WO1999002959A1

WO1999002959A1 - Porous container

Info

Publication number: WO1999002959A1
Application number: PCT/GB1998/001946
Authority: WO
Inventors: Michael Kitcherside
Original assignee: University Of Bristol
Priority date: 1997-07-07
Filing date: 1998-07-02
Publication date: 1999-01-21
Also published as: GB9808460D0; EP1007936A1; AU8228698A

Abstract

A porous container is provided which is suitable for the separation of insoluble from soluble materials. The container comprises a frame (2) supporting a mesh (4). A closure (8) engages with the frame (2) to seal the container. An orifice in the closure is covered by a mesh (12) which is treated to prevent it being wetted. This ensures that gaseous exchanges may take place during the procedures.

Description

POROUS CONTAINER

The present invention relates to a porous container. The porous container is particularly suited for use in the scientific and quality control applications where it is desired to ascertain the composition of a material or to subject a material to various reagents. Test

procedures for determining the composition of a material may require several processing steps wherein the reagents used in one step must be thoroughly removed from the container before advancing to the next step of the test.

There are many situations where it is desirable to perform a composition analysis of a substance. For example, it may be desired to find the fibre content of a foodstuff. An established laboratory technique for doing this is to place a known weight of foodstuff into a cellulose thimble. The foodstuff may then be treated with a solvent under reflux conditions in order to remove the fat therein and collected in a pre-weighed receptacle. The solvent is removed from the receptacle by evaporation thus leaving the fat behind. The receptacle containing the fat is dried and reweighed in order to determine the weight

of the fat removed.

The next step of the process is to remove the soluble proteins, sugars, starch and other non-structural carbohydrates. This is achieved using a two stage process, wherein the fat free foodstuff is quantitatively transferred into a flask and initially boiled in a weak acid for 30 minutes under reflux conditions. The contents of the flask are filtered under vacuum to separate the insoluble components which are quantitatively removed from the filter paper and washed back into the flask

using a pressurised jet of weak alkali solution. The residue is boiled in weak alkali for 30 minutes and then filtered under suction to remove the alkali and soluble components. The residue is then washed with hot purified water, hydrochloric acid (1% solution), further hot water washes and finally an optional solvent wash. The residue remaining on the filter is then quantitatively transferred into a crucible, dried to a constant weight and re-weighed. The reside within the crucible represents most of the structural carbohydrates and also the insoluble mineral components within the foodstuff. Finally the residue is burnt in a furnace at 600°C in order to drive off the organic matter cooled and re-weighed.

The loss in weight during ignition is taken to represent the fibre content of the foodstuff.

The above process is labour intensive, allowing only one test per flask to be performed. Furthermore it can introduce experimental uncertainty. The act of transferring the insoluble residues from filter papers may result in leaving some of the residue behind.

Using a pressurised stream of hot alkali prior to stage two constitutes a Health and Safety risk. Such techniques could be improved if the unwanted elements of the compound could be removed without giving rise to the risk of losing wanted constituents.

According to a first aspect of the present invention, there is provided a porous container, comprising a body having a porous element, and a vent comprising a porous element which does not wet in water.

It is thus possible to provide the container in which the various constituents of the sample can be removed in solution, while leaving the insoluble residue, at any given processing

step behind.

Preferably the container has a closure, such as a lid, such that a sample can be placed in the container and sealed therein. Advantageously the closure is secured so that the insoluble components are not lost from the container, but remain therein. The closure may be held in a push fit engagement with the container. Alternatively, the closure may be securely engaged with the container, for example, by means of co-operating screw threaded portions or some such other mechanical engagement mechanism.

The closure may be removable, although in a preferred embodiment the lid cannot be removed once it has been put in place. This prevents accidental opening of the container.

