The increased global trade which, among other things, has resulted in longer transports and in the introduction of safety systems (such as HACCP, Hazard Analysis Critical Control Point) in the food industry has augmented the pressure on food suppliers. Today, the con- sumers demand supplier specifications of the content of microorganisms in the products.

Grain products such as flour, grits and grains have been considered as microbiologically safe products and their microorganism content has not been brought into question. However, new methods of cultivating grain with a smaller amount of pesticides have resulted in a higher content than normal of microorganisms such as bacteria, yeast and mould. Also other causes, such as a wet harvest period, can increase the content of microorganisms in grain.

New fields of application in the food industry, such as complete mixes with all the ingredients in dry form, require a considerably lower level of microorganisms and a very low water activity. The normal level of micro- organisms in grain products excludes such use.

Traditionally, no attempts have been made to reduce the number of microorganisms in grain products. The grains are only roughly screened when polished by being thrown against a rough surface.

US-A-4,631,380 discloses a system for treating mate- rials using microwave radiation. The system comprises a generally pentagonal casing, in which the microwave

sources are arranged in the periphery of the casing. In- side the casing, the material which is to be treated is conveyed either vertically by gravity or horizontally using a conveyor or a conveyor belt. The microwave sources operate at different frequencies, so that a dif- ference frequency of about 40 MHz is produced. For in- stance, one microwave source operates at the frequency 2450 MHz, whereas another operates at 2410 MHz. In this manner, a more efficient transmission of heat is said to be achieved.

The article"Microwave Grain Drying", Successful Farming, August 1982 also discloses the use of microwave energy in the treatment of grain in a method with a con- tinuous flow, but a complex mechanism for stirring the grain is required to ensure an even transmission of heat by the microwave energy.

In heat treatment of grain products, it is important to obtain certain predetermined temperatures and keep them over certain time intervals to ensure that a certain type of microorganism is killed or that a certain type of enzyme is inactivated. It is also important to ensure that the temperature is not too high or maintained for too long a time. If so, the grain can lose other impor- tant properties, such as flavour, physical properties and desired enzyme activity.

However, prior-art technique does not offer any methods or devices, according to which it is possible to ensure in a simple manner that each individual grain or particle in the grain product attains the same tempera- ture and is kept at this temperature over a predetermined time interval.

Thus, there is a great demand for a new, rational and economic method for heat-treating grain products in a continuous and controlled manner.

One object of the present invention is to provide a simple and reliable method for heat treatment of grain so that it attains an even and predetermined high tempera-

ture to reduce the content of microorganisms, such as bacteria, yeast and mould, in the grain and to inactivate certain enzymes and proteins.

Other objects, features and advantages of the pres- ent invention will appear from the description below.

These objects are achieved by a method and a device according to the independent claims. Particularly pre- ferred embodiments are stated in the dependent claims.

Summary of the Invention In short, the present invention relates to a method and a device for heat treatment of grain. More specifi- cally, the invention relates to a method and a device for continuous heat treatment of a flow of grain products by means of microwaves to reduce the content of microorga- nisms, such as bacteria, yeast and mould, in the grain products and to inactivate certain enzymes and proteins.

The method for heat treatment of a grain product according to the present invention comprises the fol- lowing steps a) optionally regulating the water content of the grain product, b) providing a substantially uniform, continuous flow of the grain product, c) heat-treating the grain product by means of microwave energy, d) measuring the temperature of the grain product before and after the heat treatment in one or more posi- tions, e) keeping the temperature of the grain product con- stant after the heat treatment over a predetermined time interval, f) cooling the grain product after this time inter- val, and g) controlling the continuous, substantially uniform flow by the measured temperatures.

The device for heat treatment of a grain product according to the present invention comprises a vertical tube having an inlet and an outlet and a heating zone.

The heating zone comprises one or more means for gen- erating microwave energy, said means being arranged around the circumference of the tube. According to the invention, the device has one or more temperature sensors arranged in the tube between the inlet and the heating zone as well as between the heating zone and the outlet.

After the heating zone, the tube is insulated over a predetermined length, and an output means is arranged at the outlet for regulating the substantially uniform flow of the grain product through the device.

