In this article we will discuss about the cucumber fermentation process in food industries:- 1. Preparation of Pickles 2. Brining Techniques for Salt Stock Pickles 3. Microbiology 4. Fermented Dill Pickles 5. Deterioration 6. Softening 7. Gaseous Spoilage of Pickles 8. Chemical and Physical Deterioration 9. Pure Culture Fermentation Studies 10. Controlled Fermentation of cucumbers.
- Preparation of Pickles
- Brining Techniques for Salt Stock Pickles
- Microbiology of the Cucumber Fermentation
- Fermented Dill Pickles
- Deterioration of Pickles
- Softening of Fermented Cucumber
- Gaseous Spoilage of Pickles
- Chemical and Physical Deterioration of Pickles
- Pure Culture Fermentation Studies
- Controlled Fermentation of Cucumbers
Cucumber Fermentation Process
1. Preparation of Pickles:
The cucumber (Cucumis sativus), one of the oldest vegetables cultivated by man, is thought to have had its origin in Asia, perhaps India, more than 3000 years ago. It is popular both as a fresh and as a pickled vegetable and is grown widely in temperate climates although originally of semitropical origin. Successful culture of this vegetable is dependent upon avoidance of frost and drought and the control of microbial pathogens and insect pests.
Cucumbers for pickling must be grown from varieties known to have regular form, firm texture, and good pickling characteristics.
Formerly, the common pickling cucumber varieties recommended by various authorities included the Chicago pickling, Boston pickling, Jersey pickling, National pickling, Heinz pickling, Fordhook pickling, Snow’s perfection, Packer, and various other strains of lesser importance. These varieties, all open-pollinated or monoecious plants, which bear both male and female blossoms, although still available, are being replaced largely by hybrids developed to be used for once-over mechanical harvest.
These new hybrid varieties are called “gynoecious” because they have a preponderance of female flowers but are not 100% female. Now, however, most gynoecious hybrid seed must have a pollinator added. These new cultivars often have greater vigor and uniformity than the open-pollinated ones formerly grown. In addition, several of the hybrids are early maturing so they can be used to advantage in harvest scheduling.
Hybrids are used for once-over mechanical harvest and also have given good performance for hand harvesting. Michigan, North Carolina, and California, in that order, lead in total production of pickling cucumbers. California leads in yields per ha (acre) and has done so for many years. In 1976, California produced 33.3 MT per ha (14.8 tons per acre) for a total of 63,900 MT (71,000 tons). At present, Michigan is the only state to have committed itself heavily to machine harvesting of cucumbers, having harvested over 95% of its crop by machine in recent years.
According to Sims and Zahara (1978), the desirable characteristics of a variety suitable for once-over machine harvesting are- (1) relatively small vine with length not over 76 cm (30 in.); (2) relatively short internodes to obtain the maximum number of fruit-setting points; (3) concentrated fruit set and even maturity; (4) early maturity; (5) tendency for fruit to remain on the vine until removed by harvester; (6) resistance to skin and internal damage; (7) late yellowing of fruit if variety is black spined. The use of white spined hybrids has increased in recent years because their fruits do not turn yellow at the blossom end and maintain their greenness longer, thus providing a more uniform color; (8) uniform shape with a minimum of deformities produced under stress; blocky ends are preferable over pointed ones; (9) a thick wall, small seed cavity, and slow seed development; and (10) blossoms readily fall off fruit. Obviously, the same desirable criteria are valid for hand harvested cucumbers.
Pickling cucumbers are harvested while still immature. Fully grown (ripe) ones are undesirable for pickling because they become too large, change color and shape, are full of mature seeds, and are too soft for most commercial uses. Whether harvested by hand or by machine, care must be taken in picking and transporting the cucumbers to avoid undue bruising and crushing. It should be mandatory to deliver the cucumbers to the salting station or factory as soon as possible after harvest to prevent deterioration.
Too long a holding time prior to brining allows the cucumbers to “sweat.” This condition promotes the growth of undesirable softening organisms which may cause spoilage early in the brine fermentation before the pH becomes inhibitory to the pectolytic or cellulolytic microorganisms, which are nearly always found to be present on the cucumbers at time of harvest.