It also prevents degradation of performance or possible cross-contamination due to re-use. The inventor has found that certain materials, such as polypropylene and/or polyester which makes a particularly beneficial material for construction of the container due to it's chemical resistance and ability to burn to form carbon dioxide and water, retain mineral residues within their structure which can degrade the filtration properties of the container

if it is re-used. The container is provided with a venting means for allowing gas to escape from the interior

of the container. This is advantageous because although the container is porous, surface

tension of liquid at the pores of the container inhibits the escape of gas from the container. This in turn could give rise to a build up of gas inside the container which could drive the reagents out of the container. The venting means is formed by a porous element, such as a mesh, which does not wet in water, (i.e. the contact angle between the solid and water, measured through the water lies between 90 degrees and 180 degrees) or which has been treated with or formed from materials which are hydrophobic and/or inhibit the formation of condensation.

Preferably the venting means is provided in the closure. In one embodiment, the venting means is provided in the closure and is formed from a nylon mesh treated with hydrophobic material, for example silicone or polytetrafluroethane (PTFE), in order to inhibit liquid from blocking the venting apertures or pores therein. As noted before, this stops the build up of steam in the container whilst it is being used with a boiling reagent. Furthermore, once the sample and container have completed a step, the reagent needs to

be drained. Trials with prototype containers where the closure was not treated so as to be

hydrophobic demonstrated that the container often did not drain and had to be subjected to a centrifuging step to remove the reagent therefrom. Furthermore such a container also exhibits difficulty in refilling, and consequently acts as a non-porous container. The problems caused by surface tension are readily demonstrated by the tea bag, which is a

porous paper bag. However, when hot water is poured onto a bag, the water blocks the pores by surface tension. The bag balloons as a result of gas trapped therein. Similarly, the bag does not drain properly, again because surface tension causes water to block the pores of the bag and prevent exchange of gas between the interior of the bag and the atmosphere.

Advantageously the venting means may be convex (i.e. domed) in order to inhibit liquid

from blocking venting apertures or pores therein. The domed shape inhibits liquid from collecting above the venting means. A flexible element, such as a strap may be included in the closure so as to hold the venting mesh in a domed shape. The flexible strap has a dual function such that it holds the venting means in a domed shape thereby preventing the build up of fluid on top of the closure (ie it sheds liquid) and when mechanically depressed it will return to its original position, the mechanical motion aiding removal of moisture that may have blocked the pores or apertures preventing adequate venting.

As a further alternative the lid may include a portion arranged, in use, to be pierced by a

hollow venting shaft, for example a hypodermic needle. As yet a further alternative, the length of the container may be selected such that it's opening extends a sufficient distance

above the surface of the reagents, in use, to ensure that the contents thereof do not escape.

Preferably the container is provided with a foam trap. The trap may, for example, be in the

form of a mesh or sieve, which, in use, prevents foam from rising past the trap. This

creates a foam free head space between the closure and the foam trap. This enhances

venting from the container. A circular mesh having a diameter substantially matching that of the internal diameter of the container and having a pore size of approximately 50 - 120 microns has been found to be effective.

Preferably the container is rigid. Advantageously the container comprises a frame and a mesh or sieve element. The frame provides structural rigidity to the container while the mesh/sieve element provides the medium through which fluids can be exchanged between the interior and the exterior of the container. Advantageously the container, or frame thereof, is made from polypropylene. Polypropylene is especially suited for use in laboratory work since it is resistant to most chemicals and can withstand boiling and normal oven drying temperatures. Furthermore, under the right conditions polypropylene can be burnt to give carbon dioxide and water, thereby leaving little or no reside. Another particularly beneficial property of polypropylene is that it does not retain moisture. Many synthetic materials do retain moisture. Thus the container made with a polypropylene frame dries far more quickly than a similar shaped container made of a different material. It is important that the container can be dried, since in use, a number

of weighings are taken and the presence of moisture degrades the test results.

The fact that the container burns to leave little or no residue is also important since some tests may require a dry ash weight to be obtained for the sample. The container can be

burned without needing to open it. This ensures that no sample is lost.

