Abstract
A variety of fermentation products, such as foods containing probiotic bacteria, black rice vinegar (kurosu), soy sauce (shoyu), soybean-barley paste (miso), natto and tempeh, are sold in food stores in Japan. These fermented food products are produced by traditional methods that exploit mixed cultures of various non-toxic microorganisms. These microorganisms include lactic acid bacteria, acetic acid bacteria, sake yeast, koji molds and natto bacteria. Many traditional fermented foods have been studied and their effects on metabolism and/or immune system have been demonstrated in animal and/or human cells. This review summarizes the scientific basis for the effects of these traditional food products, which are currently produced commercially in Japan.
Introduction
Traditional fermentation products, such as foods containing probiotics, black rice vinegar, soy sauce (shoyu), miso, natto and tempeh, are quite popular foods in Japan. These fermented foods have attracted attention all over the world as foods that might promote longevity. These foods are not only traditional fermented foods but also functional foods and thus experiencing a burst of popularity as health foods worldwide.
Lactic acid bacteria in traditional fermented foods
The recent study of foods containing probiotics has consistently demonstrated their positive impact on the immune functions of animal and human cells [44]. Most of the probiotic strains of bacteria studied have been isolated from dairy products in western countries [12, 59]. With our colleagues, we have isolated several strains of lactic acid bacteria from non-dairy fermented foods that are produced in Asia. Lactobacillus species are the most common bacteria isolated from traditional Asian fermentation foods, which are made from rice, beans or vegetables. One such strain, Lactobacillus plantarum L137, isolated from Burong Isda, a traditional fermented food made from rice and fish in the Philippines can hydrolyze starch, a feature rarely associated with lactobacilli from dairy products [54, 55]. Thus, strain L137 is of interest for use in food processing, in particular, for the degradation of starch and lipid. Strain L137 contains 15 plasmids [54], one of which includes a gene for a starch-degrading enzyme, amylopullulanase (Kim H. J. et al., Abst. Meet. Soc. Nogei Kagaku, Japan, 2008). We have also investigated the operon that encodes acetyl coenzyme A carboxylase (fabH-accBCDA) from L. plantarum L137, which is essential for the biosynthesis of fatty acids [24].
Levels of cytokines in peritoneal macrophages and spleen cells after treatment with various strains of heat-killed lactic acid bacteria in vivo and in vitro, respectively. a Peritoneal macrophages; b spleen cells. Results are shown for the following genera: L, Lactobacillus; S, Streptococcus; B, Bifidobacterium. Each column represents the mean of results from three assays. Data are provided from Yamamoto Y (Takeda Food Industry, Osaka, Japan)
These phenomena can be explained by the hypothesis that lactic acid bacteria activate Th1 cells and suppress Th2 cells through the actions of peritoneal macrophages. A high ratio of Th1 to Th2 cells results in induction of the production of IL-12. The newly induced IL-12 and IFN-γ together suppress the production of IgE. Suppression of IgE results in desensitization to allergens since IgE is responsible for a variety of allergies. Despite a detailed analysis of the cellular fractions of L137 cells, such as the cell membrane, plasmids, chromosomal DNA and cytoplasm, we found no significant difference in terms of induction of cytokine synthesis between the various fractions from strain L137 and those from other strains of lactic acid bacteria. Thus, several cellular functions of L137 cells or entire cells seem to be required for stimulation of the production of cytokines. Heat-killed L137 cells are now produced commercially as an immunological supplement, “Lacdent”, in Japan (House Wellness Foods, Osaka, Japan).
In addition to the L137 strain, several strains of lactic acid bacteria have been commercialized in Japan as probiotics that prevent pollen allergies. A third of the Japanese adult population suffers from pollen allergies, mostly due to Japan cedar, in springtime (Allergen Research Report of the Advisory Panel to the Ministry of Health and Welfare, Japan, 1996). One of the commercialized probiotics, which contains Lactobacillus brevis KB290, is called Labre (Kagome, Tokyo, Japan). KB290 cells were isolated from a traditional pickle made from cress, Suguki, which is cultivated in the Kyoto area. These cells induce the synthesis of IFN-α in humans [29] and increase the levels of both IFN-α and NK cells in mice [77]. The genomic sequence of L. brevis ATCC367 has been reported [36], and the genome size of KB290 cells is 0.2 megabase pairs (Mbp) larger than that of strain ATCC367 [77]. Many lactic acid bacteria have been isolated from traditional plant-derived fermented foods, such as miso, soy sauce, sake, and shochu (Japanese whiskey). A lactic acid bacterium isolated from soy sauce was identified as Tetragenococcus halophilus [60]. Leuconostoc mesenteroides and Lactobacillus sakei were isolated from by-products of sake brewing [19]. Strains of Leuconostoc citreum, which can catalyze fermentation at 4°C, were isolated from the rice koji that is used for brewing of sake [33]. However, immunological activation is not specific to lactobacilli that originate from plant sources, and have been found properties to be associated with strains from variety of source Lactobacillus paracasei strain KW3110, isolated from a dairy product, for example, stimulated the synthesis of IL-12 and repressed that of IL-4, suppressing of production of IgE against naturally fed antigens, which resulted in desensitization of mice against the antigens [11].
