed to achieve a minimum F0 value of 5 – 6 minutes. According to legislation in some countries, a minimal F0 value of 3 minutes is required (correspondent to 12 log reduction of spores of Clostridium botulinum).
As mentioned above F0 value is valid just for spores of Clostridium botulinum with specific z - value of 10 °C. Similar values can be calculated for other microorganisms but then the value will be called F value and other z - values will be used:
Note that the index 0, reserved for Clostridium botulinum spores, is omitted and
Tref = the reference temperature, in °C, at which Dref is valid
z = the temperature increase, in °C, which is required to obtain the same lethal effect at a 10th of the time (for example 10.5 °C for Bacillus stearothermophilus spores)
Dref = the decimal reduction time for the microorganism at the temperature Tref
B* and C* values
The effective working range of UHT treatments is also defined in some countries by reference to two other parameters:
Bacteriological effect: B* (known as B star)
Chemical effect: C* (known as C star)
These values are based on experiments performed by Horak (1980) with natural milk incubated at 55 °C to enumerate thermophilic microorganisms. The results were presented in the form of straight lines relating log of time with temperature for a constant sterilizing effect. These data were extrapolated to give the line that would correspondent to 9 decimal reductions of this natural thermophilic spore population - B*value 1.
B* is based on the assumption that commercial sterility is achieved at
135 °C for 10.1 sec. with a corresponding z - value of 10.5 °C. This reference process is given a B* value of 1.0.
Similarly, the C* value of 1 is based on the conditions for 3 % destruction of thiamine. This is equivalent to 135 °C for 30.5 seconds with a z - value of
A UHT process operates satisfactorily with regard to the keeping quality of the product when the following conditions are fulfilled:
C* < 1
“The fastest moving particle”
In some countries (especially the United States), particular attention is paid to the residence time in a holding cell or tube, with special reference to the holding time for the “fastest moving particle”. Depending on the flow pattern of the liquid (turbulent or laminar flow), the efficiency coefficient for milk is in the interval of 0.5 – 0.90. This involves applying a correction factor in calculations of holding times. In special cases in the US, it is reckoned that the fastest moving particle passes a holding cell twice as fast as the average particle, i.e. the efficiency coefficient (h) is 0.5. In all relevant cases in industrial installations, the equipment is designed for maintaining turbulent flow, and an efficiency factor of 0.85 – 0.90 is utilised.
Commercial sterility regulations
Commercial sterility means the absence of microorganisms capable of growing in the food at normal non-refrigerated conditions at which the food is likely to be held during manufacture, distribution and storage.
According to Codex Alimentarius Commission (WHO/FAO) the commercial sterility of low-acid food is defined as follows:
The condition achieved by application of heat, sufficient, alone or in combination with other appropriate treatments, to render the food free from microorganisms capable of growing in the food at normal non-refrigerated conditions at which the food is likely to be held during distribution and storage.
According to US Food and Drug Administration; CFR Title 21§113, the commercial sterility of thermally processed low acid food is defined as follows: The condition achieved by the application of heat which renders the food free of microorganisms capable of reproducing in the food under normal non-refrigerated conditions of storage and distribution; and viable microorganisms (including spores) of public health significance; or by the control of water activity and the application of heat, which renders the food free of microorganisms capable of reproducing in the food under normal non-refrigerated conditions of storage and distribution.
Codex Alimentarius Commission (WHO/FAO), Code of hygienic practice for milk and milk products, CAC/RCP 57-2004:
UHT (ultra-high temperature) treatment of milk and liquid milk products is the application of heat to a continuously flowing product using such high temperatures for such time that renders the product commercially sterile at the time of processing. When the UHT treatment is combined with aseptic packaging, it results in a commercially sterile product.
UHT treatment is normally in the range of 135 to 150 °C in combination with appropriate holding times necessary to achieve commercial sterility.
Verification of process
The products subjected to commercial sterilization must be microbiologically stable at room temperature, either measured after storage until end of shelf life or incubated at 55 °C for 7 days (or at 30 °C for 15 days) in accordance with appropriate standards.
European UHT milk regulations
According to EU Regulation 1662/2006 amending EU regulation 853/2004, specific hygiene rules for food of animal origin:
Ultra high temperature (UHT) treatment is achieved by a treatment involving a continuous flow of heat at a high temperature for a short time (not less than 135 °C in combination with a suitable holding time) such that there are no viable microorganisms or spores capable of growing in the treated product when kept in an aseptic closed container at ambient temperature.
