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Food pasteurization.

Food Pasteurization

Food products that have been pasteurized are referred to as semi preserved products. The pasteurization process is the application of increased - but not excessively high-temperatures, 60-85 [degrees] C, ensuring a limited extension of the shelf life period with the "fresh character" of the relevant product being maintained to a large extent.

From a microbiological point of view, this process kills yeasts, mould fungi and lactic bacteria (their spores, however, remain unaffected). After heating the packed product, an imperishability period of 2-6 weeks is obtained by rapid cooling down and subsequent storing at 4 [degrees] C.

Since pasteurization temperatures are below the boiling point of water and the product is stored at temperatures above freezing point, any overcooking or freezing is avoided and the freshness of the product is maintained.

When pasteurizing sliced bread packed in plastics bags, the aim is to kill yeast and mould fungi as they contaminate the humid interior of the bread, ie, the crumb, during the slicing operation.

The objective when pasteurizing "fresh meals", sealed in plastics trays is the same. The products that have been pasteurized in previous process steps (blanching, cooking, boiling), such as vegetables, potatoes, rice, noodles, meat and fish are often contaminated again during the filling and packing process.

For this reason, products in the trays have to be `disinfected' once more after sealing, in order to achieve the desired imperishability period of 2 - 6 weeks.

For microbiological reasons, a relatively restricted temperature range of 75-85 [degrees] C has to be observed during this post-treatment.

It is essential therefore that the temperature distribution obtained within the tray is thus relatively even. In order to minimize temperature differences, the microwave power density is reduced while the treatment period is prolonged (extended line length and/or slower belt speeds), ie, an even heat distribution is achieved via heat conduction in the product.

A prolongation of the treatment period at identical output, however, automatically implies an increase in the width or length of the line as well as a high thermal stress over a longer period of time and thus a decrease in the product quality.

With the Berstorff hybrid microwave system (German patent), these disadvantages are avoided since the microwave power is focused in accordance with the tray geometry and can be switched on and off in different positions. The edge and corner burning effect is thus completely eliminated.

In combination with computer-controlled operation of the individual magnetrons, this hybrid chamber system (multi mode plus plane wave mode) allows one to obtain for the first time higher temperatures in the product centre (centre of the tray, half product height) than at the edges and corners.

By putting the appropriate parameters into the master computer, meals consisting of different components (eg, potatoes and meat with sauce and vegetables) can be processed with a total temperature deviation of TA = 80 [degrees] C +/-5 [degrees] C at an inlet temperature of Te = 40 [degrees] C +/-5 [degrees] C and a treatment time of t = 3 minutes per 400g tray and can therefore be pasteurized without any major blowing up of the plastics film.

A special advantage offered by the combination of the hybrid chamber with the computer control is the fact that the distance between the trays - seen in conveying direction-does not present a problem. The temperature distribution obtained when processing a single tray can thus be compared to the one resulting from a continuous loading of the line and/or the conveyor belt.

Overpressure lines with horizontal sluices for sterilizing food products in sealed packings

- in 1.2Kw hybrid design, 2 - 8 tracks (eg, 76.8Kw for 500 Kg/h - with pressure, heat retention, and cooling sections similar to the line with vertical sluices.

Both microwave sterilization processes have shown positive results. Also in this case, the advantage consists in the short heating time (1.5 - 5 minutes) while the heat retention time and the pressure cooling (below 100 [degrees] C) are physically determined. In order to reduce the line length, these zones are preferably designed as tower-type sections.

Owing to the arrangement of double sluices, the product flow is as continuous as in the case of the "open" lines operating under atmospheric pressure.


As compared to the heat conduction process, the microwave treatment applied for preserving food products offers decisive advantages with regard to the product quality and has, thus, gained ground in the field of production. This applies to pasteurization as well as to sterilization tasks. The application of this new technology on an industrial level was promoted by extensive technical development of this process bearing in mind the physical constraints.

In view of the rapidly increasing use of microwave ovens for household purposes and the trend of the consumers to reduce the time spent in the kitchen in order to serve good fresh food, the fresh meal will still gain in importance. A fundamental prerequisite for this development, however, is a production method ensuring minimum quality losses during the preservation process.

The microwave heating process does also represent an alternative for small and medium-size restaurants facing problems with regard to personnel expenditure and procurement.

Apart from the production lines described above, corresponding laboratory lines are available for further product development.

Process technology applied for pasteurizing food products by microwaves

Compared with conventional pasteurization processes based on heat transfer media, the microwave treatment presents the advantage of a relatively shortheating time in continuous operation.

As far as the line design is concerned, "open", ie, continuous lines operating under atmospheric pressure, are applied for this task.

The short heating period obtained, when applying microwave energy, results in similarly positive effects as obtained with the HTST technique (high temperature, short time) for pasteurizing fresh milk with heating and cooling times of a few seconds only. As compared with the considerably longer pasteurization process via heat transfer media, improvements with regard to colour and flavour of vegetables and other heat sensitive products are achieved with this method.

Whilst, for instance, the heating process for meals weighing 400g takes 3 - 5 minutes with microwaves, 30 - 45 minutes - depending on heat transfer medium and temperature difference - are required when heating is effected exclusively via heat conduction.

