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Vol. 20, No. 12
Chemistry and the Preserve, or Jam and Jelly Industry C. P. Lathrop NATIONAL PRESERVERS ASSOCIATION, WASHINGTON, D. C.
HEMISTRY in the preserving industry is not confined to the finished preserves, jams, and jellies, but is applied more fundamentally to the fruits employed and their capacity for transformation into products of the desired commercial character. Distinctive delicate fresh-fruit flavors, bright natural fruit colors, and firm consistency are present in the finished products only when the fruits used possessed them. The chemistry of preserving involves the procuring of fruits when they are richest in these natural ingredients and their protection and preservation through all the commercial processes required to convert them into preserves and jellies. All other chemical substances, both in the fruit and in the accessory raw materials, are of value only as they serve as a food, and either t o protect and accentuate the fruit flavor, color, and appearance or to provide a convenient form for the consumer’s use. These chemical substances include the sugars, acids, pectin, water, cellulose cell structure, salts, proteins, and waxes. Detrimental contaminations, as heavy metals, spray residues, chemicals from wash waters, synthetic colors and flavors, chemical preservatives, decomposition resulting from yeast, mold, or fruit enzymic activities, chemical filter aids, and yeasts producing diastatic enzymes are definitely a part of the preserve, jam, and jelly industry. Yet the chemistry of the fruits surpasses the chemistry of all the other elements of the preserving industry in complexity and in sensitiveness to slight changes and to sudden great alterations in the nature and quantity of the chemical ingredients. Furthermore, the relatively high cost and the difficulty of keeping certain chemical compounds in the fruit from destruction make fruit chemistry of major importance.
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Chemistry of the Fruit
Information on the isolation, purification, identification, and structural formulas of fruit flavors and colors is fragmentary. Aside from a few researches on one or two varieties of a very few kinds of fruits, little is known. Among these researches are the classic one of Power and Chesnut on the chemicals constituting the flavors of the apple and peach, that of Anderson and his co-workers, on the anthrocyan pigments of the Concord grape, and the quantitative determination by Wilson and Sale of the esters of anthranilic acid in several varieties of grapes. Even if these flavors and colors were known in terms of definite chemical compounds, owing to their unstable natures and the difficulties and probably high cost of synthetic production, they no doubt would continue to be supplied exclusively as a product of Kature. The preserving chemist, in consequence, has largely only a curiosity interest in knowing what these chemical compounds are, just as the electrician is little concerned with what electricity is. Both, however, are tremendously concerned with what they will do and how they will act under all environmental conditions to which they may be subjected. Each kind of fruit is distinctive not only in anatomical structure and appearance, but in several of its chemical compounds. Each is inherently distinctive in its chemical producing abilities as living organisms. Chemical composition is modified by environment. The flavors, the colors, and even the acids in some fruits are chemically different from those in others. Nelson, in determining the nonvolatile acids in many fruits, has shown isocitric acid to be peculiar to the
blackberry, tartaric acid to the grape, citric acid to several kinds of fruits, and I-malic acid to others. All these variations in kind, quantity, and proportion of the fruit acids have a fundamental influence on the chemistry of preserve and jelly-making, as demonstrated by Tarr’s hydrogen-ion studies on the setting of pectin. They also exert different physiological effects on taste and smell, thus influencing the apparent character and the intensity of the accompanying natural fruit flavor, although actually the acids do not add to, alter, or destroy the natural flavors. These influences of the acids are determined by their proportions to the total sugars employed and the freedom of the latter from impurities that affect flavor. The salts of some fruits, like strawberries and black raspberries, inhibit the setting of pectin, due to their buffer action on the hydrogen-ion concentration. All fruit juices retard the setting action of pectin. This is largely attributed to the fruit salts, partly as salts of the common acid ion. Other substances also influence the hydrogen-ion concentration. Levulose furnished by the United States Bureau of Standards was found by our laboratory t o retard markedly the rate of setting of pectin. This, however, may have been due to a trace of impurities. In the preserving industry each kind of fruit, owing to its chemical peculiarities, is a distinct problem in