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Keywords:

  • glutamic acid;
  • low molecular weight compounds;
  • soy sauce;
  • taste interaction;
  • umami taste

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

ABSTRACT:  Soy sauce taste has become a focus of umami taste research. Umami taste is a 5th basic taste, which is associated to a palatable and pleasurable taste of food. Soy sauce has been used as an umami seasoning since the ancient time in Asia. The complex fermentation process occurred to soy beans, as the raw material in the soy sauce production, gives a distinct delicious taste. The recent investigation on Japanese and Indonesian soy sauces revealed that this taste is primarily due to umami components which have molecular weights lower than 500 Da. Free amino acids are the low molecular compounds that have an important role to the taste, in the presence of sodium salt. The intense umami taste found in the soy sauces may also be a result from the interaction between umami components and other tastants. Small peptides are also present, but have very low, almost undetected umami taste intensities investigated in their fractions.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

Soy sauce is a fermented soybean product that is consumed as a seasoning or a condiment due to its salty, tasty taste and good flavor. Commonly there are Chinese type and Japanese type soy sauces. The former is found easily in China and Southeast Asian countries, and the later in Japan (Fukushima 1981, 1989). The difference is the main ingredients used in their production. Chinese type soy sauce uses soybeans as the only ingredient, while Japanese type uses the mixture of soybeans and wheat. The different ingredients may result in the difference of taste characteristics.

Compounds responsible for food taste are commonly having low molecular weight and non volatile properties. There are 5 basic tastes, sweet, bitter, salty, sour, and umami (Kirimura and others 1969; Maga 1983; Nishimura and Kato 1988). Umami is a newly found basic taste since the finding of the specific taste receptors (Zhao and others 2003; Ozeck and others 2004). To generally understand the definition of umami taste, Ishii and O'Mahony (2007) used the broths made from kombu (a kind of seaweeds), katsuobushi (dried bonito), and shiitake mushroom to describe the umami taste perceived by Japanese and American panels. Both panels could describe it very close, it was a palatable and pleasurable taste. Actually, the taste of free L-glutamic acid at relatively low concentration in the presence of sodium salt is purely describing umami taste (Keast and others 2004). Some interactions between umami and other tasty compounds could give an intense umami taste found in some foods and soy sauce as well.

The research on umami in soy sauce has been intensely investigated in several decades ago especially on the taste of Japanese soy sauce (Takeuchi and others 1962; Oka and Nagata 1974a, 1974b), and continued in the last 5 y on Japanese and Indonesian soy sauces (Apriyantono and others 2004; Lioe and others 2004a, 2004b, 2006, 2007). These researches were conducted not only to find the key compounds of soy sauce taste but also to discover small peptides having the intense umami taste. However, the contribution of small peptides to the umami taste was then doubted by many researchers and became a controversial (Yamasaki and Maekawa 1978; Tamura and others 1989; van Wassenaar and others 1995; Hau and others 1997; Lioe and others 2006). New techniques such as ultrafiltration, gel filtration chromatography followed by high-performance liquid chromatography (HPLC) separation linked to newly developed-taste dilution analysis were used to elucidate the precise contribution of the peptides. This review study provided the previously mentioned descriptions in detail.

Soy Sauce

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

Even though in the market we can also find artificial soy sauce that is made from soy protein hydrolysates, that is, soy proteins hydrolyzed by acid or enzymes, the soy sauce being explained here is the original soy sauce produced by fermentation. Its fermentation process consists of 2 steps, that is, solid state or koji fermentation by involving molds for the 1st step, and followed by brine or moromi fermentation by involving osmophilic lactic acid bacteria and salt tolerant yeasts as the 2nd step. Generally, there are 2 different types of soy sauce, that is, Japanese type soy sauce which is produced using soybeans and wheat as the ingredients, and Chinese type soy sauce which is made from only soybeans (Fukushima 1981, 1985; Yokotsuka 1983, 1986; Judoamidjojo 1987; Röling and others 1996; Chou and Ling 1998; Apriyantono and others 2004). The former is typically found in Japan, whereas the latter is found in China and Southeast Asia, including Indonesia, Malaysia, the Philippines, Singapore, and Thailand. Because the taste research on soy sauce has much been performed on Japanese and Indonesian soy sauces, the 2 types are subjected to be described in this section.

Japanese soy sauce

Japanese soy sauce was introduced from China into Japan sometime before the 7th century (Fukushima 1989). Recently, 5 types of soy sauce exist traditionally in Japan, that is koikuchi-shoyu, usukuchi-shoyu, tamari-shoyu, saishikomi-shoyu, and shiro-shoyu (Fukushima 1981, 1985, 1989; Yokotsuka 1981, 1986; Flegel 1988).

