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Title:
METHOD OF AND SYSTEM FOR MANUFACTURING A CONCENTRATE
Document Type and Number:
WIPO Patent Application WO/2017/141075
Kind Code:
A1
Abstract:
A system for manufacturing a concentrate from a liquid source material (30) containing constituents in solution or suspension, the system comprising an evaporator (2) configured to output a liquid concentrate (10) having a concentration of at least one of the constituents higher than a desired concentration; a detector (24) configured to measure on the liquid concentrate in a flow conduit (8) and to generate a signal indicative of the concentration of the at least one of the constituents; and a metering device (16) in fluid communication with a source of metering liquid (22) and with the liquid concentrate (10). The metering device (16) is configured to meter a quantity of the metering liquid into the liquid concentrate (10) in dependence of the signal to achieve a liquid concentrate having the desired concentration.

Inventors:
RINDORF, Lars Henning (Foss Alle 1, 3400 Hilleroed, 3400, DK)
Application Number:
IB2016/050897
Publication Date:
August 24, 2017
Filing Date:
February 19, 2016
Export Citation:
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Assignee:
FOSS ANALYTICAL A/S (Foss Allé 1, Hilleroed, DK-3400, DK)
International Classes:
B01D1/00; A23C1/04; A23C1/12; A23C9/18; A23C21/00; A23L2/08; A23L2/10; A23L5/30; B01D1/18; B01D5/00
Foreign References:
GB158569A1922-06-27
US5955128A1999-09-21
GB2115302A1983-09-07
EP1187534A12002-03-20
GB2115302A1983-09-07
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Claims:
A method of manufacturing a concentrate from a liquid source material (30) containing constituents in solution or suspension; the method comprising the step of forming a concentrate by removing liquid from the liquid source material in an evaporator (2); wherein the step of forming the concentrate comprises a step (Step 1 ) of removing liquid in the evaporator (2) to provide a concentrate having a constituent concentration higher than a desired constituent

concentration; a step (Step 2) of measuring on the concentrate to determine an indication of an actual constituent concentration; and a step (Step 3) of introducing a metering liquid (22; 36) into the concentrate after the evaporator (2) in an amount dependent on the actual constituent concentration to achieve the desired constituent concentration of the concentrate.

The method as claimed in claim 1 wherein there is provided a further step (Step 4) of recovering liquid removed in the evaporator and introducing the metering liquid (Step 3) comprises introducing a portion of the recovered liquid (22) as the metering liquid.

The method as claimed in claim 1 characterised in that the step (Step 2) of measuring on the concentrate comprises obtaining spectral data from infrared spectral analysis of the concentrate and performing a chemometric analysis of the obtained spectral data to obtain the indication of the actual constituent concentration.

The method as claimed in claim 1 wherein there is provided a further step (Step 5) of atomizing the concentrate to produce a particulate concentrate. The method as claimed in claim 4 characterised in that the liquid source material (30) is a liquid milk product.

The method as claimed in claim 5 characterised in that the liquid milk product is one or more selected from the group consisting of modified whey,

unmodified whey, skimmed milk, non-skimmed milk, semi-skimmed milk.

The method as claimed in claim 5 characterised in that the step (Step 2) of measuring on the concentrate comprises performing infrared spectral analysis of the concentrate to obtain spectral data and performing a chemometric analysis of the spectral data to obtain a measure of one or more of Protein, fat, lactose, total solids (TS) and solids non-fat (SNF) content as the indication of an actual constituent concentration.

8. A system for manufacturing a concentrate from a liquid source material (30) containing constituents in solution or suspension, the system comprising an evaporator (2) configured to output a liquid concentrate (10) having a concentration of least one of the constituents higher than a desired

concentration; a detector (24) configured to measure on the liquid concentrate and to generate a signal indicative of the concentration of at least one of the constituents of the liquid concentrate; and a metering device (16) in fluid communication with a source of metering liquid (22; 36) and with the liquid concentrate, the metering device (16) configured to meter a quantity of the metering liquid (22; 36) into the liquid concentrate in dependence of the signal to achieve a liquid concentrate having the desired concentration of at least one of the constituents.

9. The system according to claim 8 wherein the detector (24) is spectral

instrument configured to generate spectral data from input optical energy.