Sufficient to ensure that the products remain microbiologically stable after incubating for 15 days at 30 °C in closed containers or for seven days at 55 °C in closed containers or after any other method demonstrating that the appropriate heat treatment has been applied.
Chemical and bacteriological changes at high heat treatment
When milk is kept at a high temperature for a long time, certain chemical reaction products are formed, which results in discoloration (browning). It also acquires a cooked and caramel flavour, and there is occasionally a great deal of sediment. These defects are largely avoided by heat treatment at a higher temperature for a shorter time. It is important that the optimum time/temperature combination is chosen to enable satisfactory spore destruction while keeping heat damage to the milk to a minimum.
Figure 9.4 shows the relationship between the sterilizing effect and browning reaction. The A line represents the lower limit of time/temperature combinations that cause the milk to turn brown. Line B is the lower limit of combinations for complete sterilization (destruction of thermophilic spores). The regions for in-container sterilization and UHT treatment are also marked in the figure.
The figure shows that while the two methods have the same sterilizing effect, there is a great difference in the chemical effects; the browning reaction and destruction of vitamins and amino-acids. At higher temperature for shorter time the changes are much smaller. This is the reason why UHT milk tastes better and has a higher nutritive value than in-container sterilized milk.
Taste is a very subjective factor, but it is quite clear that the taste of UHT-treated milk has improved over the years. Many people find it impossible to tell the difference between good UHT milk and pasteurized milk.
As was mentioned in chapter 2, it appears that it is possible to differentiate pasteurized, UHT and sterilized milk by their lactulose content. The lower the heat load has been, the lower the lactulose content and the more premium quality of the processed product.
Ever since UHT-treated milk was introduced on the market, the quality and primarily the taste and odour have been discussed. Initially, UHT-milk was almost as white as ordinary pasteurized milk, but the product had a cooked taste and odour. The cooked off notes are caused by structural changes of protein in milk and are absolutely harmless but are not well accepted by consumers in some countries. There have been many efforts to obtain a flavour closer to that of ordinary pasteurized milk, and these efforts continue.
In this context it is important to mention that the temperature at which the milk is organoleptically tested has a big influence on the result. At refrigeration temperature, around 5 – 7 °C, the UHT flavour will be suppressed. Therefore, when, for instance, a comparison is made between the influence of various methods of UHT treatment, the organoleptic evaluation should be carried out at 20°C after the samples have been stored at 20 °C for various periods, say 2, 4 and 6 weeks.
Tests carried out in this way show that significant differences exist between direct and indirect methods, the latter exposing the milk to a higher temperature load. However, in principle there is no pronounced difference between the two direct methods (steam injection and steam infusion).
Another term used in connection with UHT treatment to characterise the quality of the treatment is the shelf life of the product. This is defined as the period during which a product can be stored without the quality falling below a certain acceptable minimum level. The concept is subjective – shelf life can be very long if the standards set for product quality are low.
The physical and chemical limiting factors of shelf life are incipient gelling, increase in viscosity, heavy sedimentation and cream lining. The organoleptic limiting factors are deterioration of taste, smell or colour.
When studying any type of food process, it is important to consider the nutritional aspects. Extensive research has been carried out on the effect of heat treatment on milk.
The heat effect of UHT treatment on the constituents of milk can be summarised as beside:
There are no changes in the nutritional value of fat, lactose and mineral salts, but there are marginal changes in the nutritional value of proteins and vitamins.
The major protein in milk, casein, is not affected by heat treatment. Denaturation of whey proteins does not mean that the nutritional value (in terms of biological value, digestibility and availability of lysine) is lower in UHT milk than in raw milk. Although sterilized milk has a lower biological value of proteins (0.85), the nutritional value reported for UHT milk (0.90) does not differ significantly from that of raw milk (0.91).
The small loss of the essential amino acid lysine causes the marginal changes. However, it has been shown that about 0.4 – 0.8 % of the lysine is lost, and this figure is the same for pasteurized milk. The corresponding value for in-container sterilized milk is 6 – 10 %.