To minimize spoilage during fermentation, it is important to remove all unsound, decomposed, broken, or crushed cucumbers. Sorting to remove all crushed or broken, defective, and distorted cucumbers (wilt, rot, crooks, nubbins, etc.) should be done before bringing to minimize spoilage during fermentation. Sorting is followed by size grading unless the cucumbers are to be fermented field run. Mechanical graders are used to separate the cucumbers into 4 or more sizes. Final size grading and sorting is done after fermentation.
Three types of pickled cucumbers are made. They include fresh pack (also called fresh cure, home style, and other names) which, at most, are held in salt brine for only as long as 2 days, then packed in jars or cans and pasteurized; salt stock pickles from which a variety of processed products are made; and fermented dill pickles. The two latter kinds undergo complete lactic acid fermentation whereas the fresh pack pickles undergo, at best, a marginal fermentation unless held in brine for 24 hr or more.
It is estimated that 40 to 50% of the annual harvest of cucumbers is made directly into fresh pack or pasteurized products including whole dills, dill spears, dill chips, sweet slices, etc. The remainder of the crop is converted into fermented salt stock pickles or fermented dills by lactic acid fermentation. The cured salt stock is desalted and processed into various staple pickle products including sweet and sour pickles, mixed pickles, processed dills, sliced pickles, relishes, etc.
2. Brining Techniques for Salt Stock Pickles:
There are 2 general methods for preparing salt stock pickles for fermentation:
1. Dry Salting:
The dry salting procedure is not used extensively for cucumbers at present because of its tendency to yield soft, flabby, shriveled pickles that do not fill out properly when processed. However, dry salting is used for other produce, especially cauliflower, red bell and pimiento peppers, salt-cured ripe olives, and is the procedure of choice for sauerkraut.
For cucumbers, dry salting is done after first adding salt brine to cover the bottom of the tank to a depth of at least 30.5 cm (12 in.) to form a cushion, thus preventing bruising, breaking, or crushing the fresh cucumbers when they are dumped into the tank. Dry salt is added at the rate of about 22.5 kg (50 lb) for every 450 kg (1000 lb) of small cucumbers and 29.25 kg (65 lb) for every 450 kg (1000 lb) of large cucumbers.
When full, the tank is covered with a circular, slatted, wooden head until there is room for about 15 cm (6 in.) of brine above the cover. The slatted head is then secured with heavy cross timbers held at the ends with clamps. For convenience in handling, the slatted cover may be 2 semicircular pieces for large tanks or even 3 pieces when the largest diameter tanks are involved. If the brine formed by osmosis does not cover the cucumbers or cover when the tank is closed, 40° salometer brine is added to the desired level. The brine should be re-circulated a day or two after the tank is filled in order to equalize the concentration of salt throughout the brine. On long storage the brine may be increased slowly until it is about 60° salometer.
In the brining industry the concentration of salt is expressed in degrees salometer which is % saturation of NaCl by weight. A saturated solution of pure sodium chloride (100° salometer) contains 26.359 g at 15.5°C (60°F). Thus, a salometer reading of 10° is equal to 2.64% NaCl by weight (rounded to the nearest tenth). Salt hydrometers are calibrated so that readings will cover several ranges of salt- low, medium, and high. Hydrometers also are available that are calibrated in % salt by weight.
2. Brine Salting:
Most picklers use the brine salting technique for fermenting cucumbers rather than the dry salting procedure just described. A “low” or a “high” brine process may be used. The low brine has a salt concentration of 25° to 30° salometer, whereas the “high” brine contains 40° or more salt by hydrometer.
The cucumbers are handled for the dry salting process except that brine is used to cover the produce. The tanks are headed in the same way if they are of wood or concrete construction.
Recently, molded plastic and fiberglass tanks have been found useful for replacement of the wood or concrete containers lost by attrition. These plastic and fiberglass containers have several distinct advantages. They are not subject to the usual biological degradation of wood or chemical corrosion of concrete; they do not have to be maintained during the off season, as do wooden ones, to keep them from developing leaks which sometimes require extensive coopering to repair.
The drain valves are plastic (polyvinylchloride), as are all other piping, so metal corrosion and resultant contamination of the cucumbers is eliminated. The greatest advantage of these newer containers, however, is that, if they are properly designed, the closures are nearly airtight so that former problems with loss of acidity caused by growth of aerobic yeasts are greatly reduced.