itself. The stemming of grapes prior to heating for juice extraction is necessary to get rid of the astringent tannin of the stems that is detrimental to the flavor, As with other fruits, this heating is required also to burst and soften the cell tissues to make a more complete extraction of the flavor, color, sugars, acids, etc., and to coagulate and precipitate the protein and gummy colloidal substances that interfere with the juice clarification. Supersaturation with cream of tartar of grapes and freshly extracted grape juice, and its normally slow partial precipitation by crystallization in the stored juice or jelly, require low refrigeration temperatures for the juice to hasten the process from several months t o a few days. Apples possess starch, which in cooking becomes changed to the soluble colloidal form that makes juice and jelly cloudy. This haze is removed from apple juice by the use of diastatic enzymes that transform the starch to soluble dextrins and dextrose. Currants generally are excessively high in acid and cause “weeping,” unless special attention is taken to keep the solids low and the texture not too firm. The high acidity masks part of the currant flavor. Chemically this high acidity cannot be sufficiently reduced without injuriously altering the hydrogen-ion concentration through the effect of the salt of the common ion formed. Breeding new varieties of currants lower in acidity must be undertaken to solve the difficulty. The valuable characters which lead t o the selection of certain commercial varieties of one fruit may cause trouble in others. For example, the firm texture of the cherry skin and of the peach and blackberry flesh so retards the penetration of the sugar in cooking for preserves as to make it difficult to promptly equalize the specific gravity of the fruit with the surrounding sirup, to prevent shriveling by plasmolysis or floating in the warm sirup when poured into retail containers. Packing pitted cherries with sugar and storing them for a few months under refrigeration obviates part of the difficulty due to the slow osmotic equilibrium with the sugar during the interval. An important advantage also is the superior
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preservation of the flavor and color. Peaches and black- after season, that the selection of one variety over another berries are softened by a preliminary short boil with water. for its flavor, color, firmness, or other feature is one of the Strawberries are usually tender and likely to break in cooking. quickest and surest ways for the betterment of preserves and Cold packing in sugar firms them, aids in the prevention of jellies. This selection can be made only after an exhaustive their floating in the preserves a t filling and pasteurization series of experiments over a period of years. The ultimate temperatures, and preserves their delicate, unstable flavors choice is based on the quantity or proportions of certain and colors better than any other known treatment. Also, chemical ingredients, some of which may not even be known. unless firm fruits are carefully cooked, although the outer The United States Department of Agriculture and several portion, the surrounding sirup, and the inner surfaces of the state agricultural experiment stations are conducting excontainer in which they are hermetically sealed may be abso- tensive breeding experiments in an attempt t o create new lutely sterile, the cell enzymes in the inner portions may not varieties of fruits that mill be more valuable commercially be killed. The inner cells of the fruit soon break down and in one or more particulars and not less desirable in others. the liberated enzymes start a destructive fermentation, pro- The new variety can then be asexually reproduced indefinitely, ducing flavors and products foreign to a normal fruit preserve. t o supply the commercial needs for a better-quality fruit. In The exterior of the preserve appears normal. terms of chemistry, this may be more pectin, more acid, more The flavors and colors of some kinds of fruit are more sugar, or more fruit salts, or a different proportion of one t o unstable and more easily damaged than those of others. All another, or the combination of chemicals that produce flavor are readily injured by holding a t high temperatures, particu- or color may be changed to give a slightly different effect. larly if exposed t o the oxygen of the air, the usual condition. Our laboratory has selected the United States Department of Not only has each kind of fruit its own chemical and physi- Agriculture strawberry No. 659 and the purple raspberry Nos. cal characteristics, but great variations occur in the quantity 161 and 326, and the Young dewberry as being superior to and proportions of the chemical constituents in the same kind present commercial varieties for preserving purposes. The fruit plant is the chemical factory for the manufacture of fruit. These variations are due to different stages of maturity, to environmental differences of soil and climate, to the of the chemical compounds largely used in the