Principles of Milling Control
2. The surface tension of juices in general is below that of pure sucrose solution. The reading is thereby inflated
Despite these acknowledged limitations the Brix spindle is used as the routine instruments in most sugar countries.
For many years there have been two standard temperatures for the calibration of Brix spindles, 20°C and 27.5°C. The former is an international standard temperature for many purposes; the
55
latter is a practical recognition of the fact t h a t the cane sugar industry belongs to the tropics and the mean t e m p e r a t u r e there is nearer to 27.5 t h a n to 20CC.
Saccharimeters are invariably calibrated at 20CC, a n d this has influenced m a n y in favour of the same reference t e m p e r a t u r e s for the Brix spindle. T h e introduction of air conditioning into m a n y sugar laboratories h a s m a d e it possible to work at or near 20°C and this t e m p e r a t u r e will p r o b a b l y supersede 27.5°C in the long run.
T h e alternative principle for the determination of Brix is the m e a s u r e of refractive index by refractometer. This basis of determina- tion is steadily gaining favour. The technical advantages of the refractometer over the Brix spindle a r e :
(1) In general the change in refractive index caused by the substitution of soluble impurities for sucrose is less, in terms of Brix, t h a n the corresponding change in density, also in terms of Brix. M o r e simply, refractometer Brix in general gives a better measure of total dissolved solids t h a n densimctric Brix.
(2) T h o u g h not insensitive to solids in suspension, the re- fractometer is much less affected thereby than the Brix spindle.
(3) T h e refractometer is not affected by surface tension.
T h e chief disadvantage of t h e familiar A b b e type of refrac- t o m e t e r is that its limit of precision is a b o u t 0.0002 in refractive index, equivalent to 0.15 in Brix. However this is no longer of any- real importance. T h e precision refractometer designed by Bausch a n d L o m b specially for the Hawaiian sugar industry m o r e than satisfies the requirements as to range a n d accuracy. It is o u t s t a n d i n g in its class, but expensive. If there h a s to be a c o m p r o m i s e financially, there are now several a d e q u a t e refractometers in a m o r e m o d e s t price category.
T h e Brix of a juice, whether densimetric or refractometric, is determined directly, b u t syrup a n d p a n p r o d u c t s are customarily diluted for the purpose. The influence of suspended matter is n o t usually of any consequence but, for the sake of the refractometer, samples should be at least finely screened. Filtration or centrifuging is better.
Previous editions of this b o o k devoted some space to t h e question of degree of dilution. It is well k n o w n that the Brix of p a n p r o d u c t s , determined by spindle on diluted material a n d calculated back to the undiluted basis, increases with increasing dilution. The Brix of final molasses, based on dilution may be 3° higher t h a n the Brix based on 1 + 1 dilution. These dilutions are a b o u t the limits in practice.
56
Noel Deerr concluded that, for the purposes of m u t u a l l y consistent purity figures, Brix should be determined at a b o u t t h a t c o n c e n t r a t i o n of impurities typical of undiluted juice. No m a t t e r h o w technically correct the conclusion m a y be, it runs to a b s u r d i t y in practice, for the dilution required for final molasses would be of t h e o r d e r of 1 + 40. It h a s to be recalled t h a t t h e error of t h e final result is the p r o d u c t of the error of the actual d e t e r m i n a t i o n a n d the dilution factor. Hence, if the Brix actually measured is accurate to 0.1°, the final result at 1 + I dilution is accurate to 0.2°, b u t at 1 + 5 the error m a y be 0.6°. High ratios of dilution are excluded for this reason, a n d there is a s t r o n g tendency to a d o p t the lowest practical dilution consistent with a convenient factor—that is, 1 + 1.
Higher dilution ratios have their advocates and it is not h a r d to raise an a r g u m e n t on the subject. However it is not w o r t h a r g u - m e n t these days, for the simple solution is to a b a n d o n the Brix spindle in favour of t h e refractometer. Brix (undiluted) determined by refractometer at successive dilutions is not quite c o n s t a n t , it varies unpredictably, but it has no consistently increasing bias, a n d at no rational dilution is the error prohibitive. Of course the re- fractometric Brix still does n o t equal the dry substance, b u t the disparity is a b o u t half t h a t associated with densimetric Brix.