Some of the vitamins in milk are considered to be more or less thermostable in regard to pasteurization or UHT treatment. Among these are the fat-soluble vitamins A, D and E and some of the water-soluble group B vitamins. However, degradation of vitamin A can be much higher if the product is fortified. Other vitamins are less stable in response to heat, e.g. B9 (folic acid) and B12 (cobulamin). The time/temperature curve in Figure 9.6 shows that thiamine losses are less than 3% in UHT-treated milk, but considerably higher in in-container sterilized milk (approximately 20 – 50 %). The same relationship regarding destruction of vitamins can be found in all other heat-sensitive vitamins in UHT and in-container sterilized milk, for example B6, B12, folic acid and vitamin C. Losses of vitamin B2 and vitamin C in in-container sterilized milk may be as high as 100%.
Some of the vitamins, e.g. folic acid and vitamin C, are oxidation-sensitive, and their losses occur mainly during storage due to a high oxygen content in the milk or in the package. However, milk is not a good source of vitamin C and folic acid, as the content is far below the recommended daily intake.
Generally speaking, losses of vitamins are considerably higher when food is prepared in the home than in UHT treatment and pasteurization of milk. The general conclusion should therefore be that UHT milk and pasteurized milk are of about the same quality, while in-container sterilized milk is of inferior quality in terms of nutritional value.
Fat: No changes
Lactose: Marginal changes
Proteins: Partial denaturation of whey proteins
Mineral salts: Partial precipitation
Vitamins: Marginal losses
Production of long-life milk
Two methods are used for the production of long-life milk for ambient storage:
A In-container sterilization
B Ultra High Temperature (UHT) treatment followed by aseptic filling into packages protecting the product against light and atmospheric oxygen
Two processes are used for sterilization in bottles or cans.
- Batch processing in autoclaves, Figure 9.7
- Continuous processing systems such as:
– Vertical hydrostatic towers, Figure 9.8
– Horizontal sterilizers, Figure 9.9
The batch system can be operated by three methods:
- In stacks of crates in a static pressure vessel, autoclave, Figure 9.7
- In a cage that can be rotated in a static autoclave
- In a rotary autoclave.
The rotary methods have an advantage over the static method due to the quicker uptake of heat from the heating medium and the greater uniformity of treatment with respect to bacterial kill and colour of the finished product.
In autoclave sterilization the milk is usually pre-heated to about 80 °C and then transferred to clean, heated bottles. The bottles are capped, placed in a steam chamber and sterilized, normally at 110 – 125 °C for 3 – 40 minutes. The batch is then cooled and the autoclave filled with a new batch. The principle is the same for cans.
Batch sterilization in autoclaves is a technique that is used more often for canned solid foods than for liquid products. The fact that sterilization takes place after bottling or canning eliminates the need for aseptic handling, but on the other hand, heat resistant packaging materials must be used.
Continuous systems are normally preferred and these are operating at higher capacities. For continuity of operation, the design of machines for continuous production depends on the use of a pressure lock system through which the filled containers pass from low pressure/low temperature conditions into a relatively high pressure/high temperature zone. After this, they are subjected to steadily decreasing temperature/pressure conditions and are eventually cooled with chilled or cold water.
There are two main types of machines on the market for continuous sterilization, differing basically in the type of pressure lock system used.
- The hydrostatic vertical bottle sterilizer
- The horizontal rotary valve-sealed sterilizer
Hydrostatic vertical sterilizer
This type of sterilizer, often referred to as the tower sterilizer, Figure 9.8, basically consists of a central chamber maintained at sterilizing temperature by steam under pressure, counterbalanced on the inlet and discharge sides by columns of water giving an equivalent pressure. The water on the inlet side is heated and the water on the outlet side cooled, each at a temperature adjusted to give maximum heat uptake/abstraction compatible with avoidance of breakage of the glass by thermal shock.
In the hydrostatic tower the milk containers are slowly conveyed through successive heating and cooling zones. These zones are dimensioned to correspond to the required temperatures and holding times in the various treatment stages.