preserving character of the season in different years, and to inherent dif- industry. Breeding improved varieties of fruits is similar to the development of a new machine or a new process in the ferences in varieties. Fruits produce their maximum flavor and color just after chemical factory, to put out a new or cleaner product or to the hard ripe stage, during the softening and mellowing period. give a higher yield. Even fruit of the most desirable variety, taken a t its best This softening comes from the solution of the pectin between the cell walls by the cell enzymes. The enzymes then pro- development, may undergo many chemical changes, largely gressively decompose and destroy such pectin in solution. detrimental, between the time of harvest and the time of Thus the increasing development and intensity of fruit utilization in the preserve or jelly factory. flavors and colors is accompanied by a steady reduction in Chemistry of Preserving Processes pectin. The modern chemistry of the preserving industry is to secure fruits valuable primarily for their flavor, color, texDecomposition of the chemical constituents of fruit is due ture, and appearance a t the full ripe stage, when such are to several causes. The destructive agencies are the fruit present in maximum quantity and quality, and then supply enzymes, liberated by the cutting or bruising of the tissues the natural pectin deficiency thus created with pectin from and cells, the fungus diseases, molds, yeasts, and bacteria fruits rich in it and picked in the earlier stages of ripeness with which the fruit becomes infected, oxidation of the colors when the pectin is a t its maximum and while the undesirable and flavors by constant contact with the oxygen of the air interfering flavor of the high-pectin bearing fruit has not been with or without the aid of enzymes or other agencies, action fully developed. There is also a loss of fruit acids during the of metals, preservatives, or other contaminating substances, later stages of ripening. If this loss constitutes an acid and the destructive metabolism of the fruit itself, which condeficiency for the best preserving requirements of that fruit, tinues until the tissues and enzymes are killed by the temit is supplied in purified form from the fruits rich in them, peratures of cooking or by the exhaustion of the food malike the lemon, grape, and Japanese quince. Some of this terials in the fruit. In the normal metabolism process the loss in fruit acid may be more apparent than real, as the chemical compounds undergo a series of changes, with the usual increase in sugar content during ripening may offset evolution of heat and loss of fruit weight. The rate of these some of the otherwise acid taste. chemical reactions is markedly influenced by temperature. Variations in rainfall, sunlight, and temperature in different Consequently, mechanical refrigeration a t low temperatures sections, and in the same section in different years, exert such is, in large measure, employed to prolong during transporan influence upon the chemical composition and character of tation and storage the original high qualities of the fruit. fruits as to require great care in both their selection and At the same time refrigeration retards the destructive reactreatment. Rainy seasons, in addition to their conditions tions on the chemicals in the fruits by the other agencies favorable for destructive fungus attacks on the fruits, have a mentioned. softening effect on fruit texture, with an accompanying lowerThe methods of harvesting. packing, shipping, and storing ing of pectin content. Kitrogenous fertilizers with straw- vary with nearly every kind of fruit. These differences in berries, unless very sparingly and intelligently applied, treatment have been evolved because one or more chemical usually cause a softening in texture and reduced flavor and constituents are particularly susceptible to injury from one color, particularly pronounced in a wet year. or more of the destructive agencies. I n general, the thinner Variations in quantity and proportion of chemical ingre- the cell walls the more rapid the destructive metabolism dients not due to environmental differences are encountered changes, the more perishable the fruit, the smaller the indiamong fruits of all kinds. These variations are inherited vidual shipping containers required, the more urgent the and asexually reproduced without deviation from the original need for prompt low refrigeration temperatures, and the parent plant. Each variety, wherever grown, has a charac- greater the frequency of bruising. Peaches and apricots teristic appearance, flavor, taste, texture, etc., due t o the are so readily discolored by their own oxidative enzymes a t kinds, proportions, and quantities of some of its chemical in- all cut surfaces that their storage under refrigeration, even gredients. So surely dependable are these differences, season with sugar, prior to canning is impractical. Canning of
