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Preserved Foods - Part 3

( Originally Published 1939 )



When a foodstuff has dried out to a greater or less extent, it can be stored for long periods without spoiling from microbic attack (such as souring, molding, and putrefying). In the absence of sufficient water, microorganisms cannot maintain their normal metabolic processes but either die off or form resistant spores. Even enzymic changes are slowed down. Moreover, removal of water reduces storage requirements, shipping charges, and packaging costs by decreasing bulk. These considerations led to high hopes, especially in the period during and immediately after the World War, for the establishment of a dried-food industry. However, these hopes have not been fulfilled 1 except in the case of certain fruits, a few vegetables, eggs, some meat and fish, and large amounts of skimmed milk. Factors probably most largely contributing to this situation are trends in the home to reduce time and labor of preparation of foods for the table, the poor condition in which these products have reached the consumer, and the unsatisfactory quality of the dried product compared to that of canned, refrigerated, and frozen products.

Where the climate provides intense solar heat, low humidity, and relatively long rainless periods, large amounts of fruits are sun-dried. Artificial drying in tunnels or other special drying equipment is known as dehydration, although dried apples are often termed evaporated apples, and dried milk is called milk powder. Dehydration is applied mostly to vegetables.

Before fruit is dried, it is treated in various ways to facilitate its drying and to improve its appearance and quality. In the first place, the product must be sorted as to soundness and quality. Sorting tables must be large enough or traveling belts must travel slow enough and be sufficiently well lighted to enable workers to see each fruit on all sides. This is more readily done before drying than afterwards. Bruised tissue is especially susceptible to discoloration and decay, and is more difficult to dry. Uniformity of ripening, size, and variety are necessary to give a finished product of uniform quality, and to make plant operations economical. Some fruits are washed, others are peeled, pitted, or seeded, some are given a dip in a lye solution to facilitate peeling and subsequent drying, and others are scalded in water or steam. Fruit may be cut in halves or quarters or slices.

Flesh of such fruit as apples, apricots, peaches, and pears discolors when it is cut open or peeled, and exposed to air. This darkening can be prevented by treating the product with fumes of burning sulphur, or by dipping it in water solutions of alkaline bisulphites (or several other chemicals). For many years, dried fruits, sun dried as well as dehydrated, have been given this treatment with sulphur fumes. The sulphur dioxide preserves the natural color and flavor of the fruit, kills any insect infestation, helps reduce microbial flora, pre-vents souring and attacks by insects during drying, and facilitates the escape of moisture. Inasmuch as a large part (one-third to one-half) of the sulphur dioxide disappears as such during 6 months' storage, the original dosage must be large. Experience has shown that fruit must carry an initial charge of 2000-3000 p.p.m. to allow for a residual content great enough to protect it from discoloration during prolonged storage. The amount of sulphur used and the length of time required are determined to some extent by the degree of ripeness of the fruit.

Vegetables are not sulphured. They are subject to deteriorative changes after they are dried, unless causative enzymes are inactivated and oxygen removed from tissues before the products are dried. This is done by blanching the material with boiling water or steam. Blanching prevents darkening and discoloration by inactivating the enzymes; it preserves or "sets" the natural color; it coagulates some of the soluble constituents; and it kills the protoplasm, thereby accelerating escape of moisture during drying. It is not prolonged enough to constitute cooking for the table.

The treated and prepared products to be sun-dried are spread in thin, even layers on trays, usually on the ground, and exposed to the direct sunshine. The temperature may be 120 F. or higher. Several days to a week or more may be required to complete the drying. During this time, the product is subject to contamination by dust and other foreign material, and also to insect infestation, unless it has been sulphured. It is protected from short rain showers by covers.