With the use of plastic sheeting to cover the brine in open tanks, the problem of control of film yeasts in cucumber fermentations has, in recent years, been reduced to a minimum. Sheet plastic (polyethylene) may be used with sauerkraut or as done with cucumbers and olives held in open tanks in California.
In the latter case, the plastic film is floated on the surface of the brine over the false head and secured to the inside of the tank with pliable wood slats nailed so that the plastic is held in place at the surface of the brine. This arrangement will provide nearly anaerobic conditions unless the plastic has imperfections or the slats are improperly placed; Complete, or nearly complete, anaerobiosis can be attained by using “Sealtite” (a wax used widely in the wine industry) to seal the plastic cover to the sides of the tank.
Once the tank of cucumbers has been filled, the cover secured, and brine added, there is a rapid development of microorganisms in the brine. In general, no attempt is made to control the microbial populations of the brines so the cucumbers undergo a “spontaneous” fermentation.
The natural controls of the microbial population of the fermenting cucumbers include the concentration of salt in the brine, the temperature of the brine, the availability of fermentable materials, and the relative numbers and types of microorganisms present on the cucumbers and in the brine at the start of the fermentation. The rapidity of the fermentation is directly related to the temperature of the brine and its concentration of salt.
The initial brine strength will vary, depending upon the individual pickling company. In the past, higher concentrations of salt were used because high salt levels were believed to retard spoilage. Now, with increasing frequency, cucumbers are fermented in brines in the 5-8% NaCl range.
At this concentration of salt, the sequence of species of lactic acid bacteria approximate that already described for the sauerkraut fermentation with the exception that the species of Leuconostoc never predominate the initial stages of the fermentation, even at 5% salt.
At 8% salt these species may not be detected at all. The other lactic acid bacteria, Pediococcus cerevisiae, Lactobacillus brevis, and Lactobacillus plantarum, occur in most, if not all, fermentations made in the range of 5 to 8% salt. Pediococcus cerevisiae is less salt resistant so sometimes it is absent in the brine at the higher concentration (8%). The same is true with Lactobacillus brevis.
During the primary stage of fermentation, a great many unrelated bacteria, yeasts, and molds have been isolated. All are widely distributed in nature and, at the beginning of the fermentation, far outnumber the desirable lactic acid bacteria in uncontrolled fermentations.
The primary stage of fermentation, therefore, is the most important phase of the pickling process. If for any reason the fermentation does not proceed normally during this period, any of the unessential microorganisms may become predominant and contribute to spoilage.
The primary stage normally lasts 2 or 3 days, exceptionally as long as 7 days, or even more. During this period, the numbers of lactic acid bacteria increase rapidly, both fermenting and oxidizing yeasts increase significantly, and the extraneous and undesirable forms decrease rapidly and may disappear entirely. At the same time, a steady increase in total acidity and a corresponding decrease in the pH of the brine are observed.
In low-salt brine stabilized at about 5% NaCl, a mixture of the low-acid- tolerant species of Leuconostoc and the high-acid-tolerant species of Lacto-bacillus and Pediococcus predominate in the intermediate stage of fermentation. If the fermentation is normal, the extraneous and undesirable bacteria have completely disappeared by the end of 10 to 14 days, although yeasts are still present in significant numbers. There is a further increase in total acidity and the pH value has also decreased more.
The data of Etchells and Jones (1943), shown in Fig. 6.1, aptly demonstrate the changes in populations of coliform bacteria, acid-forming organisms, and yeasts found in natural fermentations of brined cucumbers in salt concentrations of 20°, 40°, and 60° salometer, respectively. It is seen that the coliform bacteria arid other Gram-negative species are readily inhibited in brine fermentations having 40° salometer salt or less because of prompt development of acid by the lactic acid bacteria.
However, in brines at 60° salometer salt, the lactic bacteria and the coliform salt-resistant type of Aerobacter (represented by the dotted line in Fig. 6.1) and yeasts compete for the fermentable materials and may produce large quantities of gas (CO2 and H2) and cause a high percentage of hollow stock (bloaters). It is interesting that the salt-resistant type of Aerobacter also occurs in olive fermentations and has been identified by Foda and Vaughn (1950) as an indole-positive type of Aerobacter aerogenes.
