Canning Fruit Juices
The technical background for various unit processes employed in canning fruit juices is reviewed. Several types of juice extractors are described. Retention of unaltcred pectin is essential in those juices containing 5-18 per cent insoluble solids to prevent separation of suspended solids in the can. I n the case of tomato juice, pectin-methoxylase present is inactivated by heat before extraction to prevent destruction of pectin. To avoid unpalatable flavors in citrus juices, extractors must be adjusted to prevent incorporation in juice of dlimonene from peel oil and the glucoside, naringin, from albedo and locular wall tissue. The thermal treatment employed i n sterilizing citrus juices must be sufficient to inactivate all pectic enzymes present j otherwise, separation of suspended solids will occur. Because of certain spore-bearing anaerobes for which tomato juice is a favorable growth medium, t h e thermal treatment necessary to assure sterilization of this juice is more severe t h a n t h a t required by other fruit juices. The application of Ball's method, widely used in the calculation of sterilizing processes for nonacid foods, to the determination of processes for tomato juice is demonstrated. The changes i n flavor which some juices undergo after canning are due, in part, to decomposition of levulose formed by acid inversion of sucrose initially present. Chemical changes in an ether-soluble fraction are also a factor. The retention of ascorbic acid in juice packed commercially i n t i n plate containers is excellent and superior to t h a t in glass containers. This is ascribed to the reduced state of the juice system when preserved i n metal containers. Data are given showing effects of stannous and ferrous ions on stability of ascorbic acid i n grapefruit juice.
TECHNICAL ASPECTS R. H. LUECIC AND R. W. PILCHER American Can Company, Maywood, 111.
URING the past ten years, a variety of canned fruit and vegetable juices has come into popular favor. However, the first attempts t o pack a fruit juice in tin containers, specifically apple cider, date back t o more than twenty years ago. I n general, these early efforts met with failure. The low corrosion resistance of the hot-rolled tin plate of high metalloid content (the only type of plate available a t that time) imposed a limited commercial life on the product, with the result that the packing of cider in tin did not prove economically feasible. About ten years ago canned tomato juice first came into commercial production after a short period of development (39). Few other foods have received such immediate and widespread consumer acceptance. Production statistics for canned tomato juice from 1933 to date are as follows (25) :
D
a
Year Casesa 1933 3,737,535 1934 5,288,664 1935 5,214,415 1936 11,546,897 Expressed as cases of all size cans.
Year 1937 1938 1939
Casesa 11,869,135 5,406,680 10,600,786
This considerable production of tomato juice has had a distinct influence on the development of other food beverages. First, the widespread use of tomato juice did much to create consumer acceptance of foods of a juice character; secondly, the popular reception accorded tomato juice stimulated canners to develop and manufacture a diverse line of fruit and vegetable juices. As a result, there is on the open market today a wide variety of food juices, vegetable as well as fruit. The 1939 production of fruit juices in the United States (57) was as follows: Juice Casesa Juice Cases* Grape 1,753,791 11,091,068 Tomato 350,000 10,560,630 Lemon Grapefruit 197,199 7,500,000 Prune Pineapple 617,239 1,740,841 Other juices6 Orange a Cases of all size cans, including glass. b Includes ap le, cherry, logan,berry, blackberry, raspberry, and strawberry juices, an$ peach, pear, aprlcot, and prune nectars, and others.
