WO2013004304A2 - Method of and system for use in gravimetric compositional determination - Google Patents

Abstract

A system (2) for the use in a process of gravimetric compositional determination by solvent extraction, the system comprising: a sample tube (4) for containing a sample having a component or components to be extracted, the sample tube (4) being formed with an elongate body portion (6) closable at one end (10) against egress of the sample; a piston (16) for releasable, slidable engagement with an internal surface (8) of the elongate body portion (6) and a multi-way valve (36) adapted to provide a selectable connection between an internal volume (14) of the sample tube (4) and two or more liquid reservoirs (42-47) containing liquids for use in the solvent extraction.

Description

Method of and System for Use in Gravimetric Compositional Determination

The present invention relates to the gravimetric compositional determination of a material using solvent extraction, particularly to the determination of the amount of one or more components of a foodstuff and most particularly to the determination of fat in milk or other dairy products.

Generally, it is the mass fraction of substances extracted into a solvent from a liquid sample that is determined. The sample may already be liquid or may be made into a liquid during a digestion process usually involving concentrated acids or alkalis at elevated temperatures. The solvent is selected to be immiscible with the sample and to exhibit a preferential affinity towards the substance or substances to be extracted. Typically the solvent will have a density such that it forms a layer above (in the direction of gravity) the sample.

For example, the Rose-Gottlieb method is generally recognised as one of the foremost gravimetric methods for the determination of fat in a wide variety of food products. In particular there exists an international standard, ISO 1211:2010(E), defining the use of the Rose-Gottleib method in the extraction of fat from milk. This method employs ammonia to break down any fat-protein bonds in milk allowing essentially all of the fat to then be dissolved by an ether solvent. In this method milk is weighed into a pre-weighed sample tube. An ammoniacal ethanolic solution is added and mixed. Diethyl ether and petroleum is then added and the mixture is preferably agitated, either by shaking, stirring, centrifuging, applying ultrasound or by any other suitable means. As the transfer of substances from the sample to the solvent occurs in an interface between the two then agitation will tend to speed up the transfer since the contact surface between sample and solvent will be increased. After (optionally) agitation the mixture is allowed to settle and the aqueous and the ether layers separate, with the ether layer lying above the aqueous layer. The ether layer that now contains fat is then removed to a pre-weighed receptacle. More ether is then added to the sample tube which is again agitated. The ether layer is again removed and added to the first extract. Typically this is repeated once more to ensure as complete a fat extraction as reasonably possible and the combined three extracts are dried and weighed. The weight is then compared to the starting weight of the sample to determine weight-percent Total Fat.

As will be appreciated this Rose-Gottleib method, in common with many other known solvent extraction methods, is labour intensive and involves the handling of or exposure to dangerous fluids.

An automated Rose-Gottleib system is described by Alan R. Matheson and Patrick Otten in pages 13 to 19, American Laboratory, March 1999. This incorporates a rather complicated concentric hypodermic needle assembly for the transfer of solvent into and out of vertically held sample tubes. Solvent passes into and out of the sample tube via an inner needle whilst air passes via the outer needle of the concentric arrangement. During extraction compressed air at around 20 psi is forced through the outer needle into a head-space above the sample in the sample tube. This air pressure is then used to force the solvent through the inner needle to a collection vessel. During the transfer the location of the inner needle just below the surface of the solvent layer must be carefully and precisely maintained until it lies just above the solvent/sample interface.

According to a first aspect of the present invention there is provided a system for the use in a process of gravimetric compositional determination by solvent extraction, the system comprising: a sample tube for containing a sample comprising a component or components to be extracted and extraction liquids, the sample tube having an elongate body portion being formed with a first end closable against egress of the sample; a piston for releasable, slidable engagement with internal the elongate body portion; and a multi-way valve adapted to provide a selectable connection between internal the sample tube and two or more liquid reservoirs, one of which is advantageously a removable receptacle for receipt of a fat containing solvent.

As the sample tube is removable from the piston then weighing of sample into the sample tube can be done more accurately. Being removable, the sample tube and receptacle is each easier to clean between sample extractions. This permits their re-use with a reduced likelihood of crossover contamination between samples.

The closable end of the sample tube may be formed as a sealed end and the piston formed with a conduit for coupling internal the sample tube with the multi-way valve. This permits removal of the less dense, upper solvent layer from the sample container without having either to first remove the other layer or without having to invert the sample tube.

The piston may be configured with a surface opposing the sealed end of the sample tube shaped to funnel the contents of the sample cup towards the conduit as the piston moves to reduce the internal volume of the sample tube which is available to its contents. Preferably the surface is concave and the conduit has an opening at an apex of the concavity.