In many cases the milk is pre-treated in a pre-sterilizing plant similar to a UHT plant. The milk is heated to 135 °C or higher for a few seconds and then cooled to 30 – 70 °C (depending on the material of the bottle – as a rule plastic bottles require the lower temperature), and transferred to clean, heated bottles before it is treated in the hydrostatic tower. Pre-sterilization can take place in an indirect or direct plant. The main reason for pre-sterilization is either to decrease the number of spores that will be finally removed by the second sterilization in the container, or achieve in principle the same sterility level in the pre-sterilization step as in the UHT plant. Thus the second sterilization will remove only the microorganisms that entered the product due to the non-aseptic filling process (F0 = 1 – 2 minutes). Both pre-sterilizations are applied in order to lower the heat load in the heating tower and thereby reduce the unwanted chemical and organoleptic changes and get closer to the quality obtained by the UHT process followed by aseptic filling.
The time cycle of a hydrostatic sterilizer is approx. one hour, including 3 – 30 minutes for passage through the sterilizing section at 115 – 125 °C.
The hydrostatic sterilizer is suitable for sterilization of bottles made of glass or plastic.
The rotary valve-sealed sterilizer, Figure 9.9, is a comparatively low-built machine with a mechanically driven valve rotor, through which the filled containers are passed into a relatively high pressure/high temperature zone, where they are subjected to sterilizing temperatures of the order of 132 – 140 °C for 10 – 12 minutes. With an overall cycle time of 30 – 35 minutes, a capacity of 12,000 units per hour can be achieved.
The rotary valve-sealed sterilizer can be used for sterilization of plastic bottles and glass bottles, as well as flexible containers made of plastic film and plastic laminates.
Another system that ought to be mentioned in this context is the horizontal continuous rotating autoclave for evaporated milk in cans. The sterilizer design comprises three cylindrical vessels, each containing a helical strip attached to a roller inside the vessel. A number of channels are formed so that the cans are forwarded along the roller during processing and simultaneously rotated. This type of sterilizer is also equipped with a double detector system – one at the exit of the pre-heater and the other at the end of the pressure cooler – making it possible to detect non-sterile cans.
In a modern UHT plant, the milk is pumped through a closed system. On the way it is pre-heated, high-heat treated, homogenized, cooled and packed aseptically without any re-infection. Low-acid liquid products (pH above 4.5 – for milk more than pH 6.5) are usually treated at 135 – 150°C for a few seconds,
by either indirect heating, direct steam injection or steam infusion. High-acid products (pH below 4.5) such as juice are normally heated at 85 – 95 °C for 15 – 30 seconds. All parts of the system downstream of the actual high-temperature heating section are of aseptic design to eliminate the risk of re-infection.
Compared to traditional sterilization in hydrostatic towers, UHT treatment of milk saves time, labour, energy and space. UHT is a high-speed process and has much less effect on the colour and flavour of the milk. However, regular consumers of autoclave-sterilized milk are accustomed to its “cooked” or caramel flavour and may find the UHT-treated product “tasteless”.
The UHT processes
UHT is a technique for preserving liquid food products by exposing them to brief, intensive heating. This treatment destroys the microorganisms in the product and makes it commercially sterile.
The commercial sterility applies only as long as the product remains under aseptic conditions, so it is necessary to prevent re-infection by packaging the product after heat treatment in previously sterilized packaging materials under aseptic conditions. Any intermediate storage between treatment and packaging must take place under aseptic conditions. This is why UHT processing is also called aseptic processing.
Development of UHT
Experiments on sterilization of milk in bottles had been carried out by Louis Pasteur, but it was not until around 1960, when both aseptic processing and aseptic filling technologies became commercially available, that the modern development of UHT processing started. UHT-treated milk and other UHT-treated liquid food products are now accepted worldwide, but this has not always been the case.
The first UHT plants operated on the principle of direct steam injection. Compared with the in-container sterilization plants, the new UHT plants soon gained a reputation for producing an excellent flavour. The first indirect plants were introduced on the market some ten years later.
Research and development have been intense since UHT was first introduced. Modern plants deliver a superior product with the colour and nutritional values practically unchanged.
UHT treatment is a continuous process, and its application is therefore limited to products that can be pumped. UHT treatment can be applied to a wide range of dairy and food products. The list shown is not exhaustive. Many other liquid food products are likely to be of great interest to dairies in the future.
