HK1113063B - Ohmic heating system with circulation by augers - Google Patents

Description

The present invention relates to an ohmic heater for food compositions, including soups.

In the food industry, ohmic heating is developed for heating liquid or solid food, as this technique allows a rapid rise in temperature while preserving the organoleptic qualities of the food.

Ohmic heating allows food to be heated by passing an electric current. The resistance of the product to the flow of electricity causes the temperature to rise.

The Joule heating technique is well known, for example, ohmic heating devices comprising a tubular central channel at either end of which electrodes are placed, punched to allow a fluid to be introduced into the tube and collected. The columns used are usually composed of a tube of insulating material (pyrrex glass or plastic) in which the product to be heated circulates between the two electrodes. These two electrodes are both perpendicular to the channel and to the general direction of flow of the fluid.

For example, Japanese patent 2004-290094 describes a plant for sterilising foodstuffs with a first preheating tube, powered by a pump, the output of which is a second ohmic heating tube. One of the problems encountered in the use of ohmic heating for the heat treatment of large-piece foods such as potatoes or cooked dishes is the heterogeneity of heating of the compounds contained in the food product resulting in overcooking of some compounds to the detriment of their organoleptic qualities. This heterogeneity of heating is partly due to a difference in the residence time of the liquid phase and particles in the ohmic heating column.The control of the residence time of the compounds in the ohmic heating column requires control of the flow of food into the column. In the above solution, the liquid phase moves faster than the solid particles in the first tube and then in the intermediate pipe and then in the second tube. To take this into account, it is necessary to set the heating time in the second tube at a fixed time according to the speed of movement of the fastest phase in all equipment including the first tube, the connecting pipe and the second tube,This ensures that the fastest moving phase is sterilised throughout the plant, but at the expense of preserving the nutritional and organoleptic qualities of the slowest phase.

Furthermore, after preheating, the two phases enter the sterilization tube at different temperatures, which exacerbates the above phenomenon.

The difference in the time of stay is explained on the one hand by a problem of sedimentation, which is accentuated by a low viscosity of the liquid phase. As such, until very recently, most known ohmic heating columns were vertical with a large section.

Although these improvements have avoided the phenomena of vertical sedimentation of the parts in the liquid phase, a heterogeneity in the residence times of the different components in the heating columns is still observed and can be explained by fluid mechanical phenomena.

The dispersion of the residence times can be related to a laminar flow of the product. A liquid product in laminar regime has a dispersion of residence times up to 2 in Newtonian liquids. The product in contact with the walls has a speed almost zero while the one in the heart of the vein can go twice as fast as the average flow rate of the liquid. This phenomenon in food compositions formed of a heterogeneous mixture of a liquid phase and solid particles is greatly limited by the content of the important pieces of the products to be processed.

The second phenomenon of fluid mechanics is the slip velocity . In a suspension of particles, the carrier fluid tends to move faster than the particles it carries. This phenomenon results in a larger average time of presence of the particles than that of the liquid.

To address these drawbacks and in particular to permit heating of a continuous product flow, to uniformise the residence time of the food product compounds and to preserve the organoleptic qualities of products heated by an ohmic heating column, the invention concerns, according to its most general acceptance, a product heating system, in particular a food composition consisting of a heterogeneous mixture of a liquid phase and solid particles, comprising at least a heating pipe of tubular section of electrically insulating materials and having at its two ends a heating electrode, the two electrodes being connected to a non-ringed source of electrical power. The second heating chamber is characterised by a power supply system for the supply of fuel to a pump in which the fuel is discharged into a heating tube, which is not connected to the heating tube, and which is connected to the second heating chamber by means of a direct current supply to the pump.

The advantage of the endless screw is that it has a full core that serves as a rotating axis.

The advantageous method of construction is that the pitch of the endless screw is twice the size of the side of the larger pieces.

One variant is that the endless screw motor is fitted with a frequency changer to vary its rotational speed.

Preferably, the endless screw is made of a non-abrasive plastic material.

The advantage is that the viscosity of the food product is between 250 and 1500 millipascals/second.

Another variant is that the food has a particle content of between 30 and 80%.

Depending on the preferred method of manufacture, the foodstuff has a homogeneous conductivity with a difference between the liquid phase and the particles of 1 to 3.