The advantage of the present invention is that the method and the device according to the invention ensure in a simple, economic and rational manner that each indi- vidual grain or particle in the grain product attains the same temperature and is kept at this temperature over a predetermined time interval.

Detailed Description of the Invention The method according to the present invention com- prises the above-described steps.

The invention provides a method in which a con- tinuous substantially uniform flow of said products is treated using microwave energy.

The expression"substantially uniform, continuous flow"refers to such a product flow through a heat treat- ment zone that the product is treated in an equivalent manner per unit of volume and that the radiation inten- sity will be the same in each cross-section. This is important since all the individual constituents, grains or particles, constituting the grain product are, as far as possible, to be heated to the same temperature and to be kept at this temperature over a predetermined time in- terval. The flow rate is substantially constant, but there may be some variation around a constant mean value

without departing from the scope of the present inven- tion. In particular in the case of flour, whose flow tends to stop and obstruct the conduits, it is important for the flow to be continuous and substantially uniform.

In a preferred embodiment, the invention is particu- larly adapted for a process in a medium-sized mill. The microwave energy is very suitable to provide an even heat treatment of each unit of volume in a rapid flow of grain products. A microwave frequency of 2450 MHz is used.

The grain flow can be extended in the horizontal or in the vertical direction, but is preferably substan- tially vertically extended to facilitate the passage.

The substantially uniform flow of the grain product is in most cases at least 100 kg/h, preferably at least 200 kg/h and most preferably at least 300 kg/h, and is preferably less than 15,000 kg/h, more preferably less than 12,000 kg/h and most preferably less than 10,000 kg/h, but is regulated on the basis of the tem- peratures measured in the grain after heating.

The grain products are preferably heated at an energy level and for a period of time to an inner tem- perature in the grain of at least 70°C, more preferably at least 90°C and most preferably at least 100°C. Further- more, the temperature in the grain should be less than 140°C, preferably less than 130°C and most preferably less than 120°C.

The expression grain products and grain refers to everything from entire or partial grains, grits, beans, peas, spices to flour and/or powder.

In a preferred embodiment, the grain product for microwave radiation consists of raw products for animal feed.

In a preferred embodiment, the water content of the grain product is regulated to a water content of at least 5% by weight, preferably at least 12% by weight and most preferably at least 15% by weight, and preferably less

than 25% by weight, more preferably less than 22% by weight and most preferably less than 20% by weight.

In a particularly preferred embodiment, the grain product is heated at an energy level and for a period of time to a temperature which is sufficiently high to reduce the microorganism content of the grain product.

This temperature is preferably at least 90°C, preferably at least 100°C. Preferably, the grain is kept at this temperature for a period of time of 1-20 minutes, more preferably 5-12 minutes.

The expression microorganisms refers to bacteria, yeast and mould.

In an embodiment of the invention, it is possible to control the properties of the grain products by heat treatment of the grain in a certain temperature interval and for a certain period of time, thus reducing the en- zyme activity of the grain and/or changing the function of the proteins. It will thus be possible to develop products with specific properties for new and old fields of application. It is, for instance, possible to denatu- rate certain enzymes by microwave energy, thereby for in- stance increasing the storage stability of the products.

In a particularly preferred embodiment, the grain is heated at an energy level and for a period of time to a temperature which is sufficiently high to reduce or eli- minate the enzyme activity of the grain. This temperature is preferably at least 100°C.

According to a preferred embodiment of the inventive method, the amylase activity of the grain is reduced to prevent the decomposition of starch. This increases the storage stability of the products in combined final prod- ucts.

According to another embodiment of the method ac- cording to the present invention, oats are heat-treated by means of microwaves, instead of as traditionally by means of steam, before rolling. The treatment serves to inactive enzymes breaking down fat, so-called lipases, to

reduce the oxidation of fat in the oats during storage.

When steaming, the oats are treated by blowing steam into a tower. The steam treatment is not an optimal process, since the oats are treated for a long time at a tempera- ture favourable for the growth of microorganisms.

