Storage of Synthetic Citrus Powders

STORAGE OF

Synthetic Citrus Powders Caramelization by Citric or Tartaric Acid and Various Sugars at High Temperatures WILLIAM EDWYN ISAAC Government Low Temperature Research Laboratory, Cape Town, Union of South Africa

Lemonade and orangeade powders containing a high proportion of citric acid undergo caramelization when stored at 98" and 110' F. if they are packed in containers (bottles and cans) which prevent the escape of water vapor. Under these conditions citric acid loses water of crystallization which brings about the hydrolysis of sucrose. The liberated fructose is then caramelized. Caramelization occurs with sorbose, fructose, or any sugar liberating fructose on

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HE maintenance of troops in hot climates has brought to the fore many problems of food storage in the tropics. This problem arose in the consideration of artificial lemonade as a potential source of ascorbic acid in cases of vitamin C deficiency. The very small amounts of ascorbic acid lost on storing certain lemonade powders a t 90" F. (32" C.) for 6 weeks indicated that much longer storage periods a t this temperature were possible for lemonade powders containing a large amount of sugar, or containing tartaric acid and a smaller proportion of sugar. (In the latter type, more sugar is added when the lemonade is prepared for drinking.) Such lemonade powders not only retained nearly all the added ascorbic acid but were in good physical condition after 7 weeks a t 90" F. It was deemed advisable, however, to carry out tests a t higher temperatures on powders with a high ratio of acid to sugar, which would mean adding more sugar t o the lemonade or orangeade for drinking.

hydrolysis. Aldohexoses and sugars liberating aldohexoses on hydrolysis do not caramelize in the presence of citric acid. Caramelization in orangeade and lemon ade powders containing citric acid held at high temperatures can be prevented by using anhydrous citric acid or by substituting glucose for sucrose. Caramelization does not occur (in the absence of added water) if d-tartaric acid is substituted for citric acid.

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pale yellowish discolorations in mixtures containing tartaric acid represent slight caramelization. The results obtained from these preliminary tests indicated that a comparison of the storage behavior of powders containing citric acid with those containing tartaric acid was important. In the final series of tests, samples of the various powders were stored for 8 weeks a t 98" and 110" F. (37" and 43" C.) in cardboard boxes, glass bottles, and sealed cans. The boxes and cans were lined with Waxtex waxed paper. The storage rooms were dark, and had a rapid air circulation and low relative humidity. The relative humidity of the room a t 98" F. mas about 25 per cent and that a t 110" F., about 15 per cent. Tests were conducted in duplicate; the agreement was usually close, and the divergence was never important. The recipes for the various powders are given in Table I. No coloring matter was added. CARAMELIZATION BY CITRIC ACID

STORAGE CONDITIONS

The most outstanding result was the effectiveness of citric acid in producing caramelization. It was again found that if tartaric acid was used, caramelization did not take place although some of the powders were discolored. Lemonades L-8, L-2, L-3 and L-4 (Table 11) are arranged in order of increasing proportion of citric acid to sugar; the degree of caramelization increases with an increasing proportion of citric acid. This result, which was general throughout the tests, indicates the effectiveness of citric acid in producing caramelization. Caramelization was EFFECTOF STORAGE TEMPERATURE. affected also by storage temperature. There was much more caramelization a t 110" than at 98" F. (Table 11).

Preliminary tests were made on orangeade and lemonade powders stored in tins for a month a t 110" F. (43" C.). Some of these powders contained citric acid and others tartaric acid. Unless otherwise stated, citric acid containing water of crystallization was used. When removed from storage, the powders showed varying degrees of discoloration. caking, and hardening; some were dark brown, amorphous, and consolidated. The light broivn discolorations apparently represent stages in the changes resulting finally in the dark brown, hard, amorphous mass. The term "caramelization" will refer here only to the latter condition. The powders containing tartaric acid did not caramelize although some were discolored. There is no evidence to indicate whether or not the 470