The development and quality control of these food juices have brought many problems of a technical nature. The purpose of this discussion is to present the technical background for the various unit processes employed in the production of the fruit juices and to enlarge upon the technical aspects of a few problems now current in the industry. Since the unit processes employed in tomato juice canning resemble more closely those of the fruit juices rather than those of the vegetable juices, tomato juice has been somewhat empirically included in this discussion. Extraction
Obviously, the type of juice extractor employed varies with $he fruit being processed, although the trend is toward con-
tinuous and automatic operation in all types of extraction equipment. For tomato juice the screw type extractor has almost replaced the pulper type formerly employed. I n the screw type principle, a cylindrical or tapered screw operating in a cylindrical or conical screen forces the fruit against an adjustable pressure cone a t the discharge end. The pressure thereby created forces the juice through 0.51-0.71 mm. (0.02G0.028 inch) perforations in the screen. The waste pomace is discharged through the annular aperture surrounding the pressure cone. The pulper, which is still used in juice production, likewise employs a perforated screen cylinder, but replacing the screw is a set of two or more longitudinal paddles, mounted on a central shaft and revolving close to the interior surface of the screen. The paddles are pitched slightly to induce a forward movement of the pulp within the screen cylinder. By a combined beating and squeezing action the juice is expressed from the fruit cells and forced through the screen into a collecting trough. A new type of extractor recently made available operates somewhat on the principle of a rotary air pump. Inside a vertically mounted screen cylinder is an eccentric shaft carrying a series of oscillating radial fins, the outer ends of which are maintained in close contact with the screen by spring tension. The chopped fruit is fed into the pockets formed by the fins between the shaft and screen. In turning through 180' the size of the pocket is considerably reduced, owing to the eccentricity of the shaft, and the juice is thus expressed through the perforations in the screen. The screw type extractor is widely used for pineapple and other juices as well as for tomato juice. 292
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Tomato juice is preferred with a relatively high content of insoluble suspended solids. I n order to prevent unsightly separation of the suspended solids during sterilization of the juice and subsequent storage, it is necessary to retain unchanged in the juice a considerable proportion of the pectin initially present in the raw fruit (about 0.15 per cent); without the important colloidal properties of pectin, coagulation and separation occur quickly. The presence of pectic enzymes in tomatoes has led to the use of the hot-break method of juice extraction wherein the fruit is heated to inactivate the pectic enzymes before expression of the juice. Kertesz (18) has demonstrated the presence of pectin-methoxylase in ripe tomatoes; this enzyme catalyzes the cleavage of methoxy groups from the pectin nucleus, producing methanol and pectic acid. By this method of measurement, Kertesz ($20) found a pectin-methoxylase activity in fully ripe tomatoes of 115195 units per gram and points out that at an activity of 180 units, complete demethoxylation of the pectin present in juice cold-pressed from ripe tomatoes a t pH 4.2 should occur in less than 4 minutes a t room temperature. The relatively insoluble gelatinous pectic acid does not possess the powerful peptizing properties of pectin. It follows, therefore, that for satisfactory retention of the suspension the conversion of pectin to pectic acid between extraction and sterilization of the juice must be avoided. In practice this is accomplished by passing the crushed fruit through a steam-jacketed or waterjacketed tube before extraction. Kertesz (19) found that the pectin-methoxylase of tomato juice is completely inactivated by heating to 80' C. (176' F.) for 45 seconds. At 60' C. (140' F.) no activity was lost in 2 minutes. Another advantage claimed for the hot-break method of extraction is a reduction in losses of ascorbic acid, in part due to lowered solubility of atmospheric oxygen in the juice a t the elevated temperatures employed ($29). Some packers have successfully canned cold-pressed tomato juice by homogenizing at 800-1000 pounds per square inch (56.2-70.3 kg. per sq. cm.) pressure after extraction. Homogenization, by reducing the mass of the suspended solids, usually provides a satisfactory suspension. A machine which combines the functions of comminution and extraction and which has recently found application in
