Representative sampling is of the utmost importance. If a sample does not truly represent the average composition of the material, the most carefully conducted analysis will be of no value. In fact it could be misleading to the operating personnel.
Where the operation is continuous it is best to collect a continuous sample, always in proportion to the quantity of the material being sampled. Automatic sampling devices which fulfill these conditions should be used wherever practical. If continuous sampling is impossible, proportional samples should be taken at regular, frequent intervals.
All sampling devices must be kept clean and in perfect working order to prevent contamination of the sample or irregularities in sampling. Automatic sampling devices should be designed so that:
1. The sample taken is representative and proportional to the total quantity of material.
2. They are as self-cleaning as possible and can be easily cleaned.
3. They will not be subject to mechanical failure.
4. Evaporation or moisture absorption will not occur.
Manual intermittent sampling and composition are to be preferred to a continuous or automatic device which is subject to contamination or mechanical failure.
In the case of all weighed products, the sample should be taken at the point where the product is weighed and in the same condition.
Sample Devices and Containers
To ensure that the sampler itself does not become a source of infection, causing decomposition of the samples it must be kept thoroughly clean, with frequent steriliza- tion. Cleaning by means of steam jets is usually the most convenient and efficient method. This should be done several times per day.
Ideally, sample containers should be seamless, stainless steel, copper, enamel or plastic. Containers for different types of juices should be readily distinguishable.
Preservatives
A preservative must be added to the samples in compositing.
Brix and Ash Determination
Samples to be used for Brix and Ash determination may be preserved with mercuric chloride. 0.5ml of a saturated alcohol solution of the salt per litre of juice should be added. An alternative which is not as effective as mercuric chloride is formalin (40% for- maldehyde) 0.3 to 0.5ml per litre should be added to the sample.
Both formalin and mercuric chloride should not be used in samples taken for reducing sugar analysis because they are copper reducing.
Precaution
Mercuric chloride is poisonous. It should not get on the hands or mouth and excess of samples treatment with mercuric chloride should not be returned to process.
Pol determination
Samples for Pol determination are most effectively preserved by means of lead sub- acetate (Home's dry lead). 20 g per litre of juice is the amount which should be used.
Lead subacetate has a small but sometimes Negligible effect on the refractometer solids reading.
Addition of Preservative
The amount of preservative added to the sample should be in proportion to the amount of sample to be composited. For example, if 100 ml is to be composited, 2 g of the dry lead salt should be shaken. A similar procedure should be used for mercuric chloride.
All the preservative required for the complete sample should never be added before compositing has started.
Freezing
Freezing may be used to preserve samples for an extended period of time, but mercuric chloride should be added to prevent deterioration.
Bagasse collecting
All samples should be taken across the full width of the discharge chute and to the full depth of the blanket. Bagasse loses water easily, especially with hot imbibition, and also deteriorates very rapidly due to the action of the enzymes and micro-organisms.
Agitation causes mechanical segregation of the different particles. Provided moisture changes are recognised, a quick acting shutter in the bagasse elevator through which the whole of the bagasse conveyed by one lath is dropped through a closed chute into a cylindrical container can be used. The contents of the container can be mixed by a suitable manually or mechanically operating mixing gear.
If hand sampling is resorted to, this must be done systematically, at short distances across the entire blanket and down to the bottom of it, without any tendency towards selection, and, most important of all, at regular frequent intervals.
A convenient hand sampling method is to fashion two pieces of light 3 in. x 4 in.
board into a 'V' shaped trough, which should be as long as the last mill is wide. Two people are necessary to operate this sampler effectively. Samples are taken by holding the trough in the bagasse discharge from the last mill and withdrawing it after it is filled (which is almost instantly) by an upward motion so that the trough is equally full along its whole length.
Preparation (Compositing/Analysis)
The sample container must be completely emptied out on to a clean dry surface. The sample is thoroughly mixed by hand, making sure that the hands are clean and dry and that the operation is conducted without loss or gain of moisture on the part of the sample. It is then heaped into a cone-shaped pile and divided vertically into quarters.
Two opposite quarters are discarded and the process repeated until a suitable size of sub-sample is obtained.