Species of Leuconostoc are fostered by low temperatures (7.2°-10°C or 45°-50°F) and low salt concentrations (2.5-3.7%) according to Pederson and Albury (1950, 1954). Etchells et al. (1975) believe that this species would not normally be encountered in commercially brined salt stock cucumber fermentations at 6-8% salt and 24°-29°C (75°-85°F).
There nearly always pre exceptions that “proves the rule,” however. One of the present author’s first encounters (unpublished) with species of Leuconostoc involved approximately 150 barrels (190 liter or 50 gal. capacity) of refrigerated dill pickles being produced for the delicatessen market.
Most of the barrels of fermenting cucumbers had very slimy brine. The brine temperatures varied from 6° to 8CC (43° to 47°F) and the salt concentrations were in the range of 4-6% NaCl. On examination, it was determined that the predominating bacteria belonged to the species Leuconostoc mesenteroides.
In retrospect, it is believed that the low brine temperature countered the effect of the salt concentration to permit L. mesenteroides to dominate the brines of the refrigerated dill pickles, even at 6% NaCl. Even so, it is agreed that Leuconostoc mesenteroides is not an integral part of the lactic acid bacteria populations of normal salt stock fermentations in the range of 5 to 8% salt and always is absent in brines having more salt.
Pediococcus cerevisiae, Lactobacillus brevis, and Lactobacillus plantarum are responsible for the final stage and the completion of the lactic acid buildup in uncontrolled fermentations in brines containing salt stabilized in the range at 5 to 8%. All 3 species are found when the cucumbers are fermented at less than about 8% salt.
However, the activity of P. cerevisiae is severely restricted at this salt concentration and ceases to proliferate when the pH falls to about 3.7. This leaves only the 2 species of Lactobacillus left to complete the fermentation. At the end, the total acidity may reach as high as 0.9%, calculated as lactic acid, and have a pH value as low as 3.3, providing oxidative yeasts are held in check by anaerobiosis.
4. Fermented Dill Pickles:
Genuine dill pickles differ from the other well-known dill pickles (fresh cure and processed dills) because they are the product of bacterial fermentation in dill flavored, spiced, salt brine. They owe their distinctive flavor and aroma to the products of fermentation of the lactic acid bacteria and to the blending of flavor and aroma of dill herb and spices that were added to the brine.
The larger sizes of cucumbers are generally used for preparing fermented dill pickles. They are washed and placed in suitable containers, together with the requisite amount of dill weed (generally cured in vinegar, salt brine) and dill spices, and brined.
Dill pickles are generally fermented in low-salt brine of 5% or even less NaCl, but some use up to 7 or 8% brine. The use of vinegar to help retard the growth of undesirable microorganisms by decreasing the pH value of the brine is a common practice. The optimum temperature for the fermentation is between 21° and 26.7°C (70° and 80°F).
The fermentation is usually active for 3 to 4 weeks and an additional curing period of 3 to 4 weeks is considered essential. During this period of 6 to 8 weeks, the flesh of the pickles becomes entirely translucent. The brine contains about 0.5 to 1.2% total acidity calculated as lactic acid.
In addition, there is a small amount of volatile acid (acetic), ethanol, and other minor products produced by the lactic acid bacteria and yeasts present during the fermentation. The pH values range from about 3.3 to 3.5 if the pickles have been held under nearly complete anaerobiosis. Otherwise, if oxidative yeasts persist, the pH values will be higher due to the loss of acid utilized by them.
Formerly, it was the almost universal practice to use 190 liter (50 gal.) barrels for the fermentation, although a few picklers did use small, wooden tanks. Production now must be considered bulk fermentation, for most fermented dills are made in wood tanks or plastic or fiberglass receptacles containing 0.9 MT (1 ton) or more of cucumbers.
“Overnight,” “Refrigerated,” or “Icebox” dill pickles are similar to genuine dills with the important exception that they are stored at a low temperature (38°-45°F) where a slow lactic acid fermentation produces (at the end of 6 months), a total acidity of only about 0.3 to 0.6% calculated as lactic acid.