Foods may be dehydrated in kilns or in tunnel driers. Kilns may be several stories high, with grating floors which allow heated air from the basement to rise freely: These air currents dry the product, which is spread out on the gratings. Tunnel driers or dehydraters are long, horizontal tunnels built to allow the food to be moved forward against a current of heated air at a temperature of 150 F. or more. Cabinet or compartment driers are arranged for trays of food to be stacked vertically. When the heated air rises from below, the products on the lower shelves dry out first, necessitating shifting of the trays. Some driers fan the air across the trays. Other types of mechanical driers are used for special purposes, as for example, steam-heated rolls, drying chambers, and trays or belts in modification of the tunnel type. A much better control of the temperature, humidity, sanitation, and other drying conditions is possible by dehydration than by kiln drying or by sun drying.

The material from the drying equipment is never uniform; some portions are overdried and others still contain too much moisture. This condition is remedied by holding the product in bulk in a warm room or bin for several weeks and frequently mixing it. This is really a continuation of the drying process. As a result of this curing or conditioning, as it is called, the wetter portions give up their moisture to the drier portions to yield a uniform quality. Bin mold and insect infestation are very likely to develop at this stage.

Temperatures used for drying different products vary with different foods. Some products, like cabbage, are dried at the start at a temperature of about 120 F. which is raised as drying progresses. Fruits are dried to a residual moisture content of about 20 to 24 percent to insure against spoilage, as well as to comply with official standards. This leaves the product spongy and elastic. The proper point is determined by compressing a handful. No moisture should be yielded, and individual pieces should fall apart when the pressure is released. Vegetables are dried until they are hard and inelastic, and these qualities should not be lost in the conditioning treatment. Drying may last 6 hours or more, at temperatures of 1500 F. or above. The final moisture content for vegetables should be no more than 8 to 10 percent, but there are no federal standards for these.

Fruits are usually packed in wooden boxes, lined with several layers of paraffined or waxed paper. The dried product is forced into the box under a press in order to form a solid block. This prevents penetration of air through the mass and facilitates uniformity in moisture. Vegetables are packed in small individual containers whose contents can be quickly used up without risking spoilage in the opened package.

Dried products are very susceptible to insect infestation .4 The orchard type results from attacks of insects before the fruit is harvested. It often manifests itself by pockets of gum which imbed insect excreta. Another common type is that in which the insect and its excreta may be found in the vicinity of the stone. Storage infestation occurs after harvesting and cutting. Forms most commonly encountered are larvae of moths, sawtooth grain beetles, confused grain beetles, and dried fruit beetles. These storage insects work on the outside of the fruit and also burrow into the fleshy portions, leaving their telltale excreta. They are often quite small, and their detection requires careful search with a hand lens. Unless great care is exercised in curing, storing, and particularly in packing, insects succeed in getting into the food. Adequate plant screening and cleaning, and impervious packaging, may keep them out, but often these preventive measures are not wholly effective. Resort is made to fumigation. Fumigants mostly used are hydrocyanic acid gas, carbon bisulphide, and carbon tetrachloride. Dry heat, ranging from 125 to 130 F. and applied for several hours, will kill all insects attacking dried fruits. Moist heat is not so effective).

Decay may be caused by molds, and to a less extent by yeasts and bacteria. Brown mold rot is one of the most common causes of decay in such stone fruits as peaches, apricots, plums, prunes, and cherries, and occasionally in other fruits. Decay areas caused by mold are very difficult to discover after fruit is sulphured and dried, but the presence of mold is revealed by mold filaments.

A fermented condition is indicated by more or less darkened areas of the flesh. Such portions are more difficult to dry than darkened areas caused by other factors and, accordingly, may be more sticky than sound portions. This condition is caused by packing the fruit too moist. It should be stored in a relatively cool, dry place.

Fruit which has been knocked or shaken from the trees to the ground may have picked up dirt which is embedded in the flesh through the broken skin. On account of the moist nature of the cut fruit, the hands of the workers who pare the fruit become very dirty, and this may contaminate cut surfaces.

Some drying yards are burnt over to remove stubble, and the dry, wind-blown char may adhere to fruit surfaces. Dry yards are also located near highways, barns, and chicken yards, and are subject to direct contamination with wind-blown animal refuse. The most . common practice in sun-drying is to place the trays directly on the ground with one end raised. When the trays are stacked for finishing of drying, the dirt adhering to the bottoms may drop onto the fruit in the tray beneath. It is difficult to correct all these conditions entirely because dehydration, particularly of peaches and apricots, has not been proved entirely successful, and recourse must be made to sun-drying in the open.