The sub-sample is then tightly packed into a 16 oz. capacity dry clean sample container with a tight cover. It is then transferred to a closed container containing preservative. A suitable type container is shown in Figure 4.1.
Fig. 4.1. Bagasse sample container
When possible the timing of the sampling should be such as to permit the sample of bagasse to relate to the sample of the Last Expressed Juice.
The containers must be thoroughly cleaned and disinfected after each use. The time interval between sampling and analysis should not exceed three hours.
Preservative
Four (4) drops formalin or four (4) drops 10/l (vol/vol) ammonia (Sp. Gr. 0.380) and chloroform mixture in each 16 oz bottle full of prepared sample.
Frequency
Samples should be taken not less frequently than once every 30 minutes.
FILTER CAKE Sample Collecting
Samples should be taken across the entire width of the rotary filter or if the mud is discharged into a container for removal, such as a trolley, a sample withdrawn by a tabular "trier" long enough to penetrate the full depth of the container, may be used.
The triers should be withdrawn at least three times to each load and the samples stored in clean, glass screw-capped bottles.
Preparation (Compositing/Analysis)
The sample container must be completely emptied onto a clean dry surface. It is then reduced to small pieces either by using the dry hands or by means of a dry spatula or spoon. Precautions must be taken to avoid loss or gain of moisture. The sample is thoroughly mixed. It is heaped into a cone shaped pile and divided vertically into four quarters. The two opposite quarters are discarded and the process repeated, until a suitable size of sub-sample is obtained.
Preservative
Two (2) drops formalin or two (2) drops of 10/1 (vol/vol) ammonia (Sp. Gr. 0.880) and chloroform mixture in each 16 oz. bottle filled with the prepared sample.
Frequency
Preferably every half hour, hut, in any case, not less frequently than one every hour, JUICES
They should be collected in covered containers of a suitable size. All juice sampling devices and containers should be kept absolutely clean and sterile. Seamless copper cylinders, 24 in. x 6 in. (diameter) fitted with a conical copper lid in which small holes have been drilled should be adequate. See Figure 4.2.
Fig. 4.2. illustrating container recommended for sampling juice
FIRST EXPRESSED JUICE Sample Collection
Any device which samples the juice other than proportionately across the full width of the first roller is unsatisfactory. A collecting tray, fitted with a centrally located spout discharging over a collecting can closed by a perforated, conical lid will meet the require- ments of sampling.
Preparation (Compositing/Analysis)
At the end of each 30 minutes the full can is replaced by an empty one. The full can is conveyed immediately to the laboratory and the entire catch of juice is screened. The juice is screened to remove quantities of suspended solids through a 40 mesh screen. In the case of samples for refractometer brix, the juice is screened through an 80—100 mesh screen. After screening the sample must be thoroughly mixed by stirring.
After mixing is completed, the sample is ready for analysis or for compositing. The making up of composite samples is a process which requires extreme care and accuracy.
It is essential that exactly the correct amount of sample should be placed in the composite bottle each time. The correct amount of sample to be added should be plainly marked on the label of the composite bottle so that no error can arise. The measurement of the volume of sample should be carried out with as much care as is used when making up to the mark in a volumetric flask.
Preservative
0.5 ml saturated alcoholic mercuric chloride per litre.
Frequency
Preferably every half hour.
MIXED JUICE Collection of Sample
It is essential that the sample be proportional. This can be achieved by hanging a perforated, conical topped copper can under the juice discharged from the juice scale.
Preparation (Compositing/Analysis)
A full sampling collecting can is replaced by an empty one. The full can is then conveyed immediately to the laboratory and the entire catch is screened and mixed. The exact quantity of sample required for the composite is carefully measured out and added to the composite jar. The can is emptied, washed with hot water, placed over a steam jet (if available) and dried ready for use.
Preservative
0.5 ml. saturated alcoholic mercuric chloride per litre of sample.
Frequency
The container should be changed at least every 30 minutes. Shorter intervals are to be preferred.
LAST EXPRESSED JUICE Collection of Sample
Samples are usually taken manually by means of a small copper can attached to the end of a stick long enough to reach across the width of the mill, and, as it is particularly difficult to do this in a large mill. The greatest flow of juice is usually at the ends of the roller because some of the juice extracted by the feed roller flows to the discharged side of the mill in the space between the ends of the trashplate and the top of the roller flanges, needless to say this juice should not be sampled, but only the lesser flows along the length of the roller which are extracted by the discharge roller.
