The sugars of commerce may be conveniently classified into two varieties, viz., sucrose (cane sugar or saccharose) and dextrose (grape sugar or glucose). The former, which is the kind almost exclusively employed for domestic uses, is chiefly obtained from the sugar cane of the West Indies and American Southern States (Saccharum officinarum), and, in continental Europe, from the sugar beet (Beta vulgaris). A comparatively small quantity is manufactured in the United States from the sugar maple (Acer saccharinum), and from sorghum (Sorghum saccharatus).

Cane Sugar (C12 H22 O11).—Among the more important chemical properties of cane sugar are the following:—It dissolves in about one-third its weight of cold water—much more readily in hot water—and is insoluble in cold absolute alcohol. From a concentrated aqueous solution it is deposited in monoclinic prisms, which possess a specific gravity of 1·580. Cane sugar is characterised by its property of rotating the plane of a ray of polarised light to the right; the rotary power is 66°·6. Upon heating its solution with dilute mineral acids, it is converted into a mixture termed “invert sugar,” which consists of equal parts of dextrose and levulose. The former turns the plane of polarised light to the right, the latter to the left; but owing to the stronger rotation exerted by the levulose, the combined rotary effect of invert sugar is to the left, i. e., opposite to that possessed by cane sugar. Invert sugar exhibits the important property of reducing solutions of the salts of copper, which is not possessed by pure cane sugar. Cane sugar melts at 160°; at a higher temperature (210°) it is converted into a reddish-brown substance termed caramel. When subjected to the action of ferments, cane sugar is first transformed into invert sugar, then into alcohol and carbonic acid, according to the reactions:—

(a) C12 H22 O11 + H2O = 2 C6 H12 O6.
(b) C6 H12 O6 = 2 CO2 + 2 C2 H6O.

The varieties of cane sugar usually met with in commerce are the following:—

1. Loaf sugar, consisting either of irregular fragments, or (more often) of cut cubes.

2. Granulated sugar.

3. Soft white sugar.

4. Brown sugar, varying in colour from cream-yellow to reddish-brown.

Molasses is a solution of sugar, containing invert sugar, gummy matters, caramel, etc., which forms the mother-liquor remaining after the crystallisation of raw cane sugar; the name “syrup” being commonly applied to the residual liquor obtained in the manufacture of refined sugar.

Dextrose (C6 H12 O6), occurs ready-formed in grape juice, and in many sweet fruits, very frequently associated with levulose; it is also contained in honey, together with a small amount of cane sugar. As already mentioned, it constitutes an ingredient of the product obtained by the action of acids and ferments upon cane sugar. For commercial purposes, glucose is prepared by treating grains rich in starch, with dilute acids. In France and Germany, potatoes are used in its manufacture; in the United States, Indian corn or maize is almost exclusively employed. The processes used consist substantially in first separating the starch from the grain by soaking, grinding, and straining, then boiling it, under pressure, with water containing about 3 per cent. of sulphuric acid, neutralising the remaining acid with chalk, decolorising the solution by means of animal charcoal, and concentrating it in vacuum pans. In the United States thirty-two factories are engaged in the manufacture of glucose, which consume about 40,000 bushels of corn daily, their annual production having an estimated value of 10 millions of dollars. In commerce, the term grape sugar is applied to the solid product, the syrup or liquid form being known as glucose. The chief uses of starch sugar and glucose are in the manufacture of table syrups, and as a substitute for malt in the brewing of beer and ale. Their other most important applications are as a substitute for cane sugar in confectionery, and in the preparation of fruit jellies; as an adulterant of cane sugar, as an admixture to genuine honey, and as a source for the preparation of vinegar.

Dextrose is soluble in 11/5 part of cold water, and is much more soluble in hot water. It has a dextro-rotary power of 56°. When separated from its aqueous solution, it forms white and opaque granular masses, but from an alcoholic solution, it is obtained in well-defined, microscopic needles, which fuse at 146°. Two parts of glucose have about the same sweetening effect as one part of cane sugar.[54] It does not become coloured when mixed with cold concentrated sulphuric acid, which distinguishes it from sucrose; on the other hand, its solution is coloured dark-brown if boiled with potassium hydroxide, another distinction from cane sugar. Dextrose is capable of directly undergoing vinous fermentation, and, like invert sugar, it possesses the property of reducing alkaline solutions of copper salts, especially upon the application of heat.