COMMINUTING MACHINE WITH
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commercial production of various vegetable juices, notably carrot juice, may offer considerable advantages in the manufacture of pulpy fruit juices. In this machine the prechopped raw product is passed successively through several pairs of comminution rings. After each stage of comminution the material being treated is forced against screens, and a separation of the liquid and fibrous fractions is thus effected. The machine has a large capacity and with most products provides a good percentage yield of juice. The extractors most commonly used for citrus juices are designed to operate on unpeeled fruit. In most designs the fruit is halved by being forced against a sharp knife. The halves are caught in semispherical cups and are gently pressed against a semispherical or conical member or burr, conforming to the shape of the halved fruit, to express the juice. I n soma types of equipment the burr rotates and has a serrated surface. I n one type of citrus extractor widely used in Florida, the fruit is quartered and the quarters are forced between two rolls, one convex and the other concave. I n expressing citrus juices from unpeeled fruit, great care must be taken in the construction and adjustment of the extractor to avoid introduction of undesirable flavoring principles into the juice. For example, in the grapefruit there resides, primarily in the albedo and locular wall tissue, a bitter water-soluble glucoside, naringin (6, 40, 41). Too severe pressing or abrasion of the naringin-bearing tissues will permit some of the glucoside to be leached out by the juice after liberation from the juice vesicles. The possibilities of contamination with naringin are greatest in immature fruit. Higby (18) recently reported another principle in California navel oranges, isolimonin; when extracted with the fruit juice, he stated that it is converted to an intensely bitter lactone form. Isolimonin apparently occur8 in the albedo, the center fibrovascular bundle, and in the locular wall. Another important factor in the expression of citrus juice is the control of peel oil extraction. Directly underlying the thin cuticle on the outside of the peel is an epidermal layer of cells containing numerous oil vesicles. The citrus oils contain about 90 per cent d-limonene. On storage, juices containing excessive quantities of this oil develop an objectionable turpentine flavor. Some pickup of peel oil is unavoidable in cutting the fruit and a little is not serious, but undue pressure on, or laceration of, the peel during extraction must be avoided. This is especially true when slightly frost-bitten fruit is used, since the oil from such fruit is expressed with comparative ease. The o i l content of citrus juices should never exceed 0.030 per cent, and with proper equipment and precaution this value can be maintained below 0.01 per cent. For some juices, such as apple, cherry, and grape which are packed in a brilliantly clear state or with relatively low insoluble-solid c o n t e n t , t h e familiar rack-and-frame press is utilized. Maceration or comminution of the Courtesy, Sohwart Enoineerdng Company fruit must precede the ROTORE L E V A TAND ~ D HALFOF SCREEN REMOVED pressing operation.
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Courtesy, Food Machinery Corporation
PADDL TYPE ~ PULPER
While the figures may vary, average yields for the more widely used juices may be listed as follows: Juice Tomato Grapefruit Pinee ple F l o r i g orange
Gallons per Ton of Fruit 210-215 90- 95 130-140 90-110
Approximate Yield of Juice, % 87.6-89.7 37.5-38.6 54.2-58.4 37.5-45.9
Sweening Since little of a technical nature is involved in the screening operation, only passing reference will be made to it. The canner employs this operation to control the size and quantity of the insoluble suspended solids in his juice. I n general, a brilliant "polished" juice from which the suspended solids are eliminated lacks the full flavor and aroma of a juice containing an appreciable percentage of insoluble matter. Many of the flavoring principles appear to reside in the chromatophores incorporated in the juice as it is expelled from the juice vesicles. Some characteristic volume percentages of suspended solids as determined by centrifuging according to the tentative standards of the Agricultural Marketing Service of the U. S. Department of Agriculture for canned grapefruit juice, effective October 1, 1939, are as follows: Suspended S o l i d s Volume % 24 -33 Tomato 7 -13.5 Orange 13 -17 Pineap le 15 -18 Apple ?pp?py) 3.5-11 Grauefruit n Determined by oentrifuging a 50-00. sample 10 minutes at 1313-1320 p. m. in a 15-inch-diameter centrifuge. Juice
I.