Advantageously the mesh or sieve is made from polyester and/or polypropylene or some other material (synthetic or otherwise) resistant to chemical attack by the chemicals used

in the test procedure in which the container will be used. Polyester is commercially available in the form of thin porous sheets. Polypropylene can absorb oils. This can be a useful feature in some scientific or quality control tasks. The container can be provided with a porous insert which exhibits preferential uptake of one or more of the components

that may be emitted from the sample during the test. Alternatively or additionally, the material of the mesh may be modified. Thus a container having a polypropylene mesh is suitable for entrapping oil released from a sample therein. The oil may be recovered from the mesh by solvent extraction. Thus the materials of the mesh and, optionally, the frame may be selected so as to beneficial for the particular test procedure with which the container will be used

Advantageously the mesh has a pore size of less than 100 microns . Meshes with a pore size of less than 50 microns and of less than 30 microns , have given good test results. Preferably the mesh has a pore size of substantially 20 microns since this has been found to prevent loss of solids whilst still allowing reasonable solvent flow to and from the capsule. However smaller pore sizes may be used, for example, a pore size of 1 to 5 microns.

The mesh elements may be attached to the inside of the container frame. This stops crevices or corners being formed into which the sample could clump and thereby not be acted upon by the reagents. Advantageously the container is substantially cylindrical. Additionally or alternatively,

an end wall of the container may also be porous.

According to a second aspect of the present invention there is provided a method of testing

the fibre content within an item, comprising the steps of placing the item within a container

according a first aspect of the present invention, and then performing one or more of the

following steps;

1 Solvent extraction of the fat and reweighing the container;

2 Boiling in acid, and then removing the acid solution and soluble components from

the container and, optionally weighing;

3 Boiling in alkali, and then removing the alkali solution and soluble components

from the container and, optionally weighing;

4 Boiling in detergent, and then removing the acid solution and soluble components

from the container and, optionally weighing the container;

5 Soaking in strong acid or strong alkali acid, and then removing the acid/alkali

solution and soluble components from the container and, optionally, weighing the container;

6 Washing in weak acid, and then removing the weak acid solution and soluble components from the container and, optionally weighing the container;

7 Washing in water, and then removing the water and soluble components from the container and, optionally weighing the container;

8 Washing in solvent, then removing the solvent and soluble components from the

container and, optionally weighing the container;

9 Drying the container to remove moisture and weighing the container;

10 Burning the container to remove organic material from within the container and

weighing.

The present invention will further be described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a perspective view of a container constituting an embodiment of the present

invention; and Figure 2 is a table comparing test results of fibre measurements in various products using the prior art method (Weende Method) described hereinbefore, and using the container constituting an embodiment of the present invention in accordance with the method described hereinafter;

The container illustrated in Figure 1 is a cylindrical container body (1) having a polypropylene frame 2 providing support for a polyester mesh 4, the mesh encircles the fame 2 and also forms a first end wall 6 of the container. The container is provided with a lid 8, which again comprises a polypropylene frame 10 which engages with the frame 2 of the container. .An orifice in the end wall of the lid 8 is covered with a hydrophobic mesh 12 which ensures that the lid remains permeable to gas. The mesh 12 is, in this embodiment, formed into a dome. The domed shape also helps keep pores in the mesh clear of moisture since it stops a layer of water from collecting above the mesh. The mesh may be treated, for example, with silicone or PTFE to make it non-wetting. This helps prevent condensation from forming to a sufficient extent to block the pores of the venting

means.

The lid has a strap 13 which keeps the mesh in the domed shape. The lid also has outwardly extending fingers 14 which are inclined with respect the local surface of the lid and which cooperate with recesses or an internal rim in the frame to form latches holding the lid securely to the frame. Thus the lid can be locked in place, thereby inhibiting or

preventing its subsequent removal from the container. In use, a sample (not shown) is introduced into a pre-dried and pre-weighed container. The

lid is affixed to the container, thereby trapping the sample therein. The container is

reweighed to obtain the weight of the sample. If desired, an inorganic filtration aid may also be introduced into the container.