Composition. The criticisms of McCance and Lawrence (see page 405) concerning the distinction that should be made between the unavailable and the available carbohydrates of plant foods is applicable to the dried products. The proximate analyses of dried fruits in Table XLVII are taken from the compilation of McCance and associates.

The proximate composition of dried legumes, as presented in Table XLVIII is tabulated largely from the work of Atwater and Bryant. The figures for the carbohydrate content are not very useful because they do not distinguish between unavailable carbohydrate (roughage) and available carbohydrates which are the soluble and the nutritively valuable sugars and starch. The fuel value is not very useful because the figures are largely based on the carbohydrates, some of which are not metabolized and others are lost in the cook-water. The iron content in a number of dried products is shown in Table XLIX, compiled from the work of Stiebeling. McCance has pointed out that seeded raisins carry a higher iron content than was present in the original fruit on a comparable solids basis, indicating a pick-up in the operations. Dried apricots and peaches have been reported (McCance) to be very effective in curing nutritional anemia.

Inasmuch as these dried fruits are eaten uncooked or as stewed fruit, there is no loss of the iron in any discarded cook-liquors.

Several of the dried fruits contain appreciable amounts of some of the vitamins, as shown by the figures in Table L, compiled mostly from the tables of Daniel and Munsell.

Effect of sulphur. It is customary to treat fruit, particularly apples, apricots, peaches, and peas, with sulphur dioxide or sulphites. Vegetables are not sulphured in commercial practice. Opinion has been divided as to its healthfulness. The earlier work reported that it was harmful when ingested, but the more recent work shows that it is entirely harmless in small amounts. For example, both canned and dried sulphured apricots are reported by Saywell to produce alkaline urines and decreases in excreted ammonia and total acidity, thereby indicating that the sulphur compound does not appreciably affect the normal acid-base relationships of the ingested food. Its use cannot conceal inferiority by masking the presence of decomposition or decay because blemishes in the fruit can be seen in spite of the sulphuring. However, there does seem to be a relation between the amount of sulphites present and the quality of the fruit, as Halliday and Noble indicate in Table LI. Sulphites can prevent spoilage in fruit held at a higher moisture content than is possible without them, although protection from such abuse is accorded by the standards for moisture, enforced by the U. S. Food and Drug Administration.

The purpose for using sulphites is to preserve the natural color of the fruit. Although this varies according to conditions, practice has found that apricots, peaches, and pears need a dosing of 2000 to 3000 p.p.m. of sulphur dioxide to maintain them in merchantable condition during their commercial life.' Dried peaches and apricots have been shown to lose one-half of their sulphur dioxide content during 6 months' storage at 20 C. (68 F.), and about half is removed during cooking. The small amount that remains on the fruit after storage has been shown to protect the vitamins A and C from destruction during open-kettle cooking.

Other chemical treatment. Lye-dipping to remove the skins of fruit and otherwise to prepare it for satisfactory drying has raised a question as to the effect on the nutritive value. The work of Morgan and associates and others shows that its use in preparing prunes and raisins is inert or decidedly beneficial in the preservation of vitamins A, B, and C.

Oxidation destroys some of the vitamins. Therefore, dehydration with its higher temperature treatment for a short period is less harmful than sun-drying at lower temperatures but extended over a longer period.

Dried fruits and vegetables are sometimes fumigated to kill insect infestation. Williams states 13 that, although such highly toxic gases as hydrocyanic acid, cyanogen chloride, sulphur dioxide, ethylene compounds, and others are used, there have been no recorded cases of poisoning from residual traces of these gases. It was considered that ventilating the food after treatment allowed all traces of these gases to dissipate. However, in 1936 a case of illness was traced to the consumption of raisins which contained 2700 p.p.m. of hydrocyanic acid. A survey of the fumigating practices and analyses of 850 samples showed that most of them ranged from 200 to 800 p.p.m. of this gas. The U. S. Food and Drug Administration states that the use of this gas as a fumigant for fruit held in storage has now been discontinued.

