Metabolism Characteristics of Lactic Acid Bacteria and the Expanding Applications in Food Industry

Abstract

Lactic acid bacteria are a kind of microorganisms that can ferment carbohydrates to produce lactic acid, and are currently widely used in the fermented food industry. In recent years, with the excellent role of lactic acid bacteria in the food industry and probiotic functions, their microbial metabolic characteristics have also attracted more attention. Lactic acid bacteria can decompose macromolecular substances in food, including degradation of indigestible polysaccharides and transformation of undesirable flavor substances. Meanwhile, they can also produce a variety of products including short-chain fatty acids, amines, bacteriocins, vitamins and exopolysaccharides during metabolism. Based on the above-mentioned metabolic characteristics, lactic acid bacteria have shown a variety of expanded applications in the food industry. On the one hand, they are used to improve the flavor of fermented foods, increase the nutrition of foods, reduce harmful substances, increase shelf life, and so on. On the other hand, they can be used as probiotics to promote health in the body. This article reviews and prospects the important metabolites in the expanded application of lactic acid bacteria from the perspective of bioengineering and biotechnology.

Keywords: degradation; expanding applications; lactic acid bacteria; metabolism characteristics; products.

FIGURE 1

Decomposition of protein and metabolism of amino acids (Smit et al., 2005). Proteolysis in lactic acid bacteria is initiated by cell envelope proteinase (CEP), which degrades proteins into oligopeptides. The second stage of protein degradation is the transfer of dipeptides, tripeptides, and oligopeptides into cells. Three transport systems have been found in lactic acid bacteria, namely oligopeptide, dipeptide and tripeptide transport systems (Opp, DtpP, and DtpT, respectively). Finally, the Pep family hydrolyzes dipeptides, tripeptides, and oligopeptides into amino acids. The metabolism of amino acids includes deamination and decarboxylation. The deamination reaction produces various α-carboxylic acids, which are involved in various metabolisms in lactic acid bacteria cells. The amino acid decarboxylation reaction produces biogenic amines, which mainly includes the transport of amino acids into the cell, decarboxylation, and transport outside the cell after being converted into biogenic amines. Transamination of amino acids leads to the formation of alpha-keto acids. Alpha-keto acids can be converted to aldehydes by decarboxylation. Aldehydes are converted to alcohols or carboxylic acids by dehydrogenation. The direct dehydrogenation of alpha-keto acids leads to the formation of hydroxy acids. 1: cell envelope proteinase, 2: peptidases, 3: biosynthetic enzymes, 4: dehydrogenase, 5: aldolases, 6: lyases, 7: acyltransferases esterases, 8: dehydrogenase, 9: aminotransferases, 10: decarboxylase, 11: deiminases decarboxylase, 12: dehydrogenase complex, 13: biosynthetic enzymes.

FIGURE 2

Homolactic fermentation and heterolactic fermentation. Lactococcus spp. performs homolactic fermentation, while Lactobacillus and Leuconostoc spp. perform heterolactic fermentation. (1) In the process of pure lactic acid fermentation, lactic acid bacteria use glucose as a carbon source to produce pyruvate through glycolysis, and then produce lactic acid under the action of lactate dehydrogenase. In theory, 1 mole of glucose produces 2 moles of lactic acid. (2) In lactic acid bacteria of the heterolactic fermentation type, glucose can be decomposed into lactic acid, ethanol, CO₂ (in Leuconostoc, etc.) through the phosphoketolase (PK) pathway. In theory, 1 mole of glucose produces 1 mole of lactic acid. (3) Through the pentose phosphate (PP) pathway, glucose 6-phosphate was converted into carbon dioxide, ribulose 5-phosphate and NADPH (Eiteman and Ramalingam, 2015; Stincone et al., 2015).

FIGURE 3

Pathways of sugar metabolism and exopolysaccharides (EPS) biosynthesis in Streptococcus thermophilus (Laws et al., 2001). In Streptococcus thermophilus, exopolysaccharides is synthesized through a series of complex intracellular enzyme interactions. It is exported to the extracellular environment in the form of macromolecules through a special lipid carrier. The biosynthetic pathway of exopolysaccharides includes sugar entry into the cytoplasm, sugar-1-phosphate synthesis, polysaccharide synthesis and exopolysaccharides export. 1: fructokinase; 2: UDP-galactose-4-epimerase; 3: galactokinase; 4: galactose-1-phosphate uridylyltransferase; 5: UDP-glucose pyrophosphorylase; 6: glucokinase; 7: N-acetylglucosamine-1-phosphate uridyltransferase; 8: glucosamine-6-phosphate deaminase; 9: glucose-6-phosphate isomerase; 10: glucosamine-1-phosphate N-acetyltransferase; 11: β-glalactosidase; 12: mannose-6 phosphate isomerase; 13: α-phosphoglucomutase; 14: phosphoglucosamine mutase; 15: phosphomutase; 16: sucrose-6-phosphate hydrolase; 17: dTDP-glucose pyrophosphorylase; 18: UDP-glucose 6-dehydrogenase.

FIGURE 4

Citric acid metabolism (Gänzle, 2015). The citric acid in lactic acid bacteria is converted into succinate, lactate, acetate, and ethanol or acetylacetone through the intermediate metabolite oxaloacetate. 1: citrate lyase; 2: malate dehydrogenase; 3: fumarate hydratase; 4: succinate dehydrogenase; 5: oxaloacetate decarboxylase; 6: malolactic enzyme; 7: lactate dehydrogenase; 8: acetolactate synthase; 9: acetolactate decarboxylase; 10: pyruvate formate lyase; 11: acetaldehyde dehydrogenase; 12: alcohol dehydrogenase; 13: phosphotransacetylase; 14: acetate kinase.