If the sample is a food product and it is desired to make a determination of Crude Fibre content of the food, a weight of food (optionally de-fatted) is placed in the container. The soluble sugars, starches, proteins and other non-structural carbohydrates are removed from the sample by a two stage process. In the first stage the sample is boiled in acid under reflux conditions and then washed in to remove the acid and soluble matter before the second stage of boiling in alkali.

At the completion of each of these stages a portion of the proteins and non-structural carbohydrates are suspended in solution while most of the structural carbohydrates and insoluble minerals remain within the container. The porous nature of the container allows

the soluble fraction to be flushed from the container, while retaining the solid matter. Thus

the container effectively functions as a filter paper or sinta as well as a containment vessel.

After the steps of boiling in acid and alkali have been completed the container may be

flushed with purified water, a weak acid, then more purified water washes and then with

a solvent (optional), air dried, placed in a pre-dried and weighed glass beaker or crucible, oven dried and weighed. This weight represents the weight of the container, fibre and insoluble minerals including the filter aid, if any.

Finally the receptacle containing the container and it contents are placed in a furnace and

ignited at 600°C to vaporise the organic matter. The container will also be destroyed

during this process. However, under these conditions the container will burn to form

mainly carbon dioxide and water thereby leaving only the inorganic residue behind. The

receptacle and the mineral matter is weighed and the organic portion attributed to the food

product is reported as Crude Fibre.

In the embodiment illustrated, the vent was formed of a fine nylon mesh treated with silicone. Comparative tests have demonstrated that untreated nylon gets wet and the pores block, whereas treated nylon does not wet (it has a contact angle of approximately 109°)

and this prevents the pores of the mesh from blocking.

Figure 2 shows various determinations of fibre in various materials. Each material was

tested using the prior art method described at the beginning of this patent application and

an equivalent method using the container. The results for the prior art, Weende method,

and this invention are presented in pairs of columns. The left hand column of each pair the

experimental results whereas the right hand column contains the average result, together

with the standard deviation in parenthesis. As can be seen for dairy nuts, the prior art

method gave an average fibre weight per 100 grams as 9.72 with a standard deviation of

0.52, whereas the same test using the container of the present invention gave results giving an average fibre of 10.03 grams per 100 grams with a standard deviation of 0.04. In

general, a greater experimental accuracy is achieved when using the container constituting

an embodiment of the present invention, and this is manifested in a reduced standard

deviation.

It is thus possible to provide a porous container for analytical use which functions both as

a container and as a filter element. This enables the transfer of materials to be eliminated,

thereby giving rise to increased experimental accuracy. The use of the container also

increases the capability of the laboratory to perform many tests simultaneously.

This container can be used in any determination of acid detergent fibre, neutral detergent fibre, acid detergent lignin and other analytical methods such as starch, water soluble carbohydrates and most methods which require the separation of soluble from insoluble

fraction.

The container may, in use, be held within a rack or carousel which aids the placement and

removal of one or more containers into and out of beakers containing reagents used in

composition analysis and the like.

The carousel may advantageously allow the containers to undergo a limited amount of

vertical movement. The container may be used to perform a number of tests. An example of a test for food

fibre is set below:

1 For each test, weigh 1 - 1.5 grams of sample into a pre-dried and weighed

container, fit cap and insert the container into a carousel.

2 Independently determine the Dry Matter content of each test material using the

standard oven drying method.

3 Measure out 400 mis. of sulphuric acid ( 1.25%w/v) . Transfer to the first extraction

beaker.

4 Lower the first carousel gently into the beaker of solution and mix by rotating the

carousel for two minutes.

5 Place the beaker on a preheated hot plate, replace condenser and bring to the boil

(setting full), reducing the hot plate setting when 90°C is reached. This procedure is

repeated for each set of tests.

6 After 30 minutes from the point of boiling, remove each beaker from the hot plate,

attach the adapter tool and remove the carousel from the beaker and empty the containers

of liquid. 7 Discard the acid and solubles within beaker and fill with hot purified water. Lower

the carousel into the water ensuring that all container refill. Mix by rotation and repeat the

washing procedure (step 6) once more finally leaving the containers and beaker free of

liquid.