April, 1943

INDUSTRIAL AND ENGINEERING CHEMISTRY

471

EFFECT OF ESSENTIAL OILS. The proportion of acid and TABLEI. RECIPESOF ARTIFICIALORANQEADE AXD LEMONADE sugar in the basic recipes of the orangeade and lemonade POWDERS" powders were different and thus only a limited number of ORANQEADE POWDERS WITH SUQAR IN FORM OF SUCROSE samples with different essential oils, but with approximately 0-1 Citric acid 4 5 . 5 grams the same ratio of acid to sugar, are available for comparison. Sucrose 136.6 grams Oil of bitter orange 1 . 4 cc. Table I11 indicates that the nature and amount of essential oil make little difference. Same as 0-1, plus oil of sweet orange 1 . 4 cc. 0-2 The result of the following experiment was in agreement 0 . 7 cc. Same as 0-6, plus oil of sweet orange 0-3 with this conclusion: Five per cent water was added to two 0 . 7 cc. Same as 0-1, plus oil of sweet orange 0-4 lots of powder L-4 (equal amounts of sucrose and citric acid, 1 . 4 cc. Same as 0 - 6 , plus oil of sweet orange 0-5 together with oil of lemon) and to two similar lots of powder 0-6 Citric acid bo. 8 grams without oil of lemon. These mixtures were put into glass (basic Sucrose 1 3 6 . 6 grams bottles with screw-on aluminum lids and kept for a week at 1 . 4 cc. recipe) Oil of bitter orange 130" F. (54" (2.). At the end of that time the mixtures with Citric acid 23 grams 0-7 136 grams Sucrose and without oil of lemon were all completely caramelized. Oil of bitter orange 1 . 4 CC. EFFECT OF TYPEOF PACKAGE. The degree of caramelizaTartaric acid 23 grams 0-8 tion was also affected by the type of container. Thick card136 grams Sucrose Oil of bitter orange 0 . 7 cc. board boxes lined with waxed paper were superior to bottles and cans; in only two cases did caramelisation occur in cardLEMONADE POWDERS L-1 Tartaric acid 136 grams ' board containers although there were discoloration and vary272 grams (basic Sugar (sucrose) ing degrees of caking and hardening, especially when the recipe) Oil of lemon 5 . 7 cc. powders contained citric acid. The two exceptions were L-2 to L-8 have the basic recipe of L-1 with the acid content varied, as powders 0-6 and 0-7 a t 110' F. which showed only slight follows: caramelization. Powders packed in bottles and especially Citric acid L-2 136 grams those in cans became caramelized even if other conditions Citric acid L-3 204 grams 272 grams Citric arid L-4 were favorable. L-5 272 grams Tartaric acid Tartaric acid 204 grams L-6 The somewhat greater degree of caramelization in lemon1,Tartaric acid .7 68 grams ade and orangeade powders packed in sealed cans may be Citric acid .L-8 68 grams related to a catalyzing of the reaction by the can itself. HowORANGEADE POWDERS WITH SUGAR I N FORM O F GLUCOSB ever, the difference is more likely to be due to the complete Tartaric acid *GO-1 45 grams prevention of water vapor loss from a sealed tin; also the G1ucose 136 grams Oil of bitter orange 1 . 4 cc. powder was wrapped in waxed paper and was thus not in .GO-2 Citric acid 45 grams direct contact with the can. Caramelization was only Glueose 272 grams slightly more apparent in cans than in bottles. Oil of bitter orange 1 . 4 cc. .GO-3

Citric acid Glucose Oil of bitter orange

90 grams 136 grams 1 . 4 cc.

$GO-4


90 grams 272 grams 1 . 4 cc.

GO-5


68 grams 136 grams 1 . 4 eo.

80-6


65 grams 272 grams 1 . 4 eo.

a Two ounces of basic recipes 0-6 pr L-1 together with l l ( a or.ll/n pounds .of su ar were made up to 6 pints with water. The essential oils were dis-

solve! in a small amount of alcohol and thoroughly mixed with the sugaracid mixture. The powders were spread out to dry before acking. The substitution of citric acid for tartaric gives a better drlnk gut about one and a half times (lemonade) to twice (orangeade) as much 'citric acid must be used, according t o tasting tests made in this laboratory.