At the o t h e r end of the Brix scale are the low figures c h a r a c - teristic of cane extracts, bagasse extracts a n d imbibition fluids.
F o r m e r l y it was necessary to have recourse to the p y e n o m e t e r a n d as the m e a s u r e m e n t of Brix by this device is tedious and exacting, these d e t e r m i n a t i o n s were avoided if possible. M o d e r n refracto- meters yield reliable results so easily t h a t the m e a s u r e m e n t of low Brixes is no longer a p r o b l e m , b u t suspended solids m u s t be removed from the sample.
Dry Substance.
D r y substance a n d moisture are c o m p l e m e n t a r y quantities a n d are invariably determined concurrently by a quantitative drying.
M a n y of the n o r m a l constituents of sugar factory materials are subject to d e c o m p o s i t i o n at elevated t e m p e r a t u r e s . When this p h e n o m e n o n is of low significance, drying is carried o u t at 100°C or higher; if decomposition h a s to be kept to a m i n i m u m , the p r o d u c t is dried at a b o u t 60°C u n d e r v a c u u m .
Some p r o d u c t s , being viscous fluids, release m o i s t u r e very reluctantly. In such cases, to assist e v a p o r a t i o n , t h e area of exposed surface is artificially increased by the addition of q u a r t z s a n d or by a b s o r b i n g the p r o d u c t into f i l t e r p a p e r .
Dry Substance in Cane.—Knowledge of t h e dry substance and m o i s t u r e contents of cane is desirable for the direct deter- mination of pol a n d Brix in cane and the indirect d e t e r m i n a t i o n of fibre in cane.
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T h e cane m u s t first be c o m m i n u t e d by a fibrator, h a m m e r mill, cutter grinder or like m a c h i n e . E v a p o r a t i o n a n d loss of juice m u s t be avoided, so totally enclosed machines are to be preferred.
A sample of not less t h a n 1 kg is dried in a Spencer type oven at a b o u t 105°C. T h e o p e r a t i o n is simple a n d results are highly reproducible.
Dry Substance in Bagasse.—A convenient a n d desirable s t a n d a r d weight of bagasse for drying is 500 g a n d t h e drying canisters a n d the Spencer oven should be p r o p o r t i o n e d accordingly.
Final bagasse, having a low c o n t e n t of dissolved solids m a y be dried safely at up to 130°C, b u t for bagasses richer in sugar it is desirable to be m o r e conservative, and 110CC is a g o o d t e m p e r a t u r e for drying bagasses in general.
Dry Substance in Filter Cake.- -Weigh n o t less t h a n 5 g of the cake into a tared dish or t r a y a n d d r y at 100-105°C. R o t a r y filter cake, being rich in fibre, dries readily. If the fibre content of the p r o d u c t is low, e v a p o r a t e m o s t of the water below 100°C a n d then raise t h e t e m p e r a t u r e to n o r m a l .
Dry Substance in Sugars.—The drying of sugars is normally carried out in a l u m i n i u m or glass dishes, a b o u t 2 inches in diameter,
I inch high, with lids. A catch weight of sugar a b o u t 5 g is t r a n s - ferred to t h e tared dish, sealed w i t h o u t delay, a n d weighed.
T h e r e are m a n y s t a n d a r d conditions for drying. I C U M S A specifies a t e m p e r a t u r e of 60°C, a pressure n o t exceeding 50 mm of mercury, an air bleed, a n d a final loss in weight n o t exceeding 1 mg per hour.
O t h e r s t a n d a r d s are 3 h o u r s at 103-105°C, 5 h o u r s at 98-99°C a n d 20 minutes in a Spencer oven at 110CC. This list is not ex- h a u s t i v e ; it merely indicates t h a t there is a tendency to prefer, for routine use, a rapid m e t h o d that yields acceptable results. M o s t sugars leaving the factory are subject to analysis by a t r a d e l a b - o r a t o r y , a n d one obvious expedient is to a d o p t the same p r o c e d u r e as the t r a d e l a b o r a t o r y .
