Sauerkraut Fermentation: Process, Microbiology, Defects and Spoilage | Industrial Microbiology

In this article we will discuss about the sauerkraut fermentation:- 1. Introduction to Sauerkraut 2. Process for Sauerkraut Fermentation 3. Microbiology of the Sauerkraut Fermentation 4. Defects and Spoilage of Sauerkraut.

Introduction to Sauerkraut:

The use of cabbage (Brassica oleracea) as a food antedates known re­corded history. Sauerkraut, a product resulting from the lactic acid fermen­tation of shredded cabbage, is literally acid (sour) cabbage. The antecedents of sauerkraut differed considerably from that prepared at present. At first the cabbage leaves were dressed with sour wine or vinegar.

Later the cabbage was broken or cut into pieces, packed into containers, and covered with verjuice (the juice expressed from immature apples or grapes), sour wine, or vinegar. Gradually the acid liquids were replaced by salt and a spontaneous fermentation resulted.

One may speculate that sauerkraut manufacture comparable to the method used today developed during the period of 1550 to 1750 A.D. although cabbage has been known and used commonly for about 4000 years. Those readers particularly interested in the historical evolution of the sauerkraut fermentation should consult Pederson (1960, 1979) and Pederson and Albury (1969).

Originally sauerkraut was made only in the home because it provided a means for utilizing fresh cabbage which otherwise would spoil before it could be used Now the commercial production of sauerkraut has become an important food industry. Even so, a significant quantity is still produced in the home, particularly in rural and suburban areas where home vegetable gardens still exist.

Cabbage varieties best suited for growth in the major production areas are used early, midseason, and late types are grown. Varieties formerly used such as Early Flat Dutch, Late Flat Dutch, Early Jersey Wakefield, and others have been replaced in part by new cultivars which have been bred to be well-adapted to mechanical harvesting and at the same time inherently contain less water, thus reducing the generation of in-plant liquid wastes. Mild-flavored, sweet, solid, white-headed cabbage is the choice as it makes a superior kraut.

Process for Sauerkraut Fermentation:

Properly matured sound heads of cabbage are first trimmed to remove the outer green broken or dirty leaves. The cores are cut mechanically by a reversing corer that leaves the core in the head. Then the cabbage is sliced by power-driven, rotary, adjustable knives into long shreds as fine as 0.16 to 0.08 cm (1/16 to 1/32 inches) in thickness.

In general, long, finely cut shreds are preferred, but the thickness is determined by the judgment of the manufacturer. The shredded cabbage (known also as slaw) is then conveyed by belts or by carts to the vats or tanks for salting and fermentation.

Salt plays a primary role in the making of sauerkraut and the concen­trations used are carefully controlled. According to the legal standard of identity the concentration of salt must not be less than 2%, nor more than 3%. As a result most producers use a concentration in the range of 2.25 to 2.5% of salt. Salt is required for several reasons.

It extracts water from the shredded cabbage by osmosis, thus forming the fermentation brine It suppresses the growth of some undesirable bacteria which might cause deterioration of the product and, at the same time, makes conditions favorable for the desirable lactic acid bacteria. Salt also contributes to the flavor of the finished sauerkraut by yielding a proper salt-acid ratio (bal­ance) if the cabbage is properly salted.

The use of too little salt causes softening of the tissue and produces a product lacking m flavor. Too much salt interferes with the natural sequence of lactic acid bacteria, delays fermentation and, depending on the amount of over-salting, may produce a product with a sharp, bitter taste, cause darkening of color, or favor growth of pink yeasts.

Uniform distribution of salt throughout the mass of shredded cabbage cannot be overemphasized. In some factories the slaw is weighed on con­veyor belt lines and the desired amount of salt is sprinkled on the shreds by means of a suitable proportioner as it moves along the conveyor to the vat.

In other plants hand-carts are used to carry the shredded cabbage to the vat. Some prefer to salt the weighed cabbage in each cart. Others transport the slaw in carts which are weighed occasionally to check the capacity. The shreds are then dumped into the vat, distributed by forks, and then salted with a specific weight of salt.

The variations of salt concentrations in the brines covering kraut have been thoroughly investigated by Pederson and Albury (1969) and discussed by Pederson (1975, 1979). No mention of recirculation of the brines to gain uniformity in concentration of salt was noted.

It would seem that this method of ensuring uniform salt distribution in sauerkraut brines would be as effective as it is in the olive industry. Only small alterations in tank or vat design would be required to make it possible to completely recirculate the brine, pumping from the bottom and discharging at the surface.

Brine begins to form once the shreds are salted, and the tank is closed once it has been filled to the proper level. Formerly, the slaw was covered with a thick layer of outer leaves and then fitted with a wood cover (head) which was heavily weighted. Within a few hours the brine had formed and the fermentation had started. The head then was fixed in position in much the same manner as with pickle or olive tanks.