UHT plants are often flexibly designed to enable processing of a wide range of products in the same plant. Both low-acid products (pH > 4.5) and high-acid products (pH < 4.5) can be treated in a UHT plant. However, only low-acid products require UHT treatment to make them commercially sterile. Spores cannot develop in high-acid products such as juice, and heat treatment is therefore intended only to kill yeast and moulds. Normal high-temperature pasteurization (85 – 95 °C for 15 – 30 seconds) is sufficient to make high-acid products commercially sterile.
UHT plants are fully automatic and have four operating modes: plant pre-sterilization, production, AIC (Aseptic Intermediate Cleaning) and CIP (full Cleaning In Place). Safety aspects must be a prime consideration in the design of a UHT plant. The risk of supplying an unsterile product to the aseptic filling machine must be eliminated. Interlocks in the control programming provide security against operator errors and tampering with the process. It is for example, impossible to start production if the plant is not properly pre-sterilized or if the system loses sterility during production.
All sequences involved in starting, running and cleaning the plant are initiated from a control panel, which contains all the necessary equipment for control, monitoring and recording of the process.
Various UHT systems
There are two main types of UHT systems on the market: direct and indirect.
In the direct systems, the product comes in direct contact with the heating medium, followed by flash cooling in a vacuum vessel, homogenization and eventually further indirect cooling to packaging temperature.
The direct systems are divided into:
- Steam injection systems (steam injected into product), Figure 9.10
- Steam infusion systems (product introduced into a steam-filled vessel), Figure 9.11
It is also possible to combine direct heating and indirect cooling without subsequent flash cooling. In this case, the condensate created by the steam while heating remains in the product.
In the indirect systems the heat is transferred from the heating media to the product through a partition (plate or tubular wall).
The indirect systems can be based on:
- Plate heat exchangers, Figure 9.12
- Tubular heat exchangers, Figure 9.13
- Scraped surface heat exchangers, Figure 9.14
Furthermore, it is possible to combine the heat exchangers in the indirect
systems according to product and process requirements.
- Fresh and recombined liquid milk
- Concentrated milk
- Dairy creams
- Flavoured milk drinks
- Fermented milk products (yoghurt, buttermilk, etc.)
- Whey-based drinks
- Ice cream mix
- Desserts (custards and puddings)
- Protein drinks
- Soy drinks
- Baby foods
- Fruit and vegetable juices
- Beverages such as tea and coffee
- Toppings and creams based on vegetable fat
- Nutritional solutions
General UHT operating phases
These operating phases are common to all UHT systems and are therefore not described under each system.
Before start of production the plant must be pre-sterilized in order to avoid re-infection of the treated product.
The pre-sterilization involves:
- Hot water sterilization so that the minimum temperature necessary (normally 125 °C) will be reached at the last point in the line that must be sterile. Minimum time for the hot water sterilization is 30 minutes from the moment the relevant temperature has been reached in the whole aseptic part of the plant.
- Adjustment of the plant to conditions required for production.
The production phases vary according to the different processes and are described below.
Aseptic intermediate cleaning
Aseptic Intermediate Cleaning (AIC) is a useful tool in cases where a plant is used for very long production runs. A 30-minute AIC can be carried out whenever it is necessary to remove fouling in the production line without losing aseptic conditions. The plant does not have to be resterilized after AIC. This method saves downtime and permits longer production runs.
The full CIP cycle takes 70 to 90 minutes and is normally carried out immediately after production.
The CIP cycle for direct or indirect UHT plants may comprise sequences for pre-rinsing, caustic cleaning, hot-water rinsing, acid cleaning and final rinsing, all automatically controlled according to a pre-set time/temperature program. The CIP program must be optimised for different operating conditions in different plants.
Direct UHT plants
UHT processing means commercial sterility to ensure food safety and long shelf life at ambient temperature. It entails heating the product to a specific temperature for a specific length of time. The higher the temperature, the shorter the time required to destroy microorganisms. The more rapidly the product can be heated and then subsequently cooled down again, the less impact the process has on the chemical changes in the product, such as changes in taste, colour and even to some extent, nutritional value. The most effective way of achieving rapid heating is to mix high temperature steam directly with the product, followed by flash cooling in a vacuum vessel. This is called a direct system.
Flash cooling is an operation, which as well as cooling, also involves deaeration and deodorisation of the treated product. In addition, deaeration secures higher homogenization efficiency and the deaeration will also positively influence the storage stability of the processed product in terms of preventing oxidation during storage.
