FI90386B - Method for generating smoke for smoke treatment of foodstuffs - Google Patents

Abstract

The invention concerns a method and an arrangement for generating smoke for smoke treatment of foodstuffs. The smoke is developed by means of a technique in which combustion takes place from above, the smoke being used only when there is a charcoal layer of 0.5 to 100 mm on the fuel stack, which has arisen during combustion of the fuel. The charcoal layer acts as a regulator of the combustion air flow, the combustion being allowed to take place at a temperature which is advantageous as regards the development of odours and hazardous substances. The layer of charcoal also acts as a selective filter and removes hazardous substances from the smoke. <IMAGE>

Description

90386

The present invention relates to a method for generating smoke for use in smoking foodstuffs

Smoking is a method of improving the preservation and taste of foods such as meat and fish. However, a disadvantage of this method of food processing has been found to be that the burning or heating of wood to produce smoke produces harmful substances in addition to the desired flavoring substances, some of which have been found to be carcinogenic.

The problem of contaminants has been recognized for a relatively long time and several approaches have been proposed to address it. In general, these known methods are based on the treatment of the smoke formed, which seeks to separate the flavoring substances from the smoke on the one hand and the contaminants into their own fractions on the other hand.

One such known method is based on cooling surfaces placed in the passage of smoke, on which it is desired to condense the harmful substances contained in the smoke. In part, such methods work in the desired way, as contaminants such as tar remain on the sealing surfaces. Other contaminants are also concentrated, but the effectiveness of the method is deficient, as only some of the contaminants are removed and, on the other hand, the desired flavorings are lost, so that smoking must be correspondingly more intense in order to achieve the desired flavor.

More advanced methods are based on the finding that the flavoring substances contained in the smoke are generally water-soluble and the contaminants, respectively, are generally water-insoluble. This finding has allowed for a kind of "indirect" smoking. The desired flavorings have been separated from the smoke by water washing. This aqueous fraction is then further treated by various physical and / or chemical methods to obtain the most harmless but aromatic liquid smoke fraction possible. For the smoking effect, the foods to be smoked are soaked in that liquid smoke fraction to impart flavor to them. The products can then be subjected to further light smoking in a conventional manner to obtain the desired appearance. The result can be considered to be, at least in principle, a substantially cleaner product which has undergone a complex treatment process.

Similarly, methods are also known in which the smoke fraction separated from the smoke by water washing is treated to such an extent that a dry product is obtained. This can then be used as a spice to "smoke" the food. These dry methods may also involve light smoking in the usual manner to improve the appearance of the product.

Recent research on smoking has sought to find solutions to the problem of harmful substances by focusing on the process of smoke formation itself and finding ways to minimize the formation of harmful substances without losing flavor. In these studies, one of the essential factors for the composition of the smoke produced by the burning of wood is the burning temperature. Combustion temperatures slightly below 700 ° C, i.e. about 650-700 ° C, have been found to be the optimum range. In this case, too, the formation of contaminants cannot be completely avoided, but it has been found to be the best possible for flavorings and contaminants in the combustion temperature range in question. This flavoring and harmful I; 3 The ratio between 90386 substances deteriorates sharply at temperatures above 700 ° C, while at temperatures below 700 ° C the change in the ratio is more gentle. (Potthast: Advances in Food Reseach, Vol. 29, 1984).

One essential factor in keeping the combustion event to remain in that optimum range is the correct dosing of the combustion air. In addition to the fact that the oxygen in the air affects the actual combustion event, the air also participates in the secondary reactions immediately after the combustion event, which play a role in the final composition of the flue gas.

This state-of-the-art approach to reducing the contaminant problem must be considered correct, and the benefits it provides can be supplemented by older known methods, such as smoke condensation.

As a result of the tests performed, it has been found that the target combustion event, and the events immediately following it, are associated with factors which, if properly controlled, have a significant effect on the composition of the smoke obtained, low levels of contaminants and high levels of flavorings.

As a starting point for achieving favorable results, the right combustion technology has been found. Experiments have shown that in order to achieve proper results, smoke development must be based on the so-called upper combustion technology, i.e. the use of a combustion chamber in which a substantial part of the combustion air is led to the fuel through the burnt material layer on top of the combustion layer. Although the use of the upper fire technique to develop smoke for smoking can be considered per se known from, for example, German Patent No. 867,947, this technique involves many aspects which can affect the quality of the smoke and which have not been elucidated e.g. in the above publication.

