Food fermentation by lactic acid bacteria for the prevention of cardiovascular disease

In this post we will concentrate on the components that are naturally produced by food micro-organisms and (could) provide protection against cardiovascular diseases. Most attention is focused on the B vitamins, which play a direct role in homocysteine metabolism and low-calorie sugars that can be used in low-carbohydrate, low-calorie diets. All the food components described in this post can be produced during food fermentations and, as such, represent a natural way of food enrichment.

Fermentation is one the oldest methods of food processing and can be considered as a desirable microbial activity in foods. This biochemical activity is caused by the enzymes of the fermenting micro-organisms. Fermented food products such as bread, beer, wine, soy sauce, yogurt, and cheese have been known for a long time. Traditionally, fermentation was used as a conservation technique for raw food material. After the fermentation, the fermented product has a much longer shelf-life than the unfermented product. Alternative conservations means were salting or drying. However, some products, e.g. dairy products and (alcoholic) beverages, could only be conserved by fermentation. During the fermentation process by food grade (lactic acid) bacteria or yeasts, food safety can be improved by the production of (lactic) acid or ethanol by these micro-organisms during growth on the raw food material.

Consequently the pH of the food matrix is decreased or the solvent concentration increased, resulting in a micro-environment that prevents or limits the growth of undesirable and/or pathogenic bacteria, yeasts, or molds. Well-known examples are the production of yoghurt or sourdough bread. Another food preservation property of fermentation occurs through the direct production by the fermenting micro-organisms of compounds with anti-bacterial or anti-fungal properties. For example lactic acid bacteria have the capacity to produce specific proteins or peptides, bacteriocins, that may directly interfere with the cell membrane of other (harmful) micro-organisms. One of such bacteriocins is nisin, which can be produced by Lactococcus lactis (Hurst, 1966). Nisin is present in some fermented dairy products, and is also produced and merchandized as a natural additive for the preservation of all kinds of food products (Cleveland et al., 1991).

Nowadays, food safety and shelf-life can be guaranteed by refrigeration or by direct addition of preservatives to the (processed) food. Nevertheless, there is still a high demand for fermented foods because of their added value in terms of consistency, color, and especially flavor and taste. The current small price difference between natural and chemically produced flavors, combined with the consumers’ increasing demand for more natural (`green’) products, has increased the interest of the food industry in fermentation for flavor improvement of food products. Good examples are some flavor forming capacities of lactic acid bacteria. Proteases and peptidases, which constitute the proteolytic system of lactic acid bacteria (Kunji et al., 1996) are responsible for the release of amino acids. Subsequent amino acid degradation is important for the formation of flavor compounds such as aldehydes, thiols, and keto acids in fermented dairy products. The degradation routes of amino acids generally involve different reactions, including deamination, transamination, decarboxylation, and cleavage of the amino acid side chain. Aromatic, branched-chain, and sulfurous amino acids, in particular, are precursors of compounds with, respectively, floral, cheesy, and sulfur flavors. Such compounds have been found in various cheeses, including semihard cheeses such as Gouda and Cheddar. Sugar conversion via glycolysis can also lead to the production of flavor compounds such as diacetyl and acetaldehyde, the main flavor components in butter (Hugenholtz et al., 2000) and yoghurt (Chaves et al., 2002), respectively.

Fermentation can also contribute to the nutritional value of the fermented food by increasing the bio-availability of nutrients or by the production of vitamins and other nutritional components by the fermenting micro-organisms. In foods of plant origin, such as cereals, oilseed, roots, and others, antinutritional factors (ANF) may inhibit efficient digestion and absorption processes. This is relevant not only for humans, but also for animals. An example of an ANF is phytic acid that forms complexes with metal ions and in this way limits the availability of these ions for uptake in the gastrointestinal tract. Several lactic acid bacteria, molds and yeast produce the enzyme phytase that degrades phytic acid during fermentation. Other ANF that can be removed from the raw food material by micro-organisms are galacto-oligosaccharides, which may cause flatulence and cramps, protease inhibitors, such as trypsin inhibitors, which limit the degradation of proteins and oligosaccharides, and antinutritional glycosides, for example linamarin, a cyanogenic glycoside present in bitter cassava.