MECHANISM OF CARAMELIZATION

Further experiments were carried out to determine the role of moisture, whether the action of citric acid was due to its acidic nature, and whether caramelization in sugar-acid mixtures was peculiar to sucrose or a derivative of sucrose. These experiments were carried out in duplicate with the exception of the xylose-citric acid, the sorbose-citric acid, and the raffinose-citric acid mixtures. IMPORTANCE OF MOISTURE. Five per cent distilled watei was added to powder L-3, and samples were stored a t 110" F. in the three types of container. At the end of a week all three powders were caramelized. Although caramelization was not so severe as that in the powders with lower moisture content stored for a longer period, it showed that carameliza-

TABLE 11. COMPARISON O F CITRIC AND TARTARIC ACIDSI N

PRODUCING CARAMELIZATION IN LEMONADE POWDERS (STORED I N GLASS CONTAINERS FOR 8 WEEKB)

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Acid Content of Powder

Powder

1/5

No. L-8

1/3

L-2

2/5

L-3

l/Z

L-4

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Powdera with Citric Acid Condition of powder

98' F.

110' F.

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Light brown: raked; Medium brown: caked; no caramelization slight caramelization Light medium brown, 5 0 4 6 % caramelized; hard compact lump: remainder, light some diffuse caramelibrown zation Same as L-2, but about Complete or nearly 20% caramelized complete caramelization Light medium brown; Completely caramelabout 50% caramelized ; very hard ized

-Powders Powder

No. L-7

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with Tartaric Acid Condition of powder 98' F. 110' F.

L-1

Very slight caked Same as L-7

yellow:

L-6

No discoloration:

Very pale yellow; slightly caked Pale yellow: some caking

Same as L-7

slightly caked

L-5

Same as L-6

Same as L-1


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TABLE 111. EFFECT O F ESSENTIAL OILS Type of Container Glass Tin Glass 4

Temperature, O F.

98 98 110

Lemonade Powder L-3 (5.7 cc. lemon oil in 476 grams) Light medium brown; 20% caramelization About 307, caramelization Approx. complete cnrameliaation

ON CARAMELIZATION O F S U G a R C I T R I C

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0-3 (r.4 cc. bitter 0.7 00. sweet oil) Light medium brown;

+

about 20% caramelization Yellowish brown: diffuse caramelization 80-100% caiamelized

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ACIDL f I X T U R E

Orangeade PowdersQ, 227 Grams 0-5 (1.4 cc. of bitter 1.4 cc. sweet oil)

+

7

0-6 (1.4 cc. bitter oil)

Medium brown; about 25Y0 caramelization

25-60% caramelization

Medium brown; about 50% caramelization lOOy0 caramelized

Medium brown; about. 66% caramelized 50-10070 caramelized

Products containing orange oil as likely to discolor even when there is no caramelization.