The effect of the carbon layer on the combustion event can be considered indirect, because when using the upper fire technique, the air entering the combustion zone has to pass through the carbon layer formed on the fuel surface, in countercurrent to the smoke leaving the combustion zone. The carbon layer thus forms a flow restrictor for the combustion air, which has a direct effect on the intensity of combustion, which in turn has an effect on the combustion temperature and the composition of the smoke formed thereby.

On the other hand, the resulting carbon layer affects the flue gases through its considerable absorption effect. Carbon has been found to retain smoke by selecting ingredients, with the choice apparently based on the molecular weights and polarity of the substances. However, experiments have shown that the choice is advantageous in that compounds found to be potent, such as benzo-α-pyrene, have been found to adhere more carbon to phenolic compounds classified as aromatic compounds.

For these two events, the thickness of the layer has proven to be an essential factor for the carbon layer formed on the surface of the fuel, which factor apparently has a complex effect on the overall smoke evolution event.

If we first consider the effect of the carbon layer on the flow of combustion air, it is clear that the effect of slowing down the flow increases as the carbon layer increases. When fuel is ignited, the carbon layer is non-existent, so that the passage of combustion air to the combustion zone

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5 90386 there are no obstacles. At this stage, the combustion takes place wildly from the point of view of the formation of smoke, i.e. mainly by roaring, whereby the combustion temperature easily becomes too high for the formation of various substance compounds. Smoke produces little aroma, but a lot of harmful substances. However, if the fuel, and its grain size, is chosen correctly, which will be returned to later, the combustion will level off relatively quickly after ignition as the carbon layer begins to form on the surface of the fuel.

In terms of combustion, a functional carbon layer can thus be considered to have a certain minimum thickness in order to have a sufficient damping effect on the intensity of combustion. Experiments have shown that this minimum thickness is found to be about 0.5 mm, somewhat depending on the degree of roughness of the fuel used. Coarse fuel may require a minimum thickness of about 2 mm from the carbon layer to achieve the desired combustion event. Once that carbon layer is formed, the combustion evens out and the open fire disappears, whereupon the combustion continues by boiling. By visual observation, a clear increase in smoke development can also be seen at this stage. Measurements have also shown that the temperature of the combustion zone is at this stage in the optimum range for smoke composition, i.e. temperatures below 700 ° C, where flavors are preferably formed relative to contaminants and the ratio is not very dependent on temperature changes.

It can also be considered obvious that the thickness of the carbon layer has a clear effect on the absorption efficiency of the layer. As the layer increases, the absorption efficiency increases.

However, experiments have shown that there is also a certain maximum value for the desired effect with the thickness of the carbon layer. As combustion progresses and the carbon layer grows thereafter, smoke development has been found to decline. Likewise, a change in the composition of the smoke can already be sensory altered, which factors speak of substantial changes in the combustion event, and possibly also in the post-combustion events.

From the point of view of combustion itself, it is obvious that as the carbon layer grows, it forms such a large limiting factor for the flow of combustion air that the combustion zone does not receive enough air to maintain combustion as desired. Combustion slows down, as does the combustion temperature. In this case, the amount of smoke obtained is limited, and on the other hand, its composition changes in an unfavorable direction due to too low a combustion temperature, i.e. the amount of flavoring substances relative to the contaminants is reduced.

On the other hand, the thickness of the carbon layer can also be considered in terms of absorption. With regard to absorption, a certain upper limit is also encountered in the efficiency of the carbon layer, the thicker carbon layer no longer significantly increases the absorption of pollutants from the flue gases. The carbon particles in the upper layers fill up and lose their power. The absorption is thus limited only to a certain limit above the combustion layer.

On the contrary, an over-thick layer of carbon can also have detrimental effects on the flue gas cleaning efficiency. The carbon layer forms a fairly efficient reaction surface for the flue gas components with the oxygen flowing through the carbon layer on the one hand and through the carbon layer on the other hand. As a result of these reactions, substantially more harmful substances may be generated and released into the flue gas than was originally present in the flue gas. This post-reaction situation can also be substantially affected by the fact that the upper carbon layers of the filtration layer can start to glow in the incoming combustion air stream, whereby it is possible that a substantial part of the smoke flavorings are destroyed.

The maximum thickness of the carbon layer on top of the combustion zone has been estimated by experiments to be about 100 mm, although the grain size of the fuel can set the preferred maximum thickness limit well below, about 20-50 mm. The latter value is due, for example, to the fuel with excellent combustion properties, the majority of the granules of which were in the range of 125 to 2000 / im. With a clearly coarser fuel, the maximum thickness of the carbon layer can be somewhat higher than the above-mentioned 100 mm.