In another particularly preferred embodiment of the present invention, the grain product consists of peas and beans which are heated at an energy level and for a pe- riod of time to a temperature which is sufficiently high to increase the digestibility of beans and peas and to inactivate growth-inhibiting substances and substances with unpleasant taste. The soya bean, for instance, con- tains growth inhibitors, such as trypsin and urease, and the activity of these inhibitors can be reduced by 75%.

Another embodiment of the invention aims at com- pletely or partially reducing the gluten vitality of the grain to adjust its properties to various ready-made mixes, for instance for sponge cakes and pancakes, which require completely different types of properties of the flour. Preferably, the grain is heated at an energy level and for a period of time to a temperature which is suffi- ciently high to eliminate the gluten vitality of the grain. This temperature is preferably at least 90°C.

One embodiment of the invention relates to a method of producing a flour with cold-swelling starch. This is effected by moistening wheat grains before the microwave treatment and subsequently grinding the treated grain.

In a preferred embodiment of the present invention, the grain is cooled after the microwave treatment by an air flow being blown over the grain, which removes water steam and condensate. The cooling of the grain reduces the water activity of the grain. By reducing the tempera- ture and the water activity, the storability of the prod- uct increases.

Preferred embodiments of the inventive device will now be described, inter alia, with reference to the ac-

companying drawings. These drawings are not to be con- sidered as limiting the scope of the present invention.

Fig. 1 is a cross-sectional view of a preferred em- bodiment of the device according to the present inven- tion.

Fig. 2 is a cross-sectional view of another prefer- red embodiment of the device according to the present invention.

The device according to the present invention com- prises a vertical tube 1 having an inlet 2 and an outlet 3 and a heating zone 4. The heating zone 4 comprises a portion of the vertical tube 1, in the periphery of which and around the circumference of which one or more means 5 are arranged for generating microwave energy. According to the invention, one or more temperature sensors 6 are arranged in the tube 1 between the inlet 2 and the heat- ing zone 4 as well as between the heating zone 4 and the outlet 3. The vertical tube 1 is also insulated with in- sulation 7 in a section of a predetermined length after the heating zone and in the direction of the outlet 3.

Furthermore, at the outlet 3 an output means 8 is ar- ranged for regulating the substantially uniform flow of the grain product through the tube 1.

In a preferred embodiment of the inventive device, the output means 8 is a sluice valve 8a (see Fig. 1) with an inlet 9 having the same cross-sectional form as the vertical tube 1. That is to say that if the vertical tube has a circular cross-section with a certain radius, the sluice valve inlet has the same circular cross-section with the same radius, and so on. The speed of the sluice valve 8a is controlled by a control device 10 using the measured temperature value or values as input variables, said control device 10 subsequently transmitting a signal to a motor 11 that drives the sluice.

In another preferred embodiment of the inventive device, the output means 8 is a screw 8b (see Fig. 2), which is preferably arranged at least in the outlet 3 but

can extend from the outlet 3 through a part of or through the entire vertical tube 1 (see Fig. 2) and whose speed is also controlled by a control device 10 using the meas- ured temperature value or values as input variables, said control device 10 subsequently transmitting signals to a motor 11 that drives the screw 8b.

It is important for the product to be conveyed and heat-treated in a closed conduit or a closed tube or system so that the product will not be contaminated again. In addition, the cooling must be effected with a controlled air flow (right temperature, right quantity and a high degree of purification) to avoid re-contamina- tion.

Examples The present invention will now be illustrated by some Examples. These Examples are not to be considered as limiting the scope of the present invention.

In the Examples, various kinds of grain products, entire grains or parts thereof as well as flour are heat- treated by microwave radiation according to one or more preferred embodiments of the present invention.

The microwave device which was used had 18 radiation sources and was of the type described in WO 92/11497. The grain was conveyed and heated in a polypropylene tube having a diameter of 20 cm, which tube crossed the micro- wave device. The temperature of the product before and after the treatment was measured by thermocouples.

Example 1 Hulled oats were treated according to the inventive method to inactivate lipase activity. Before treatment, the oats had a lipase activity of 17 ml 0.05 M Na- ethylate/g (titration analysis).