Great care should be taken not to collect feed roll juice, some of which falls on the opposite side of the trashbar to the last roll juice a few inches away.
Sampling should be taken bearing in mind the timing of the sampling of the bagasse, so that both samples are related.
Preparation (Compositing/Analysis)
The sample is taken immediately to the laboratory and the juice is screened and mixed. The exact quantity required for the composite sample is carefully measured out and added to the composite jar.
Preservative
0.5 ml saturated alcoholic mercuric chloride per litre of sample should be added.
Frequency
Not less frequent than every 30 minutes, CLARIFIED JUICE
Collection of Sample
The juice should be sampled continuously, e.g. by placing a pet cock or small pipe with a valve in the discharge side of the pump or in the pipeline conveying the juice.
Preparation (Compositing/Analysis)
The sample should be taken to the laboratory preferably every hour. The entire catch is mixed and a sample withdrawn for compositing.
Preservative
0.5 ml of saturated alcoholic mercuric chloride per litre of sample.
Frequency
The composite sample should be analysed every three hours.
MASSECUITES(A.B.C.)
Each grade of Massecuites should be sampled manually during discharge from the pan, at approximately 25, 50 and 75% of its volume. The first sample should be taken from the first flow. It is important to realise that the composition of massecuites varies from point to within the pan, due to imperfect circulation.
Preservative
Massecuite samples need no preservative.
Frequency
Every strike of each grade.
MOLASSES (A and B)
A representative sample may be taken from the tank. It should be taken from several places because the molasses does not mix readily.
MOLASSES (Final)
Final molasses may be sampled by taking a bleed-line from the discharge line of the final molasses pump, with an adjustable pet cock feeding a small trickle into a sample can which is replaced by another at the curing of each 'C' strike. The pet cock should be regularly cleared with a piece of wire to make sure its bore is not restricted.
Preservative
Molasses samples need no preservative.
Frequency
Every A, B and C strike. Where continuous crystallisation is practised the Final molasses samples are taken every two hours.
SYRUP
This should be sampled continuously by placing a pet cock or small pipe with a valve in the pipe line conveying the syrup. The sample should be taken to the laboratory every hour.
Preservative
Syrup samples need no preservative.
Frequency
The composite sample should be analysed every six hours.
SUGAR (a) Bagged Sugar Sample Container
The container should preferably be constructed of metal and provided with a tight fitting lid. It should be seamless, preferably of stainless steel or copper. Before use, all containers should be thoroughly cleaned and dried.
Collection and Frequency of Sample
A small scoopful of sugar is taken from at least each fourth bag at weighing or a small trier inserted into each fourth bag after closing and the sugar withdrawn placed in the sample container.
Preservative None is required.
(b) Bulk Sugar
Sample Container and Sampling
A small rotating paddle and a suitable container may be mounted to scoop a portion, off a conveyor belt or out of a stream of sugar dropping into the sugar bin. Sugar cascading off the end of a belt may be made to fall over an auger rotating in an upward turned, half pipe of diameter similar to that of the auger, or a small bucket elevator may be used to sample the sugar travelling in the main conveyor, and, the sample further subsampled by falling over the cone of a typical grain sampler.
When sugar is received into or discharged from trucks or other similar containers, a small volume of sugar obtained by inserting an open, cylindrical can into the falling sugar, at least once per truck, is adequate. The sampling can withdraws 135 gms of sugar per 10 ton truck. Sugar samples being withdrawn in exposed situations should be placed immediately into screw-capped jars or other containers.
Preservative
None is required for sugar, but sugar samples must be stored in airtight bottles. Screw- capped lids should have a sealing rubber gasket or be sealed by plastic adhesive tape. The bottles should be completely filled.
Procedure and Frequency
For the routine control of sugar factories, samples should represent between five and ten tons of sugar.
Sugar from each source entering a bulk sugar store will have a sample withdrawn from each truck load, and then samples composited in such a way that the composite sample represents the weekly deliveries from that source.
Outgoing sugar from bulk stores should be sampled in such a way that a composite sample represents no more than 500 tons.
CHAPTER V