That citric acid brings about caramelization under conditions where tartaric acid does not is related to the fact that citric acid contains water of crystallization while ordinary Sugar + 0 5 7 Sugar + 1% d-tartaric acid is anhydrous, The water of crystallization of Type of Sugar Added Water" Added Mater citric acid supplied a t least part of the moisture necessary for Container 9 8 O F. llOo F. 98" F. 110' F. 9 8 O F. llOo F. caramelization at 98' and 110" F., as indicated by the efflor1.9(5) 2.1 2.4 Cardboardbox 1.9 2.4 2.9 0.74 0.76 0.85 Glass bottle 0.6 1.2 0.8 escence of citric acid crystals during the discoloration of the powders which occurred before obvious caramelization had taken place. It was also shown that samples of the citric acid used gave off water within a day when enclosed in dry tion will take place in each type of container if enough water stoppered test tubes incubated a t 104' F. (40" C). is present. To test the theory that caramelization in citric acid-sucrose It was also shown that moistened sugar lost more water mixtures stored a t high temperatures depended upon the from the cardboard than from the glass containers under the liberation of water of crystallization, a mixture of sucrose and conditions of rapid air circulation and low relative humidity anhydrous citric acid containing 40 per cent citric acid (proin the storage rooms (Table IV). portions of powder L-3) was stored in bottles a t 110" F. for These two experiments indicate that insufficient moisture a month. For comparison, a similar bottled mixture conaccumulated in the cardboard containers to permit the initiataining ordinary crystallized citric acid (control) was stored. tion of caramelization. After 2 weeks the control showed extensive caramelization This result was conclusively shown later by the fact that while the mixture containing anhydrous citric acid showed lemonade and orangeade powders packed in cardboard boxes nbne. At the end of a month the control was almost comcompletely caramelized when kept for about 6 months under pletely caramelized, but the anhydrous citric acid-sucrose ordinary room conditions in a damp suburb of Cape Town. mixture still showed no trace of caramelization. The same powders had not caramelized in storage at 98" F. TESTS WITH GLUCOSE.A set of orangeade powders was TYPEOF ACID. Mixprepared in which glucose monohydrate was tures were made of equal substituted for sucrose. parts of citric acid and The proportion of citric sucrose and of tartaric acid present was sufacid and sucrose. To ficient to bring about each of the mixtures was caramelization if sucrose added 5 per cent water. h a d b e e n u s e d . The They were then put into powders were stored a t glass bottles with screw110" F. in sealed cans on metal lids and kept for 8 weeks; a t the end at 130" F. for a week. of that time there was no At the end of this caramelization although period b o t h sets had in two (of six) lots there undergone caramelizawas slight discoloration tion. The citric acid(Table V). These sucrose mixture had alpowders were kept for most completely liquea further period of about fied and was dark brown 6 months a t room tem(almost black) in color. perature in a damp subThe liquefied part reurb of Cape Town. No sembled a thick, dark caramelization took colored molasses. Caraplace although the cans melization was less comhad been opened so that plete in t h e t a r t a r i c the powders were only acid-sucrose mixture: it partly covered by the h a d become a h a r d , Figure 1. Rlixtures of Equal Parts of Citric Acid and lids. glassy, solid mass of Different Sugars with 5 Per Cent Water after 4-7 Day medium dark brown Storage a t 130" F. Anhydrous glucose A . Xylose C. Galactose F. Lactose was also tested. The color. interspersed with D. Glucose G. Raffinose B. Fructose following mixtures were small whitish granules. E. Sucrose SUGAR TABLE IV. PERCENT WATERLoss FROM MOISTENED STORED A WEEKIN DIFFERENT CONTAINERS


