Lactic acid in food is a naturally occurring organic acid that plays a crucial role in both preservation and flavor development. Lactic acid fermentation is a cost-effective and energy-efficient method, often requiring minimal or no heat, making it a sustainable approach to food production. Across continents, lactic acid-fermented foods are vital components of diets, contributing significantly to global food security.
The Diverse Applications of Lactic Acid in Food Production
Lactic acid bacteria (LAB) are essential in the creation and preservation of a wide array of foods. These include:
- Fermented Vegetables: Cabbage (sauerkraut, Korean kimchi), cucumbers (pickles)
- Cereal-Based Ferments: Cereal yogurt (Nigerian ogi, Kenyan uji), sourdough bread, and non-wheat/rye breads (Indian idli, Philippine puto)
- Dairy Products: Yogurts, cheeses, and fermented milk-wheat mixtures (Egyptian kishk, Greek trahanas)
- Vegetable Protein: Protein-rich meat alternatives (Indonesian tempe)
- Sauces and Pastes: Amino acid/peptide meat-flavored products from fermented cereals and legumes (Japanese miso, Chinese soy sauce)
- Cereal-Fish-Shrimp Mixtures: Philippine balao balao and burong dalag
- Fermented Meats: Salami and other cured meats
How Lactic Acid Bacteria Preserve Food
While LAB can be fastidious on artificial media, they thrive in most food substrates. They rapidly reduce pH, inhibiting the growth of spoilage organisms. Leuconostoc and lactic streptococci typically lower pH to 4.0-4.5, while certain lactobacilli and pediococci can reach pH 3.5 before inhibiting their own growth.
Beyond lactic acid production, lactobacilli can generate hydrogen peroxide through NADH oxidation, catalyzed by flavin nucleotide. Flavoproteins like glucose oxidase also produce hydrogen peroxide, exhibiting an antibiotic effect against spoilage organisms. Lactobacilli are relatively resistant to hydrogen peroxide.
Streptococcus lactis produces nisin, a polypeptide antibiotic effective against Gram-positive bacteria, including S. cremoris. Conversely, S. cremoris produces diplococcin, which inhibits S. lactis. This competition regulates microbial populations during milk fermentation, inhibiting other Gram-positive bacteria.
Heterofermentative lactobacilli produce carbon dioxide, which acts as a preservative by displacing oxygen and creating anaerobic conditions, provided the food substrate is properly sealed.
Advantages of Lactic Acid Fermentation
Brining and lactic acid fermentation remain highly favored for vegetable processing and preservation due to their low cost and energy requirements. They also yield diverse and appealing flavors. Salt concentration influences the fermentation, limiting pectinolytic and proteolytic hydrolysis to control softening and prevent putrefaction. Furthermore, lactic acid fermentation enhances food’s resistance to spoilage and toxin development, modifies the flavor of the original ingredients, and can improve nutritional value.
For communities with limited access to canned or frozen foods, lactic acid fermentation combined with salting offers a practical preservation method, enhancing the organoleptic and nutritional properties of fresh vegetables, cereal gruels, and milk-cereal mixtures.
Examples of Lactic Acid Fermented Foods
Sauerkraut
Sauerkraut production involves lactic acid fermentation of cabbage, a common preservation method in the Western world, China, and Korea. Shredded cabbage is mixed with approximately 2% salt, which draws out liquid and provides a substrate for LAB growth. Anaerobic conditions are maintained to prevent spoilage.
The fermentation sequence typically involves Leuconostoc mesenteroides, which initiates growth and produces carbon dioxide, lactic, and acetic acids, rapidly lowering the pH. This inhibits undesirable microorganisms. Lactobacillus brevis and Lb. plantarum then complete the fermentation, with Lb. plantarum contributing to high acidity. At high temperatures or salt concentrations, Pediococcus cerevisiae may develop, further contributing to acid production.
Korean Kimchi
Korean kimchi differs from sauerkraut, having less acid and being carbonated. Chinese cabbage and radish are the main ingredients, supplemented with garlic, green onion, ginger, leaf mustard, hot pepper, parsley, and carrot.
Kimchi is a year-round staple in Korean diets, consumed three times daily. Its popularity is due to the carbonation from fermentation with natural microflora. The optimum salt concentration during kimchi fermentation is around 3%, with lower temperatures (around 10°C) preferred. The optimum acidity is 0.4-0.8% lactic acid with a pH between 4.2 and 4.5. Organisms isolated from kimchi include L. mesenteroides, S. faecalis, Lb. brevis, Lb. plantarum, and P. cerevisiae.