A housing may be provided for releasably retaining the sample tube and have accommodated therein the piston, multi-way valve and optionally also the receptacle. This has an advantage that a user’s exposure to solvents and other potentially harmful fluids may be reduced.

The housing may be adapted to receive a plurality of sample tubes and have accommodated therein a same plurality of pistons and optionally receptacles, each one of which being associated with a different individual one of the plurality of sample tubes. Multiple samples may then be processed simultaneously.

A sensor may usefully be provided to detect an interface region between solvent and sample.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings of which: Fig.1 depicts schematically a first example of a system according to the present invention; Fig. 2 depicts schematically a second example of a system according to the present invention; and Fig. 3 is a flow chart of a method according to the present invention for the determination of the fat content of milk according essentially to the Rose-Gottleib method.

Considering now Fig. 1, a first exemplary embodiment of a system 2 for the use in a process of gravimetric compositional determination by solvent extraction is illustrated. The system 2 comprises one or more, here for illustration and for ease of explanation only one, sample tube(s) 4 formed with an elongate body portion 6 having internal surface 8, a sealed flat bottom 10 and an open opposing end 12 to define an internal space 14. An individual piston 16 is provided for each sample tube 4. The piston 16 is intended to form a releasable slidable engagement with the internal wall 8 when the sample tube is docked in housing 18 (illustrated by 4' on Fig. 1). The piston 16 is movable to accurately control the volume of the space 14 delimited by an inner surface 20 of the piston 16 and the sealed bottom 10 of the sample cup 4. The inner surface 20 is, in the present embodiment, made concave such that as the volume of the space 14 is reduced the fluid contents of the sample tube may be preferentially directed towards a through conduit 22 in the body of the piston 16. An ‘O’-ring 26 or other resilient seal is provided to maintain a perimetrical fluid-tight seal with the internal wall 8 as the piston 16 slides within the sample tube 4. A piston drive unit, here in the form of stepper motor and drive assembly 28, is provided to effect the necessary movement of the piston 16 within the docked sample tube 4'.

A docking station for the sample tube 4 is, in the present exemplary embodiment, provided and here comprises a base plate 30 and a sample tube clamp 32 which is arranged for movement into and out of contact with a docked sample tube 4' so as to releasably retain it in contact with the base plate 30. In the present embodiment a shaker unit 34 is also provided, here mechanically coupled to the base plate 30, to agitate the contents of a docked sample tube 4' and thereby enhance solvent-sample mixing. This may be replaced by any suitable means for mixing the solvent and sample, such as, for example, an ultrasonic stirrer.

The through conduit 22 is connected to a multi-way valve 36 via resilient tubing 38. The multi-way valve 36 is adapted to selectively connect the through conduit 22 (and thus internal space 14) with atmosphere via tubing 40 and with two or more, here four, liquid containers 42-48. The containers 42-44 contain liquids for supply into the internal space 14 of the sample tube 4 for use in the solvent extraction process whereas the container 48 is a receptacle for liquid, in this embodiment fat containing solvent liquid, from the internal space 14. The number and type of liquid containers will depend on the extraction process for which the system 2 is to be used.

An evaporation station 50 is here provided integral with the system 2 and is adapted to contain the container 48 and comprises an oven and a fume extractor. In other embodiments the container 48 may be removable from the system 2 for insertion into a physically separate evaporation station.

An interface detector 52 is located in the liquid flow path in a position between and including the internal space 14 of a docked sample tube 4' and the container 48 and operates to detect an interface between the extraction liquid and the liquid sample layers. In the present embodiment the interface detector 52 comprises a complementary optical emitter 54 and detector 56 arranged on opposing sides of a transparent wall section 58 of the resilient tubing 38 at a location between the multi-way valve 26 and the piston conduit 22. The detector 52 is here adapted to detect changes in optical transmission properties of liquid in the tubing 28 and thereby detect the interface. Acoustic or other properties could alternatively be employed to detect this interface, such as by monitoring density changes in the liquid, and the detector 52 could be located at other positions, such as to detect the interface position in a docked sample tube 4', without departing from the invention as claimed.

An output from the detector 52 indicating the detection of the interface is made available to a control unit 60 which is configured to control the operation of the piston drive assembly 28 and position of the multi-way valve 36 in dependence on at least this output.

A second exemplary embodiment of a system 80 according to the present invention is illustrated in part in Fig. 2. The system 80 comprises one or more sample tube(s) 82 and an associated docking station(s) 84.