To enhance the functions of a probiotic strain, we developed an expression vector for L. plantarum that was based on one of its endogenous plasmids, pLTK2 [18], and included a native promoter from strain L137 [24]. As transgene, we chose the choA gene, for cholesterol oxidase, which has been widely used for degradation of cholesterol in bacteria [41]. Using this expression vector, we were able to produce a cholesterol-degrading enzyme in both L. plantarum and L. casei [25]. An edible vaccine is also been developed using L137 cells. In developed countries, more than 70% of all cases of asthma in children are caused by house dust mites. To desensitize children against mite allergens, the Del f10 gene [1], for one of the main allergens from house dust mites, was expressed in cells of L. plantarum NCL21, which is a derivate of L137. The Der f10 antigen produced by L. plantarum NCL21 had high IgE-binding activity in sera from patients and healthy persons (Yoshida M et al., Abstr. Meet. Soc. Biotechnol., Japan, p. 53, 2005). Thus, L. plantarum L137 also has potential as the source of an edible vaccine for desensitization to a mite allergen.
Black rice vinegar, kurosu
Transfer inoculation with a crepe-like skin of acetic acid bacteria. A layer of acetic acid bacteria, covering the surface of moromi, is shown as it is being transferred to a new batch of moromi (Photograph courtesy of Nishikawa Y, Tamanoi Vinegar, Nara, Japan)
Since kurosu is produced from unpolished rice, it is characterized by higher levels of amino acids and organic acids than other vinegars, with the exception of balsamic vinegars. An extract of kurosu was recently shown to suppress lipid peroxidation more effectively than extracts of other vinegars since the former has stronger anti-oxidative activity in a radical-scavenging system [50, 66]. This extract was also shown to prevent hypertension in rats, lowering their blood pressure [51]. Kurosu also exhibited anti-tumor activity in a mouse skin model of carcinogenesis and it had a suppressive effect on the growth of a variety of lines of cultured tumor cells [47, 64, 65]. Vinegars produced from sweet shochu (Japanese whisky) post-distillation slurry by Acetobacter aceti had antitumor activity when administered orally to mice in a mouse model [40]. They inhibited the activity of angiotensin I-converting enzyme and repressed the formation of advanced end products of glycation [78].
At present, kurosu vinegar is experiencing a burst of popularity as a health drink in Japan.
Fermented soybean-barley paste, miso
Miso is a common Japanese food or seasoning, and the first reference to miso appeared on the ancient Chinese text ‘Syurai’ around 700 bc. Miso is produced by fermentation of soybeans, rice or barley with koji mold (A. oryzae), which is cultivated on steamed rice or barley under solid-state conditions, and then mixed with salt. In some cases, Saccharomyces cerevisiae and lactic acid bacteria are used in addition to koji mold. Although the most common type of miso is made from soybeans, many kinds of miso are produced, with variations the ingredients, the temperature and duration of fermentation (from one week to 20 months), the salt content (5–13%), the variety of koji, and the fermentation vessel.
The typical product is a thick paste that is used in miso soup, a Japanese culinary staple, which is prepared with vegetables, tofu (pressed protein curds from steamed soybeans), and dried sardines, seaweed or shellfish. Since miso contains a high level of protein and is rich in vitamins, amino acids, organic acids and minerals, it has played an important nutritional role in Japan. More than 95% of the Japanese population enjoyed miso [23], and miso is part of the daily diet of much of the Japanese population.
An epidemiological survey suggests that people who eat miso taken soup every day reduce their risk of gastric cancer [13]. The estrogenic effects of isoflavones in soybean products might limit growth of tumors in the mouse liver [15]. A study of human pulmonary adenomatosis cells and human gastric cancer cells revealed that fatty acids, such as oleic and linoleaic acids and their esters, which are found in miso, inhibit the proliferation of these cancer cells [74]. Since miso contains vitamin E, saponin, isoflavones, lecithin, choline, and dietary fiber from soybeans; vitamin B2 from koji mold; and vitamin B12 from lactic or propionic acid bacteria, Japanese peoples believe that it is good for their health and, indeed, miso does lower cholesterol levels [14]; it might even have anti-aging effects and prevent arteriosclerosis [23].