Pickled Vegetables
Pickling, involving lactic acid fermentation, is widely used for cucumbers and other vegetables. A common method involves placing cucumbers in a 5% salt brine. The cucumbers absorb salt until equilibrium is reached. Acidity reaches 0.6-1.0% (as lactic acid) with a pH of 3.4-3.6 in about two weeks, depending on temperature.
In Malaysia, commonly pickled vegetables include cucumbers, ginger, onion, leek, chili, bamboo shoots, and mustard leaves. Young unripe fruits such as mangoes, papaya, pineapple, and lime are also pickled. In Egypt, pickled vegetables include carrots, cucumbers, turnips, cauliflower, olives, onions, and peppers, served as appetizers with meals.
Indian Idli and Dosa
Indian idli is a small, white, acidic, leavened, steam-cooked cake made by lactic fermentation of polished rice and black gram dhal (Phaseolus mungo). Dosa is a closely related product made from the same ingredients but fried like a pancake.
Idli fermentation allows bread-like products to be made from non-wheat or rye cereals without yeast. Rice and black gram dhal are soaked, ground, and combined with water and salt to form a batter. The batter is fermented in a warm place (30-32°C) overnight, during which acidification and leavening occur. L. mesenteroides and S. faecalis are essential for leavening and acid production.
Philippine Puto
Philippine puto is a leavened steamed rice cake made from year-old rice grains that are soaked, ground with water, and fermented naturally. Part of the acid is neutralized with sodium hydroxide. Puto is similar to Indian idli but contains no legumes.
Sourdough Breads
Sourdough breads involve a close relationship between yeasts and LAB. Sourdough leaven contains both yeasts and lactobacilli. The leaven is prepared by mixing wheat, rye, or other cereal grain flour with water and incubating in a warm place. LAB predominate due to acid production, while yeasts survive due to their acid tolerance.
The essential microorganisms in sourdough are a Lactobacillus sp. and a yeast, Torulopsis holmii. The lactobacillus prefers maltose, while T. holmii grows on glucose, allowing both to develop in dough where amylases hydrolyze starch to maltose.
Nigerian Ogi (Kenyan Uji)
Nigerian ogi is a smooth, sour porridge made by lactic acid fermentation of corn, sorghum, or millet. Soybeans may be added to improve nutritive value. Lb. plantarum appears to be the essential microorganism in the fermentation, while Saccharomyces cerevisiae and Candida mycoderma contribute to the flavor.
Nigerian Gari
Nigerian gari is a granular starchy food made from cassava by lactic acid fermentation of the grated pulp, followed by dry-heat treatment. Cassava tubers are washed, peeled, and grated, and an inoculum of fermented cassava juice is added. The pulp undergoes anaerobic acid fermentation, with a pH reaching 4.0 and a total acid content of about 0.85% (as lactic acid).
Philippine Balao Balao
Balao balao is a lactic acid-fermented rice-shrimp mixture, made by blending cooked rice, raw shrimp, and salt. The mixture ferments for several days or weeks. The chitinous shell softens, allowing the whole shrimp to be eaten. When packed to exclude air, sufficient acid is produced to preserve the product without high-temperature cooking.
Mexican Pulque
Pulque is a white, acidic, alcoholic beverage made by fermenting juice from Agave species. The essential microorganisms in the pulque fermentation are Lb. plantarum, a heterofermentative Leuconostoc, Sac. cerevisiae, and Zymomonas mobilis. The heterofermentative Leuconostoc produces dextrans, contributing viscosity and rapidly increasing acidity.
Egyptian Kishk, Greek Trahanas, and Turkish Tarhanas
These are mixtures of sheep’s milk yogurts and parboiled wheat, sometimes with tomato or onion added. The milk undergoes lactic acid fermentation, with a pH ranging from 3.5 to 3.8 and a titratable acidity of 1.3 to 1.8% (as lactic acid). Dried kishk and trahanas are not hygroscopic and can be stored for several years.
Conclusion
Lactic acid fermentation is a versatile and essential process in food production, contributing to preservation, flavor development, and nutritional enhancement. Its simplicity and low cost make it particularly valuable in developing countries. Embracing and optimizing lactic acid fermentation techniques can significantly contribute to food security and dietary diversity worldwide.