8 Add 400 mis of sodium hydroxide 1.25% (w/v) and lower the carousel into the

liquid.

9 Replace the extraction beaker onto the hot plate (setting full), replace the condenser and bring back to the boil, reducing the hot plate setting when 90°C is reached. This procedure is repeated at 3 minute intervals for each set of test being performed.

10 After 30 minutes from the point of boiling, each beaker is removed from the hot

plate, and the carousel removed from the beaker. All the liquids and solubles are discarded

and step 7 repeated until the solution becomes clear. Repeat with hydrochloric acid (1%)

wash, and finally with a further 2 washes of hot purified water.

11 All drained containers containing their residues are placed on tissue for a few

minutes and the place into a pre-dried and weighed tall form beakers (100 mis.).

12 The containers containing the residues are oven dried (100°C) to a constant weight,

desiccated, cooled and re-weighed and finally placed in a furnace (600°C) and ashed for a period of 4 hours, desiccated, cooled and re-weighed.

The crude fibre content can be calculated from the following equation:

% Crude Fibre in the Fat-Free Dry Matter =

(Beaker + Residue Weight - Capsule Weight) - Beaker + Ash Weight 100 x

Sample Weight x Dry Matter grams/gram

Claims

1. A porous container comprising a container body (1 ) having a porous element (4),

and a vent comprising a porous element (12) which does not wet in water.

2. A porous container as claimed in claim 1 , further comprising a closure element (8).

3. A porous container as claimed in claim 2, in which the closure element (8) mechanically engages with the container body (1) so as to secure the closure element to

close the container.

4. A porous container as claimed in claim 2, in which the closure element (8) is held

in a push fit engagement with the container body (1) so as to close the container.

5. A porous container as claimed in claim 1 in which the vent is provided in the

closure element (8).

6. A porous container as claimed in claim 5 in which the closure element is provided

with a flexible strap mechanism (13).

7. A porous container as claimed in claim 1, in which the porous element (8) of the

vent is treated with a hydrophobic agent in order to inhibit blocking of the apertures or

pores therein.

8. A porous container as claimed in any one of the preceding claims, further

comprising a foam trap.

9. A porous container as claimed in any one of the preceding claims, in which the

container body (1) is rigid.

10. A porous container as claimed in any one of the preceding claims, in which the container body comprises a frame (2) which provides structural rigidity to the container.

11. A porous container as claimed in any one of the preceding claims, in which the

container is substantially cylindrical.

12. A porous container as claimed in any one of the preceding claims, in which the

sides of the container are porous.

13. A container as claimed in any one of the preceding claims, in which the pore size

is less than 100 microns.

14. A container as claimed in any one of the preceding claims, in which the pore size

is less than 30 microns.

15. A container as claimed in any one of the preceding claims, in which the pore size

is less than 20 microns.

16. A container as claimed in any one of the preceding claims, in which the pore size

is less than 10 microns.

17. A container as claimed in any one of the preceding claims, in which the frame (2)

of the container is made of polypropylene.

18. A container as claimed in any one of the preceding claims, in which the mesh or

sieve is made from a polymer.

19. A method of testing the fibre content of an item, comprising the steps of placing

the item within the container as claimed in any one of the claims 1 to 18, and then

performing one or more of the following steps.

1 Solvent extraction of the fat and reweighing the container;

2 Boiling in acid, and then removing the acid solution and soluble

components from the container and, optionally, weighing; 3 Boiling in alkali, and then removing the alkali solution and soluble

components from the container and, optionally, weighing;

4 Boiling in a detergent, and then removing the detergent and soluble

components from the container and, optionally, weighing;

5 Soaking the container in strong acid or strong alkali and removing the acid/alkali and soluble components from the container and, optionally, weighing

the container;

6 Washing in water, removing the water and soluble components from the

container and, optionally weighing the container;

7 Washing in a solvent, removing the solvent and soluble components from

the container and optionally, weighing the container;

8 Drying the container to remove moisture and weighing the container;

9 Burning the container to remove organic material within the container and

weighing.