The rapid heating and cooling explains why direct systems deliver superior product quality and are often chosen to manufacture heat-sensitive products, such as premium quality market milk, enriched milk, cream, formulated dairy products, soy milk and soft ice mix, as well as dairy desserts and baby food.
Processing of starch-based products in a direct system has a positive effect on texture and smoothness, thus enhancing the mouthfeel.
Direct UHT plant based on steam injection and plate heat exchanger
In the flowchart in Figure 9.15, the product at about 4 °C is supplied from the balance tank (1) and forwarded by the feed pump (2) to the pre-heating section of the plate heat exchanger (3). After pre-heating to approximately 80 °C, the product then continues to the ring nozzle steam injector (4). The steam injected into the product instantly raises the product temperature to about 140 – 150 °C (the pressure prevents the product from boiling). The product is held at UHT temperature in the holding tube (5) for a few seconds before it is flash cooled.
Flash cooling takes place in the vacuum vessel (6) in which partial vacuum is maintained by a pump (7). The vacuum is controlled so the amount of vapour flashed off from the product equals the amount of steam previously injected. A centrifugal pump (8) feeds the UHT-treated product to the aseptic two-stage homogenizer (9).
After homogenization, the product is cooled to approximately 20 °C in the plate heat exchanger (3) and then continues directly to an aseptic filling machine or to an aseptic tank for intermediate storage before being packed.
If the temperature drops during production, the product is diverted into a reject tank and the plant is flushed by water. The plant must be cleaned and sterilized before restart.
Plants with capacities of 2,000 – 30,000 l/h are available.
Direct UHT plant based on steam injection and tubular heat exchanger
As an alternative to the above design, the plate heat exchanger in Figure 9.15 (3) can be exchanged for tubular heat exchangers, as shown in Figure 9.16, when products of low or medium viscosity are to be treated.
Following pre-sterilization of the plant and cooling down to about 25 °C, the milk at approx. 4 °C is routed into a tubular heat exchanger (3) for pre-heating to approx. 80 °C.
Steam injection (4) instantly raises the temperature to 140 – 150 °C. The milk is held at this temperature for a few seconds (5) before being cooled down. The injected steam is flashed off as vapour in a vacuum vessel (6), whereupon the temperature of the milk drops to 80 °C.
After aseptic homogenization (9), the milk is cooled (10) to packaging temperature, approximately 25 °C.
If the temperature drops during production, the product is diverted into a reject tank and the plant is flushed by water. The plant must be cleaned and sterilized before restart.
Direct UHT plant based on steam infusion
The main difference between this system and the steam injection system is the way the milk and steam are brought together.
The basic principle of steam infusion is to heat a product by passing it through an atmosphere of steam, as shown in Figure 9.11. The product-spreading system may vary, but the resulting milk droplet size must be uniform, so that the rate of heat transfer does not vary. If the droplet size varies, the infuser will depart from the theoretical model upon which the design is based. Otherwise, the process is similar to the steam injection system as shown in Figures 9.15 and 9.16 above.
Indirect UHT plants
In many cases, products must not only be attractive and healthy to eat and drink, but also economical to manufacture, store and distribute. The most cost-effective method of UHT processing is indirect heating – a heating method in which the processed product never comes into direct contact with the heating medium. There is always a wall in between. This technique applies to all types of heat exchangers, however in dairy applications tubular-based systems are the most common. Homogenization can be applied either before or after the final heating of the product. Homogenization before UHT treatment is possible in indirect UHT plants, which means that non-aseptic homogenizers can be used. However, an aseptic downstream homogenizer might improve the texture and physical stability of certain products that have a high content of protein, dry matter or fat.
Double homogenization, using one homogenizer upstream and one downstream, can be used to obtain premium quality and long shelf life stability for some products. This process solution is appropriate for products such as coffee cream and evaporated concentrated milk
Indirect UHT plants are a suitable choice for processing of milk, flavoured milk products, cream, dairy desserts, yoghurt drinks, concentrated milks and other non-dairy applications, such as juices, nectars and tea.
Indirect UHT plant based on plate heat exchangers
UHT plants of the indirect heating type are built for capacities up to
30,000 l/h. A typical flowchart is shown in Figure 9.17.