Dry Substance in Juice, Syrup, Massecuite and Molasses.—
Juice is t a k e n undiluted, b u t for the drying of massecuite a n d m o l - asses it is convenient to t a k e the diluted p r o d u c t p r e p a r e d for other p u r p o s e s , or to m a k e up a 1 — 1 dilution. Syrup m a y be h a n d l e d either way.
T h e classical m e t h o d s — t h e sand m e t h o d a n d Josse's f i l t e r p a p e r m e t h o d — a r e set o u t in m a n y b o o k s of reference. If syrup, molasses or massecuite is weighed undiluted it m a y be diluted cautiously later to p r o m o t e its distribution over t h e absorbent.
D r y i n g is best performed at low t e m p e r a t u r e u n d e r v a c u u m as for sugars, b u t this takes some 16 h o u r s . T h e c o m m o n practice is
5 8
to achieve results by drying at 103-105
CC for a set period which is of the order of four hours.
Tate and Lyle have designed a special vacuum oven and speci- fied a standard procedure for this application. The extender is aluminium powder. The Tate and Lyle method is probably the best available, but it is most likely to be used only as a standard of reference for establishing the constants of a rapid method. For general purposes the filter paper method is the most attractive.
Pol.
Pol is the equivalent sucrose content of a material, as measured by a saccharimeter.
A saccharimeter is calibrated at a standard temperature, 20°C, for a specific weight of sucrose, the normal weight. The normal weight of sucrose dissolved in water, made up to 100 ml at 20°C and tested in a 200 mm tube at 20°C yields a saccharimeter reading of 100°.
There is no need to recount the problems of earlier years, con- fusion over the c.c. and the ml, the Herzfeld Schonrock scale, the Ventzke scale—suffice it to say that there are still several standard scales in use, but for each of them there is an accepted normal weight. The normal weight for the International Sugar Scale is 26.000 g and this value has been adopted in this book when a normal weight is expressed in figures. Those who work on a different standard will substitute accordingly.
Like Brix, pol may be regarded as a weight of notional matter, and as such, it is not affected by temperature. However, as in the case of Brix, the result of a determination is influenced by tempera- ture. In the case of Brix, the thermal relationships of sucrose solutions are known, and the behaviour of impurities is so nearly the same as to make it reasonable to adopt sucrose temperature corrections for solutions in general. Relative to the saccharimeter, impurities in general behave nothing like sucrose, and therefore pol cannot be corrected for temperature unless either the proportion of impurities is small, or the impurities are known and can be compensated for individually.
Probably because the normal solution is made up at 20°C and the quartz wedge saccharimeter is calibrated for that temperature, it is customary to regard 20°C as the temperature at which pol should be determined. This is certainly desirable for testing sugars, and air conditioning has made it possible in many laboratories to test all samples at close to 20°C. If operations cannot be conducted at 20°C the next precaution is to see that the solution for testing is made up or tested for Brix, as the case may be, at the same tem- perature as the polarization is determined
There is a strange inconsistency between the bases of Brix and pol. Brix is a concentration, weight to weight, in vacuo, as mentioned
59
earlier. By the specification of the n o r m a l weight, pol is a con- centration, weight to weight, in air with brass weights.
Consider a solution of p u r e sucrose in water, having a Brix of 20. A n o r m a l weight (26 g) of this solution will have a t r u e weight of 26.026 g (with m o d e r a t e accuracy) a n d will therefore contain 5.2052 g of sucrose, in vacuo. T h e weight of sucrose which, dissolved in 100 ml of solution, gives a reading of 20 is 5.2034 g in vacuo or 5.2000 g in air with brass weights. Hence the n o r m a l solution under consideration, containing 5.2052 resp. 5.2018 g of sucrose would give a reading of 20.007, and this, by definition is its pol. This also is the concentration of a 20 Brix solution of sucrose in water, meas- ured in air with brass weights.