We might anticipate that the high level of salt in miso might cause high blood pressure, although one cup of miso paste soup (average 150 g) contains 1.2 g NaCl, which is less than that in other favorite Japanese foods. For example, one piece of salted salmon contains approximately 5.7 g of NaCl/piece (70 g), whereas three pieces of yellow pickled radish (30 g) contain about 7.1 g of NaCl [23].
Soy sauce, shoyu
Soy sauce or shoyu, in Japanese, is a liquid seasoning that is currently used in cooking worldwide. The varieties of soy sauce that are produced depend on the various types and ratios of raw materials used, the microorganisms employed and the conditions of fermentation [21, 79]. Many other sauces that are produced in China and Southeast Asia are made from fish or grain, and they resemble shoyu visually. However, these sauces are not categorized as soy sauce. Shoyu was first made, according to legend, by a Buddhist bishop, Kakushin, at Yuasa, Wakayama, Japan, in 1254. Traditional soy sauce is produced in wooden tanks by static fermentation for 1 to 2 years. However, nowadays, most soy sauce is produced in computer-controlled systems, from the cooking of raw materials to bottling. Moistened soybeans are cooked at high pressure and high temperature, while wheat is roasted and ground. Then almost the same amounts, by weight, of soybeans and wheat are mixed with a small amount of seed koji mold, A. oryzae or Aspergillus sojae [31]. The mash, which contains koji, NaCl [15–17% (w/v)] and water, is called moromi, and is fermented in large enamel or stainless tanks for 6–8 months at room temperature or higher. The moromi also contains halotolerant lactobacilli, for example, Pedicoccus soyae nov. sp. [60] or Tetragenococcus halophilus, and halotolerant yeasts, such as Zygosaccharomyces rouxii and Candida famata [61]. Other microorganisms, such as Aureobasidium pullulans [62] and Metschnikowia koreensis [63], which contribute to the production of polyols from starch, have also been isolated from the mash. Finally, the aged moromi is pressed and the liquid exudate is pasteurized to yield soy sauce.
Soy sauce promotes the secretion of gastric juice in humans [32]. It contains certain bioactive components in addition to its taste components, such as amino acids and polyols [56–58] and aromatic compounds, and various biological activities of soy sauce have been reported. Such activities include anticarcinogenic activities, as indicated by inhibition of benzopyrene-induced forestomach neoplasia [6, 15, 20] and antimicrobial activity against Shigella flexneri, Salmonella typhi, S. paratyphi, S. enteritidis, Vibrio cholera [2], and Escherichia coli O157:H7 [37]. Soy sauce also has antioxidant activity [3, 34]. It contains 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl–3(2H)-furanone [45] and related compounds as flavor components [20], and these compounds have potent antioxidant and anticarcinogenic activity. Soy sauce also exhibits anti-platelet aggregation activity, which may be due to β-carbolines [72]. Soy sauce decreases blood pressure [17], and inhibition by soy sauce of an angiotensin I-converting enzyme has been demonstrated [26]. Isoflavone derivatives from soy sauce can inhibit the activity of histidine decarboxylases, which produce histamine, a mediator of inflammation, allergy and the secretion of gastric acid [27, 28]. Such active compounds are formed during digestion of soybeans and wheat and by the enzymatic reactions that occur during fermentation. During fermentation and subsequent aging of soy sauce, the proteins in soybeans and wheat are completely digested to short peptides and amino acids by microbial proteolytic enzymes, and none of the allergens present in the raw materials are present in soy sauce [52, 73]. About 1% (w/v) of the polysaccharides originating from the cell walls of soybeans are present in soy sauce and these polysaccharides have potent antiallergic activities both in vitro and in vivo [30]. The oral administration of polysaccharides from soy sauce was shown to be effective as a treatment for patients with allergic rhinitis. Thus, soy sauce is not only a traditional seasoning but also a functional seasoning. The daily consumption of soy sauce in Japan has been estimated to be close to 30 ml per person [survey from the Japan Soy Sauce Brewers Association, 1988]. Thus, on average, the Japanese consume approximately 4.5 g of NaCl per day per person as soy sauce. For this reason, the soy sauce containing a reduced concentration of sodium has been developed for consumers who are prefer to limit their intake of salt.