In general, any insoluble materials passing a screen with 0.51-mm. (0.020-inch) perforations are permitted to remain in the pulpy fruit juices. With tomato juice a 0.71-mm. (0.028-inch) screen may be employed on the extractor, or in some installations a screen with larger perforations is used on the extractor and the juice then passed through a finisher
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having a 0.51-mm. screen. The finisher is built on the same principle as the pulper previously described but is somewhat smaller in dimensions. In the case of the citrus juices several methods of screening are commonly in use. Some canneries employ finishers, others rotary cylindrical screens, and still others an oscillating type of screen. Some operations combine a finisher with one of the other types. I n the production of clarified juices it is necessary to resort to special treatments. The Pectinol treatment is widely employed for apple juice (11). It takes advantage of the pectinpolygalacturonase activity of commercial pectinase preparations made from certain molds (Aspergillus sp., Monilia sp., and others). This enzyme splits the chainlike polygalacturonic acid structure of the pectins into their simpler constituents, sugars and acids, which are soluble and lacking in colloidal activity. Probably there are other accompanying enzymatic reactions. This treatment permits the insoluble solids to flocculate for easy filtration. The limitation of this method is the time required for the enzymes to complete their work. Another clarification procedure is to heat the expressed juice t o 93.3" C. (200" F.)t o coagulate the suspended matter partially, cool, treat with colloidal clay (bentonite, 0.562 gram per liter or 0.075 ounce per gallon), and filter after the addition of filter aid (Hyflo, 7.49 grams per liter or 1 ounce per gallon). Sipple, McDonell and Lueck (38) employed this method successfully on apple juice. As a result of his work on cherry juice, W. McK. Martin recommends the bentonite procedure for that product; he also advises that attempts a t Pectinol clarification of cherry juice were not successful under the conditions of his experiments. Sterilization In the canning industry the term "sterilization" is not used in the dictionary or medical sense. Rather, it implies the destruction of all microbiologicalforms, both spoilage and pathogenic, which might multiply in the food a t ordinary temperatures. It is frequently expressed as commercial sterilization. The fact that most fruit juices normally possess pH values below 4.5 greatly limits the type of microflora which can multiply therein; fortunately the types that can develop are nonspore formers and are destroyed a t relatively low temperatures. Thus, with the exception of tomato juice, the sterilization of the fruit juices is readily effected a t temperatures in the range 79.4-90.6' C. (175-195" F.). Spiegelberg (33) demonstrated the necessity for increasing the temperature attained in the sterilization of pineapple as the pH value approaches 4.5. Tanner ($4) suggested that the pulp and juice of sound citrus fruit are sterile. However, after the fruit is cut and the juice extracted, it can become burdened with microorganisms of all types. Nolte and von Loesecke (96) observed the types of organisms found in Florida citrus juices, both in the raw and pasteurized states. They conclude that while some organisms present in the raw juice can survive the minimum sterilizing temperature employed in Florida citrus canneries, 85" 6. (185" FJ,they are of a type which cannot multiply in citrus juice media. The present trend in fruit juice sterilization is towards the high-short process wherein the juice is heated in a few seconds to sterilizing temperatures in a tubular or plate-type heat interchanger and promptly filled and sealed into cans. The can is inverted immediately after closure, and the hot juice is relied upon to sterilize the container. Some canneries still practice the older procedures, such as bringing a large volume of juice to sterilizing temperature in a steam-jacketed kettle, or filling the can a t room temperature and sealing it in a vacuum closing machine followed by submersion in hot water. Sipple, McDonell, and Lueck (32) report a lack of flavor and a cooked taste in clarified apple juice sterilized in cans after
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vacuum closure. The following data show the adverse effect of the kettle-heating method on the ascorbic acid content of Florida grapefruit juice, as determined in this laboratory; for comparison, data obtained on flash-pasteurized grapefruit juice are also included: Sterilizing Method
Ascorbio Acid
Mg./cc. Kettle fist juice drawn o f f ) Kettle {last juice drawn off) High-short
0.294 0.061 0.400