Based on the above, a smoke generation method has been developed for the smoking of foodstuffs, in which the smoke is generated by burning wood or a substantially similar carbonizing material by the combustion technique and characterized by smoke from combustion only when about 0.5 to 100 mm above the combustion zone. a thick, undisturbed carbon layer resulting from fuel combustion. Particularly preferably, smoke is recovered only when the thickness of the carbon layer is in the range from 2 to 50 mm.

It has also proved to be quite significant that the carbon layer formed during combustion remains undisturbed during the utilized smoke generation phase.

The method according to the invention can be carried out either in batches, which operating model is advantageous for small-scale smoking installations. From this effortless! - 8 90386 operating model, a substantially continuous operation can also be achieved for industrial-scale smoking systems. Such a continuous but batch-based device implementation is shown in the accompanying drawing 1, which will be explained in more detail later. With regard to the combustion event, a completely continuous process is also achievable, but the carbon layer limiting phase involves events, the consideration of which will be explained later in connection with the inventory of the apparatus according to Figure 2.

There are also general considerations associated with the construction of the equipment used in the method that should be taken into account in the equipment design. First, it is noteworthy that if the combustion chamber is made of a highly thermally conductive material, such as metal, combustion occurs more efficiently in the middle of the combustion chamber than at the edges, i.e. apparently heat loss through the combustion chamber walls has an effect on the combustion event. The phenomenon is accentuated if the combustion chamber is angular in cross-section, in which case combustion occurs most slowly at the angles of the chamber. Uneven propagation of the fire front in the height direction can be considered as a detrimental factor in terms of wood consumption and result, so this fact should be taken into account in the design of the equipment to be used. It would be advantageous if at least the vertical walls of the combustion chamber were of poorly thermally conductive material, for example a ceramic material, or if it were appropriate to insulate them thermally. The problem of uneven combustion can also be reduced by a small side flow of combustion air conducted through the walls of the combustion chamber, but this flow must not be so large that the smoke development event is disturbed. Careful heating of the combustion chamber wall from the outside also promotes even combustion.

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The problem of uneven combustion can also be reduced by choosing the shape of the combustion chamber as having a horizontal cross-section.

The development of large amounts of smoke should also take into account the fact that as the combustion surface increases, flow problems may arise due to the opposite flow directions of combustion air and flue gases, which interfere with the process if air is not sufficiently passed to the fuel surface.

The following Example 1 describes the experiment performed in which the combustion event was investigated by monitoring the combustion temperature in the fuel pile.

Example 1

The experiment used a smoking oven made for household use, which had the following external dimensions: width 400 mm, depth 450 mm and height 700 mm. There was a short smoke duct at the top of the smoking area to remove the smoke. A combustion chamber was made in the lower part of the smoking furnace, into which the fuel-containing combustion box could be inserted through an opening in the wall of the furnace, which could be closed by a door. The burner was open on the outside, but closed on the walls. The cross-sectional dimensions of the box were 130 x 140 mm. The air required for combustion was led into the combustion chamber through the combustion box inlet.

In the experiment, sawdust chopped from beech was placed in the combustion box as fuel. The grain size of the crumb was as follows: 10 90386

Grain size (/ μm)% 4000 - 2000 1.1 2000 - 1000 9.4 1000 - 500 27.3 500 - 250 34.4 250 - 125 22.0 125 - 74 4.6 <74 1.2

This chip had a density of about 270 kg / m 2 and a moisture content of 7.2% by wet weight. In the combustion experiment, 200 g (wet weight) of chewing gum was used, which was poured into soup in a combustion box. The height of the heap from the middle was about 86 mm.

To monitor the temperature, temperature sensors (NiCr-Ni thermocouple, type K, wire length 0.25 mm) were placed in the middle of the box at different heights. The height of the sensors from the bottom of the box was: sensor height from the bottom, mm 1 85 2 69 3 51 4 40 5 24

At the beginning of the experiment, the chewing gum was ignited over its entire surface with a blow lamp. After the open fire disappeared from the surface of the heap, a box was placed in the combustion chamber of the smoke oven. Sufficient combustion air was supplied to the combustion through the box inlet by means of natural draft.

The experiment monitored the development of temperature at different measuring points as a function of time as the fermentation front progressed in the debris.