10. The system as claimed in claim 8 further comprising a spray dryer (6)

configured to manufacture a particulate concentrate from an input liquid concentrate wherein the input liquid concentrate is the liquid concentrate having the desired concentration of at least one of the constituents.

1 1. The system as claimed in claim 10 wherein the metering device (16) is

configured to meter the metering liquid (22;36) to achieve the desired concentration of at least one of the constituents in the liquid concentrate selected to be below a concentration at which the operation of the spray dryer (6) becomes impaired.

Description:
Description

Method of and System for Manufacturing a Concentrate

[0001] The present invention relates to a method of and system for manufacturing a concentrate, particularly to a method of and a system for manufacturing a concentrate from a liquid source material containing constituents in solution or suspension.

[0002] A wide variety of industries, such as the food, feed, dairy, beverage and pharmaceutical industries often require a concentrate to be produced from a liquid source, either as a particulate concentrate, for example in powder, granular or agglomerate form, or as a liquid concentrate. Concentration permits easier handling and cheaper shipping. For example in the beverage industry water is often removed from fruit juices to form a liquid concentrate which is later made into a drinkable product by the addition of water; in the dairy industry spray dryer equipment is used for converting a previously concentrated liquid milk or whey liquid source material into a particulate concentrate which, for example, can be reconstituted into a liquid milk product later by adding an appropriate amount of water.

[0003] Production of a concentrate from liquid source material containing

constituents, such as for example protein, fat, sugar or solids, in solution or suspension involves the removal of a predetermined amount of the liquid in an evaporator to produce a desired constituent concentration in the concentrate or a desired physical property, such as for example viscosity, surface tension or 'stickiness' of the concentrate, that is dependent on the constituent concentration. In the production of milk powder, for example, total solids concentration in the concentrate is often measured as an indication of viscosity which influences the performance of the spray dryer equipment. In the production of fruit juice concentrate, for example, the concentration of sugars in the concentrate effects the 'stickiness' of the concentrate and hence its pumping properties as well as the difficulty with which storage containers are cleaned.

[0004] It is therefore desirable to be able to control the evaporator to generate a concentrate having a known and reproducible constituent concentration. However, in practice this is difficult to achieve since evaporators tend to have a high thermal mass and therefore exhibits considerable latency in responding to changes in operating parameters.

[0005] This may prove particularly problematic in the manufacture of a particulate concentrate from a liquid source. Generally, the manufacturing process for obtaining a particulate concentrate involves the initial production of a liquid concentrate from a liquid source material containing constituents in solution or suspension and passing this liquid concentrate into a spray dryer. To obtain the liquid concentrate the liquid source material is passed through an evaporator by which a fraction of the liquid is evaporated to produce a desired constituent concentration in the concentrate. The evaporator is used because the energy consumed per unit of liquid removal is much less for the evaporator than for the spray dryer.

[0006] Clearly then, it is desirable to limit the amount liquid to be removed by the spray dryer in order to reduce the energy consumed in the manufacturing process. However, as the constituent concentration in the concentrate fed into the spray dryer increases (that is, liquid content in the concentrate decreases) a limit is reached at which the constituent concentration adversely affects spray drying. At this limit it becomes difficult to achieve adequate atomization in the spray dryer and the liquid concentrate can cause blockages in the spray nozzles of the atomizer of the spray dryer which leads to mechanical breakdown. It is important therefore to achieve a concentrate with a desired constituent component concentration which is less than, but as close as possible to, a limit at which the concentration adversely affects the spray drying process.

[0007] As previously stated, it is relatively difficult to control the evaporator and so in practice an operator sets a desired constituent concentration, typically solids content (dry matter or total solids), of the concentrate to be output from the evaporator at a level which is significantly less than is optimum with respect to energy conservation. In this manner the difficulty in precisely controlling the output of the evaporator is accommodated since the spray dryer is working significantly away from its optimal operating point. [0008] A solution to this problem for particulate concentrate production is provided in GB 2,1 15,302 where there is provided a method and

apparatus of manufacturing milk powder by concentrating a milk source material in one or more evaporation stages or 'effects' and passing the concentrate to a spray dryer for atomization and drying in which method and apparatus the sole or final evaporation effect is sufficiently close to the spray drier that the concentrate is fed into the spray dryer before

developing a maximum viscosity which would be sufficient to impair the operation of the dryer. In this manner a desired concentration of total solids can be selected to provide a concentrate having a viscosity at or even slightly above the maximum viscosity for optimal operation of the spray dryer. Since more liquid can therefore be removed in the evaporator the energy efficiency of the manufacturing process is improved. However, the requirement that the final evaporation effect be located as close to the spray dryer as possible provides strict design constraints on the

apparatus, particularly on the choice of the final evaporation effect.