These refrigerated pickles retain some of the fresh cucumber flavor and are highly prized as a food adjunct. However, they are so perishable they must be kept under refrigeration until sold for consumption. Consequently, their availability is not widespread and many people have never had the pleasure of eating them.
Bulk fermentation has become possible because of increased understanding of the need for anaerobic conditions in the fermentation. Wood tanks are fitted with a plastic film and the plastic and fiberglass containers are designed to provide anaerobiosis.
At the beginning of either type of dill fermentation, the original microbial population includes a wide variety of unrelated bacteria, yeasts, and molds. They all are widely distributed in nature and far outnumber the lactic acid bacteria at the start. However, if the fermentation proceeds in a normal fashion, the lactic acid bacteria soon predominate.
The sequence of lactic acid bacteria already described for salt stock pickles also is found in the fermenting brines of the dill pickles just described. However, because of the lower salt concentration, and in the case of the refrigerated dills, the lower temperature, Leuconostoc mesenteroides plays a more important role. The work described by Pederson and Ward (1949) and Pederson and Albury (1950) substantiate the initiation of the fermentation by Leuconostoc mesenteroides at lower temperatures and lower salt concentrations.
Once the lactic fermentation has an appreciable start, then the other species, Pediococcus cerevisiae, Lactobacillus brevis, and Lactobacillus plantarum begin to dominate the fermentation. The fermentation then is completed by the 2 species of Lactobacillus, L. brevis and L. plantarum.
Genuine dill pickles may be marketed in bulk in plastic containers of various sizes, or, as is done by some picklers, packed in glass, covered with an acidified brine, closed, and pasteurized at 74°C (165°F) for 15 min (center of jar temperature) and cooled rapidly to 37.8°C (100°F) or less. So far as is known, the refrigerated dills are always sold in bulk and held under refrigeration until consumed, because of their extreme susceptibility to spoilage.
5. Deterioration of Pickles:
Extensive study has been made of the spoilage of cucumbers during fermentation, curing, and storage. Most of the deterioration is caused by the activity of microorganisms, either by the elaboration of deteriorative enzymes or as the result of copious production of gaseous end products (carbon dioxide and hydrogen).
Chemical defects are generally confined to metallic contamination or unanticipated alteration of flavor and aroma by the use of specific chemicals or undefined congenerics used for spicing purposes. Auto-chemical and physicochemical reactions also have occurred.
The most damaging defect caused by microorganisms is tissue destruction resulting from cellulolytic or pectinolytic enzymes elaborated by a variety of organisms. Tissue destruction and loss of texture or firmness generally means nearly total economic loss to the pickler.
Gaseous deterioration, resulting in the production of internal cavities or distorted stock caused by excessive gas pressure is another common spoilage caused by microorganisms. This defect is known as “bloater” or “floater” spoilage. Affected pickles may have lens-shaped internal cavities or the locules may be slightly separated.
In severe cases the locules become completely separated and the flesh in each locule is compressed so that the interior is completely hollow and the shape then is reminiscent of a balloon. The salt stock pickles damaged by destructive gas pressure generally may be salvaged by diverting them to relish type products. However, bloater spoilage of dill pickles may mean an economic loss at the present time.
6. Softening of Fermented Cucumber:
Softening occurs when microorganisms are capable of elaborating pectinolytic or cellulolytic enzymes under the conditions of salinity, acidity, etc., which exist in pickle brines. Softening is a progressive spoilage which occurs most frequently soon after the cucumbers are brined for production of dill or salt stock pickles.
The skin of the cucumber is attacked first, usually at the blossom end. In a short time, the entire skin may be affected, become slippery and be easily removed. This characteristic first manifestation of softening has given rise to the descriptive terms “slips” or “slippery” pickles in the industry.
“Mushy” pickles result when the softening progresses into the deeper layers of cells in the pickles and more and more pectic materials, present in the middle lamella separating the individual cells of the cucumber are attacked. It is interesting that the form of the pickle may appear normal, but when pressure is applied, it turns to a mushy consistency.
Three kinds of pectolytic enzymes are produced by the bacteria, Pectin methylesterase (pectinesterase) splits off methyl groups from the pectin molecule leaving pectic acid. Polygalacturonase degrades pectic acid leaving saturated digalacturonic acid or higher oligouronides. Polygalacturonic acid trans-eliminase splits pectic acid leaving unsaturated digalacturonic acid or higher unsaturated oligouronides as the major end products.