I n the kettle method from 15 to 30 minutes are usually required to bring the juice to sterilizing temperatures, during which there is contact with the air. Another 30 minutes or more may be required to empty the kettle. During these periods a marked destruction of ascorbic acid is evident. While it is generally true that a thermal treatment adequate to prevent spoilage in a food by microorganisms is more than adequate to inactivate enzymes naturally present, such has not proved to be the case with the citrus juices. I n normal canned grapefruit or orange juice there is but a slight settling of insoluble solids; the body of the product remains clouded or opaque. This opacity is apparently due to a colloidal suspension of solids stabilized by naturally occurring pectins. On the installation of high-short sterilization equipment (flash pasteurizers), many canners have been dismayed to find a complete clarification of the main body of the juice, a marked sediment in the bottom of the can, and frequently a fluffy white curd floating on the surface of the juice. An observation of Cruess ('7) in 1914, the fundamental work of Kertesz (18, 19, 20) on pectic enzymes, and some recent observations of Joslyn and Sedky (17)have served to explain the cause of this difficulty. The latter pointed out the interdependence of time and temperature a t a given pH in the inactivation of the pectic enzymes responsible for the splitting of the natural pectin without which the insoluble solids of the juice cannot be maintained in suspension. For example, these authors show that bringing Valencia orange juice to 87.8' C. (190' F.) in 3 minutes prevented clearing during 72day storage, whereas more than 10 minutes a t 80' C. (176" F.) were necessary to prevent clearing within 36 days. Before the advent of flash pasteurizers a considerable time was taken to heat the contents of a kettle or can to sterilizing temperatures of 85' C. (185' F.) or somewhat less. The time factor was sufficient to inactivate the pectic enzymes even a t temperatures as low as 79.4' C. (175" F.). However, the old temperatures proved inadequate with the short heating times made possible by flash pasteurizers. Parks (28) conducted field investigations in grapefruit juice canneries in the Rio Grande Valley in order to establish practical control of pectic curd and clearing. Figure 1 shows that a minimum temperature of 85" C. (185" F.) must be attained in the flash pasteurizer to insure inactivation of the pectin splitting enzymes. A minimum temperature of 87.8' C. (190" F.) is recommended since many canneries have difficulty in locating sufficient conveyors to permit holding the sealed and inverted can for 2.5 minutes before it enters the cooler. Parks found that for kettle-sterilized juice a minimum of 82.2' C. (180" F.) is sufficient, provided 20 minutes are taken to bring the juice t o temperature. For juice sterilized in the can, a process of 15 minutes at 82.2' C. (180' F.) is adequate for the number 2 can. The relative activity of the several pectic enzymes in the citrus juices does not appear to have been determined. It is likely that protopectinase, pectin-methoxylase, and pectinpolygalacturonase are all present. Kertesz found but little polygalacturonase in ripe tomatoes, but the insoluble white curd in citrus juices closely resembles the lignic acid obtained by Ehrlich and Schubert (10) in the acid decomposition of the pectin nucleus. There is need for more work on the pectic
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enzymes of citrus juice patterned after the procedures used by Kertesz on tomato juice. The authors have frequently observed a jellylike separation in canned pineapple juice. This was probably gelatinous pectic acid produced by methoxylase activity in inadequately heated juice. The sterilization of tomato juice presents a special probIem. Until recently it was thought that as in other acid juices (pH below 4.5) the destruction of all microflora that could multiply in tomato juice could be effectedmerely by heating to 87.8" C. (190' F.). Hucker and Pederson (14) suggested that temperatures even as low as 82.2" C. (180' F.) may be used with satisfactory results. Canning technologists have been forced to modify radically their conceptions of the thermal treatment required to sterilize tomato juice as a result of the discovery of certain heat-resistant spore-bearing types of spoilage organisms for which tomato juice provides a favorable environment for development. I n 1931 Berry (2) isolated and identified Bacillus thermoacidurans, an anaerobic spore former capable of developing a repulsive off-flavorin tomato juice without production of gas. This type of spoilage in a canned food is particularly insidious inasmuch as the can in which it occurs shows no external manifestation. The heat resistance of this organism apparently varies with the particular strain of B. thermoacidurans and with the concentration of spores present. I n thermal death-time determinations made in this laboratory, tomato juice suspensions containing around 800,000 spores per cc. have survived 70 minutes at 100' C. (212' Fa). I n higher concentrations spore suspensions of this organism may readily survive 90 minutes at 100' C. Recently Townsend (66) isolated from spoiled tomatoes an anaerobic spore-forming, butyric acid, and gas-producing organism having a resistance in tomato juice as high as 20 minutes at 100' C.
170°e
76.7
175; 79.4
180; 82.2
185," 85
190° F. 87.8' C.
FIGURE 1. EFFECT OF TEMPERATURE ON GRAPEFRUIT JUICE From 1939 experimental pack of Marsh variety grapefruit: juice handreamed and passed through double screen, pulper, and finisher; produot flash-pasteurieed and filled a t temperatures indicated; cans inverted 2.5 minutes before cooling to 100" F . (37.5' C.).