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The results of the test are given in the following curve 1: Temperatures in boiling chips Γ .1 85 mm «2 69 mm • a 51 mm. * ** 4 0 mm ^ 24 mm 800 —-1-1-1-1-1-r — t —I-1 i Λ l - thermocouples υ 500 - 1 - ”sequence | 200 - Yv /.-- J,> 'Sk \ ”-1 100 - Curve 1 g l" U * r i i_i_i_ | -1-1-1-1-1- 0 60 120 BREAK (min)

It can be seen from the results that after the actual combustion combustion developed (measuring point 2), the temperature in the combustion zone rose slightly above 500 ° C, and at the following measuring points slightly higher. The peak value of point 4 was reached in about 80 min. At this point, however, smoke evolution had already begun to decline, and continued to decline, although the temperature at point 5 still rose, albeit at a slower rate. The obvious reason for the deceleration can be considered to be the flow resistance provided by the carbon layer formed on the surface of the heap, which slowed down the penetration of the combustion air into the combustion layer.

Experimental smokes were also performed with this device and the described combustion technique, in which fish was smoked. Benzo-α-pyrene, which indicates contaminants, was determined from the smoked products. Several experiments were performed by varying the ratios of the smoking conditions to 90386, i.e. by selecting the incineration step in which the incineration box was inserted into the smoking furnace. Most samples had benzo-α-pyrene concentrations below the detection limit, with a maximum concentration of 0.05 ug / kg.

For comparison, a smoking test was also performed in which the fuel burned in free-flowing air. In this smoking, the benzo-α-pyrene content of the product was measured to be 0.5 μg / kg.

The devices according to the invention are explained with the aid of the accompanying drawing, in which

Figure 1 shows a batch operation in principle, and

Figure 2 shows another hardware alternative, also in principle.

The accompanying drawing 1 schematically shows a batch but substantially continuously operating smoke generating apparatus for carrying out the method according to the invention.

The apparatus has a plurality of smoke generating units 2 mounted on an endless, horizontal conveyor, such as a chain conveyor. These units are closed-walled, open-top boxes with dimensions selected according to the desired smoke evolution. The cross-sectional dimensions can be, for example, about 0.5 x 0.5 m. The height dimension is not otherwise critical, but a sufficient layer of fuel must be provided in the box. For example, a height height sufficient for a smoke evolution event is about 150 mm.

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The smoke generating units are arranged to pass in the step movement of the conveyor 1 alternately through the fuel filling station 3, the ignition station 4, the smoke generating station 5 and the discharge station 6.

In the filling station 3, a suitable amount of fuel is dispensed into the smoke generating unit from the silo 7, either with a tray dispenser 8 or using a leveling scraper, which sweeps the excess fuel out of the smoke generating unit. In the next step, the smoke generating unit proceeds to the ignition station 4. Here the surface of the fuel is ignited by a flame, preferably a gas flame 9, from the entire surface of the smoke generating unit. When the fuel is ignited throughout, the ignition flame is extinguished. The smoke generating unit is held in the ignition station for a suitable time during which the surface of the fuel burns with an open flame and gradually forms the desired carbon layer on its surface.

The smoke formed during this time is led out of the equipment by a separate smoke duct 10, i.e. this smoke is not used in the smoking process. The combustion event of the ignition phase is monitored to detect the formation of a sufficient carbon layer. A fairly reliable indication of the formation of the desired carbon layer is a clear increase in smoke generation. Generally, this step takes about 1-10 minutes. After a sufficient carbon layer, i.e. at least about 0.5 mm, has been found, the smoke generating unit can be moved to smoke development stage 5. Excessive smoke evolution can be observed at the smoke generating station, depending on the grain size of the fuel, for about an hour. a carbon layer generally having a thickness in the range of 20 to 50 mm. However, with a fuel classified as coarse, the advantageous smoke development can continue up to a carbon layer thickness of about 100 mm, or somewhat above this.

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Upon combustion, at this stage, the smoke generating unit is removed from the smoke generating station and led to a discharge station 6, where carbon and unburned fuel are removed.

The process periodically circulates the above-mentioned cycle, in which case the combustion situation of the smoking station must be foreseen in the ignition phase in order to avoid interruptions in smoke generation. It is also possible to knowingly periodically use the smoke to be given in successive cycles which follow at certain intervals.

The process can be controlled by suitable automation or alternatively manually.

If the preferred maximum carbon layer thickness of the spent fuel has been found to be, for example, about 50 mm, it is advantageous to fill the smoke generating unit 2 with fuel in the filling step 3 to a layer thickness of, for example, about 70 to 80 mm. The fuel layer remaining at the bottom of the smoke generation unit after the smoke generation stage has its own significance as a recipient of tar fractions that may distill during the smoke generation stage.

The combustion air required for the smoke generation stage is led to the surface of the fuel by a blower apparatus 11, the amount of air supplied by which is preferably adjustable. Air must be dispensed evenly over the fuel surface. The use of natural draft is also possible in conducting combustion air, which should also be made adjustable.