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strawberries, on the other hand, is so damaging to color, flavors, and texture, that refrigeration with sugar is universally employed. Similar special treatments could be given for each kind of fruit. Iron and tin are particularly injurious to the fruit pigments. The compounds of the pigments with the metals become quickly oxidized on exposure to air, with a marked increase in the intensity of the discoloration. Tin turns grapes, cherries, raspberries, and blackberries a deep brilliant purple and strawberries a pale red. Iron produces dull brownish discolorations. Copper and aluminum are far less injurious and are therefore used wherever fruit must come in contact with metal. Copper being somewhat injurious to the color of grape juice, aluminum containers are generally used in making grape products. Storage containers for fruits and fruit juices are therefore usually of wood or glass, although double lacquer enamel tinned cans and earthenware are employed, the former to an increasing degree for cold-pack fruits in 3 to 5 gallon cans. Wooden containers are lined with paraffi to prevent juice absorption. The size of the container involves chemistry so far as the conduction of heat from the fruit is retarded. The 50gallon barrel commonly employed so retains heat that the respiration processes of the fruit proceed a t such a rapidly increasing rate and rising temperature towards the center of the barrel that many of the chemical constituents of the fruit, such as flavor, color, and pectin, are injured, and even the sugar is changed by fermentation if this heat accumulation is not prevented. For this purpose an initial storage temperature of 0 O F. for about 72 hours is becoming common, to remove the heat from the center of the barrel before the heat of fruit respiration can acquire momentum. A subsequent holding temperature of 15-20' F. is used. For full benefit, the sugar employed in fruit refrigeration is thoroughly mixed with the fruit and the solution in the juice secured by the osmotic effect of the sugar before freezing. For this reason small containers must not be placed in freezing storage too promptly. Also large sucrose crystals are best, as they distribute more evenly with the fruit and thus go into solution more quickly. Solid carbon dioxide (dry ice) in the center of a 50-gallon barrel of fruit has been shown t o be commercially useful when standard freezing storage accommodations are not available. The carbon dioxide reduces the oxidation changes and retards fruit respiration. The chief disadvantage in its use is the pressure, requiring delayed cooperage or venting of the barrel. Even sterile fruit, fruit juices, preserves, and jellies in hermetically sealed containers retain their color, flavor, and pectin qualities better a t low storage temperatures. Small containers, permitting the most rapid cooling, best retain the original color, flavor, and pectin, especially in the case of jams and jellies. Losses and changes of color, flavor, and pectin values in such containers are due t o the unstable nature of the products and to chemical reactions among the inert ingredients, perhaps influenced by the chemicals of the container and retarded by low temperatures. Oxidation is one of the causes of the chemical changes in the color and flavor of fruits and fruit juires, and even of jellies, jams, and preserves. The industry is just beginning t o realize the importance of this serious problem. Air entrapped in the head space of containers of cold-pack fruit and fruit juices and in the head space of hermetically sealed containers of fruits and fruit juices, preserves, and jellies exhibits these destructive changes over a period of storage. The evacuation of this air from the head space and its replacement with carbon dioxide, nitrogen, etc., may yet become the commercial practice. That heat accelerates these changes
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is demonstrated by their material retardation by storage at low temperatures. Every extra moment the hot preserves and jellies are exposed to the air during and after cooking results in a perceptib!e change and injury. Cardinal principles in preserve and jelly manufacture therefore are a very rapid cook, as low a cooking temperature as possible, a very rapid subsequent cooling, and as short an exposure to the air as is physically possible. In recent years many factories have installed the vacuum pan method of cooking, which lowers the highest cooking temperature by 60-80" F. and eliminates contact with much of the air oxygen during cooking. This perceptibly reduces the destructive oxidation losses in color and flavor and appears t o assist in establishing a somewhat more complete equilibrium in the impregnation of the sugar into the fruit. Owing to the higher cost of equipment, a large vacuum pan is used. This greater bulk requires a heating time two t o three times that of the open kettle process, thus counterbalancing some of the advantage of the vacuum heating process. Small vacuum pans nearer the size of the open kettle are needed to reduce the bulk per batch. As Kohman, Eddy, and Halliday have shown in the case of strawberries, the