Nutritive value. Dried fruits are subjected to the cumulative effects of several factors which affect the retention of their original vitamin content. Extensive investigations on this problem have been conducted at the University of California, in which fresh fruit from the same orchards was preserved by freezing storage as representative of the original condition of the fruit, tested for their original vitamin content, and then dried by the usual commercial method, and stored under known conditions." Inasmuch as the vitamins are recognized as distinct chemical entities, it is to be expected that the effects of the various treatments would be different on each vitamin.

Vitamin A was found to be retained in peaches to the extent of 80 to 100 percent, whether sun-dried or dehydrated, sulphured or unsulphured. Prunes retained from 60 to 90 percent in the sulphured and 25 to 50 percent in the unsulphured, although only a small amount of the unsulphured is manufactured. Apricots retained 25 to 50 percent in the sulphured and only 16 percent in the unsulphured, but the original amount in the fresh fruit was so large that the final values remained substantial. Vitamin A in raisins and figs was found to be definitely higher when the fruit was dehydrated than when sun-dried, and sulphuring in nearly all cases was an aid in conserving vitamin A. The variety of dates determined the amount of this vitamin to a greater degree than the horticultural or drying conditions, but their artificial ripening, fumigation, and pasteurization exerted no adverse effects. From these data and Fellers' review, it is seen that drying is harmful to vitamin A in most products, that sun-drying is decidedly more injurious than dehydration, and that the sulphur dioxide treatment is a protective factor.

Vitamin C was almost completely lost in both sun-drying and dehydration, but sulphuring was decidedly helpful in protecting the full content in peaches and apricots (when the SO2 content was above 500 p.p.m.). Prunes retained 0 to 50 percent when sulphured and not lye-dipped, but 100 percent when lye-dipped. The vitamin was completely destroyed in figs whether sulphured or not, and no vitamin C was found in dried raisins and dates. Other workers report the practically complete loss of this vitamin in onions, leeks, potatoes, carrots, and beets, although sulphured cabbage retained some. In general, the same condition obtained for vitamin C as for vitamin A in that these vitamins are protected from oxidation by means of low temperatures such as used in dehydration, and by a reducing agent, such as sulphur dioxide. As a practical matter, dried fruits and vegetables do not constitute a prophylactic against scurvy."

Vitamin B was found by Morgan and associates to be almost completely destroyed in the dried sulphured fruits, but it was retained in substantial amounts in the unsulphured products, especially raisins. However, as a whole, dried fruits are not a rich source of vitamin B. Its content in dried vegetables does not seem to have been extensively determined.

Vitamin G in dried fruits and vegetables has been studied only a little, but the indications are that it is quite resistant to change and is probably little affected by sulphuring, lye-dipping, dehydration, or sun-drying. Prunes contain it in substantial amounts." Sebrell states that there is no pellagra-preventive value to dried prunes or evaporated apples, but that dried peas have some such value.

Epidemiology. An outbreak of severe colitis was traced by Hunwicke and Grinling 16 to French packaged dried dates. The causative organism was B. coli tropicalis, very similar to Escherichia coli. Out of 11 samples examined, intestinal coliform organisms were isolated from 6, and these were from the packaged product, and not any were from the bulk dates. This indicates infection during the packing operations. Similar outbreaks of illness have been attributed to dried peas and also to codfish cakes, but the evidence indicates that the products were contaminated after cooking. Dried fruits and vegetables have a very meager epidemiology.

Microbiology. Prescott in his early work found that microbial flora on artificially dehydrated vegetables varied in numbers from 100 to more than 1,000,000 organisms per gram. Types were similar to those found on fresh fruits, in soil, and in water, and contained no pathogens. During storage, the numbers greatly decreased. He incubated laboratory samples of fruit with cultures of pathogenic organisms, dried them at 80 C. (176 F.) for 4 hours, and found that there were some survivors such as typical Eberthella typhosa and some paratyphoid organisms. A second series gave similar results. In a third series, he inoculated carrots, cabbage, and potatoes with a dozen different varieties of pathogenic organisms, including those of typhoid, paratyphoid, botulism, and enteritis, and dried them under plant conditions. He was unable to recover any of the organisms from these dehydrated inoculated vegetables. He did not consider this work conclusive.