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473

Sucrose hydrolyzes to glucose and fructose. The sample of sucrose used did not reduce Fehling solution and, unless subjected to prolonged boiling, did not give an osazone with Acid Orangeades with Glucose Orangeades with Sucrose phenylhydrazine and acetic acid. The stored Powder Content Powder Condition of powder No. Condition of powder ofiPowder No. citric acid-sucrose mixture gave a copious precipitate of cuprous oxide with Fehling solution Light medium brown; 0-7 Unaffected 1/7 GO-2 and an abundant osazone precipitate with caked and hard ...... Similar t o GO-2 1/5 GO-6 phenylhydrazine and acetic acid. Glucosazone could be distinguished under the microscope, 0-1 Light brown; caked; White; somewhat more 114 GO4 Maltose gives glucose on hydrolysis. The slight diffuse caracohesive than GO-2 sample of maltose used did not reduce a fresh melization Similar t o 0-1 0-2 solution of Barfoed reagent. After one week Light brown; caked; 0-4 a t 130" F. an additional 4 per cent of water about 33% carawas added to the maltose-citric acid mixture. At melized the end of the second week the mixturewas tested Pale yellowish brown; 1/3 GO-5 for glucose. There was some tardy reduction of decidedly cohesive .... a fresh Barfoed solution which indicated that a Medium brown; about 0 6 Similar to GO-5 2/5 GO-3 certain amount of hydrolysis had taken place. 66% caramelized Medium brown; about 0-6 Lactose hydrolyzes to glucose and galactose, 50% caramelized Barfoed reagent is not reduced by lactose but is Light yellowish brown; 0-3 reduced by glucose and galactose. Neither the caked: diffuse caraoriginal sample of lactose nor the stored lacmelization tose-citric acid mivture reduced Barfoed reagent which was, however, shown to be reduced bv a glucose-citric acid mixture. With weak acid, raffinose hydrolyzes to fructose and meliput into bottles with aluminum lids and stored for a month biose. In the presence of strong acid, melibiose breaks down a t 110' F.: further to give glucose and galactose. Fbffinose does not reduce Fehling solution and does not form an osazone with 1. Equal amounts of citric acid and sucrose (control) 2. Equal amounts of citric acid and anhydrous glucose phenylhydrazine, Hydrolyzed raffinose will give positive 3. Equal amounts of anhydrous citric acid and anhydrous results for both of these reactions; the sample of raffinose gIucose used here did not give these reactions. The stored raffinosecitric acid mixture reduced Fehling solution and yielded Before the month was out, the control was completely an osazone precipitate. Microscopic examination showed caramelized; the other two mixtures were unaffected a t the the presence of more than one type of osazone, but crystals end of the month. Mixture 3, containing only anhydrous of the glucosazone type were distinguished. constituents, was still a fine powder; mixture 2, containing STORAGE OF SUGARS AT HIGHTEMPERATURES. Samples ordinary citric acid, was compacted but not caramelized. of glucose, fructose, sucrose, and lactose were stored for 22 From this behavior and the obvious moisture on the sides of weeks a t 98" and 110" F. in glass bottles with screw-on metal the bottles, it was clear that the citric acid had given off lids. With the exception of fructose, they were in excellent water of crystallization. condition a t the end of the period, and were neither discolored This test also showed that compacting in itself is not nor caked. At 98" F. fructose had darkened from pale yellownecessarily an indication of the beginning of caramelization. ish white to straw color but showed little caking; a t 110" F. TESTSWITH OTHER SUGARS. Mixtures were made of fructose was decidedly darker in color than a t 98' F.;it was equal portions of citric acid and the sugar to be tested. Five light yellowish brown and caked, and small isolated drops of a per cent water was added to each mixture, and they were brown sirupy liquid had appeared on the sides of the bottle. kept a t 130" F. for 4-7 days. The results are given in Table SUMMARY AND CONCLUSIONS VI and Figure 1. A mixture of sucrose and citric acid containing sufficient moisture caramelizes rapidly a t high storage temperatures. Under these conditions the water of crystallization of citric T A B LVI. ~ EPFECTOF CITRICACID IN PRODUCING acid is a source of moisture. The higher the proportion of CARAMIDLIZATION OF SUGARS citric acid present, the greater the amount of caramelization R~SULT SUGAR in a given time due to the water of crystallization of the citric acid. Pentose Pale yellowish color Xylose If the water of crystallization given off by citric acid is Monosaccharides allowed to escape or if anhydrous citric acid is used, no caramelization takes place. Completely caramelized; dark The first stage in the caramelization of a sucrose-citric reddish brown sirup Aldohexoses acid mixture is the hydrolysis of sucrose. Caramelization Pale yellow Glucose results from the liberation of fructose, since a fructose-citric No discoloration Galactose acid mixture undergoes complete caramelization whereas a Disaccharides glucose-citric acid mixture does not caramelize. Suorose Almost completely liquefied. A dark brown (almost black) color Ordinary &tartaric acid is anhydrous, but in the presence Maltose of added moisture this acid also brings about inversion of No caramelization Lactose sucrose followed by caramelization. 'Trisaccharide Caramelization results only from the interaction of citric Raffinose Medium dark brown sirup acid with the ketohexoses fructose and sorbose and with those ~

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TABLE V. EFFECT OF GLUCOSE ON KEEPING QUALITY OF ORANQEADE POWDERS CONTAINING CITRICACID (PACKED IN CANSAT 98' F. FOR 8 WEEKS)

......