destruction of vitamins A and B, and particuIarIy C, is almost entirely due to oxidation, which is vastly accelerated a t high temperatures. By the elimination of much of the oxygen in contact with the preserve and jelly and also by greatly lowering the temperature in cooking, the vacuum process may prove essential in conserving the vitamin content of preserved fruits. The gel setting properties of fruit pectin are quickly reduced by high temperatures, combined with long exposure. The cause is generally attributed to a progressive decomposition of the pectin molecule by the spIitting off of methyl groups. The lower temperature of the vacuum pan would partially arrest this loss by retarding the reaction. Preserves and jellies have a sugar content a t or beyond the saturation point for sucrose. Experience shows, and the work of Jackson of the United States Bureau of Standards furnishes figures, showing that a partial inversion of the sucrose raises the saturation point for the total sugar content of sucrose, dextrose, and levulose several per cent, and makes possible the avoidance of sucrose crystallization in preserves and jellies. With a high sucrose inversion the levulose aids in keeping the dextrose from crystallizing. The partial sucrose inversion is usually a normal procedure in the cooking of the fruit and sugar in preserving, due to the inversion activity of the acid present in the fruit, the high temperature, and the length of cooking. With peaches particularly the acid content is too low t o effect sufficient sucrose inversion for the short cooking period, With the low temperature of the vacuum pan this trouble is aggravated and extends to other fruits, as blackberries. Inversion of the sucrose by the addition of fruit acid, or its partial substitution with prepared invert sugar, is necessary. Granulated sucrose remains the preeminently superior sugar for preserving. Its high state of refinement and purity preclude damage to the fruit through the introduction of contaminating chemical impurities, and sucrose itself exerts a strongly protective action on the color and flavor of fruit. It also firms the fruit if the osmotic equalization process between the fruit and the sugar is carried on largely a t a temperature of 180" F. and lower to avoid plasmolysis of the cells. Dextrose, when employed alone or in more than a very small proportion (15 to 25 per cent) with sucrose, due to the inversion of the sucrose and formation of additional dextrose is in a supersaturated condition a t the end of the cooking. Its solubility is greatly reduced as the temperature is lowered, resulting in a precipitation of the dextrose, which a t cool
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warehouse storage is very heavy and unsightly. For this reason alone its employment by itself or with more than such very small portions of sucrose as to offer little economic incentive, is impossible. Furthermore, the crystalline dextrose of commerce (corn sugar) is not identical with that obtained by the inversion of granulated standard sucrose. This is shown by the strong, bitter taste produced when commercial dextrose is cooked with fruit, a condition not experienced with dextrose produced from the pure commercial sucrose. On the other hand, TThite sucrose sirups which have not gone through the complete refining steps of the standard white granulated sucrose injure somewhat the flavor and color of fruits, but not nearly as severely as commercial corn sugar. Injury from the incompletely refined sucrose is caused by minute traces of impurities. This may also be true with corn sugar. Traces of iron in the crystalline levulose made from Jerusalem artichokes furnished by the Bureau of Standards markedly injured the color of raspberry preserves, changing it from bright red t o dull reddish brown during the first month’s storage. The color of preserves from the same variety of raspberries made with sucrose remained bright red. A sugar chemically indentical with invert sugar and chemically as free from impurities may yet be the outcome from placing crystalline dextrose and crystalline levulose on the market. When such time arrives their equal mixture will become a new source of sugar for the preserving industry. Levulose alone may also be found to serve satisfactorily in place of sucrose. Its still limited supply and incomplete refinement have prevented adequate tests. The objective with preserves, jams, and jellies is a standard, uniform product. Unfortunately, the unstable nature of many of the chemical constituents of fruits, the inability to exactly control the chemical reactions, and the wide variations in the kind, quantity, and proportions of the essential chemicals i n the original fruit cause preserves and jellies, even when made from the same kind of fruit, to vary considerably in their properties. The best that can be done is to secure fruits as close in chemical and physical composition and character as is needed or desired for the preserves and jellies and supply the natural chemical deficiencies that exist in such fruits with constituents that