MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.13. MICROBIOLOGY OF FERMENTED AND SALTED VEGETABLES

Plant-based foods can be preserved effectively through The activity of lactic acid Bacteria. This preservation method offers several distinct advantages: lactic acid suppresses the growth of undesirable microorganisms (such as putrefactive and butyric acid bacteria), eliminating The Need for chemical preservatives. The products of lactic acid and partial Alcoholic Fermentation that accumulate in fermented vegetables and fruits impart specific flavors and aromas. Depending on the raw material used, the final product is referred to as fermented or sauerkraut-style (cabbage), brined or salted (cucumbers, tomatoes), or soaked (apples, pears, etc.). Common salt is added during brining and fermentation, whereas soaking is conducted without salt.

Sauerkraut Fermentation. HEAD cabbage has been cultivated by humans for over four thousand years, with fermentation being its primary method of preservation. The process involves lactic acid bacteria, which drive Lactic acid fermentation, and Yeasts, which induce alcoholic fermentation. The cabbage and carrots are shredded and tightly packed into prepared vessels. Fermentation takes place in the presence of sodium chloride (1.5–2 %), which is added to draw juice out of the plant Tissues. To create anaerobic conditions, a weight is placed on top of the cabbage.

At 21–24 °C, fermentation typically proceeds within 6–8 days. The finished product should be crisp, possess a pronounced sour taste and pleasant aroma, and contain up to 1.25–1.7 % lactic acid.

Spontaneous fermentation in the prepared cabbage begins within just a few hours at 15–24 °C. During the initial stage of fermentation, coliform bacteria, Aerobacter cloacae, Flavobacterium rhenanus, and others proliferate, accumulating volatile acids (formic and acetic), small amounts of lactic and succinic acids, alcohol, and gases in the cabbage.

The conditions of the initial phase of cabbage fermentation are characterized by a pH of 6.2 and a sodium chloride concentration of 2.5 %. Under these conditions, after 2–3 days of fermentation, Heterofermentative lactic acid cocci—Leuconostoc mesenteroides—become dominant. By this time, the total acidity of the product rises to 0.7–1.0 % (calculated as lactic acid), and products of heterofermentative lactic acid fermentation are formed: lactic acid, ethanol, acetic acid, carbon dioxide, as well as mannitol, which gives the cabbage a bitter aftertaste.

After 4–6 days, the leuconostocs largely die off, and lactobacilli, predominantly L. plantarum, become the dominant group. They accumulate up to 1.5–2.0 % lactic acid and complete the lactic acid fermentation. Strains of L. plantarum ferment mannitol, thereby eliminating the bitter taste of the cabbage. If unfermented sugars remain in the cabbage juice (cabbage contains between 2.9 and 6.4 % sugar), heterofermentative lactobacilli of the species L. brevis begin to multiply, accumulating lactic and acetic acids up to a concentration of 2.4 %, which gives the product a sharp, tangy flavor.

The sodium chloride concentration and Temperature significantly influence the course of fermentation and the species composition of lactic acid bacteria. At an elevated salt concentration (3.5 %) and a fermentation temperature of about 30 °C, the growth of Leuconostoc mesenteroides is suppressed, and lactobacilli of the species Pediococcus cerevisiae and L. plantarum (along with enterococci such as S. faecalis) become predominant, resulting in an undesirable odor. At a reduced salt content and low temperature (below 15 °C), fermentation slows down, the sequential succession of lactic acid bacteria is disrupted, and this likewise negatively impacts the organoleptic Properties of the finished product.

To ensure the proper execution of the fermentation process in sauerkraut production, The Use of pure microbial cultures has been proposed. A dry culture preparation of L. plantarum has been developed. Cabbage fermented with pure cultures of lactic acid bacteria better retains Vitamins and exhibits reduced protein breakdown.

Upon completion of fermentation, the finished sauerkraut should be stored at temperatures not exceeding 4–5 °C without exposure to atmospheric oxygen.

Agents of cabbage spoilage may include:

✵ putrefactive bacteria — cause softening of the cabbage and impart an unpleasant putrid taste and odor;

✵ butyric acid bacteria — cause a sharp, rancid taste and odor;

✵ spore-forming pectolytic bacteria — cause softening of the product and The Development of an off-flavor;

✵ yeasts — form a surface film and cause the cabbage to become slimy.

Salting Cucumbers. Spontaneous fermentation of cucumbers is driven by the proliferation of the microorganisms naturally present on them. The direction of the fermentation process is determined by temperature, the availability of fermentable substances, salt concentration, and the species COMPOSITION OF THE microflora. A higher amount of salt (6–8 %) is added during the pickling of cucumbers compared to sauerkraut production.

The process of spontaneous cucumber fermentation consists of several stages (Table 17).

Class="center">Table 17. Microflora composition at various stages of cucumber fermentation (E. I. Kvasnikov, O. A. Nesterenko, 1975)

During the primary stage, Molds, yeasts, bacteria of the genera Pseudomonas, Flavobacterium, Achromobacter, Bacillus, and coliform bacteria are found in large numbers. Quantitatively, all these microorganisms predominate over lactic acid bacteria. Any of these undesirable microorganisms can become dominant at this stage and cause product spoilage. The primary stage is characterized by acidification of the brine and the accumulation of gases (H₂ and CO₂) resulting from Formic acid fermentation induced by coliform bacteria. Toward the end of the primary stage, the number of lactobacilli and yeasts increases, and the acidity of the brine rises.

In the intermediate stage of fermentation, homo- and heterofermentative lactic acid bacteria are dominant, alongside a high Abundance of yeasts. At the beginning of this phase, fermentation is carried out primarily by leuconostocs, whereas toward the end, more acid-tolerant lactobacilli—chiefly L. plantarum—prevail.

As the transition is made to The final stage of fermentation, the acidity of the brine increases to 0.7–1.0 % (calculated as lactic acid), and the pH drops to 3.5–3.8. At this stage, fermentation is carried out entirely by lactobacilli. The high acidity of the brine and the depletion of nutrients suppress the growth of lactic acid bacteria, bringing fermentation to a halt. Finished pickled cucumbers contain lactic acid (0.5–1.0 %), acetic acid, ethanol, traces of glycerol and mannitol, and minor amounts of Aromatic Compounds.

Microbiological spoilage of pickled cucumbers occurs when film-forming yeasts or lactic acid molds develop On the surface of the brine, resulting in the softening of the cucumbers. The formation of hollow cavities inside the cucumbers can be triggered by the proliferation of coliforms, excessively intensive growth of heterofermentative lactobacilli, or yeasts. Typically, cucumber spoilage results from violations of fermentation and storage regimes. To prevent the proliferation of yeasts and molds, a 0.1 % solution of potassium or sodium sorbate is added to the brine.