The product at about 4 °C is pumped from the storage tank to the balance tank (1) of the UHT plant and from there by the feed pump (2) to the regenerative section of the plate heat exchanger (3). In this section the product is heated to about 75 °C by UHT-treated product, which is cooled at the same time. The pre-heated product is then homogenized (4) at a pressure of 18 – 25 MPa (180 – 250 bar). Homogenization before UHT treatment is possible in indirect UHT plants, which means that non-aseptic homogenizers can be used. However, an aseptic downstream homogenizer might improve the texture and physical stability of certain products.
The pre-heated, homogenized product continues to the heating section of the plate heat exchanger, where it is heated to about 137 °C. Heating is performed by hot water in a closed water curcuit. After heating, the product passes through the holding tube (5), dimensioned for about 4 seconds.
Finally, cooling is performed regeneratively in two sequences: first against the cold end of the hot water circuit, and then against the cold incoming product. The product that leaves the regenerative cooler continues directly to aseptic packaging or to an aseptic tank for intermediate storage.
If the temperature drops during production, the product is diverted into a reject tank and the plant is flushed by water. The plant must be cleaned and sterilized before restart.
In many cases, indirect UHT plants are designed for a variable capacity between 50 and 100% of the nominal and are directly connected to a line of aseptic packaging machines. To reduce the over-processing of the product if one of the packaging machines stops, the heating section can be divided and split into subsections, a split heater.
The split heating system is illustrated in Figure 9.18. In the event of a sudden 50% reduction of the flow compared with nominal, a valve (C) is activated so that the heating medium by-passes outside the first heating section (A). The temperature of the product will thus be kept at the pre-heating temperature (75 °C) until the product reaches the second (final) heating section (B) where heating to the relevant UHT temperature takes place.
The time/temperature curves in Figure 9.19 show the difference in the heat load on the product at nominal and half capacity. The dotted line on the graph represents the temperature development in a system without split heating facilities running at 50% of nominal capacity.
Indirect UHT plant based on tubular heat exchangers
A tubular system is chosen for UHT treatment of products with low or medium viscosity that may or may not contain particles or fibres. The term medium viscosity is a diffuse concept, as the viscosity of a product can vary depending on raw material, additives and mechanical treatment. Soups, tomato products, fruit and vegetable products, certain puddings and desserts are examples of medium-viscosity products well suited to treatment in a tubular concept. Currently tubular systems are the most utilised type of UHT plants. They are also frequently utilised when longer processing times are required for ordinary market milk products.
The running time of indirect systems can be prolonged even further by installation of a stabilising holding tube, which stabilises milk proteins and thus minimises fouling in the heat exchangers and the ordinary holding tube.
The processing principle, shown in Figure 9.20, does not differ very much from the UHT plant with a plate heat exchanger described above. Plants with capacities from 1,000 up to 30,000 l/h can be built.
The tubular heat exchanger comprises of a number of tubes assembled into modules that can be connected in series and/or in parallel to offer a complete optimised system for any heating or cooling duty. This system can also be provided with a split heating arrangement.
If the temperature drops during production, the product is diverted into a reject tank and the plant is flushed with water. The plant must be cleaned and sterilized before restart.
Indirect UHT plant based on coiled tubular heat exchanger
Coiled heat exchangers are suitable for treatment of high-viscosity food products with or without particles.
A coiled heat exchanger system is based on a number of relevant heat exchangers and a typical flowchart for this process is shown in Figure 9.21. Specific hourly capacities or temperature programs cannot be stated due to the wide variation in the physical characteristics of individual products. Soups, tomato products, fruit and vegetable products, certain puddings and desserts are typical examples of products well suited for treatment in a coiled heat exchanger concept.
The Dean effect will greatly improve the mixing of the product in a coiled heat exchanger, thus reducing the needed amount of heating surface of the heat exchanger system. Together with only one in- and one outlet, the particle integrity will be on a higher level compared with other heating systems.
The product is pumped from a tank (1) by a feed pump (2) to a high-pressure pump (piston pump) and then further to the first coiled heat exchanger (4). Additional heating stages can be utilised to bring the product up to the desired temperature. Monitors located at different stages of the process check that these temperatures have been attained.
The holding tube (5) maintains the product at the required temperature for a predetermined period of time. The product is cooled with water (6) and chilled water until it reaches packaging temperature.