Moreover, this does not provide a solution for production of liquid concentrates having a known and reproducible concentration of at least one constituent where no spray dryer stage is provided.

[0009] According to a first aspect of the present invention there is provided a

method of manufacturing a concentrate from a liquid source material, such as milk or juice products, containing constituents in solution or suspension; the method comprising the step of forming a concentrate by removing liquid from the liquid source material in an evaporator; wherein the step of forming the concentrate comprises a step of removing liquid in the evaporator to provide a concentrate having a constituent concentration higher than a desired constituent concentration; a step of measuring on the concentrate to determine an actual constituent concentration; and a step of introducing a metering liquid into the concentrate after the evaporator in an amount dependent on the actual constituent

concentration to achieve the desired constituent concentration of the concentrate. [0010] This has an advantage that variations in the output from the evaporator can be more quickly accommodated to achieve a desired constituent concentration.

[001 1] In some embodiments the method further comprises a step of recovering liquid removed in the evaporator and the step of introducing a metering liquid into the concentrate comprises introducing a portion of the recovered liquid as the metering liquid.

[0012] This has an advantage that the introduced metering liquid has the same chemical makeup as the liquid of the concentrate into which it is being introduced.

[0013] In some embodiments there is provided a further step of atomizing the concentrate to produce a particulate concentrate.

[0014] According to a second aspect of the present invention there is provided a system for manufacturing a concentrate from a liquid source material containing constituents in solution or suspension, the system comprising an evaporator configured to output a liquid concentrate having a concentration of least one of the constituents higher than a desired concentration; a detector configured to measure on the liquid concentrate and to generate a signal indicative of an actual concentration of at least one of the constituents of the liquid concentrate; and a metering device in fluid communication with a source of metering liquid and with the liquid concentrate, the metering device configured to meter a quantity of the metering liquid into the liquid concentrate in dependence of the signal to achieve a liquid concentrate having the desired concentration.

[0015] These and other advantages and features will be better understood from a consideration of the following description of one or more exemplary embodiments of the method and the system according to the present invention made with reference to the drawings of the accompanying figures, of which:

Fig.1 shows, schematically, the salient elements of a system

according to the present invention for the manufacture of a particulate concentrate;

Fig.2 shows, schematically, the salient elements of a system

according to the present invention for the manufacture of a liquid concentrate; and

Fig. 3 is a flow chart illustrating the method according to

the present invention.

[0016] Considering Fig.1 , an exemplary embodiment of a system for the

manufacture of a particulate concentrate is shown as comprising a more or less conventional evaporator 2 with a number of evaporator effects 4a, 4b, 4c (here, without limiting the invention as claimed, three) and a spray dryer 6 connected by a flow conduit 8 with an output 10 of the evaporator 2, here illustrated via a buffer tank 9. As is known in the art, a separator 5a, 5b, 5c is associated with each of the evaporator effect stages 4a, 4b and 4c and serves to separate vapour and concentrate generated by its associated evaporator stage 4a, 4b or 4c. As is well known, each separator, 5a say, is configured to provide (illustrated by solid lines) the separated concentrate from its associated evaporator effect, 4a say, as input to the next evaporator effect, 4b say, and to provide (illustrated by open lines) the separated vapour as a heat source for the next evaporator effect, 4b say. In the present embodiment the final separator, 5c say, provides the separated concentrate to the output 10 of the evaporator 2. The configuration of evaporator effects and separators described above and illustrated in Fig. 1 is one of many known in the art and is not intended to limit the invention as claimed in the appended claims.

[0017] An atomizer 12 of the spray dryer 6 is located towards the top of the spray dryer tower 14 and operates to disperse incoming liquid concentrate into small droplets for drying as they fall down the tower 14. The atomiser 12 may for example comprise spinning disk atomizer or spray nozzles, both of which may be of known design. A pump 13 may be provided before the atomizer 12 to pressurize the concentrate flowing to the atomizer 12.