The smoke generating unit may also include a heat exchanger 12, which may be a condenser or a heater. The condenser can be used to target any impurities in the flue gas in a known manner. In the case of hot flue gas, the temperature of the flue gases can be raised by means of a heat exchanger.

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Ash is emptied during the emptying stage of the smoke generating units into a suitable collecting vessel 13 or removal conveyor.

An alternative apparatus for carrying out the invention is shown in the accompanying drawing 2. This apparatus is in principle continuous, although its use involves aspects requiring periodization, which will be clarified later.

The apparatus consists of a combustion chamber 15, a jacket 14 surrounding the upper part of the combustion chamber, means 16 for supplying fuel to the lower part of the combustion chamber, means 17 for supplying combustion air substantially to the combustion chamber, devices 18 for introducing smoke into the smoke chamber, devices 18 'and 18' 'for bypassing smoke hardware.

The jacket 14 of the apparatus may have a relatively simple structure, in principle a metal plate structure, since it is not subjected to substantial thermal stresses when the apparatus is used. The combustion chamber 15 itself, on the other hand, requires more structurally. Based on the above, it is preferable to make it round in cross-section, whereby its lower part 15 'may be formed of a metal tube. Instead, in the region 15 '' of the actual combustion zone, the combustion chamber should be made thermally insulated, or it can be made of a material with poor thermal conductivity, for example ceramic.

The fuel is introduced into the lower part of the combustion chamber by suitable supply equipment, such as a screw feeder 16. To conduct combustion air to the combustion chamber mouth, the apparatus may have a separate piping 17, or alternatively combustion air may be provided through suitably sized openings in the jacket 14. The combustion chamber 15 is preferably equipped with temperature sensors 20 located substantially at the height of the desired combustion zone.

Such equipment can be used as follows. Suitable finely divided wood is fed with a screw feeder 16 into the combustion chamber 15 until the combustion chamber is completely filled to the brim. The fuel surface is then ignited throughout and a sufficient amount of air is supplied to the fuel for combustion. At this stage, the flue gas generated is led out of the equipment, for example the flue gas via the branch line 18 'of the line 18, i.e. the flue gas of this ignition stage is not utilized. After the combustion has stabilized, which can be stated quite reliably e.g. of the amount of smoke evolved, the flue gas is introduced into the smoke and the combustion is allowed to continue for a certain, predetermined time (1 to 10 min) during which the fire front is found to advance in the depth direction of the above about 30 to 50 mm, in some cases about 100 mm. The flue gas bypass duct 18 'is then opened and the supply system 16 is started momentarily. The new fuel entering the combustion chamber pushes the fuel column upwards, whereby the dew layer of its edge areas collapses from the mouth of the combustion chamber to the bottom of the jacket 14 and can be removed by ash removal devices 19.

The cycle of supplying new fuel inevitably causes a flushing effect in the carbon layer on top of the combustion layer, so during flushing and the subsequent stabilization of combustion, the smoke must be discharged from the smoke system via the bypass duct 18 '. Once the combustion has stabilized, the smoke can again lead to smoking. The procedure is repeated a few times to form a suitable pile at the mouth of the combustion chamber, which is triggered in its surface layer substantially throughout I; i7 90386 with each new fuel supply. In principle, a continuous smoke generation event has been achieved with the equipment, although flotation and the subsequent stabilization of combustion cause their own interruptions in the development of usable smoke.

It may be advisable to continue the above-mentioned supply period of new fuel at least in some supply cycles until such time as a fresh layer of fuel is applied to the combustion plant up to the surface. In this case, the tar fraction is also removed from the equipment, which is distilled from the fuel above the fire front and enriched in the fuel below. After such a feeding period, the pile must be re-ignited as described above.

Contrary to the above, the apparatus may also be provided with a suitable scraper which removes the Kars bed from the mouth of the combustion chamber after each of the fuel supply cycles. Even then, it is advisable to follow the above-mentioned bypass when utilizing smoke.

The operation of the device can be controlled relatively reliably manually by monitoring the development of smoke and adjusting the fuel supply depending on it. The operation can also be automated relatively simply by placing temperature sensors 20 in and immediately in the vicinity of the combustion zone, which can be used to determine the length and frequencies of the fuel supply cycles to be maintained by simple logic equipment to keep the combustion zone at the desired distance from the combustion zone.

It is also essential for the performance of the method and the operation of the equipment that the fuel used is correctly selected. Useful fuel is e.g.