Now, however, a sheet plastic cover is used. This cover is much larger in area than the top of the vat or tank itself. The plastic sheeting is placed firmly against the top of the shredded cabbage with the edges draped over the sides of the container to form an open bag. Then enough water or preferably salt brine is placed in this bag so that the weight of the liquid added forces the cabbage shreds down into the brine until the brine covers the surface of the uppermost shreds. Unless the shreds are completely covered with brine, undesirable discoloration together with undesirable flavor changes will occur. This newer method of covering and weighting provides nearly anaerobic condi­tions, particularly after fermentation becomes acid and quantities of carbon dioxide are produced. Precautions to avoid pin holes or tears in the plastic are mandatory if aerobic yeast growth is to be avoided.

With the old method of closure film forming yeasts always were a problem and if the scum was not removed at intervals a yeasty flavor was imparted to the kraut. Pichia membranaefaciens yeast strains, in particular, voraciously oxidize lactic acid contained in salt brines. Other genera also may be involved and besides destroying acid also contribute to yeasty flavor.

By the time the tank or vat is filled with the salted shreds and weighted, brine has formed and fermentation has started in a sequence of bacterial species responsible for the lactic acid fermentation.

Microbiology of the Sauerkraut Fermentation:

Although the lactic acid fermentation was described by Pasteur in 1858 and much work had been done in the intervening years with various lactic bacteria from cabbage and cucumber fermentations, it was not established that a definite sequence of bacterial species of lactic acid bacteria were responsible for the fermentation of either vegetable until 1930 when Peder­son first described the lactic acid bacteria he observed in fermenting sauer­kraut.

Pederson found that the fermentation was initiated by the species Leuconostoc mesenteroides. This species was followed by gas-forming rods and finally by non-gas-forming rods and cocci. Since 1930 additional studies by Pederson and Albury (1954, 1969) have firmly established the impor­tance of Leuconostoc mesenteroides in initiating the lactic fermentation of sauerkraut.

Also they more closely identified the species and sequence of the other lactic acid bacteria involved. Now it is accepted that the kraut, fermentation is initiated by Leuconostoc mesenteroides, a heterofermentative species, whose early growth is more rapid than other lactic acid bacte­ria and is active over a wide range of temperatures and salt concentrations.

It produces acids and carbon dioxide that rapidly lower the pH, thus inhibit­ing the activity of undesirable microorganisms and enzymes that may soften the shredded cabbage. The carbon dioxide replaces air and creates an anaerobic condition favorable to prevention of oxidation of ascorbic acid and the natural color of the cabbage. Also carbon dioxide stimulates the growth of many lactic acid bacteria. It also may be that this species provides growth factors needed by the more fastidious types found in the fermentation.

While this initial fermentation is developing, the heterofermentative species Lactobacillus brevis and the homofermentative species Lactobacil­lus plantarum and sometimes Pediococcus cerevisiae begin to grow rapidly and contribute to the major end products including lactic acid, carbon dioxide, ethanol, and acetic acid. Minor end products also appear.

These are a variety of additional volatile compounds produced by the various bacteria responsible for the fermentation, by auto-chemical reactions, or the intrin­sic enzymes of the fermenting cabbage itself. Hrdlicka et al (1967) reported the formation of diacetyl and acetaldehyde, the primary carbonyls formed during cabbage fermentation.

Volatile sulfur compounds are major flavor components of fresh cabbage according to Bailey et al. (1961) and Clapp et al. (1959) and also of sauerkraut. However, according to Lee et al. (1976), the major portion of the volatiles of sauerkraut is accounted for by acetal, isoamyl alcohol, n-hexanol, ethyl lactate, cis-hex-3-ene-l-ol, and allyl isothiocyanate. Of these, only the latter two have been identified as major constituents of fresh cabbage.

These latter authors concluded that although these two compounds define the character of cabbage products (kraut) they do not contribute significantly to the determination of its quality. They further believe that the fresh and fruity odor of such compounds as ethyl butyrate, isoamyl acetate, n-hexyl acetate, and mesityl oxide are probably more important in determining the acceptability of sauerkraut.

Temperature is a controlling factor in the sequence of desirable bacteria in the sauerkraut fermentation at a salt concentration of 2.25%. At the optimum of 18.3°C (65°F) or lower the quality of the sauerkraut is generally superior in flavor, color and ascorbic acid content because the hetero­fermentative lactic acid bacteria exert a greater effect.

According to Pederson and Albury (1969) an average temperature of about 18°C (65°F) with a salt concentration of 2.25% may be considered normal in the kraut-producing areas of the United States. At (or near) this temperature, fermentation is initiated by Leuconostoc mesenteroides and continued by Lactobacillus brevis and Lactobacillus plantarum, the latter species being most active in the final stages of fermentation.

Under these conditions a final total acidity of 1.7 to 2.3% acid (calculated as lactic acid) is formed, and the ratio of volatile to nonvolatile acid (acetic/lactic) is about 1 to 4. The fermentation is completed in 1 to 2 months, more or less, depending upon the quantity of fermentable materials, concentration of salt, and fluctuations in temperature.