A variety of bacteria, yeasts, and molds are known to produce pectolytic enzymes The Gram-positive types of Bacillus include the following species- B. subtilis, B. pumilus, B. polymyxa, B. macerans, and B. stearothermophilus; all have been studied and their pectolytic enzymes described by Vaughn and his associates.
The Gram-negative types, involving several genera and including Achromobacter, Aerobacter, Aeromonas, Escherichia, Erwinia, and Paracolobactrum, also have been found to contain strains having pectolytic activity.
All of the bacterial species and genera have been shown to degrade pickles, making them first slippery and then mushy in texture when tested in vitro with sterilized cucumbers. The limiting pH for the activity of the bacterial pectolytic enzymes is in the range of 5.0 to 5.5.
It is concluded, therefore, that representative pectinolytic species of the preceding genera of bacteria may cause softening of cucumbers, either salt stock or dills, if:
(1) The pectinolytic bacteria pre-dominated the microbial populations of the cucumbers and their brines.
(2) The pH of the brined cucumbers was in the desirable range for softening—pH 5.5 or above. It is known that some of the softening bacteria can raise the pH of the brines unless the initial values are inhibitory.
(3) The desirable lactic fermentation is retarded or arrested in some manner so that the pH values of the brined cucumbers remain relatively high for several days.
(4) The brine concentration is in the range of 5 to 8% NaCl.
A variety of different yeasts and molds also have the ability to decompose pectinous substances. The first reports of pectolytic activity by yeasts apparently were made by Cruess and Douglas (1936) and Roelofsen (1936) in July and October, respectively. However, it was not firmly established until 1951 that yeasts did possess pectolytic activity.
The work of Luh and Phaff with Saccharomyces fragilis was reported then, and later confirmed by Roelofsen (1953) in a reiteration of his 1936 publication, originally published in an obscure journal in the Dutch language. A few other yeasts are known to be pectinolytic.
Vaughn et al. (1969A) described 3 species of Rhodotorula that produced polygalacturonase and, again, in 1972 described 2 other species of Saccharomyces that could decompose pectinous material. All of the yeasts produced polygacturonases that were active in the acid range well below the pH value 5.5 thought to be limiting for the bacterial enzymes.
Salt concentrations above become limiting for the growth of some of the yeast strains. This may be one reason why, in the past, more yeasts causing softening have not been recovered from cucumber brines. So far as is known, yeasts produce only polygalacturonase.
A variety of molds produce softening enzymes including both cellulolytic and pectolytic types. The molds are known to produce pectinesterase, polygalacturonase, and pectin-trans-eliminase. These enzymes have been carefully described by Phaff (1947) who worked with Penicillium chrysogenum and by Edstrom and Phaff (1964) who used Aspergillus fonsecaeus to describe pectin-trans-eliminase purification and properties.
The fungi include representatives of the genera Alternaria, Aspergillus, Cladosporium, Dematium, Fusarium, Geotrichum, Mucor, Myrothecium, Paecilomyces, Penicillium, Phoma, and Trichoderma. Etchells et al. (1955) demonstrated that molds grow and secrete the softening enzymes into the cucumber flowers. The introduction of the fresh or dried flowers containing the enzymes, together with the cucumbers to which they are attached, provides the spoilage factor when the brine is added.
Tanks filled with small cucumbers retaining a high percentage of flowers or with experimentally added flowers possess high enzyme activity, and the pickles usually become soft or inferior in firmness. Losses caused by contaminated flowers can be greatly reduced by draining the original cover brine and replacing it with new brine. This apparently reduces the amount of softening enzyme so that softening becomes negligible or nil.
Draining the cover brine had been a satisfactory method for control of the softening problem since .1954 until recent state and federal regulations concerning disposal of salt severely restricted its use. Studies then were directed toward a search for inhibitors of pectolytic and cellulolytic enzymes.
This search, involving isolation of an inhibitor from various plant sources, was successful. The forage crop, Sericea lespedeza was a particularly good source of the inhibitor and Bell et al. (1965 A) reported that 50-100 ppm of a crude extract from this source would block softening of no. 1 size cucumbers.