may be commercially available in a refined state from other sources rich in them. Thus a deficiency in fruit acid may be supplied by citric acid from lemons, by tartaric acid purified from cream of tartar found in grapes, or by Z-malic acid found in the Japanese quince. A deficiency in pectin is supplied by adding pectin, usually derived from apples or citrus fruit. The sugar deficiency, which always exists, is supplied from sources already mentioned. Other deficienciesin flavor, color, and appearance of fruit tissues cannot be supplied. They remain a defect in the final product and explain some of the wide variations in the quality and attractiveness of preserves and jellies. These deficiencies in chemical ingredients are classed as such largely because, through defects in quality or quantity, they make impossible the best results. Most of the chemicals in fruits and preservedproducts perform more than one function. The chemicals of color and flavor obviously have only one primary function of importance, and from the economic side give the sharpest and most valuable distinction between pure preserves and jellies on the one hand and cheap imitation concoctions of pectin, acid, sugar, and water, with synthetic colors and flavors on the other. Sugar gives thickness and body to the product, preserves the color, flavor, shape, and general appearance of the fruit, and largely determines the percentage yield. It also reduces the destructive action of yeasts, molds, etc. It functions with the acid and pectin to give a gel condition, and
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blends with the acid to physiologically bring out the full fruit flavors present. The acid aids in the inversion of part of the sucrose and in setting the pectin, and balances the sweet taste of the sugar. Tarr has shown the effect of the acid on the pectin is in direct relation to the hydrogen-ion concentration. Our laboratory finds that the effect of one fruit acid over another and the quantity required t o blend with sugar in its influence on flavor is in close correlation with their known hydrogen-ion dissociation. Yet each kind of acid affects the sense of taste differently, thereby producing a somewhat different effect on the apparent flavor. Furthermore, the use of buffer salts having a common ion t o the acid to reduce the hydrogen ion below that required t o set the pectin appears t o have little effect on the relation of the acid to the sugar, so far as apparent flavor is concerned. Pectin in the fruit cell walls aids in keeping the shape and appearance of the fruit in the preserve. Pectin in solution in the fruit juice sugar sirup aids in thickening the sirup and is the chemical on which the gel condition primarily depends. The fruit salts, and perhaps the proteins, resins, etc., serve as buffer agents in slowing down the speed of pectin setting by affecting the hydrogen-ion concentration. The chemistry of the interrelationship of sugar, pectin, acid, and water, voluminously reported in the chemical literature, has been valuable in explaining previous mystifying experiences in the preserve plant. Summary
The chemistry of the preserve, jam, and jelly industry is an intelligent selection of the best variety of fruit a t its most desirable stage of development, the subsequent prevention or retardation of the alteration of its chemical constituents, chiefly through oxidation or decomposition reactions, by controlling environmental factors of temperature, time, and exposure to harmful chemical substances, as oxygen or metals, or to infection with living organisms, and, as far as commercially available, the supplying, in a highly purified condition, the chemical deficiencies in the fruit used, as determined by its failure to function otherwise to the degree necessary for the production of a product of the character commercially demanded. The major chemical problems requiring solution in the preserving industry are: (1) A more complete knowledge of the essential chemical substances, and their variations, with the causes and possible remedy for such variations. (2) Creation by plant breeding and commercial prcduction of fruits more perfectly supplied with the chemical characteristics found most important for the production of uniformly attractive, pleasing, and serviceable preserves and jellies. (3) Prevention economically of the oxidation changes in fruits and juices during storage, during manufacture, and during storage. (4) A thorough study of the vitamin content of fruits and the effect of different preserving treatments upon them. ( 5 ) The corrosion of metal containers is a matter of major importance to the preserving industry. Fortunately, Dr. Kohman of the Xational Canners Association is ably determining many of the causes of this distressing condition. We sidestep the issue when possible by using glass or wooden instead of metal containers. Japanese Production of Rayon Shows Rapid Growth-Japan produced 12,500,000 pounds of rayon in 1927, representing an increase of 127 per cent over the 1926 output, the Department of Commerce has been advised.