W h e n samples have densities of the same order as t h a t of sucrose the buoyancy effects cancel out and the pol coincides with the concentration based on true weights. Steps have been taken recently to express the n o r m a l weight as a true weight of sucrose.
T h e whole m a t t e r is distinctly academic and the defects of old established practice are negligible.
T h e r e is a further point in relation to pol that deserves m e n t i o n . T h e s t a n d a r d formula relating polariscope reading to pol is—
T h e 99.718 is the weight in g r a m s of 100 ml of water, weighed in air with brass weights at 20CC. F o r the purposes of a formula which is general as to t e m p e r a t u r e , t h e s.g. should be s.g. t/20°C, where / is the t e m p e r a t u r e of the solution as tested. Since specific gravity figures are available for only a few particular t e m p e r a t u r e s , it is expedient to m a k e use of the observed Brix to derive the specific gravity. If t e m p e r a t u r e effects on the Brix spindle itself are ignored (the error being tolerable) every Brix reading represents a definite density at a n y t e m p e r a t u r e ; moreover, the reading of a 20°C spindle in a solution at t°C, when applied in a table relating Brix to s.g.
20/20°C, will yield the s.g. t!20°C of the solution.
Hence, the second formula is general as to t e m p e r a t u r e if the t e r m "s.g. 2 0 / 2 0 ° C " is interpreted as " t h e result obtained by apply- ing the observed Brix of the solution in a table relating Brix to s.g.
20/20°C, "
It is o p p o r t u n e to mention t h a t , if the Brix is determined by refractometer, the observed refractometer Brix serves just as well as the observed spindle Brix for the p u r p o s e of determination of pol.
H o w e v e r it m u s t be r e m e m b e r e d t h a t a spindle assumes the tem- p e r a t u r e of the test solution b u t t h e test solution assumes t h e
60
t e m p e r a t u r e of a refractometer. It m a y be necessary to adjust a refractometer Brix reading to the t e m p e r a t u r e of the solution as tested in the polariscope.
Clarification for Pol Determination.- - By far the most c o m m o n l y used reagent for the clarification of sugar p r o d u c t s in general is basic lead acetate also called sub-acetate of lead. T h e dry reagent is familiarly k n o w n as H o m e ' s dry lead after the specific formulation prepared' by D r . H o m e . However, the several c o n t e m p o r a r y speci- fications of basic lead acetate do not agree with Dr. H o m e ' s formula, and it seems that the n a m e for general use should be merely " d r y l e a d " .
F o r clarification w i t h o u t dilution dry lead stands almost alone.
T h e only c o m p e t i t o r is dry neutral lead acetate which has the a d v a n t a g e of not interfering appreciably with the rotation of fruc- tose; however it is so inferior as a clarifying agent that it is not used unless the effect of dry lead on fructose must be avoided.
Dry lead m a d e into a standard solution in water becomes t h e clarifying agent called wet lead. In m a n y cases, when there is no b a n on dilution, wet lead is used in preference to dry lead, as it is easier to control the q u a n t i t y a d d e d and the clarifying effect is generally better.
F o r cane juices a n d p r o d u c t s of like composition which may be polarized undiluted, dry lead is the preferred clarifying agent.
T h e pol subsequently determined is essentially the pol of the liquid c o m p o n e n t . When pol is determined by diluting one or m o r e normal weights of sample to 100 ml wet lead is preferred. T h e pol is essentially the p o ! of t h e sample material including any insoluble m a t t e r which it m a y contain, a n d is subject to an error caused by the fact t h a t any insoluble m a t t e r a n d precipitate occupy part of the
100 ml final volume so t h e volume of solution is less than 100 ml.
A special case is the Schmitz m e t h o d wherein 100 ml of a juice are t a k e n , clarified with wet lead, a n d diluted to 110 ml A factor of 1.1 is applied to the polariscope reading, the result being taken as the reading of an undiluted juice. T h e pol determined is t h a t of the liquid p h a s e , subject to the error caused by insoluble m a t t e r a n d precipitate.