Finally, the cooled product is pumped to an aseptic buffer tank (not shown) which provides a buffer volume between the continuous process line and the packaging system.
Failure to meet the pre-set values automatically opens a return valve to direct the product to a reclaim tank.
Indirect UHT plant based on scraped surface heat exchangers
Scraped surface heat exchangers are a suitable type for treatment of high-viscosity food products with or without particles.
A scraped surface system is based on a number of relevant heat exchangers and a typical flowchart for this process is shown in Figure 9.22. Specific hourly capacities or temperature programs cannot be stated owing to the wide variation in the physical characteristics of individual products.
The product is pumped from a tank (1) by a feed pump (2) to the first scraped surface heat exchanger (3). Additional heating stages (4) can be utilised to bring the product up to the desired temperature. Monitors located at different stages of the process check that these temperatures have been attained.
The holding tube (5) maintains the product at the required temperature for a predetermined period of time. The product is cooled with water (6 and 7) and chilled water (8 and 9) until it reaches packaging temperature.
Finally, the cooled product is pumped to an aseptic buffer tank (not shown), which provides a buffer volume between the continuous process line and the packaging system.
Failure to meet the pre-set values automatically opens a return valve to direct the product to a reclaim tank.
The aseptic tank, in Figure 9.23, is used for intermediate storage of UHT-treated dairy products. Product flow and service media connections are placed in its valve and control module. An aseptic tank can be used in many ways in UHT lines, depending on plant design and the capacities of the various units in the process and packaging lines. Two examples are shown in Figures 9.24 and 9.25.
- Simultaneous packaging of two products. The aseptic tank is first filled with one product, sufficient to last for a full shift of packaging. Then the UHT plant is switched over to another product, which is packed directly in the line of packaging machines.
- One or more aseptic tanks included in the production line thus offer flexibility in production planning.
Direct packaging from a UHT plant requires recirculation of a minimum extra volume of 300 litres per hour to maintain a constant pressure to the filling machines. Products that are sensitive to reprocessing cannot tolerate this and the product must then be fed via an aseptic tank with the required constant pressure. One of the major advantages of an aseptic tank is that the product is only processed once, and at optimal conditions without any recirculation. This will always secure consistent, and best, product quality.
The optimum arrangement of UHT plants, aseptic tanks and aseptic packaging machines must thus be decided for each individual process.
Aseptic packaging has been defined as a procedure consisting of sterilization of the packaging material or container, filling with a commercially sterile product in an aseptic environment, and producing containers that are tight enough to prevent recontamination, i.e. that are hermetically sealed, Figure 9.26.
For products with a long non-refrigerated shelf life, the package must also give almost complete protection against light and atmospheric oxygen in order to protect the nutritional value and product sensory. A milk carton for long-life milk must therefore be of high-quality carton board sandwiched between layers of polyethylene plastic.
The term “aseptic” implies the absence or exclusion of any unwanted organisms from the product, package or other specific areas. “Hermetic” is a term used to indicate suitable mechanical properties to exclude the entry of bacteria into the package or, more strictly, to prevent the passage of microorganisms and gas or vapour into or from the container.
UHT pilot plants
Special pilot plants are available for testing small quantities of new, interesting products. In these plants it is possible to study the effects of varying technological parameters related to the UHT process, such as temperature programs, holding times, heating method (direct or indirect) and deaeration vs. no deaeration, in addition to homogenizing pressures and temperatures. Many technological parameters are related to the product such as recipes, ingredients, pre-treatment, etc.
These product parameters are just as important as the process parameters, and successful development of a new UHT product requires that all of them are studied together. At the same time, the pilot plant can be used to study heat-related properties of the product such as stability, sensitivity, and heat resistance of spores.
Many laboratories in the food and dairy industry have installed UHT pilot plants for product development. Such plants are also found in schools, universities and other scientific institutions that are interested in food and dairy technology. Some manufacturers of UHT plants also have pilot plants for research and trials with customers’ products.
The complete UHT plant can consist of one module for indirect heating in plate heat exchangers and additional modules for direct heating and homogenization. The flow chart in Figure 9.27 illustrates a pilot plant for indirect heating in plate heat exchangers, or alternatively in a tubular heat exchanger and additional modules for direct heating and homogenization of the product, either upstream (non-aseptic, 5a) or downstream (aseptic, 5b).