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414

sugars which yield fructose on hydrolysis (sucrose and raffinose). This result suggests that caramelization is a property of the ketohexoses and of sugars which yield ketohexoses on hydrolysis. Neither of the two aldohexoses tested (glucose and galactose) nor the aldopentose xylose caramelized in the presence of citric acid. This was also true of the disaccharides (maltose and lactose) which yield aldohexoses on hydrolysis. Fructose is susceptible to discoloration on heating, especially in acid solution. Dehydration takes place and results in the formation of compounds (hydroxymethylfurfural, hymatomelanic acid) m-hich readily undergo polymerization to form dark colored substances ( 1 ) . Even in the absence of citric acid, fructose darkens when stored a t high temperatures in the dark; the higher the temperature, the greater the amount of darkening. It thus seems that in artificial lemonade and orangeade powders containing a high proportion of citric acid and stored a t high temperatures, the citric acid gives off water of crystallization which, if not allowed t o escape, brings about hydrolysis of sucrose to glucose and fructose. Then the fructose undergoes caramelization which, once started, liberates water for further inversion and caramelization. The higher

Vol. 35, No. 4

the storage temperature and the greater the proportion of citric acid, the greater the amount of available water of crystallization and, consequently, the gieater the amount of caramelization during a given storage p.eriod, provided the moisture does not escape from the contniner. An artificial lemonade or orangeade powder intended to be kept under tropical conditions should contain a greater proportion or all of the sugar t o be ultimately added. If a powder of smaller bulk is required, either glucose (preferably anhydrous) can be substituted for sucrose or tartaric acid can be used in place of citric, or both of these substitutions can be made. Powders should be dry before packing. If glucose is substituted for sucrose, allowance should be made for its being less sweet. Its relative sweetness has been variously evaluated from 50 to 73.4 per cent of that of sucrose ( 2 ) . Citric acid gives a pleasanter drink than tartaric acid, but about one and a half times to twice as much citric acid is needed. LITERATURE CITED

(1) Joslyn, M. A,, IND.ENG.CHEM., 33,305-14 (1941). (2) Low, B., “Experimental Cookery”, 2nd ed., New York, John Wiley & Sons, 1937.

Effect of Acetylation on Water-

Binding Properties of Cellulose RAG STOCK JOHN C. BLETZINGER Sterling Pulp and P a p e r Company, E a u Claire, \Tis.

HE pronounced affinity of cellulose for water-i. e., its hygroscopicity and ability to smell-is chiefly attributed t o its abundance of free hydroxyl groups. If part or all of these groups are replaced by ester groups, hygroscopicity and swelling ability decrease. For example, Sheppard (16) found that the w-ater-binding capacity of cellulose acetate decreased with increasing acetyl content and that a fibrous triacetate took up only 9 per cent water compared with 16 per cent for the unacetylated cotton fiber. The forces by which the water molecules are held to the hydroxyl groups of the cellulose have been ascribed t o secondary valences in the older literature (10, 21), whereas today they are believed to be hydrogen bonds (7). No doubt the same forces are operative when cellulose fibers are beaten in the presence of water, as a step preparatory to the formation of a sheet of paper. As beating progresses, the water-binding capacity of the cut, bruised, and fibrillated fibers increases, evidently due to a considerable increase of their external surface. As a result, an increasing number of free hydroxyl groups are made accessible to wateri. e., t o “hydration”, as this phenomenon is known to the paper manufacturer. Hydration, combined with the mechanical disintegration exerted by the beating device, develops

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the physical strength properties inherent in the cellulosic material so treated. After a certain time of beating, depending upon the nature of the material and the efficiency of the beater, the fibers acquire their maximum strength properties. When the sheet is formed on the paper machine or a handsheet-making device by the mechanical removal of the water and subsequent drying, strong forces of surface tension are developed between the interfaces of hydrated adjacent cellulose fibers and fibrils, by which their surfaces are drawn more closely together. As this process continues, the hydroxyl groups release more and more of their water molecules and eventually approach one another so closely that the secondary valences (or hydrogen bonds) satisfy one another directly, and bonding between fiber and fibrillae surfaces is accomplished (2, 21). I n the light of Sheppard’s results, it is conceivable that both hydration and bonding of the fibers would be seriously impeded if part or all of the free hydroxyl groups in cellulose were replaced by hydrophobic ester groups. A direct prodf could thus be furnished for the assumption that the water binding of cellulose fibers during beating and the bonding of fiber surfaces during sheet formation are a function of their

Storage of Synthetic Citrus Powders

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