The water content of the oats was regulated to 16.5% by weight and fed into the device according to the present invention. The microwave effect was 11.5 kW, and the tem-

perature of the oats was measured before and after the heat treatment. At an inner temperature of 101°C, a com- plete inactivation of lipase was obtained, and the flow through the device was controlled by the control device 10 so that this temperature was obtained after the heating zone 4. The flow was 184 kg/h.

Example 2 Wheat was treated according to the inventive method to reduce a-amylase activity.

The water content of the wheat was regulated to 16.3% by weight and fed into the device according to the present invention. The microwave effect was 16.2 kW, and the tem- perature of the wheat was measured before and after the heat treatment. At an inner temperature of 105°C, a reduc- tion was obtained from an initial amylase activity of 1.6 k (a) (k (a) = the velocity constant for the hydrolysis of starch) (falling number 250 s, measured according to ICC 107) to an amylase activity of 0.6 k (alpha) (falling number 450 s). The flow through the device was controlled by the control device 10 so that the temperature in ques- tion was obtained after the heating zone 4. The flow was 200 kg/h.

Example 3 Wheat was treated according to the inventive method to denaturate gluten. Initially, the wet gluten content was 24% by weight.

The water content of the wheat was regulated to 13.5% by weight and fed into the device according to the present invention. The microwave effect was 14.3 kW, and the tem- perature of the wheat was measured before and after the heat treatment. At an inner temperature of 91°C, a com- plete denaturation of gluten was obtained. The flow was 240 kg/h.

Example 4 Wheat was treated according to the inventive method to reduce the microorganism content. The initial amount of mould and yeast fungi in the wheat was 103-104 per gram of wheat.

The water content of the wheat was regulated to 19.7% by weight and fed into the device according to the present invention. The microwave effect was 16.2 kW, and the temperature of the wheat was measured before and after the heat treatment. At an inner temperature of 90°C, a product was obtained with <101 mould and yeast fungi per gram of wheat (measured according to NMKL 98: 3). The flow through the device was controlled by the control device 10 so that the temperature in question was obtained after the heating zone 4. The flow was 160 kg/h.

Example 5 Rye was treated according to the inventive method to reduce the microorganism content. The initial amount of mould and yeast fungi in the rye was 103-104 per gram of rye.

The water content of the rye was regulated to 12. 1% by weight and fed into the device according to the pres- ent invention. The microwave effect was 16.2 kW, and the temperature of the rye was measured before and after the heat treatment. At an inner temperature of 80°C, a prod- uct was obtained with <101 mould and yeast fungi per gram of rye (measured according to NMKL 98: 3). The flow through the device was controlled by the control device 10 so that the temperature in question was obtained after the heating zone 4. The flow was 204 kg/h.

Example 6 Oats were treated according to the inventive method to reduce the microorganism content. The initial amount of mould and yeast fungi in the oats was 103-104 per gram of oats.

The water content of the oats was regulated to 14.2% per weight and fed into the device according to the pres- ent invention. The microwave effect was 16.2 kW, and the temperature of the oats was measured before and after the heat treatment. At an inner temperature of 101°C, a prod- uct was obtained with <101 mould and yeast fungi per gram of oats (measured according to NMKL 98: 3). The flow through the device was controlled by the control device 10 so that the temperature in question was obtained after the heating zone 4. The flow was 184 kg/h.

Example 7 Various spices were treated according to the inven- tive method to reduce the microorganism content. The ini- tial amount of mould and yeast fungi in the spices was 103-104 per gram of spices.

The water content of the spices was regulated to 13.7% by weight and fed into the device according to the present invention. The microwave effect was 16.2 kW, and the temperature of the spices was measured before and after the heat treatment. At an inner temperature of 99°C, a product was obtained with <101 mould and yeast fungi per gram of spices (measured according to NMKL 98: 3) with a minimum loss of flavour and aroma. The flow through the device was controlled by the control device 10 so that the temperature in question was obtained after the heating zone 4. The flow was 160 kg/h.

The method and device according to the present in- vention can be used for a number of purposes, as evident from the description above. By the method and the device according to the invention, it is possible to ensure in a simple, economic and rational manner that each individual grain or particle in the grain product attains the same temperature and is kept at this temperature over a pre- determined time interval.