The nutritional value of fermented food products is also greatly enhanced, compared with raw food material, by the production of nutraceuticals, such as antioxidants, vitamins, low-calorie sugars, and oligosaccharides by the fermenting micro-organisms (Hugenholtz et al., 2002). Compounds, like glutathione, or the 3-carotenoid lycopene, have antioxidative properties and can prevent oxidative stress that may damage tissue DNA. These compounds cannot be produced by humans, and should be present in the diet in sufficient concentrations.

Vitamins are also essential for human health. Yeast and lactic acid bacteria are a particularly rich source of antioxidants and vitamins, not only because of the uptake and subsequent accumulation of these vitamins in the microbial cells, but also because of the biosynthetic capacity of these micro-organisms to produce such compounds. More recent applications of fermentation is for the production of probiotics, fermented foods that contain living lactic acid bacteria that exert a positive effect in the intestine after consumption. Research has focused on probiotic bacteria such as lactobacilli and bifidobacteria, that can survive the transport through the gastrointestinal tract and that can colonize the small or large intestine. In most cases fermented, probiotic foods are yoghurt-like drinks with a high concentration of these lactic acid bacteria. Some beneficial attributes of probiotics in relation to cardiovascular disease, such as the cholesterol-lowering activities, are briefly discussed towards the end of this post. (For a recent overview of the effect of probiotics see the proceedings of the Montreal International Symposium on probiotics and health; Roy, 2002.)

In the remainder of this post we will focus on the fermenting capacities of lactic acid bacteria and present an overview of the production of B vitamins, on the production of low-calorie sugars and some probiotic effects related to cardiovascular disease, all present in fermented foods produced by lactic acid bacteria.

Lactic acid bacteria are Gram-positive, non-spore-forming bacteria and are naturally present in raw food material and in the human gastrointestinal tract. They have a long history of use by humans for food production and food preservation. The lactic acid bacteria group include rod-shaped bacteria, such as lactobacilli, and sphere-shaped bacteria, such as streptococci, lactococci, pediococci, and leuconostocs. Lactic acid bacteria are widely used as starter cultures for fermentation in the dairy, meat and other food industries. Their properties have been used to manufacture products such as cheese, yogurts, fermented milk products, beverages, sausage, and olives. Lactic acid bacteria have a relatively simple metabolism and can easily serve as cell factories for the production of flavor compounds (e.g. diacetyl), thickeners (e.g. exopolysacharides), antioxidants (e.g. lycopene), vitamins (e.g. folate), antibiotics (e.g. lantibiotics), oligosacharides, and many more primary or secondary metabolites. Lactococcus lactis is by far the most extensively studied lactic acid bacterium and displays a relatively simple and well-described metabolism where the sugar source is converted mainly to lactic acid. Over the last decades a number of elegant and efficient genetic tools have been developed for this starter bacterium. These tools are of critical importance in metabolic engineering strategies that aim at inactivation of undesired genes and/or (controlled) overexpression of existing or novel ones. In this respect, especially the nisin-controlled expression (NICE) system for controlled heterologous and homologous gene expression in Lactococcus lactis has proven to be very valuable (de Ruyter et al., 1996).

The design of a metabolic engineering strategy requires a proper understanding of the pathways that are manipulated and the genes involved, preferably combined with knowledge about fluxes and control factors. Most of the metabolic engineering strategies so far applied in lactic acid bacteria are related to primary metabolism and comprise efficient rerouting of the lactococcal pyruvate metabolism to end products other than lactic acid, including ethanol, diacetyl, and alanine, resulting in high production of both natural and novel end products. Metabolic engineering of more complicated pathways involved in secondary metabolism has only recently begun by the engineering of exopolysaccharide and folate production in L. lactis (Boels, 2002; Sybesma, 2003).