Fellers examined 79 samples of American dried fruits and found 8 samples to be contaminated with lactose-fermenting organisms, al-though none were identified as Escherichia coli. Sixty of these samples carried no demonstrable yeasts (on nutrient agar). A few anaerobes were present in about half of the samples, and cocci and molds were quite abundant. Smeall 19 incubated dates with typhoid bacillus and found that the organism was still viable on the sixty-eighth day. He studied the flora of Tunis dates and, although he found no definitely pathogenic organisms, he did identify Streptococcus faecalis, and found it viable on the seventy-fifth day.

Clague sprayed suspensions of Escherichia coli onto grapes, carrots, and spinach. He found that these organisms were killed by dehydrating the grapes but they survived drying on the carrots and spinach when the food was blanched before the contamination; on the other hand, they were killed when blanching followed the spraying. Artificial drying (dehydration) effectively killed the yeasts and greatly reduced the number of molds and bacteria on fruits. Dehydration was not so effective in reducing the number of microorganisms on the vegetables as on the fruits. The types of organisms on dried foods were mostly Gram-positive, sporulating bacteria, and molds, although Gram-negative bacteria and yeasts were occasionally found. There were no proteolytic organisms on 16 samples of commercially dried vegetables. Some lactose fermenters were present, but they were not the Escherichia coli type.

In view of the occasional incrimination of dried fruit in disease outbreaks, its evident contamination in packing, and the wide variety of its microbic flora, Fellers studied the possibility of correcting this condition by pasteurizing the food. He coated dates, figs, raisins, and prunes with cultures of Escherichia coli. A heat treatment of 160 F. (71 C.) for 30 minutes, at a relative humidity of 75 percent or more, was effective in destroying all these organisms. The Eberthella typhosa was used on dates, and all the organisms were destroyed after exposure to 170 F. (77 C.) for 25 minutes or 160 F. (71 C.) for 30 minutes. Storage tests were made on the dates, prunes, raisins, and figs which had been inoculated with Escherichia coli. On holding at these temperatures, colon organisms died on the prunes in 15 days, and on the raisins, dates, and figs in 30 days.

The common practice of sulphuring fruit, dipping it in boiling lye solution for a few seconds, treating it with boiling water or steam for a few minutes, dehydrating it at 140-160 F. (60-71 C.) for several hours, or drying it in direct sunlight at 120 F. (49 C.) for several days, individually and cumulatively reduces microbic flora to a very considerable extent. Storage of the desiccated product under proper conditions of low humidity still further causes organisms to die off.


Standards. The U. S. Department of Agriculture has issued the following definitions and standards for several of the more commonly used dried fruits:

Evaporated Apples. Peeled, cored and sliced apples from which the greater portion of the moisture has been evaporated. The finished product contains not more than 24 percent of moisture.

Dried Apricots. Halved and pitted ripe apricots from which the greater portion of the moisture has been evaporated. Before packing, the dried fruit is commonly processed by washing. The finished product contains not more than 26 percent of moisture.

Dried Peaches. Halved and pitted ripe peaches from which the greater portion of the moisture has been evaporated. Before packing, the dried fruit is commonly processed by washing. The finished product contains not more than 26 percent of moisture.

Dried Prunes. Whole ripe prune plums from which the greater portion of the moisture has been evaporated. Before packing, the dried fruit is commonly processed by treatment with boiling water or steam. The finished product contains, in the fleshy portion, not more than 25 percent of moisture.

Types of spoilage. Various dried fruits and dried food products have been condemned for being moldy and fermented. In some instances, in addition to the presence of insect infestation and filth, excessive spray residues are found. Wormy dried figs, apples, cherries, dates, currants, and olives have been seized. Dried fruits and vegetables may also contain too much moisture and excessive amounts of sulphites. Undue quantities of sulphur have been used to rejuvenate old and decomposed stock. The U. S. Government has set no tolerance for sulphur dioxide in dried food products, although some states have established restrictive limits or even permit none. Zinc has been a contaminant from the drying trays. Arsenic and lead may be present from spraying of the fruit.

