178 BEET-SUGAR TECHNOLOGY

JUICE PURIFICATION. FUNDAMENTAL CHEMISTRY 179 of the theoretical baaes of juice purification _as is possible at present, There are very few papers in the literature of value in this respect, which are directly connected. with sugar-beet juices. Most of such information must be sought in publications which normally do not deal with beet-sugar prob�

lems. This makes it difficult to draw valid conclusions, and many of those submitted here will surely have to be corrected in the light of new, mOTe direct experiences.

It is proposed to first give a short description of the more important sub­

stances involved in juice purification, and then to sketch the mechanism of the elementary operations into which beet-juice purification with lime and carbonic acid can properly be separated. Finally, some of the more important conditions will be given which are' believed to be necessary if certain results, such as rapid filtrations and Jow colors are to be achieved.

The limited space, however, does not permit either a thorough treatment of the different subjects, Or mention of all the influences of the beets, soil, climate, conditions of juice extraction, etc., on juice purification. The literature cited is also far from complete, and does not give an accurate accounting of the relative importance of the work of the various authors.

IMPORTANT SUBSTANCES IN JUICE PURIFICATION Water. Water comprises 80 to 85 percent of the weight of the beets and the juices, and can thus properly be considered as the solvent, repre­

senting 44 to 47 moles of H20 per kg of juice. Its simple and small molecule with large dipole moroent, high dielectric constant, and the possibility of ionizing both as an acid,

H20 "", H+ + OH- ""' 2H+ + 0- (1) and base,

H20 + H+ "'" H,O+ (2)

make it an excellent solvent, facilitating the hydration by secondary valences and the electrical dissociation of the solvents. It has a very marked tendency to form cybotactic structures, or uniform molecular patterns in long chains, which may serve to "fill up" even very fragile large molecules, making possible the existence of solutions of colloidal substances, such as native proteins and pectins.

Its own dissociation constant _is rather low, and very

mark�y

^affected by te'mperature :

pKH,o � pOH + _pH � 14.92 at O°C., 13.25 at 5I)°C., and 12.24 at 100°C.

It is for this reason that the pH, er pK - pORI is not a good measure of the pOR at higher temperatures: for a given pHI the pOR is �m..i;l,ller,. the higher the temperature.

180 BEET.SUGAR TECHNOLOGY Water is a sufficiently strong acid,

(F) (011)

� 10-"." at 25°0.

(H,O)

(3)

to influence the dissociation of weak acids and bases, as they are found In beet juices, and "it must therefore always be considered in the equilibria est.ablished.

'5

� �� __ L-� __

�����

^__ ������

TEMPERATURE. °C Figure 7-1. pL�o and temperature.

The great excess of water in juices enables it to take part easily in

bydra­

tion :reactions, as for example,

H,O + sucrose ....

�vert

^{sugar, (4)}

H,o + RCONH, ... ROOOH + NH.. (5) H,o + CO, ... H,cO.,

H,O + CaO ... Ca(OH)"

some of which are very important.

(6) (7)

� Sucrose. Sncroee is rather _unstable, chemically, being easily hydrated, especi&\Jy in acid or alk&line solutions, to glucose and fructoes, which are even more reactive and decompose into acids and colored

producte .

. It is

i

mportant thst in the

preeence

of heat and

high

o.Ikalinity it "Can foDn oxaJic acid in ths evaporators or vl.cuum pans,

and thus result

ⁱⁿ

cal­

� """,",te scale.'

"

JUICE PURIFICATION. FUNDAMENTAL CHEMISTRY 181 The great number of hydrophilic OR groups it contains make it easily soluble in water, though its molecular weight,' 342, is rather high. These hydroxyl groups can be esterified, and, &8 examples, may form tbe so-caJIed sucrocarbonates during the ga.'lsing of limed juices.

The hydroxyl groups are further able to dissociate ionicaJIy, so that sucrose will behave 88 a. polybasic acid in alkaline solutions,· and can fonn different salts, or saccharates.

The pK. is 12.7 at 25'0., 12.3 at 40'0., and-by an extrapolation of doubtful validity-11.6 at 80'0. pK, is 13.1 at 25'0.

OH

Figure 7-2. Structural COnDgur&tiOD. of sucrose, �ucopyranoByl�-fructofuranoside.

Glucose

and

Fructose.

These monosaccbarides are chem.icaJly more reactive than sucrose, and thus frequently undergo further destruction BO rapidly that they can be found only in traces, even if large amounts of su­

crose have been inverted. They contain 00 and OOR groups which are easily oxidized; hence the term, reducing sugar •. In warm alkaline solution they split rapidly into a great number of acids and colored decomposition products. One molecule of glucose or fructose usuaJIy gives two molecules of acids; thus one molecule of BUCl'08<j yields four molecules of acids, and can thus form two equivaJants of lim� salts. The decomposition products in aJkaline solution are more highly colored at lower a.1kalinities; even traces of invert sugar which decompose in second carbonation juices can ca.use 8.

considerable increase in oolor. The most desirable destruction of invert sugar ooming from

the

heet i. thus done by heeting a raw juice. which

bee

been limed at low temperatures. Figure 7..g summarizes the results ... •

The carbonyl groups of glucose and fructose are able to react with the amino groups of amino acids, tbiough M&illard's' or the browning ....

