A test tube approximately 10 mm in diameter and 80 mm in length is filled with a sample of the water and inverted for a few seconds, allowing most of the water to drain. Upon replacing the test tube in an upright position approximately 0.2 ml of water will accumulate in the bottom of the tube. To this is then added one to two drops of 4% alcoholic alpha-naphthol solution followed by 15 to 20 drops of concentrated sulphuric acid. On mixing the contents of the tube by shaking, the development of a violet colour within 30 seconds is indicative of the presence of sugar.
Laboratory Manual for S. Afr. Sugar Factories 1962 — 57
C H A P T E R VIII
A N A L Y S I S O F P R O D U C T S
a. Sucrose % cane: T h e m e t h o d used to determine sucrose % cane depends mainly on the size of the parcel of cane to be analysed.
i. G E N E R A L F A C T O R Y M E T H O D : T h e w e i g h t of s u c r o s e in a quantity of cane crushed in a period which begins and ends with an empty mill train is determined by adding the weights of sucrose in the mixed juice a n d the bagasse p r o d u c e d from t h a t quantity of cane. This m e t h o d is generally used by t h e factories to assess the weight of sucrose in cane crushed in a weekly period. T h e m e t h o d is also applicable for shorter periods but the results are less accurate owing to t h e difficulty of assessing the weights of mixed juice a n d bagasse corres- p o n d i n g to a relatively small quantity of cane.
ii. J A V A R A T I O M E T H O D : T h e p e r c e n t a g e of sucrose in a n individual consignment of cane is assessed by a different m e t h o d . T h e consignment is crushed a n d the sucrose (po1) percentage of the first expressed juice is determined. Sucrose
% cane is found by multiplying sucrose % first expressed juice by the so-called Java ratio. This factor is t h e average ratio of sucrose % cane to sucrose (pol) % first expressed juice determined for the weekly period in which the consignment is crushed. O n e of the d r a w b a c k s of this m e t h o d is t h a t the juice content of the cane of the individual consignments is n o t taken into account. T h e m e t h o d is applied for cane p a y m e n t purposes.
i i i . T E S T M I L L A N A L Y S I S : Owing t o the considerable varia- tion in the composition of t h e units constituting a parcel of cane, a satisfactory m e t h o d of assessing its sucrose content by obtaining a representative sample has n o t yet been found, in spite of extensive attempts carried out in m a n y countries including South Africa.
T h e following test mill analysis of a sample is carried out when the greatest accuracy is n o t required. T h e sample, after being weighed, is crushed in a test mill. Usually, only the weight of the bagasse p r o d u c e d is determined a n d the weight of the juice is calculated as the difference between the 58 — 1962 Laboratory Manual for S. Afr. Sugar Factories
weight of the cane a n d t h e weight of the bagasse. If facilities exist for the weighing of b o t h bagasse a n d juice this should be d o n e . A n y shortage from t h e total weight of t h e cane is then divided between t h e bagasse a n d the juice in the ratio of 1:2.
T h e juice is analysed in the o r d i n a r y way for brix a n d pol (sucrose)—see u n d e r Mixed juice, chapter VIII, 5; the analysis of the bagasse however is m o r e complicated.
T h e bagasse is shredded or c h o p p e d , the o p e r a t i o n being conducted as quickly as possible to avoid loss of moisture.
T h e p o l is then determined as u n d e r Final bagasse, altering the a m o u n t of water taken to allow for the weight of fibre in bagasse, so as to give an approximately half n o r m a l solution.
N.B. In the case of chopped bagasse, care should be taken that the final sample is representative of the trash, rind and pith originally present.
T h e n pol % cane =
Fibre % cane =
where Fibre % bagasse = D r y m a t t e r % bagasse—brix % bagasse
and brix % bagasse = X 100.