According to the present invention a liquid metering device 16 is provided and is configured to meter an amount of liquid into the concentrate after the evaporator 2. In the present embodiment the metering device 16 is provided in fluid communication with the flow conduit 8 between the output 10 of the evaporator 2 and the atomizer 12. The liquid metering device 16 in the present exemplary embodiment comprises a T connector 18 placed in-line with the flow conduit 8 and having its leg connected to a regulator valve 20, thus enabling liquid to be metered into the concentrate. In the present embodiment an input conduit 22 is provided to supply liquid recovered from the evaporator 2 to the regulator valve 20 for introduction into the concentrate. According to the present embodiment, vapour separated by the final separator 5c passes into a condenser 32 and the condensed liquid is flowed through the input conduit 22 for metering into the concentrate. In other embodiments the liquid to be metered may be obtained from a separate source, usefully with a known chemical composition, or from another separator 5a or 5b of the evaporator 2.

[0018] A detector 24 is configured to make measurements, preferably in-line

measurements on the concentrate in order to determine an indication of an actual constituent concentration in the concentrate and to provide these measurements as an input to a controller 26 which is configured to control the metering device 16 in order to regulate the amount of metering liquid introduced into the concentrate in dependence on the measurements made by the detector 24. The detector 24 may, as illustrated, be located to measure on the concentrate after introduction of the liquid and thereby provide the measurements as a part of a feedback control loop of the controller 26. The detector 24 may alternatively be located to measure on the concentrate before the introduction of the liquid and thereby provide the measurements as a part of a feed-forward control loop of the controller 26.

[0019] The detector 24 is selected in dependence on the characteristic(s) of the concentrate which is known to influence the operation, critically the breakdown, of the spray dryer 6. For example, the detector 24 may be a detector selected to measure directly the physical characteristic of interest or may be selected to measure one or more constituents of the liquid concentrate, the concentration of which is known to effect the physical characteristic of interest or may be selected to measure one or more constituents as an indication of the actual constituent concentration of the concentrate. An optical refractometer may be employed as the detector 24 to measure the total solids as an indication of the actual constituent concentration which affects that viscosity of the concentrate. Most usefully, since physical properties, such as viscosity, of the concentrate are dependent on its chemical composition, the detector 24 may be a device for measuring one or more chemical components of the concentrate, such as fat and/or protein, an indication of its relevant physical property, for example viscosity. In addition or as an alternative, the concentration of one or more chemical components of the concentrate may be employed as an indication of the actual concentration of the concentrate.

In the present embodiment the detector 24 is a spectral instrument configured to generate spectral data from input optical energy, such as infrared, particularly near infrared optical energy, as intensity indexed against a measure of its wavelength. Thus the detector 24 essentially comprises an infrared probe 25 and a spectrometer or interferometer 27. The probe 25 is configured to couple infrared radiation into liquid

concentrate in the flow conduit 8 downstream of the liquid metering device 16 and to receive the coupled infrared radiation after its interaction with the liquid concentrate. The probe 25 may terminate internal of the flow conduit 8 or may terminate external of the flow conduit 8 at a suitably transparent portion of the conduit 8. The spectrometer or interferometer 27 is optically coupled to the probe 25 to generate the spectral data from the received, coupled infrared radiation and is provided with connection to a data processor 29 (here illustrated as a component of the controller 26) which is configured to receive the spectral data from the spectrometer or

interferometer 27 and to determine the concentration (absolute or relative) of one or more of the constituents of the concentrate. This determination is done here by applying one or more chemometric models (one model for each constituent to be determined) to the spectral data received from the spectrometer or interferometer 27, each of which model is constructed, in a known manner, to provide a mathematical link between spectral features and an amount of a specific constituent of the concentrate in the flow conduit 8.

[0021] An example of the operation of a system according to Fig. 1 for use in the manufacture of milk powder will now be described. An input 28 of the evaporator 2 is here in fluid communication with a store 30 of liquid source material which, in the present example, is a milk product such as a one of raw milk, modified whey, unmodified whey, skimmed milk, non-skimmed milk, semi-skimmed milk, typically having a standardised fat to solids nonfat (SNF) ratio.

[0022] To increase profitability, an operator of the system seeks to maximize total solids (TS) concentration (that is to minimize liquid content) at the atomizer 12 through maximizing the TS content of the concentrate leaving the evaporator 2. There is a known risk that the TS concentration could exceed a critical value at which the spray dryer 6 breaks down with resulting high cost associated with cleaning, poor product quality, and production stoppage amongst other things.