C a n e juice which h a s suffered deterioration m a y n o t r e s p o n d satisfactorily to t h e clarifying effect of dry lead. T h e addition of up to four d r o p s of strong a m m o n i a solution m a y m a k e the necessary difference. If not, Herles' reagent should be tried. Herles' reagent is a m i x t u r e of t w o s o l u t i o n s :
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A. Dissolve 100 g s o d i u m h y d r o x i d e in 2 litres of water.
B. Dissolve 1 kg of lead nitrate in 2 litres of water.
Check t h a t on mixing equal volumes of the t w o , the reaction of the mixture is acidic. If alkaline, dilute the caustic soda solution as required to yield an acidic mixture.
T a k e either t w o n o r m a l weights or a m e a s u r e d 50 ml of t h e juice in a 100 ml flask, a d d 5 ml of solution B, 5 ml of solution A, dilute to near 100 ml, s h a k e a n d m a k e up to t h e m a r k . If 5 ml of each solution are insufficient, m o r e of each m a y be used up to a practical limit of 15 ml If t w o n o r m a l weights were taken, the p o l is half the polariscope r e a d i n g ; if 50 ml were t a k e n , double t h e reading a n d calculate the pol as for an undiluted juice.
Herles' reagent a p p e a r s to be the best clarifying agent available.
If it fails there is little h o p e of achieving dependable results.
Pol Determination.
Cane.—See later u n d e r the Analysis of C a n e . Bagasse.—See later u n d e r the Analysis of Bagasse.
Juices.—Cool, if necessary, to r o o m t e m p e r a t u r e or 20~C, clarify with dry lead, filter a n d polarize. D e t e r m i n e t h e observed Brix, a n d using this and the polariscope reading, derive the pol from Schmitz's Table. T h e s t a n d a r d a d d i t i o n of dry lead is 1 g per 100 ml for r a w j u i c e ; less is re- quired for diluted or clarified juices.
Filter Cake.—In view of t h e error created by the insoluble p o r t i o n of filter cake it is c u s t o m a r y to achieve a r o u g h c o m p e n s a t i o n by t a k i n g 25 g as the n o r m a l weight. T a k e 50 g of the cake, a d d water a n d mix to a p a s t e with a stirring r o d . W a s h t h e mixture into a 200 ml flask a n d a d d wet lead as required for clarification ( a b o u t 10 ml).
M a k e up to 200 ml, mix, filter a n d polarize in a 400 mm t u b e . T h e pol is half t h e polariscope reading.
Pan Products.—Syrup a n d all grades of massecuite a n d molasses are tested for pol by n o r m a l weight m e t h o d s . T h e strength of solution m a d e up ranges from n o r m a l , in t h e case of syrup, to as low as one-fifth n o r m a l for s o m e specimens of final molasses. A solution of one n o r m a l weight in 300 ml is p o p u l a r for intermediate p r o d u c t s .
If d r y lead is used for clarification, the q u a n t i t y r e q u i r e d ranges from 1 g per n o r m a l weight for syrup up to 8 g p e r n o r m a l weight for final molasses. W e t lead is easier to use a n d is generally m o r e effective. T h e q u a n t i t y ranges from 2 ml to a b o u t 15 ml.
After clarification the solution is m a d e up to v o l u m e , mixed, filtered a n d polarized. T h e polariscope r e a d i n g
6 2
must be corrected for the normality of the solution. In- soluble matter and the lead precipitate cause errors which are ignored. There is also some error due to the effect of dry lead on the rotation of fructose, an error which nat- urally becomes more serious as the proportion of fructose rises.
An attempt is usually made to eliminate this error, in the case of final molasses. After the filtration, 50 ml of the filtrate are taken in a 50-55 ml flask, 2 ml of acetic acid (1 vol. 96 per cent acid -f- 4 vol. water) are added, and the volume made up to 55 ml with water. This solution is mixed and polarized. The polariscope reading must be corrected for the dilution as well as the normality. The acidification restores the rotation of the fructose.