The sample tube 82 is formed with an elongate body portion 86 having internal surface 88, a sealable first end 90 and an open opposing end 92 to define an internal space 94. The elongate body portion 86 is provided with a section 96 that extends beyond the sealable first end 90 to form a support by which the sample tube 88 may be maintained upright when on a vertical surface. An openable closure 98 such as a valve or piercable septum, perhaps formed as part of a removable cap to allow it easy replacement and re-use of the sample tube 82, seals the first end 90.

The docking station 84 comprises a ring 100 inside which a sample tube 82' is to be docked and a support 102 fixedly attached to the ring 100 to retain a piston 104 in place above a docked sample tube 82', here by securely retaining a piston drive unit 106, such as a one employing a stepper motor, to which the piston 104 is operably connected. The piston 104 is movable by the drive unit 106 to slidably engage about its periphery with the internal surface 88 of a docked sample tube 82'and move to accurately control the volume of internal space 94 available to hold a liquid.

An actuator 108 is mechanically coupled to the ring 100 by means of a rotatable and reciprocally translatable shaft 110. The actuator 108 is operable to selectively provide rotational and translation movement of the shaft 110 which transmits the same to the ring 100 so as to respectively rotate and agitate a docked sample tube 82'. Rotation is necessary to ensure that the solvent layer containing the component(s) to be extracted is properly located with respect to the closure 98 for removal. As the sample tube 82 is necessarily docked with its open end 92 upwards (to prevent spillage) and since the solvent used is typically less dense than the sample then, in these circumstances, the docked sample tube 82' must be rotated through substantially 180⁰ when the solvent layer is to be removed.

In common with the first exemplary system 2 which is illustrated in Fig. 1 the system 80 of the present embodiment also comprises resilient tubing 38 and a multi-way valve 36 connected to one end of the tubing 38. The other components of the system 2 are also comprised in the system 80 of the present embodiment but, as they are intended to operate as provided for in that system 2 they are not illustrated in the system of Fig. 2 nor will they be described further.

Usefully and as illustrated in the present embodiment the opposite end of the tubing 38 is here provided with a connector 112 which is intended to cooperate with the openable closure 98 of a docked sample tube 82' to open the closure 98 and thereby establish a liquid connection with the internal space 94. For example, when the closure 98 is formed as a piercable septum the connector 112 may be a hollow needle. In another embodiment the closure may, for example, be a ball valve naturally urged towards a closed position and the connector 112 may comprise a protrusion to engage with and urge open the ball valve.

A method of gravimetric determination of fat in milk according to the Rose-Gottlieb technique and using the system according to the present invention will now be described with reference to the flow chart illustrated in Fig. 3 and the system according to Fig. 1.

Initially, at step 302, a sample is prepared by accurately weighing (typically to nearest 1 mg) between 10g and 11g of milk directly or by difference into a sample tube 4, in this example an undocked sample tube 4. An ammoniacal solution is then added to the weighed sample and mixed. This may be performed manually but in the present example is performed automatically. Here the sample tube 4' may be docked in the docking station 30,32; the piston 16 engaged with the internal wall portion 8 of the docked tube 4' and slid along the elongate body 6 to a location just above the upper surface of the milk sample to define a minimum volume of the internal space 14. At this time the multi-way valve 36 is connected to couple the internal space 14 to atmosphere/waste via conduit 40. The multi-way valve 36 is then operated to couple a first reservoir 42 holding the ammoniacal solution with the internal space 14 and the piston 16 is withdrawn sufficiently to cause a required amount (typically 2ml) of the ammoniacal solution to be sucked from the reservoir 42 and into the docked sample tube 4'. The shaker unit 34 is operated to effect a mixing of the contents of the sample tube. Ethanol (here around 10ml) is added by selectively coupling a second reservoir 44 containing ethanol to the internal space 14 via the multi-way valve 36 and withdrawing the piston 16 along the elongate body an appropriate amount. The shaker unit 34 is operated and the mixture shaken. An aqueous dye is, in this example, then added to the mixture to aid in interface detection at a later stage. Congo red solution is the preferred dye according to the ISO standard 1211:2010(E) but this is carcinogenic and any aqueous dye may be substituted provided that it does not affect the determination. Suitable dyes may be Cresole Red or Carmine, should a red colour be desired. To do this the multi-way valve 36 is operated to couple a third reservoir 45 containing an aqueous Congo red solution ( typically 1g of Congo red dissolved in around 100ml water) and the piston 16 withdrawn sufficiently to effect transfer of an appropriate amount of the Congo red solution from the third reservoir 45.