Laboratory examination. The chemical analytical procedure for dried fruits is similar to that for fruits and fruit products.

The analysts of the U. S. Food and Drug Administration sample bulk dried peaches, apricots, and nectarines by collecting representative portions of at least 100 pieces.' Boxed or packaged fruit is sampled by taking several pieces from each package. Each piece is carefully examined by the naked eye, sometimes aided by a small pocket lens, and classified as to degree of insect infestation, moldiness, and contamination with dirt.

Inspection control. Inasmuch as most of the dried fruits and vegetables are produced on the Pacific Coast, it is impossible for local control officers over the country at large to compel sanitary plant practices in the production of this food. However, examination of dried fruit will reveal the presence of moldy, dirty, or insect-infested stock. Manufacture of jellies and other foods from dried fruit products, especially dried apple pomace or pulp, is practiced widely throughout the country, and often it is done under very poor sanitary conditions. The fact that it is known in the trade as "apple waste" tends to create an indifference and carelessness among the workmen in the observance of proper sanitary practices.

Dried fruits are often offered for sale in opened boxes set on the sidewalks. All such boxes should be raised at least 18 inches from the footway to protect the contents from contamination from animals. Such products cannot be washed before they are eaten, and therefore should not be handled by the unwashed hands of salesmen or be accessible to flies or other vermin. Their stickiness, especially when opened and exposed to air, facilitates their contamination with miscellaneous street filth which freely blows about. The increasing tendency to package these products in transparent wrappers or in cartons is a sound, sanitary measure. Some are pasteurized, and this treatment should be encouraged.


1. P. F. Niellons and associates, U. S. Dept. Agr. Departmental Bul. 1335, 1925.

2. P. F. Nrcaons and W. V. CauEss, Ind. Eng. Chem., 24, 649 (1932).

3. P. F. NIcHoLS, Am. J. Pub. Health, 24, 1129 (1934).

4. B. J. HOWARD, Publication 4, Microanalytical Division, U. S. Food and Drug Administration, May, 1935.

5. R. A. MCCANCE and associates, Med. Res. Council, Spec. Rept. Ser. 213, Lon-don, 1936.

6. W. O. ATWATER and A. P. BRYANT, U. S. Dept. Agr. Office Exp. Sta. Bul. 28, revised, 1906.

7. H. K. SrIEBELING, U. S. Dept. Agr. Circular 205, 1932.

8. E. P. DANIEL and H. E. MuNSELL, U. S. Dept. Agr. Miscel. Pub. 275, 1937.

9. L. G. SAYWELL, J. Nutrition, 6, 397 (1933).

10. E. G. HALLIDAY and I. T. NOBLE, J. Home Econ., 28, 15 (1935).

11. A. F. MORGAN, Am. J. Pub. Health, 25, 328 (1935).

12. C. R. FELLERS, Mass. Agr. Exp. Sta. But. 338, 1936.

13. C. L. WILLIAMS, Am. J. Pub. Health, 23, 561 (1933).

14. Report of the Chief of the Food and Drug Administration, 1937, p. 5.

15. W. H. SEBRELL, Pub. Health Repts., 49, 754 (1934).

16. R. F. HUNWICKE and G. N. GRINLING, Lancet, I, 1071 (1928).

17. S. C. PRESCOTT, J. Bacteriol., 5, 109 (1920).

18. C. R. FELLERS, Am. J. Pub. Health, 20, 175 (1930).

19. J. I. SMEALL, Brit. Med. J., II, 917 (1932).

20. J. A. CLAGUE, Food Research, 1, 45 (1936).

21. "Service and Regulatory Announcements," Food and Drug 2, fifth revision, November, 1936.

22. Methods of Analysis, Association of Official Agricultural Chemists, 4th ed., 1935.

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