The chief commercial varieties of American glucose are the following:—

Maltose and levulose are isomers of dextrose. The former is prepared by the action of malt or diastase upon starch. It has a dextro-rotary power of 150° and its property of reducing copper salts is only about 60 per cent. of that of dextrose. It is converted into the latter compound upon boiling with dilute sulphuric acid. Levulose, as previously stated, is formed, together with dextrose, from cane sugar by treatment with dilute acids or with ferments. It turns the plane of a ray of polarised light to the left, its rotary power varying considerably at different temperatures.

Lactose, or milk sugar, has already been referred to under the head of Milk. It is isomeric with cane sugar, possesses a dextro-rotary power (58°·2), and undergoes fermentation when mixed with yeast, and reduces alkaline copper solutions, but in a different degree from glucose.

Many of the substances frequently enumerated as being used to adulterate sugar are at present very seldom employed. The usual list includes “glucose” (often meaning invert sugar), sand, flour, chalk, terra alba, etc. Loaf sugar is almost invariably pure, although its colour is sometimes improved by the addition of small proportions of various blue pigments, such as ultramarine, indigo, and Prussian blue. The presence of ultramarine was detected in about 73 per cent. of the samples of granulated sugar tested in 1881 by the New York State Board of Health. Tin salts[55] are also occasionally employed in the bleaching of sugar and syrups. Granulated sugar is asserted to be sometimes mixed with grape sugar, and powdered sugar has been found adulterated with flour and terra alba; but the varieties which are most exposed to admixture are the low grades of yellow and brown sugar, in which, however, several per cent. of invert sugar are normally present. Sand, gravel, and mites form a rather common contamination of raw sugar. From the year 1876 to 1881, 310 samples of commercial sugar were examined by the public health authorities of Canada, of which number 24 were reported as containing glucose, and 11 as of doubtful purity. Of 38 samples of brown sugar recently analysed by Dr. Charles Smart, of the National Board of Health, 9 were adulterated with glucose. From the investigations of A. L. Colby, Analyst to the New York State Board of Health, it was found that of the 116 samples examined, the white sugars were practically pure; whereas, of 67 samples of brown sugar, 4 contained glucose. Of 16 specimens of brown sugar, tested by a commission appointed by the National Academy of Sciences in 1883, 4 contained about 30 per cent. of this body.[56] Many varieties of sugar-house syrups, and the various forms of confectionery, are very extensively adulterated with artificial glucose.

The average sugar-house syrup has the following composition:—

Dr. W. H. Pitt, in the Second Annual Report of the New York State Board of Health, gives the following analysis of grocers’ mixed glucose syrup, and of confectioners’ glucose:—

American Grape Sugar Co.’s Syrup.

Confectioners’ Glucose.

It is stated that a large proportion of the American maple syrup and maple sugar found on the market, consists of raw sugar, flavoured with the essential oil of hickory-bark, for the manufacture of which letters patent have been granted.

Analysis of Sugar.—The examination of sugar is ordinarily confined to the estimation of the water, ash, and determination of the nature of the organic matters present. The proportion of water contained in a sample is found by drying it for about two hours in an air-bath, at a temperature of 110°. Moist and syrupy sugars, such as muscovadoes, are advantageously mixed with a known weight of ignited sand before drying. The ash is determined either by directly incinerating a few grammes of the sugar in a tared platinum capsule, or by accelerating the process of combustion by first moistening the sample with a little sulphuric acid. In this case the bases will naturally be converted into sulphates, and a deduction of one-tenth is usually made from the results so obtained, in order to reduce it to terms of the corresponding carbonates. The proportion of ash in raw cane sugar varies somewhat, but it should not much exceed 1·5 per cent. Its average composition, as given by Monier, is as follows:—

Insoluble mineral adulterants are readily separated by dissolving a rather considerable amount of the sample in water and filtering. In this manner the presence of sand, terra alba, and foreign pigments may be recognised.