Koji
Traditional method for cultivation of koji with Aspergillus oryzae. A brewing artisan, Toji, is inoculating spores of koji mold on steamed rice (left panel) and the koji mold is cultivating in wooden containers (right panel) (Photograph courtesy of Miyashita N, Yaegaki Bioindustry, Himeji, Japan)
The genome of A. oryzae [35] is 25% longer than that of the model fungus A. nidulans and the pathogenic fungus A. fumigatus [49]. The genome of A. oryzae has 50% more genes that are involved in the formation of secondary metabolites than the latter two species. These results might explain why A. oryzae not only has high enzyme productivity but also has an unusually strong ability to convert starches, proteins and lipids to a variety of bioactive compounds, as mentioned earlier in the discussion of individual fermented products. The genomic sequence also confirmed that A. oryzae cannot generate aflatoxin, a strong carcinogen that is produced, for example, by Aspergillus flavus and Aspergillus parasticus [39].
Natto
Recently, the Western world has started to pay attention to natto, which is a fermented product made from soybeans cultured with Bacillus subtilis (synonym, B. subtilis subsp. subtilis; former name, Bacillus natto). Natto has been popular in Japan for more than 400 years. Natto has a high nutritive value and is easily digested. In addition, natto has antibacterial effects. In the olden days, food poisoning was more common in Japan and people ate natto in efforts to prevent cholera, typhoid and dysentery. Approximately three quarters of the Japanese population eat natto at least once a week and half eats natto once every 3 days on average [16]. To prepare natto soybeans are immersed in water and then steamed. The steamed soybeans are inoculated with spores of B. subtilis. The original source of the natto bacterium was rice straw. Traditionally and, sometimes, nowadays, natto is made from steamed soybeans covered with rice straw, without the inoculation of bacteria. During for approximately 20 h fermentation, the starch and proteins of the soybeans are converted to a mixture that contains amino acids, vitamins, and enzymes.
Natto contains saponin and isoflavones, which come from soybeans, as well as the fibrinolytic enzyme, vitamin K2 and dipicolinic acid, which are generated by natto bacteria. The concentration of vitamin K2 increases to 124 times that in the soybeans at the start of the fermentation by natto bacteria [76]. Vitamin K2 stimulates the formation of bone. After the growth of bacteria ceases (after approximately 15 h-fermentation), natto becomes sticky as a result of formation of poly-γ-glutamic acid [16], which also stimulates absorption of calcium. These effects of natto might help to prevent osteoporosis in older women in Japan. The polymer is marketed as a dietary supplement in Ajinomoto, Tokyo, Japan (Fujishima Y., Abstr Third Annu World Cong Indust Biotechnol Bioproc, p. 65 Toronto 2006). A fibrinolytic enzyme, designated nattokinase [69], not only breaks up blood clots (thrombi) but also breaks down built-up fibrin that has been associated with heart disease [10, 68]. No other enzyme came close to nattokinase in studies of 173 different foods that were examined for their potential thrombolytic activities [76]. Dipicolinic acid, found in natto, has anti-bacterial activity against E. coli O157 and against Helicobacter pylori [53, 70], the causative agent of stomach ulcers. Natto has attracted attention all over the world as a food that might promote longevity.
Tempeh
Tempeh is a traditional Indonesian fermented soybean-based food that is now being produced in Japan because of its superior nutritive qualities and metabolic regulatory functions [38, 46]. Traditional tempeh is produced from boiled and dehulled soybeans that are fermented with Rhizopus sp. After fermentation for 1–2 days, the resulting tempeh is pressed into blocks that are either yellow or black. The color depends on the species of soybean and mold used. Recently, improved methods for tempeh production have been developed that raise levels of γ-aminobutyric acid [4, 5] and isoflavones [46] in the final product. γ-Aminobutyric acid has antihypertensive effects. By adding soybean germ (hypocotyls) that contains large amounts of isoflavones, Nakajima et al. [46] prepared a new isoflavone-enriched tempeh. This granular fermented soybean-based food might be useful as a nutritional supplement for the elderly [71]. Isoflavones have estrogen-like functions and, thus, may be expected to alleviate symptoms of osteoporosis after menopause [7] and to suppress the onset of arteriosclerosis because of it improves the metabolism of lipids, such as cholesterol [9, 67].
These variety of traditional fermented foods consumed in Japan probably contributes to Japanese longevity, which is the world’s greatest. Although genetically modified (GM) soybeans and varieties of grain have been approved by the Japanese Ministry of Agriculture, their products must be labeled as GM foods. Consumers are, however, not eager to accept such products. In Japan, therefore, all fermented foods are still produced, for the most part, from non-GM ingredients.
