Lactic acid bacteria (LAB) are a phylogenetically diverse group of Gram-positive, catalase-negative, non-sporing cocci and rods that produce lactic acid as the primary end product of carbohydrate fermentation. The principal genera of food significance include Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus thermophilus. LAB are generally recognized as safe (GRAS) and have a long history of use in food preservation and fermentation. They are fastidious, requiring complex growth factors including amino acids, vitamins, and nucleotides.
LAB are classified based on their fermentation pathway. Homofermentative LAB (Lactococcus, Pediococcus, S. thermophilus, and many Lactobacillus species) produce primarily lactic acid via the Embden-Meyerhof-Parnas (EMP) pathway, converting one glucose molecule into two molecules of lactate. Heterofermentative LAB (Leuconostoc and certain Lactobacillus species) use the phosphoketolase pathway, producing equimolar amounts of lactic acid, ethanol, and carbon dioxide. This distinction is critical for predicting the sensory characteristics of fermented products: homofermentative cultures yield a clean, sharp acidity, while heterofermentative cultures contribute additional flavor complexity and gas production.
Many LAB produce antimicrobial compounds beyond organic acids, including hydrogen peroxide, diacetyl, and bacteriocins. Nisin, produced by Lactococcus lactis subsp. lactis, is the most extensively characterized bacteriocin and is approved as a food preservative (E234) in numerous countries. Nisin acts on the cell wall precursor lipid II, disrupting cell wall synthesis and forming pores in the cytoplasmic membrane. It is particularly effective against Gram-positive bacteria and spore formers. Other bacteriocins include pediocin (Pediococcus) and sakacin (Lactobacillus sakei).
Starter culture production involves propagation in sterilized media under controlled pH and temperature, followed by concentration via centrifugation or membrane filtration, and preservation by freezing (with cryoprotectants such as glycerol or sucrose) or freeze-drying. Commercial starter cultures are evaluated for acidification rate, phage resistance, ability to produce desired flavor compounds, and robustness during production and storage. Applications span dairy (cheese, yogurt), meat (fermented sausages), vegetables (sauerkraut, pickles), and sourdough bread. Lactic acid bacteria are the foundation of fermented dairy and fermented vegetable production. Yeast fermentation can occur alongside LAB in certain products, and some LAB species are used as probiotics.