The determination of the character of the organic constituents of commercial sugars is effected, either by chemical or by physical tests, and, in some instances, by a combination of these methods. The presence of such adulterants, as flour or starch, is very easily detected upon a microscopic examination of the suspected sample.

If cane sugar, containing grape sugar, is boiled with water, to which about 2 per cent. of potassium hydroxide has been added, the solution acquires a brown colour.

Upon mixing a solution of pure cane sugar with a solution of cupric sulphate, adding an excess of potassium hydroxide, and boiling, only a slight precipitation of red cupric oxide takes place. Under the same conditions, grape sugar at once produces a copious green precipitate, which ultimately changes to red, the supernatant fluid becoming nearly or quite colourless. A very good method for the quantitative estimation of grape sugar when mechanically mixed with cane sugar, is that of P. Casamajor. It is executed by first preparing a saturated solution of grape sugar in methylic alcohol. The sample to be tested is thoroughly dried, and then well agitated with the methylic alcohol solution, in which all cane sugar will dissolve; any grape sugar present remains behind, and upon allowing the mixture to remain at rest for a short time, forms a deposit which is again treated with the grape sugar solution, and then collected upon a tared filter, washed with absolute methylic alcohol, and weighed. Glucose and invert sugar are usually quantitatively determined by means of Fehling’s solution.

As this preparation is liable to decompose upon keeping, it is advisable to first prepare cupric sulphate solution by dissolving exactly 34,640 grammes of the salt in 500 c.c. of distilled water, and then make up the Rochelle salt solution by dissolving 68 grammes of sodium hydroxide, and 173 grammes of Rochelle salt in 500 c.c. of water, the solutions being kept separate. When required for use, 5 c.c. each of the copper and Rochelle solutions (corresponding to 10 c.c. of Fehling’s solution) are introduced into a narrow beaker, or a porcelain evaporating dish, a little water is added, and the liquid brought to the boiling point. The sugar solution under examination should not contain over 0·5 per cent. of glucose. It is cautiously added to the hot Fehling’s solution from a burette until the fluid loses its blue colour (see p. 37). The number of c.c. required to completely reduce 10 c.c. of Fehling’s solution, represents 0·05 gramme of grape sugar. The foregoing volumetric method is sometimes applied gravimetrically by adding a slight excess of Fehling’s solution to the sugar solution, collecting the precipitated cupric oxide upon a filter and weighing, after oxidation with a few drops of nitric acid; or, it may be dissolved, and the copper contained deposited by electrolysis, in which case the weight of copper obtained, multiplied by 0·538, gives the equivalent amount of glucose. The proportion of cane sugar in a sample of raw sugar can be determined by first directly estimating the proportion of invert sugar contained by means of Fehling’s solution, as just described. The cane sugar present is then inverted by dissolving one gramme of the sample in about 100 c.c. of water, adding 1 c.c. of strong sulphuric acid, and heating the solution in the water-bath for 30 minutes, the water lost by evaporation being from time to time replaced. The free acid is next neutralised by a little sodium carbonate, its volume made up to 200 c.c., and the invert sugar now contained estimated by Fehling’s solution. The difference in the two determinations represents the glucose formed by the conversion of the cane sugar; 100 parts of the glucose so produced is equivalent to 95 parts of cane sugar.

Commercial cane sugar is, however, generally estimated by the instrument known as the saccharimeter or polariscope.