Unfortunately, there has been difficulty in obtaining approval for use of the inhibitor under commercial conditions so Etchells et al. (1975) have turned to studies involving reclaiming the salt brines for recycling. (It seems to this author, at least, that the simplest method for reclaiming the brine would be the use of heat to inactivate the enzymes, followed by precipitation, flocculation, and filtration procedures, etc., commonly used for purification of water supplies.)
7. Gaseous Spoilage of Pickles:
Gas-producing microorganisms, now known to cause gaseous deterioration of pickles, represent a number of genera of yeasts and bacteria. Although it was suspected earlier, the first substantial evidence that gaseous spoilage was caused by microorganisms was presented by Veldhuis and Etchells in 1939.
They found that hydrogen was produced in significant quantities in the fermentations at 60° Salometer brine and in some, but not all, fermentations at lower concentrations of salt. They also isolated, but did not identify, an organism which produced significant amounts of hydrogen (probably an Aerobacter—author’s comment).
Somewhat later Jones et al. (1941) and Ftchells and Jones (1941) suggested that gaseous fermentation by unidentified yeasts was the cause of “floater” spoilage. Still later Etchells et al. (1945) presented evidence that coliform bacteria of the genus Aerobacter caused the formation of hydrogen in pickle fermentations.
Yeasts active in natural pickle fermentations are undesirable, either because they cause copious gas formation or consequently increase bloater formation, or because they utilize lactic acid and, if uncontrolled, cause a rise in pH to the point where spoilage bacteria can renew their, activities.
The fermenting (subsurface) yeasts have been identified as belonging to the genera Brettanomyces, Hansenula, Saccharomyces, and Torulopsis by Etchells et al. (1961). The oxidative (surface or film) yeasts nave been identified as belonging to the genera Candida, Endomycopsis, Debaryomyces, and Zygosaccharomyces by Etchells and Bell (1950). Mrak and Bonar (1939) considered the genus Debaryomyces to be responsible for film formation on salt stock pickles.
Lactobacillus brevis, a gas-forming lactic acid bacterium, was shown by Etchells and Bell (1956) to cause floater formation in vitro. More recent work by Etchells and his associates has unravelled most of the unknown factors in the complete explanation of “bloater” spoilage.
They found that when unheated large-sized cucumbers are brined, serious bloating occurs even when Lactobacillus plantarum is the fermenting species. It was found that when unheated cucumbers are brined, respiration of the fruit liberates enough carbon dioxide into the brine so that, when combined with the small amount produced by L. plantarum, it is sufficient to cause bloater damage.
Other gas-forming bacteria found in the initial stage of the cucumber fermentation are known to cause in vitro spoilage in pasteurized 5% brine cucumbers. These include the Bacillus polymyxa-macerans group of aerobic bacilli and the gas-forming pseudomonad, Aeromonas liquefaciens.
Thus, it is seen that all of the gas-forming microorganisms found in pickle brines, desirable as well as undesirable, may at one time or another be responsible for producing enough carbon dioxide and/or hydrogen to cause gaseous spoilage.
Nitrogen purging of the fermenting brine is used to reduce undesirable levels of carbon dioxide that otherwise might result in bloater formation.
8. Chemical and Physical Deterioration of Pickles:
The main cause of non-biological chemical deterioration of pickles is the direct addition of undesirable chemicals to the brines or the pickles themselves. The changes taking place may affect the appearance and color or the flavor and aroma of the pickles.
Contamination with copper and iron formerly was the most common type of chemical deterioration. Vinegar or lactic acid which was used for preparation of brines or liquors for pickles formerly was frequently heavily contaminated with copper and iron.
Other metallic contamination with these two chemicals resulted from contact of corrodible equipment (valves, pipe lines, etc.) made from alloys containing copper and iron with the comparatively corrosive brines used for pickling. Stainless steel and plastic components have eliminated this problem.
Copper replaces magnesium in the chlorophyll and pheophytin contained in the pickles and causes them to turn an unnatural, artificial green to blue green color. Only 5 to 10 ppm of copper in the brine or liquor is enough to spoil the pickles.
Iron is involved in the blackening of pickle brines and pickles. Black brine in pickle fermentations, instead of always involving sulfate or reduction of protein sulfur compounds, in all probability frequently is the reaction of iron with poly-phenolic compounds to form complex iron “tannates” which are black, bluish-black, or greenish-black in color.