[0023] It is noted that TS is not a unique indicator for spray dryer 6 breakdown.

Breakdown occurs when the spray dryer 6 is fouled by wet or sticky powder or powder formation ceases. The concentrate exits the atomizing nozzle of the atomizer 12 as droplets which are dried into powder. It is well known that viscosity, in particular among others, determines the droplet size. Too fine droplets results in suboptimal product quality and high operating expenses. Too large droplets result in atomizer fouling and poor quality powder. Another parameter of importance is, for example, the surface tension of the concentrate. Both viscosity and surface tension are affected by the chemical composition of the milk, including TS. In particular, protein or lactose concentration, absolute or relative (fractional change), could also or alternatively be measured as indicators of break down. However, in the following TS measurements will be employed as the break down indicator since this is what is typically desired to be maximised at the atomizer 12. [0024] It is well known, that the energy consumption efficiency per evaporated kg of water of the evaporator 2 is several times superior to that of the spray dryer 6, and it is well known that the energy consumption of the evaporator 2 and the spray dryer 6 in a milk powder plant is a considerable expense. It is also well known that productivity of the plant markedly affects operating economy. If the water evaporation capacity of the atomizer is the limiting factor in the plant then increasing TS will increase production in the atomizing step.

[0025] As an example of the advantage of the invention it is assumed that break down in the spray drying process occurs at a TS limit of 52%. The residual water content of the milk powder after the atomizer 12 is about 5%, and the corresponding TS is 95%. The evaporator 2 provides a feed of milk concentrate of about 40% TS, lower than the breakdown limit 52%. The TS is increased by the atomizer 12 to 95% in the final product. In order to mitigate temporary peaks in TS at the output of the evaporator 2 the TS must be well below that of the breakdown. Additionally, seasonal or supplier variations in the physical and chemical properties of the liquid source material affects the sprayability of the powder. Every percentage of evaporated water has a cost that is much higher, as a typical example 10 times higher, for the atomizer 12 than for the evaporator 2. An optimal solution is controlling the evaporator 2 such that it provides just below the critical TS value of 52%. However due to the large thermal mass of the evaporator 2 there is considerable latency in it responding to changes in operating parameters which may also lead to regulatory oscillations. It is therefore considerably difficult, if not practically impossible, to make such control of the evaporator 2. According to the method of the first aspect of the present invention, as illustrated by the flow diagram of Fig. 3, a solution is proposed which is, at Step 1 , to operate the evaporator 2 to produce a concentrate with a higher than desired TS (or other suitable constituent concentration such as non-fat solids, protein, fat, lactose) value; at Step 2, measure on the concentrate produced by the evaporator 2 using a detector 24 to obtain an indication of an actual constituent (here TS) concentration and, at Step 3, to mix a metering liquid, usefully, in some embodiments of the present invention and as illustrated at Step 4, liquid taken from the evaporator 2, before it is fed into the atomizer 12 in dependence on the indication of the actual amount of constituent in order to provide a concentrate of optimum or near optimum constituent (by way of example, here TS) concentration value of 52% (being an example of a desired constituent concentration) at the atomizer 12 where, at Step 5, it is dried to form a particulate concentrate. The desired value constituent concentration (by way of example here TS concentration) can always be maintained by the system since the response time of such an arrangement is very short (few seconds).

[0026] Usefully, in some embodiments of the present method measuring (Step 2) on the concentrate may be done using a spectrometer or interferometer 27 to generate infrared, particularly near infrared, spectral data from input infrared optical energy which has interacted with the concentrate and applying, in a known manner, one or more chemometric models (one model per constituent to be determined) to the generated spectral data in order to determine an indication of an actual amount of the constituent (by way of example, here TS) in the concentrate after the evaporator 2.

Measuring in this manner has an advantage that more than one

chemometric model may be applied to the same spectral data in order to obtain an indication of the actual concentration of more than one of the constituents.