It has to be acknowledged that the pol of final molasses is of very little significance except for internal comparisons, and there seems to be very doubtful virtue in going to some pains to restore the rotation of fructose. There would be more merit in suppressing the rotation of the glucose, if this were feasible.
Professor J. A. Lopez Hernandez has published claims that the rotations of the reducing sugars are sup- pressed by the addition of a solution of sodium borate.
It is true that the rotations of the reducing sugars are greatly lessened, but the quantity of borate required to render the reducing sugars ineffective causes a significant reduction in the rotation of the sucrose.
It seems that no standard addition of sodium borate can be depended upon to yield a pol consistently of the same order as sucrose, but the principle of proportioning the borate solution approximately to the concentration of reducing sugars has not yet been properly studied and may yield gratifying results.
The accepted formula for the correction of a sugar pol for temperature is—
p
2 0= p
t- 0.0003 S(t— 20) — 0.004 R (t — 20) Where
p
2 0= pol corrected to 20°C
Pt = pol at t°CS — per cent sucrose in sample
R = per cent reducing sugars in sample (assumed to be
invert sugar)
t = temperature of solution.
For general purposes S is taken to be 100 and R is usually neglected, so the more familiar formula is—
P
20 = Pt+ 0.03 (t — 20) 63T h e two t e m p e r a t u r e correction formulae stated a b o v e are specifically for quartz wedge compensated saccharimeters, a n d e m b o d y corrections for b o t h the sugar a n d the instrument. A polarimeter-saccharimeter h a s no t e m p e r a t u r e co-efficient of its o w n , a n d when such an i n s t r u m e n t is used, the value of the S t e r m should be halved, t h a t is, 0.0003 becomes 0.00015. F o r corrections from below 20°C, use 0.0002.
In previous editions of this b o o k it was argued t h a t , in t h e absence of air conditioning in the l a b o r a t o r y , pol, in general, w o u l d be determined at the prevailing t e m p e r a t u r e a n d m u s t be recorded as d e t e r m i n e d ; there- fore the same should apply to sugars. This is quite a p - p r o p r i a t e for m a g m a and remelt sugars, b u t shipment sugars normally are checked by a t r a d e laboratory where the pol is determined at or corrected to 20 C, a n d this is the p r o c e d u r e obviously to be followed.
Sugar.—Mainly for commercial reasons, the d e t e r m i n a t i o n of the pol of raw sugar has become highly specialised, a n d the equipment a n d m e t h o d are specified in minute detail. T h e full statement occupies a b o u t eight pages of print, a n d only a bare outline will be given here.
M i x the sample with a m i n i m u m of exposure to the air and rapidly weigh out the n o r m a l weight within 0.002 g.
W a s h with a b o u t 60 ml of water into a 100 ml flask g r a d u a t e d correctly within 0.02 ml. Dissolve the sugar a n d add 1 ml of wet lead. Mix, a d d water to just below the neck and swirl the flask to p r o m o t e mixing. Stand aside for 10 minutes to stabilise t e m p e r a t u r e . A d d water to bring the volume exactly t o the g r a d u a t i o n m a r k . M i x a n d f i l t e r , taking precautions against evaporation. Polarize in a 200 mm tube accurate to 0.03 m m . T a k e five readings of the polariscope, to 0.05, a n d average the reading to 0 . 0 1 . O p e n the polariscope tube a n d take the t e m p e r a t u r e of t h e contents.
Whether t h e t e m p e r a t u r e be 20°C or otherwise it is customary to arrange as well as possible t h a t the m a k i n g up of the solution and the testing in the polarimeter a r e conducted at one t e m p e r a t u r e , a n d the t e m p e r a t u r e correc- tion formulae q u o t e d are based on this premise. If the t e m p e r a t u r e of m a k i n g the solution, tm, differs from the t e m p e r a t u r e of reading the r o t a t i o n , tr, c o m p e n s a t i o n m a y be achieved by a d d i n g to the result the correction 0.03 (tr — tm).
Sucrose.
Regrettably, the d e t e r m i n a t i o n of sucrose is still a lengthy a n d exacting operation. By c o m m o n consent the p o l of bagasse
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