In order to convey an intelligent idea of the physical laws which govern the practical working of the polariscope, it will first be necessary to refer to the subject of the polarisation of light. The transformation of ordinary into polarised light is best effected either by reflection from a glass plate at an angle of about 56°, or by what is known as double refraction. The former method can be illustrated by Fig. 1, Plate X., which represents two tubes, B and C, arranged so as to allow the one to be turned round within the other. Two flat plates of glass, A and P, blackened at the backs, are attached obliquely to the end of each tube at an angle of about 56°, as represented in the figure. The tube B, with its attached plate, A, can be turned round in the tube C without changing the inclination of the plate to a ray passing along the axis of the tube. If a candle be now placed at I, the light will be reflected from the plate P through the tube, and, owing to the particular angle of this plate, will undergo a certain transformation in its nature, or, in other words, become “polarised.” So long as the plate A retains the position represented in the figure, the reflected ray would fall in the same plane as that in which the polarisation of the ray took place, and an image of the candle would be seen by an observer stationed at O. But, suppose the tube B to be turned a quarter round; the plane of reflection is now at right angles to that of polarisation, and the image will become invisible. When the tube B is turned half-way round, the candle is seen as brightly at first; at the third quadrant it disappears, until, on completing the revolution of the tube, it again becomes perfectly visible. It is evident that the ray reflected from the glass plate P has acquired properties different from those possessed by ordinary light, which would have been reflected by the plate A in whatever direction it might have been turned.

If a ray of common light be made to pass through certain crystals, such as calc spar, it undergoes double refraction, and the light transmitted becomes polarised. The arrangement known as Nicol’s prism, which consists of two prisms of calc spar, cut at a certain angle and united together by means of Canada balsam, is a very convenient means of obtaining polarised light. If two Nicol’s prisms are placed in a similar position, one behind the other, the light polarised by the first (or polarising) prism passes through the second (or analysing) prism unchanged; but if the second prism be turned until it crosses the first at a right angle, perfect darkness ensues. While it would exceed the limits of this work to enter fully upon the theoretical explanations which are commonly advanced concerning the cause and nature of this polarised, or transformed light, it may be well to state here that common light is assumed to be composed of two systems of beams which vibrate in planes at right angles to each other, whereas polarised light is regarded as consisting of beams vibrating in a single plane only. If, now, we imagine the second Nicol’s prism to be made up of a series of fibres or lines, running only in one direction, these fibres would act like a grating and give free passage to a surface like a knife blade only when this is parallel to the bars, but would obstruct it if presented transversely. This somewhat crude illustration will, perhaps, serve to explain why the rays of light which have been polarised by the first Nicol’s prism are allowed to pass through the second prism when the two are placed in a similar position, and why they are obstructed when the prisms are crossed at right angles, it being remembered that in a polarised ray the vibrations of the beams of light take place in a single plane.

Suppose we place between the two Nicol’s prisms, while they are at right angles, a plate cut in a peculiar manner from a crystal of quartz, we will discover that rays of light now pass through the second prism, and that the field of vision has become illuminated with beautiful colours—red, yellow, green, blue, etc., according to the thickness of the quartz plate used. On turning the second Nicol’s prism on its axis, these colours will change and pass through the regular prismatic series, from red to violet, or the contrary, according to the direction of the rotation produced by the intervening plate. Quartz, therefore, possesses the remarkable property of rotating the plane of polarisation of the coloured rays of which light is composed; and it has been discovered that some plates of this mineral exert this power to the right, others to the left; that is, they possess a right or left-handed circular polarisation. Numerous other substances, including many organic compounds, possess this quality of causing a rotation—either to the right or left—of a plane of polarised light. For example, solutions of cane sugar and ordinary glucose cause a right-handed rotation, whilst levulose and invert sugar exert a left-handed rotation. The extent of this power is directly proportional to the concentration of the solutions used, the length of the column through which the ray of polarised light passes being the same. It follows that on passing polarised light through tubes of the same length which are filled with solutions containing different quantities of impure cane sugar, an estimation of the amount of pure cane sugar contained in the tubes can be made by determining the degree of right-handed rotation produced; and it is upon this fact that the application of the polariscope in sugar analysis is based. The optical portions of the most improved form of the polariscope—that known as the Ventzke-Scheibler—are represented by Fig. 2.