With the short-term microwave heating, the fresh green colour as well as the flavour of broccoli, for example, remain unchanged, whereas the longer heating time with heat transfer media causes an olive or brownish colouring. At the same time, the extended treatment period affects the flavour and freshness of the product.

A physical criteria for microwave heating is the temperature distribution within the product that is primarily determined by the dielectric factors, the thickness and the geometry of the individual components.

In this context, the tray geometry is of special importance. In the case of conventional microwave line designs implying a concentration of the electromagnetic fields, the product is overheated at the edges and especially in the corners of the tray (eg, 15 - 20 [degrees] C hotter than the product in the centre of the tray).

In order to compensate for these negative effects, microwave ovens for household application are generally provided with turning tables. In case of microwave production lines, a similar compensation can be obtained by transporting the tray on a conveyor belt through the line.

Microwave lines for pasteurizing food products

Line Design

Berstorff microwave hybrid chamber lines are of modular design:

- Standard hybrid chamber length L = 2.1.m

- Standard set of magnetrons P = 1.2Kw with microprocessor card

- Standard track arrangement Laboratory lines:

1 hybrid chamber module. 6 - 24 magnetrons (1 - 4 tracks)

microwave power P = 7.2 - 28.8Kw

output m = 100 - 400 Kg/h at T = 40 [degrees] C Pilot lines

1 - 2 hybrid chamber modules. 12 - 48 magnetrons (2 - 4 tracks)

microwave power P = 14.4 - 57.6Kw

output m = 100 - 800 Kg/h Production lines (Figs. 1 and 2)

2 - 8 hybrid chamber modules, 24 - 192 magnetrons (2 - 4 tracks)

microwave power P = 28.8 - 230Kw

output m = 400 = 2400 (3200)Kg/h

The following foods products have already been successfully treated with this process:

- fresh meals (ready-cooked meals, also those consisting of several components)

- dairy products

- pasta products (on trays or in bags)

- pizza bases (also with shortened fermenting time)

- bread and cakes

- cereal products (muesli products and bars)

- spices (thermal stability being of prior importance)

- nuts (vermin killing)

- cacao beans (roasting)

Sterilization of food products by applying microwave energy

In the case of conventional sterilization processes in autoclaves, sterilization by microwaves also implies the use of pressure vessels due to the overpressure that is required for physical reasons. Since corresponding production lines are usually operating continuously, double sluice systems are arranged at the inlet and outlet of the pressure vessel, and between the temperature retention and cooling sections.

Like in the case of microwave pasteurization lines, the product sealed in the tray is continuously guided through the pressure line on a conveyor belt system. For passing the sluices, small tray groups (5 - 20 trays, depending on the line size) are separated via speed increase conveyors and thus adapted to the sluice cycle.

For microwave sterilization in the pressure vessel, the computer-controlled hybrid chamber technology is applied with similarly positive results as for microwave pasteurization.

The technical difference compared with the "open" pasteurization line, is the necessity to convey the sealed tray within the pressure vessel through a heat retention and cooling section in order to cool the product down to below 100 [degrees] C in the overpressure section in front of the outlet pressure sluice. Such cooling is required to avoid bursting the tray due to the internal steam pressure.

In order to comply with these process requirements, the length of the pressure lines has to be adapted accordingly. A reduced length can only be obtained by subdividing the microwave section into an "open" section for heating the product from the inlet temperature to approximately 90 - 95 [degrees] C, a pressure section for increasing the temperature from 90 - 130 [degrees] C and a heat retention and cooling section of vertical design with dense product conveyance. It is thus possible to realize an acceptable line length even for high outputs.

Microwave power required in the pressure vessel can be further reduced by an additional steam injection in the pressure chamber, accelerating the heating process.

Microwave pressure line design

Laboratory lines

Table chamber design for basic tests with 1 - 2 trays

- with 2 magnetrons of 1.2 kW, hybrid design

- with master computer, colour monitor, keyboard, printer and disk


- for operation under overpressure, atmospheric pressure or vacuum

- with reversible conveyor belt Pilot lines (fig. 3)

3.5 m pressure vessel, D-300 mm, p max - 4 bar, p min - 100 mbar

- with 6 - 24 magnetrons of 1.2 kW, hybrid design

- with master computer, colour monitor, keyboard, printer and disk


- with complete conveyor belt system, flat or trough-shaped

- with vertical sluices for bulk goods

- with horizontal sluices for trays

- for outputs of up to 100 kg/h Production lines

Overpressure lines with vertical sluices for sterilizing free-flowing and pumpable products

- in 1.2 kW hybrid design, 2 - 4 tracks in the pressure vessel with master computer, colour monitor, keyboard, disk memory and hard-copy printer

- with heat retention section in the pressure vessel

- with direct relief behind the heat retention section

- with direct temperature control at determined positions in conveying direction

- for production with open trays, the outlet sluice has to reach into the aseptic section.

PHOTO : Fig. 1 Chamber with microwave power directly focused on the product to be treated

PHOTO : Fig. 2 Production line for the pasteurization of ready-cooked meals

PHOTO : Fig. 3 Pilot line for processing under vacuum, atmospheric pressure or overpresure
COPYRIGHT 1990 Food Trade Press Ltd.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Food Trade Review
Date:Jun 1, 1990
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