, tions, resulting in

highly-colored products

and're1ease of 00. sometimes noted in foaming of low-purity materi&ls.

Raffinose. Thie sngar is chemically ,...ther unreactive, snd seems to pass through an the prooessing operations snd into the molasses. It is strQngly dextrorotstory, and its presence may result in mJsleading apparent-purity

182 BEET-SUGAR TECHNOLOGY

determinations. Its presence likewise influences the shape of sucrose crystals.

Acids with Soluble Lime Salts.

Acids which give easily soluble lime salts are, from the point of view of juice purification, of lesser importance.

They increase the ionic strength of juices, thus influencing the dissociation constants of such substances as carbonic acid. If present as free acids, they give rise to the formation of lime salts. Their pK, if low, is also re­

sponsible for less efficient deljrning in second carbonation.s Oxyacids are able to form complex calcium salts, and may thus also increase the lime

'"

9

o

u

0.05 Q,.[

ALKAUN ITY % C.O 0.2

Figure 7 �3. Calor produced by destruction 'of invert sugar. The influence of alkalinity.

(VaSatko-Kasjanov, 1938).

content of carbonated juices. In alkaline, limed and carbonated juices, they act as buffers, affe'cting the pH-alkalinity relation of the juices. They are, through their cations, strongly melassigenic,9.

la

and make it difficult to produee high-quality white sugars directly from beet juices, because of high ash content and hygroscopicity.

Acids with Insoluble Lime Salts.

Examples of these acids are sul­

furic, phosphoric, oxalic, tartaric, and citric. These are, in the form of po­

tassium and sodium salts, very important as source _of the so-called "natural . alkalinity," formed by the schematic reaction,

2Na+ + An-+ Ca++ ₊ 20H-₌ CaAn + 2Na+ + 20H- (8) where An-is the anion of such an acid and CaAn its insoluble lime salt.

The NaOH or KOH so formed is the natural alkalinity, and plays a very

JUIOE PURIFIOATION. FUNDAMENTAL OHEMISTRY 183 important role in the precipitation of Ca from soluble lime salts in the second carbonation.

The natural aJkalinity has, however, an unfavorable effect on the pre4 liming, or precipitation of proteins, as it creates a high pH even at a. low calcium concentration. Thls therefore necessitates a high final alkalinity in first carbonation.

Amino Acids.

These acids, which are chielly glutamic and aspartic, seem to have a series of undesirable efiects. In the preliming, they peptize the precipitate of proteins with lime, and render it less complete.ll

They exist normally in solution as ampholytes. There is, for each one, a certain pH region-near 3 .0 for aspartic and 3.25 for glutamic acid-in which the ba.sic group ·e.nd only one of the acid groups are ionized, so that the total positive and negative charges are equal. In this isoelectric state, the amphoions behave as if un-ionized, and are generally least soluble.

Amino acids react with Ca.-H- to form complex catioos, (CaAn+),12· 11 thus increasing the total amount of lime in the juices. Carbonic acid is rea.dily bound by them in the form of carbaminatesu which are changed into car­

bona.tes relatively slowly. They act as very efficient buffers, and lower the pH of second carbonation juices, which meahs also a lowering of the (COi)' and therefore a less efficient deliIning. Dark colored products ms.y be formed, through reaction of their amino groups with the CO of the re­

ducing sugars according to Maillard's reaction.' They are melassigenic, either by themselves, or through their cations. Deterrninations of appa.rent purity are made inaccurate because of their optical activity, and through changes in optical activity they may simulate sucrose losses.

Amides.

These include glutamine and asparagine. Their CONH, group is easily sa.ponified, giving ammonia

sa.1J;s,

which, in turn, give off free NH.

through alkalinity, heat and boiling. This, in turn, means liberation of free acid, which can form lime salts, cause a loss in aJka.linity, and even bring about inversion of sucrose and corrosion.1IIi

The lOBS of alkalinity may prove advantageous by protecting the evapora­

tors from calcium carbonate scale formation," and in facilitating the boiling and crystallizing of low purity materials.

Proteins and Peptides.

Proteins form colloidal solutions, whieh in the native state, are rather stable. They ma.y, however, be denatured:

that is, lose their regular space arrangement by extreme conditions of pH, heat, and mechanicSl treetment. SOlutions of such proteins are f

�

\ativelY unstable; and can precipitate.

Proteins are polyelectrolytes, having a large number of acidic and ba.sic groups of very different pK. Their state of ionization depends on the pH of the solution. They have, like amino acids, a certain region of pH where their total positive and negative charge is zero, and in this isoelectric state