At a second step 304 solvent extraction is performed. In the present example a fourth reservoir 46 of diethyl ether is selectively coupled to the internal space 14 via the multi-way valve 36 and the appropriate amount (here 25ml) of the ether is transferred as the piston 16 is withdrawn further within the elongate body 6 of the docked sample tube 4'. The shaker unit 34 is then operated to shake the mixture in the sample tube 4'. After shaking light petroleum is added to the mixture by coupling a fifth reservoir 47 of light petroleum to the internal space 14 via the multi-way valve 36 and withdrawing the piston 16 sufficiently to transfer the appropriate amount (here 25ml) and the mixture is mixed again using the shaker unit 34. The agitation is then stopped and the mixture is allowed to separate into two layers – an upper fat containing solvent layer and a lower aqueous layer.

The multi-way valve 36 is operated to couple the internal space 14 of the docked sample tube 4' with the container 48 and the piston 16 is moved to reduce the volume of the internal space 14. This forces the upper fat containing layer through the conduit 22 in the piston 16 and into the container 48. In order to avoid transfer of aqueous layer into the container 48 the output of the interface detector 52 is monitored using the control unit 60. The optical transmission from source 54 that is detected by the detector 56 will change as the aqueous dye (here Congo red) containing aqueous layer passes the transparent wall section 58. When this change is registered by the control unit 60 the unit 60 sends control signals to operate one or both of the multi-way valve 36 and the piston drive assembly 28 to avoid transfer of aqueous layer to the container 48. For example after a predetermined time from registering the colour change (dependent on the speed of the piston 16 and the length of conduit 38 between the multi-way valve 36 and the transparent wall section 58) the multi-way valve 36 is switched by a signal from the control unit to divert the contents of the conduit 38 to the atmosphere/waste line 40 and a further signal is generated to halt the movement of the piston 16.

The above extraction step is repeated twice more. The second extraction is carried out with the transfer of smaller amounts of ethanol (now 5 ml), diethyl ether (15 ml) and light petroleum (15ml). The third extraction is performed without addition of ethanol but using the same amounts of ether and light petroleum as used in the second extraction. This third extraction is optional and is often omitted when the milk sample has a fat content of less than 0.5% mass fraction.

At a third step a gravimetric determination of the fat content in the container 48 is performed. Optionally the container 48 may be removed from the housing 18 for the performance of this step. In the present example the container 48 is located within the integral evaporation station 50 so that obtaining fat residue for weighing may be performed automatically by the system. The oven of the station 50 heats the container 48 and its contents to drive off the extracted solvent which is safely collected by the fume extractor of the station 50. The container 48 is then removed from the oven and allowed to cool to the temperature of the room containing the weighing equipment. The container 48 is then weighed and the weight of the extracted fat may deduced from a prior knowledge of the weight of the empty container 48.

Claims (11)

1. A system (2;80) for the use in a process of gravimetric compositional determination by solvent extraction, the system comprising: a sample tube (4;82) for containing a sample having a component or components to be extracted, the sample tube (4;82) being formed with an elongate body portion (6;86) closable at one end (10;90) against egress of the sample; a piston (16;104) for releasable, slidable engagement with an internal surface (8;88) of the elongate body portion (6;82); and a multi-way valve (36) adapted to provide a selectable connection between an internal volume (14;94) of the sample tube (4;82) and two or more liquid reservoirs (42-47) containing liquid for use in the solvent extraction.
2. A system (2) as claimed in Claim 1 characterised in that the sample tube (4) has the closable end (10) as a permanently sealed end and in that the piston (16) is provided with a through conduit (22).
3. A system (2) as claimed in Claim 2 characterised in that the piston (16) is provided with a surface (20) opposing the sealed end (10) when slidably engaged with the sample tube (4) formed with a concavity having an apex at which one end of the through conduit (22) is located.
4. A system (2) as claimed in Claim 1 characterised in that the reservoirs (42-47) include at least one reservoir (46) for liquid solvent.
5. A system as claimed in Claim 4 characterised in that the at least one reservoir (46) for liquid solvent contains an ether based solvent.
6. A system as claimed in Claim 6 characterised in that the reservoirs include at least one reservoir containing an ammoniacal solution.
7. A system as claimed in Claim 1 characterised in that there is included a receptacle (48) for a liquid extract from the sample tube (4) connected to the multi-way valve (36) for selective connection to internal volume (14).
8. A system (80) as claimed in Claim 1 characterised in that the closable end (90) of the sample tube (82) is formed with an openable closure (98).
9. A system (80) as claimed in Claim 8 characterised in that the elongate body portion (86) of the sample tube (82) is configured to extend beyond the closable end (90) to form a support (96).
10. A system (80) as claimed in Claim 8 characterised in that there is provided a connector (112) cooperably engagable with the openable closure (98) to establish a liquid connection with the internal volume (94) on engagement.
11. A system (80) as claimed in Claim 10 characterise in that the openable closure (98) comprises a piercable septum and in that the connector (112) comprises a hollow needle.

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