For some time it has been known that the rate at which the center of a can of tomato juice can be brought to sterilizing temperatures is amazingly slow, considering the liquid character of the product. From work in 1939 Stevens and Berti of this laboratory report a rate of heat penetration for tomato juice which is even slower than previously determined rates. Apparently, under processing conditions where the can is strictly immobilized, tomato juice assumes the properties of a gel, and heating is by conduction only, just as in a heavybodied food such as pumpkin. A gel structure prevents the development of convection currents within the mass of food which, when present, always provide relatively rapid heating. The mechanisms of heating involved in the thermal sterilization of canned foods where gel formation may be a factor is discussed at length in a recent paper by Jackson and Olson (16). Stevens of this laboratory has found that in heat
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penetration measurements on tomato juice, even gentle agitation of the can greatly increases the rate of heat penetration. Until recently technical agencies associated with the canning industry have not been in agreement on the thermal treatment which should be given tomato juice. With the present 100" C. (212" F.) thermal processes, the successful tomato juice canner has had to observe strict plant sanitation. Special attention must be given to the cleansing of machinery or equipment in which heat-resistant spoilage organisms may lodge, multiply, and form serious foci of infection. Attention is now being directed to the establishment of processes which will assure destruction of the spoilage types under all conditions and thus place tomato juice on the same footing as the nonacid foods. This will probably require methods of controlled agitation of the can, and/or temperatures in excess of 100" C. Consideration is also being given to the high-short principle employing temperatures as high as 135" C. (275" F.). The establishment of thermal processes for acid foods on a sound scientific basis, as has been done for the nonacid foods, is complicated by the difficulty in isolating suitable test organisms and maintaining a known thermal resistance in these organisms. Ball (1) developed a mathematical method for process calculation which is now widely used for the nonacid foods. Ball's formulas combine all pertinent physical and bacteriological data for any specific product, and permit rapid sdnd convenient calculation of lethal processes. Ball's formulas have been simplified by the nomograms of Olson and Stevens (87). Working with an inoculum of the Townsend organism (36) having a resistance in tomato juice somewhere between 10 and 13 minutes at 95" C. (203" F.), Stevens and Berti of this laboratory found that the process requirement for a tomato juice as determined by an inoculated pack in number 2 cans checked well with the calculated process using the known heat resistance of the above organisms and the rate of heat penetration for the particular tomato juice. Both still and agitating types of process were studied; the data obtained are presented below. Although further investigations of this type should be made, possibly on other organisms, the agreement between the calculated and the required processes as determined by the inoculated pack is exceptionally close in this instance :
(I
6
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chemical method, as similar juice packed in glass and stored in the dark. Following these two papers, Tressler and Curran (36) offered data to support their conclusion that ascorbic acid is not lost more rapidly from glass bottles than from tin cans, provided both types of containers are completely filled; this condition, of course, is never attained commercially. These authors also offeras an explanation of the decrease of ascorbic acid in tomato juice durihg storage, the possibility that dissolved or entrapped oxygen accounts for the effects noted. These investigators did not study the effect of variable headspaces in tin containers. Reynolds (52) reported that metallic tin added to glass-packed tomato juice would yield a product higher in ascorbic acid than the juice packed without added tin. This author suggests that the results may have been due to nonascorbic acid indophenol-reducing substances. I n all these reported studies, the titration technique was exclusively used in the estimation of ascorbic acid. Tressler and Currant subjected their samples to the hydrogen sulfide reduction procedure prior to titration. I n 1934 and 1935 R. W. Pilcher and J. F. Feaster tested in this laboratory the applicability of the Bessey and King ( 3 ) modification of the Tillman technique to the estimation of ascorbic acid in canned foods, where interference with the titration by iron and tin salts in the food is an active possibility. Packs of tomato juice from the same 1000-gallon holding tank in a cannery were made in tin and glass containers; the normal head spaces for these two types of containers mere maintained -approximately 20 cc. (0.7 ounce) in cans and 11 cc. (0.4 ounce) in bottles. The plain No. 300 cans (14 fluid ounces) and the 4-fluid-ounce bottles were processed to the same container center temperature, 93.3-95.0" C. (200-203" F.). The cased cans and bottles were stored in a dark basement and subjected to bioassay by the standard technique and to titration by the Bessey and King method, using 8 per cent acetic acid as the extracting medium. Fresh cans and bottles were opened daily for the feeding tests, and chemical estimation of ascorbic acid was made at stated intervals on freshly opened samples. After 8-month storage a t room temperature, representative samples. from the tin and glass packs were composited and analyzed for iron and tin. Spectrograms made on the ash from these samples indicated that the only major difference between the samples as far as metallic
Process Time a t 100" C., Min. Proosss Method Requireda Ca1cd.b Agitating > 6 < 8 8 Still >45