Oxygen is required to oxidize iron to the ferric state and to keep this reaction progressing. This complex reaction starts at the surface and extends downward as the oxygen penetrates. Iron sulfide, however, will form from reduced (ferrous) iron, so sulfuretted pickle brines are nearly uniformly black from the top to the bottom of the brine and, presumably, have the rotten egg odor which identifies H2S.
Rahn (1913) was one of the first to investigate the problem of black brines in pickles. He found that the black color resulted from the production of iron sulfide caused by bacterial reduction of sulfate in gypsum (CaSO4) to H2S, which then reacted with iron to produce the black iron sulfide.
Fabian et al. (1982) confirmed the work of Rahn and extended it to include a discussion of the possible liberation of hydrogen sulfide in the course of protein decomposition by bacteria. Although hydrogen sulfide production was well documented, the bacteria responsible were not identified.
9. Pure Culture Fermentation Studies:
Etchells et al. (1964) reported the results of their study of pure culture fermentation of brined cucumbers, an investigation which was of great importance to the pickling industry. This work led to the concept for the development of “In-Container” or “Ready to Eat” pickled products of a variety of kinds.
The processes for production of the ready to eat dill pickles and other products were described in a public service patent by Etchells et al. (1968 B). The process involved heat-shocking and aseptic packing of the product into sanitized containers, followed by covering with brine pasteurized at 76.7°C (170°F) and cooled to 4.4°C (40°F) and inoculation with pure cultures of lactic acid bacteria which resulted in a controlled fermentation.
However, in attempting to adapt the pure culture concept to tanks (2.25 to 13.5 MT or 2.5 to 15 tons capacity) used for bulk fermentation of cucumbers, many problems were encountered economically which could not be resolved, However, a bulk fermentation procedure has since been described which is practical and if the essential steps are followed will ensure against losses by spoilage organisms of whatever kind.
10. Controlled Fermentation of Cucumbers:
There are a number of steps in the process developed by Etchells and associates which are new. Some others have been practiced in the industry for at least 50 years. None the less, all of the steps aid in advancing our knowledge of the science and technology of cucumber fermentation.
Since it was impractical to destroy the contaminating organisms with heat, as was done in the pure culture process described above, thorough washing and in-container chlorination have been used to reduce the initial contamination by microorganisms, Chlorine (about 80 ppm) is added to a 25° salometer brine and used as a cover brine. The chlorinated cover brine is carefully acidified with acetic acid (food grade) or its equivalent of 200 grain vinegar. The cover brine is chlorinated again, about half a day after adding the cover brine.
Salt is added to the cover brine to maintain the original brine strength, which otherwise would be diluted by the water content (approximately 95%) of the cucumbers. The amount of salt added to the cover brine at the outset, and after 1 or 2 days, will depend upon the size of the cucumbers being brined.
After the initial salt addition has become equilibrated, but before the second salt addition and around 3 or 4 hr before the addition of the starter culture(s), sodium acetate is added to buffer the cover brine. This buffering is done to ensure that all of the fermentable sugars will have been utilized during the active stage of the lactic acid culture. Otherwise, acid-tolerant, fermenting yeasts could later ferment the residual sugar and produce levels of carbon dioxide perhaps sufficient to cause some gaseous spoilage.
Inoculations may be made with the 2 species of homo-fermentative lactic acid bacteria; P. cerevisiae and L. plantarum or else L. plantarum alone. It is most important that the starter cultures selected be able to give maximum performance under the conditions optimum for fermentation. They must grow well at 23.9°-29.4°C (75°-85°F), not be retarded by 6 to 8% salt and produce a minimum amount of carbon dioxide. The strains of P. cerevisiae must not inhibit L. plantarum if the 2 species are to be used together in starting the fermentation.
Nitrogen purging is started as soon as the tank of cucumbers is headed, brined, and acidified. The type and rate of purging will depend upon schedules for different sizes of cucumbers and container capacities. Etchells et al. (1975) report, “It has been found repeatedly that restricting a buildup of carbon dioxide in the brine during the entire fermentation, using the controlled process, prevents bloater formation.”