The invention will be better understood by reading the following description, referring to the attached drawings for non-limiting examples of embodiments where: Figure 1 shows a schematic view of a heating system according to the present invention. The heating system of the invention consists of a hollow tube (1) in an insulating material with a supply pipe (2) at one end leading radially into the tube (1) and at the other end a pipe (3) for the heated product to exit. The tube (1) contains a screw (4) with a helical groove (5) enclosing a tubular core (6). The core (6) is driven by a motor (7) which drives the screw in rotation, and this motor is controlled by a frequency variable to allow adjustment of the screw's rotation speed and enslavement according to temperature changes measured at the tube outlet and possibly other parameters from sensors installed on the sterilization line.

Ring electrodes (8, 9) are provided upstream and downstream of the screw to produce ohmic heating of the materials introduced into the tube. The feed pump (10) is connected directly to the ohmic heating tube to prevent any loss of charge between the pump and the endless screw. To ensure a steady supply and constant flow within the tube (1), the drive motor of the pump (10) and the drive motor (7) of the screw (4) are controlled synchronously by a control circuit.- What? The consecutive segment of the helical grooves forms a longitudinal partition of the tube, which limits the differences in the speed of movement of the different constituents of a heterogeneous mixture introduced into the tube. The fastest particles have an average speed of movement roughly equal to the average speed of the slowest particles, the variations being limited by the presence of two consecutive screw segments. The heating, which is a function of the current intensity, the resistance of the compound and the passage time through the tube, is therefore constant regardless of the nature of the constituents. The tube shall consist of a temperature probe (11) placed near the tube outlet.an electrical signal is provided by a control circuit to control the speed of the screw. For example, the diameter of the tube is 125 mm. The pitch of the helical groove is 100 mm. It depends on the size of the solid pieces present in the mixture to be sterilized. Optimally, the pitch is greater than 2L, where L defines the length of the largest piece. The pitch is preferably between 2L and 4L. The electrodes are supplied with an alternating current with a frequency of 15000 to 30000 hertz and a voltage of 1500 to 5000 volts per metre.The operating range is between 20 and 155°C. The screw is made of a non-abrasive plastic material. Several tubes may be used in series for, for example, temperature-level sterilisation. The sterilised product is then cooled to 40°C by passing through a cold water exchanger. The viscosity of the foodstuff to be sterilised in such a plant is between 250 and 1500 millipascals/second, and the particle content is between 30 and 80%. The conductivity is preferably less than 10 milliSiemens/cm and more than 0.01 milliSiemens/cm at 25°C. In the case of meat offal,The electrical conductivity is between 1 milliSiemens/cm and 7 milliSiemens/cm.

Claims (10)

1. An apparatus for the ohmic heating of products, more particularly a food composition formed of an heterogeneous mixture of a liquid phase and solid particles, including at least a heating pipe with a tubular section (1) made of electrically insulating materials and both ends having an annular electrode to produce an ohmic heating of the products introduced into the heating pipe, both electrodes being connected to a power supply, the heating device being supplied by a feed pump (10) driven by a first motor, characterized in that said feed pump directly opens into said heating tube (1), said heating tube (1) including an endless screw (4) composed of a non conductive material, driven by a second motor (7) controlled to provide an flow rate in the heating chamber.
2. An apparatus according to claim 1, characterised in that the endless screw (4) is provided with a solid core used as a rotation axis.
3. An apparatus according to claim 1 or 2, characterised in that the thread of the endless screw (4) is greater than twice the size of the side of the largest pieces.
4. An apparatus according to one of claims 1 or 3, characterised in that the motor (7) of the endless screw (4) is provided with a frequency variator so that the rotation speed thereof can vary.
5. An apparatus according to one of claims 1 to 4, characterised in that the endless screw (4) is composed of a non abrasive plastic material.
6. An apparatus according to one of claims 1 to 5, characterised in that it includes a food composition having a viscosity comprised between 250 and 1,500 milliPascal/second.
7. An apparatus according to one of claims 1 to 6, characterised in that it includes a food composition having a particle content comprised between 30 and 80%.
8. An apparatus according to one of claims 1 to 7, characterised in that it includes a food composition having an homogeneous conductivity including a difference between the liquid phase and the particles from 1 to 3.
9. An apparatus according to one of the preceding claims, characterised in that the thread of the endless screw (4) is comprised between 2L and 4L, where L defines the length of the largest piece.
10. An apparatus according to one of the preceding claims, characterised in that said second motor is controlled for ensuring a flow rate in the heating chamber synchronous with the supply flow.