[0027] The construction of a calibration model for use in the method according to the present invention is well known in the art and generally requires a calibration or training data set, which includes reference values for the constituent of interest for prediction, and the measured spectral data attributes corresponding to these properties. Data is collected from a number of samples, including concentrations for a constituent of interest for each sample (the reference) and the corresponding infrared spectrum of that sample. Multivariate calibration techniques such as partial-least squares regression, or principal component regression (and near countless other methods) are then used to construct a mathematical model that relates the multivariate response (spectrum) to the concentration of the constituent of interest, and such a model can be used to efficiently predict the concentration of the constituent of interest in new samples.

[0028] As an advantageous side effect of increasing TS in the concentrate fed to the atomizer 12 is that high TS in the atomizer gives a high production of milk powder. If we assume an unchanged flow rate to the atomizer 12 then a powder production increase of 15% is achieved. Given the cost of operation and purchase of powder production plants, this production increase will improve the economics of the plant beyond that of the energy consumption reduction.

[0029] In a second exemplary embodiment of the present invention which is

illustrated in Fig. 2 the system of Fig.1 is modified to manufacture a liquid concentrate. In this embodiment the spray dryer 6 is replaced with a storage tank 34 for liquid concentrate flowing out of the evaporator 2. Elements which are common between the embodiments of Fig.1 and Fig.2 are identified using the same reference numerals. The multi-stage evaporator 2 has an input 28 for fluid communication with a liquid source material (IN) and an output 10 through which liquid concentrate passes into a flow conduit 8 which terminates at the storage tank 34.

[0030] The liquid metering device 16 is disposed between the output 10 and

storage tank 34 to meter quantities of liquid into the liquid concentrate in dependence of a measured degree of constituent concentration, which for fruit juices for example may be sugar concentration, of the concentrate. As the concentration of the sugar increases the concentrate becomes increasingly sticky. Increased stickiness causes problems in pumping the concentrate and for cleaning between different batches.

[0031] As illustrated in Fig.2 the liquid to be metered into the concentrate may be liquid recovered from the evaporator 2 or may be liquid of known composition from an external source 36 and fed through a conduit 38 to the liquid metering device.

[0032] The detector 24 is provided to measure on the liquid concentrate and to provide these measurements as an input to the controller 26, which controller 26 operates to control the metering device 16 to regulate the amount of liquid introduced into the concentrate in dependence of the measurements. Where the liquid to be metered originates from the evaporator 2 (as illustrated in Fig. 2 by the solid lines) then the

composition of the liquid may be variable and it is useful if the detector 24 is located to measure on the concentrate downstream of the liquid metering device, after addition of the liquid to be metered. Thus the detector 24 and controller 26 provide a feedback control of the liquid metering device 16. Where the liquid to be metered originates from a source 36 of liquid of known composition the detector 24 may usefully be located upstream of the liquid metering device 16. In this configuration the controller 26 may be provided with information regarding the composition of the liquid to be metered from the source 36 and be configured to operate the liquid metering device 16 to provide a metered amount of liquid into the liquid concentrate calculated from the measurements made by detector 24 and the information regarding the composition of the liquid from source 36. Thus the detector 24 and controller 26 provide a feedforward control of the liquid metering device.

The system according to Fig.2 may be operated to manufacture fruit juice concentrate (or other liquid concentrate) in a manner hereinafter described with respect to the method illustrated in Fig. 3 to achieve a liquid concentrate having, for example, a desired sugar concentration. Fruit juice is input into the evaporator 2 as liquid source material and at Step 1 the evaporator 2 is operated to output a liquid concentrate having an actual concentration that is higher than the desired concentration. The actual concentration of the liquid concentrate is, at Step 2, monitored via measurements performed by the detector 24, which here comprises a known optical spectrometer or interferometer 27, and a signal representing the actual (absolute or relative) concentration (for example derived from the application of a chemometric model to spectral data obtained from the optical spectrometer or interferometer 27) is passed to the controller 26. The controller 26 processes the signal representing the actual

concentration and develops a control signal for output to the liquid metering device 16 which, at Step 3, is operated via control signals from controller 26 to control the metering of a metering liquid into the liquid concentrate in dependence on the monitored actual concentration in order to achieve a liquid concentrate having the desired concentration. In embodiments where the metering liquid is from an external source 36 (such as water when manufacturing liquid fruit juice concentrate) then usefully the control signal from the controller 26 may also be dependent on the known composition of that metering liquid.

It will be appreciated that in a system according to the present invention operating according to the method of the present invention control of the evaporator 2 need not be tightly regulated as its output is rapidly compensated for according to the control regime described herein.