The light from a gas burner enters at the extremity of the instrument and first passes through the “regulator A,” which consists of the double refracting Nicol’s prism a and the quartz plate b, it being so arranged that it can be turned round its own plane, thus varying the tint of the light used, so as to best neutralise that possessed by the sugar solution to be examined. The incident ray now penetrates the polarising Nicol’s prism B, and next meets a double quartz plate C (3·75 millimetres in thickness). This quartz plate, a front view of which is also shown in the figure, is divided in the field of vision, one half consisting of quartz rotating to the right hand, the other half of the variety which rotates to the left hand. It is made of the thickness referred to owing to the fact that it then imparts a very sensitive tint (purple) to polarised light, and one that passes very suddenly into red or blue when the rotation of the ray is changed. Since the plate C is composed of halves which exert opposite rotary powers, these will assume different colours upon altering the rotation of the ray. After leaving the double quartz plate the light, which, owing to its passage through the Nicol’s prism B is now polarised, enters the tube D containing the solution of cane sugar under examination; this causes it to undergo a right-handed rotation. It next meets the “compensator” E, consisting of a quartz plate c, which has a right-handed rotary power, and the two quartz prisms d, both of which are cut in a wedge shape and exert a left-handed rotation. They are so arranged that one is movable and can be made to slide along the other, which is fixed, thus causing an increase or decrease in their combined thickness and rotary effect. The ray of light then passes through the analysing Nicol’s prism F, and is finally examined by means of the telescope G, with the objective e and ocular f. Fig. 3 gives a perspective view of the Ventzke-Scheibler polariscope. The Nicol’s prism and quartz plate which constitute the “regulator” are situated at A and B, and can be rotated by means of a pinion connecting with the button L. The polarising Nicol’s prism is placed at C, and the double quartz plate at D. The receptacle h contains the tube P filled with sugar solution, and is provided with the hinged cover h´, which serves to keep out the external light while an observation is being taken. The right-handed quartz plate and the wedge-shaped quartz prisms (corresponding to c and d, Fig. 2) are situated at G, and at E and F, and the analysing Nicol’s prism is placed at H. When the wedge-shaped prisms have an equal thickness coinciding with that of the quartz plate c (Fig. 2) the left-handed rotary power of the former is exactly neutralised by the right-handed rotary power of the latter, and the field of vision seen at I is uniform in colour, the opposing rotary powers of the two halves of the double quartz plates C (Fig. 2) being also equalised. But if the tube, filled with a sugar solution, is placed in the instrument, the right-handed rotary power of this substance is added to that half of the double quartz plate which exerts the same rotary effect (the other half being diminished in a like degree), and the two divisions of the plate will now appear of different colours. In order to restore an equilibrium of colour the movable wedge-shaped quartz plate E is slid along its fellow F by means of the ratchet M, until the right-handed rotary power of the sugar solution is compensated for by the increased thickness of the left-handed plate, when the sections of the plate C will again appear uniform in colour. For the purpose of measuring the extent to which the unfixed plate has been moved, a small ivory scale is attached to this plate, and passes along an index scale connected with the fixed plate. The degrees marked on the scale, which are divided into tenths, are read by aid of a mirror s attached to a magnifying glass K. When the polariscope is in what may be termed a state of equilibrium, i. e. before the tube containing the sugar solution has been placed in it, the index of the fixed scale points to the zero of the movable scale.

In the practical use of the Ventzke-Scheibler saccharimeter the method to be followed is essentially as follows: 26·048 grammes of the sugar to be tested are carefully weighed out and introduced into a flask 100 cubic centimetres in capacity; water is added, and the flask shaken until all crystals are dissolved. The solution is next decolorised by means of basic plumbic acetate, its volume made up to 100 cubic centimetres, and a little bone-black having been added if necessary, a glass tube, corresponding to P (Fig. 3) which is exactly 200 millimetres in length, and is provided with suitable caps, is completely filled with the clear filtered liquid. This is then placed in the polariscope, and protected from external light by closing the cover shown at h´. On now observing the field of vision by means of the telescope, it will be seen that the halves into which it is divided exhibit different colours. The screw M is then turned to the right until this is no longer the case, and absolute uniformity of colour is restored to the divisions of the double quartz plate C (Fig. 2). The extent to which the screw has been turned, which corresponds to the right-handed rotation caused by the sugar solution, is now ascertained on reading the scale by the aid of the glass K. The instrument under consideration is so constructed that, when solutions and tubes of the concentration and length referred to above are used, the reading on the scale gives directly the percentage of pure crystallisable cane sugar contained in the sample examined. For instance, if the zero index of the fixed scale points to 96°·5 on the movable scale, after uniformity of colour has been obtained, the sample of sugar taken contains 96·5 per cent. of pure cane sugar. The results given by the polariscope possess an accuracy rarely, if ever, attained by any other apparatus employed in the determination of practical commercial values.[57]