At higher temperatures, as would be expected, they found that the rate of acid production was faster. For example, at 23°C (73.4°F) a brine acidity of 1.0 to 1.5% (calculated as lactic acid) may be observed in 8 to 10 days and the sauerkraut may be completely fermented in about 1 month.

At a still higher temperature of 32°C (89.6°F), the produc­tion of acid generally is very rapid with acid production of 1.8 to 2.0% being obtained in 8 to 10 days. As the temperature increased, they observed a change in the sequence of lactic acid bacteria. First, the growth of Leuco­nostoc mesenteroides was retarded and Lactobacillus brevis and Lactobacil­lus plantarum dominated the fermentation. At higher temperatures the kraut fermentation became essentially a homofermentation dominated by Lactobacillus plantarum and Pediococcus cerevisiae.

As a result, the quality attributes of flavor and aroma deteriorated and the kraut was reminiscent of acidified cabbage because of the large quantity of lactic acid and little acetic acid produced by the homo-fermentative species. They also observed that sauerkraut fermented at higher temperatures would darken readily and, therefore, should be canned as quickly as possible after the fermenta­tion was completed.

An extremely important observation they made was that kraut could be successfully fermented even when started at the low temperature of 7.5°C (45.5°F). Leuconostoc mesenteroides can grow at lower temperatures than the other lactic acid bacteria involved in the fermentation. At this low temperature (7.5°C or 45.5°F) an acidity of 0.4% (as lactic acid) is produced in about 10 days and 0.8 to 0.9% in less than a month.

This amount of acidity coupled with saturation of the mass of kraut and brine with carbon dioxide is sufficient to provide the conditions necessary for preservation and later completion of the fermentation provided that anaerobiosis is maintained throughout the period of latency. When the kraut mass warms enough, the fermentation then is completed by the lactic acid bacteria of the genera Lactobacillus and Pediococcus, known to grow poorly if at all at 7.5°C (45.5°F).

Thus, it may require 6 months or more before the fermentation is completed. Such kraut is generally of superior quality because it remains cool and is not subjected to high temperature during-fermentation. In good commercial practice this variation in temperature permits the processor to maintain a supply of new, completely fermented sauerkraut throughout most of the year.

Precedent for the recommendation by Pederson and Albury that sauer­kraut be fermented at not over 18.3°C (65°F) had already been recorded by Parmele et al. (1927), Marten et al. (1929), and others.

Defects and Spoilage of Sauerkraut:

Abnormalities of sauerkraut, although varied, with few exceptions can be and generally have been avoided by application of scientific knowledge already available to the industry. For example, the simple expedient of providing anaerobiosis has eliminated most of the problems involving dis­coloration (auto-chemical oxidation), loss of acidity caused by growth of, molds and yeasts, off-flavors and odors (yeasty and rancid) caused by exces­sive aerobic growth of molds and yeasts, slimy, softened kraut caused by pectolytic activity of these same molds and yeasts, and pink kraut caused by aerobic growth of asporogenous yeasts, presumably members of the genus Rhodotorula.

Stamer et al. (1973) described the induction of red color in white cabbage juice by L. brevis while studying the effects of pH on the growth rates of the 5 species of lactic acid bacteria commonly associated with the kraut fermen­tation. L. brevis was the only species which produced such color formation in white cabbage juice and did so only when the juice was buffered with either calcium carbonate or sodium hydroxide.

No color development occur­red when the pH of the juice (3.9) was not adjusted or when the pH of the juice was raised to 5.5 and the juice sterilized by filtration before it was re-incubated. Therefore, red color formation was caused by L. brevis and did not arise as the result of chemical or inherent enzymatic reactions of the juice.

It remains to be seen whether this interesting phenomenon will be ob­served in industrial kraut fermentations. Since color induction by L. brevis was found to be pH dependent it seems unlikely to be found in normal kraut fermentations but could easily result from accidental addition of alkali to the shredded cabbage during salting.

Slimy or ropy kraut has been observed for many years. It is generally caused by dextran formation induced by Leuconostoc mesenteroides and is transitory in nature. This species prefers to ferment fructose rather than glucose. Therefore, in the fermentation of sucrose, the fructose is fermented leaving the glucose which interacts to form the slimy, ropy, water-insoluble dextrans.

These vary from an almost solid, gelatinous mass to a ropy slime surrounding the bacterial cells. These variations are easily demonstrated by growing L. mesenteroides in a 10% sucrose solution containing adequate accessory nutrients. The fermenting kraut may become very slimy during the intermediate stage of fermentation but with additional time the dex­trans are utilized by other lactic acid bacteria. Thus, it is imperative to distinguish between dextran induced slimy kraut and permanently slimy kraut caused by pectolytic activity. The former condition certainly is not a defect but should be considered a normal step in a natural progression.