This m e t h o d is generally used by the factories to assess the fibre percentage of the cane crushed in a weekly period.
ii. D I R E C T M E T H O D : The fibre percentage o f a sample o f 50-100 lb. of cane can be determined with sufficient accuracy as follows: T h e sample is sub-sampled as described in chapter V, 2, b. T h e sub-sample is passed t h r o u g h a suitable l a b o r a t o r y disintegrator or shredder a n d all the shredded material is collected in a large bucket. W h e n the material has been well mixed 100 g are weighed out in duplicate into tared 500 ml beakers. T h e samples are transferred q u a n t i - tatively to strong linen bags which are t h e n tied with string.
T h e bags a r e placed in briskly boiling water for an h o u r a n d Laboratory Manual for S. Afr. Sugar Factories 1962 — 59 X 100.
b. Fibre % cane
i. G E N E R A L F A C T O R Y M E T H O D : T h e fibre p e r c e n t a g e of a quantity of cane crushed in a period which begins a n d ends with an empty mill train is determined by substituting the relevant d a t a in the expression,
then in cold running water for another hour. D u r i n g the second h o u r the bags should be squeezed at intervals. Finally the bags are pressed to eliminate surplus water and are dried at 110-130°C to constant weight. T h e bags are cooled in a desiccator before weighing. T h e fibre is then removed from the bags and the bags are weighed empty. T h e percentage of fibre in cane is found by subtracting the weight of the empty bag from the weight of the bag + fibre in each case.
iii. I N D I R E C T M E T H O D : I f a n experimental mill i s available, the fibre percentage of a sample of cane can be assessed by a m e t h o d similar to t h a t described in 1, b, i above. T h e weighed sample is crushed and the purity of the expressed juice and the weight, pol and moisture content of the bagasse is determined. Pol % bagasse and moisture % bagasse are determined by the m e t h o d s described under 2, a and 2, b below. T h e m e t h o d of calculating the final result is similar to that described in 1, b, i above.
2. Final b a g a s s e
a. Pol % bagasse: Since the error m a d e is insignificant, sucrose % bagasse is generally assumed to be equal to pol % bagasse and this latter value is determined. T h e a p p a r a t u s for the determination of pol in bagasse is described in chapter IV, 2, n. 520 g of u n c h o p p e d bagasse are weighed out into a digester and 3,775 g of h o t water containing 80 ml of a 5 % sodium c a r b o n a t e solution are added.
These quantities have been selected so t h a t the resulting extract is a half n o r m a l solution, as explained below.
The average moisture content of Natal bagasse being 53 % and its brix content being 3.25%, its average fibre content F is 43.8%. Assuming 30% brix-free water to be present, the amount of juice in bagasse available to mix with the water added is:
520 x = 225 g (approximately 225 ml) hence:
4000 — 225 — 3775 ml water required to bring the extract volume to 4000 ml.
T h e perforated plate is placed on the bagasse a n d pressed below the surface of the liquid. T h e cover and condenser are attached a n d the water is b r o u g h t to the boil. After boiling continuously for one hour, a sample of the solution is w i t h d r a w n a n d cooled to r o o m t e m p e r a t u r e . It is clarified with the m i n i m u m a m o u n t of powdered lead sub acetate necessary for clarification and polarized in a 400 mm tube. T h e reading gives the percentage pol (sucrose) in bagasse directly.
b. Moisture % bagasse: 100 g of bagasse are weighed into trays 20 cm by 20 cm by 2 cm m a d e with suitable gauze b o t t o m s a n d dried for 8 h o u r s at 1 0 0 - 1 0 5 ° C .
60 — 1962 Laboratory Manual for S. Afr. Sugar Factories
A l t h o u g h a n o r m a l drying oven can be used, a Spencer type oven is r e c o m m e n d e d . In order to reduce the time required for an analysis, a special type of oven in which heated air is blown t h r o u g h the sample placed on a tray provided with a metal gauze b o t t o m is used in some factories. Special care is required to ensure that fine particles are n o t blown o u t of the tray and t h a t the openings of the gauze are n o t blocked. W h e r e greater accuracy is required, samples of 1000 g m u s t be used for the analysis. A p p a r a t u s for this p u r p o s e is on the m a r k e t .