The proportion of grape sugar intentionally added to cane sugar can also be determined by the use of the polariscope, certain modifications being observed in its application. As previously stated, cane sugar is converted into a mixture of dextrose and levulose, termed invert sugar, by the action of dilute acids. While the rotary effect of dextrose upon the plane of a ray of polarised light is constant at temperatures under 100°, that exerted by levulose varies, it being reduced as the temperature is increased; hence it follows that at a certain temperature the diminished levo-rotary power of the levulose will become neutralised by the dextro-rotary effect of the dextrose, i.e. the invert sugar will be optically inactive. This temperature has been found to approximate 90°. Since dextrose is not perceptibly affected by the action of weak acids, it is evident that by converting cane sugar into invert sugar and examining the product by the polariscope at a temperature of about 90°, the presence of any added dextrose (glucose) will be directly revealed by its dextro-rotary action. This is accomplished by a method suggested by Messrs. Chandler and Ricketts,[58] which consists in substituting for the ordinary observation tube of the polariscope a platinum tube, provided with a thermometer, and surrounded by a water-bath, which is heated to the desired temperature by a gas burner (Plate X. Fig. 4). The sugar solution to be examined is first treated with a little dilute sulphuric acid, then neutralised with sodium carbonate, clarified by means of basic plumbic acetate, filtered, and the polariscopic reading taken at a temperature of 86° to 90°.

Since the results given by the foregoing method represent pure dextrose, it is necessary to first ascertain the dextro-rotary power of the particular variety of glucose probably employed for the adulteration of the sugar under examination, and then make the requisite correction. This process for the estimation of glucose is especially advantageous, in that the optical effect of the invert sugar normally present in raw cane sugars is rendered inactive.

It is sometimes desirable to determine the relative proportions of the organic constituents which are present in commercial glucose. These usually consist of dextrose, maltose, and dextrine, all of which possess dextro-rotary power, but not in the same degree; that of dextrose being 52, that of maltose 139, and that of dextrine 193. An estimation of the amount of each can be made by first ascertaining the total rotary effect of the sample by means of the polariscope.[59] This is expressed by the equation

P = 52 d + 139 m + 193 d',

(1)

in which P is the total rotation observed. Upon now treating the solution of glucose with an excess of an alkaline solution of mercuric cyanide (prepared by dissolving 120 grammes of mercuric cyanide and 25 grammes of potassium hydroxide in 1 litre of water), the dextrose and maltose contained in the sample are decomposed, leaving the dextrine unaffected. A second polariscopic reading is then made, which gives the amount of dextrine present, that is

P' = 193 d',

(2)

from which the proportion of dextrine is calculated.

Subtracting the second equation from the first, we have

P - P' = 52 d + 139 m.

(3)

Both dextrose and maltose reduce Fehling’s solution, the total reduction (R) being the reducing per cent. of the former (d) added to that of the latter (m). The reducing power of maltose is, however, only 0·62 as compared with dextrine, therefore

R = d + 0·62 m.

(4)

Multiplying by 52, we have

52 R = 52 d + 32·24 m,

and subtract from (3), which gives

P - P' - 52 R = 106·76 m,

(5)

whence

m = P - P' - 52 R 106·76

(6)

d = R - 0·62 m

(7)

and

d' = P' 193.