Koikuchi-shoyu is produced using an equal amount of wheat and soybeans. This soy sauce is characterized by a strong aroma and a deep reddish-brown color. About 85% of the total soy sauce production in Japan belongs to this type. The other types of soy sauce, which are made from the same composition of the ingredients as koikuchi-shoyu, are usukuchi-shoyu and saishikomi-shoyu. Usukuchi-shoyu has a lighter, red-brownish color, and milder aroma than those of koikuchi shoyu. These properties are reached by applying a different condition of fermentation so that the color development can be reduced. Saishikomi-shoyu has a characteristic of full aroma and bodied taste, because it uses raw soy sauce (without pasteurization) instead of the salt solution in the 2nd step fermentation (Yokotsuka 1960, 1981, 1983, 1986; Whitaker 1978; Fukushima 1981, 1985, 1989).

Tamari shoyu is produced using a large amount of soybeans and 10% or less of wheat. This type of soy sauce lacks aroma, but has a greater viscosity and a darker brown color than those of koikuchi-shoyu. In contrast, shiro shoyu is made using a very high ratio of wheat to soybeans and has a very light yellow to tan color because its fermentation is done under conditions which can strongly prevent color development (Yokotsuka 1960, 1981, 1986; Fukushima 1981, 1985, 1989). The chemical composition of 5 types of shoyu or Japanese soy sauce is presented in Table 1.

Table 1—.  Chemical composition of the 5 types of Japanese soy sauce (shoyu)a and Indonesian soy sauce.b
TypeNaCl % w/vTotal nitrogen % w/vReducing sugars % w/vAlcohol % v/vpH
Koikuchi-shoyu17.61.55 3.82.24.7
Usukuchi-shoyu19.21.77 5.50.64.8
Tamari-shoyu19.02.55 5.30.14.8
Saishikomi-shoyu18.62.39 7.5Trace4.8
Shiro-shoyu19.00.5020.2Trace4.6
Indonesian soy sauce12.6 2.3 3.40.1-

Microbiological and biochemical changes in the 2 steps of shoyu fermentation are different from each other. In the 1st step of shoyu fermentation, that is koji fermentation, all types of shoyu used a purely cultured starter of Aspergillus oryzae or Aspergillus sojae to ferment the mixture of cooked soybeans and roasted wheat (Yokotsuka 1960, 1981, 1983, 1986; Yong and Wood 1977; Whitaker 1978; Hesseltine and Wang 1980; Fukushima 1981, 1985, 1989; Flegel 1988; Chou and Ling 1998). In addition to it, tamari shoyu also used Aspergillus tamarii (Yokotsuka 1986; Flegel 1988). The koji fermentation period is about 2 to 3 d at temperature below 40 °C (Yokotsuka 1960; Hesseltine and Wang 1980). This step is marked by the production of extracellular enzymes from the koji molds that consist of protease and carbohydrase complexes to break down the proteins and carbohydrates contained in the raw ingredients into small molecular weight peptides, amino acids, and sugars for the subsequent brine fermentation (Yong and Wood 1977; Whitaker 1978; Hesseltine and Wang 1980; Fukushima 1981, 1985, 1989; Yokotsuka 1983, 1986; Flegel 1988). Glutamine and glutamic acid are obviously released by the action of the protease complex (Flegel 1988). Glutamic acid is known to be the major component of soybean and wheat proteins (Fujiwara and others 1962a; Kuroshima and others 1969; Fukushima 1989). The release of glutamic acid may give a tasty taste or an umami taste of soy sauce (Nishimura and Kato 1988). In the koji fermentation, the production of glutaminase, that is an enzyme for converting glutamine into glutamic acid, is more substantial than in the subsequent brine fermentation (Kuroshima and others 1969; Fukushima 1989). The result of koji fermentation is called koji, which has a moisture content of about 26% (Hesseltine and Wang 1980).

In the 2nd step fermentation, that is moromi fermentation, an equal amount of koji is mixed with brine solution having a salt concentration range of 22% to 23% w/v to obtain moromi with a salt concentration range of 17% to 18% w/v. However, in saishikomi-shoyu, raw soy sauce is used instead of a brine solution to obtain a thicker taste of soy sauce. The high salt concentration is believed to limit the growth of undesirable microorganisms. The microbes of this fermentation step are Pediococcus halophilus (a lactic acid bacterium) which grows and produces lactic acid at the earlier stage to drop the pH of moromi from 6.5 to 7 to less than 5; Zygosaccharomyces rouxii or Saccharomyces rouxii (a salt-tolerant yeast), which grows accompanying the decrease of the moromi pH and gives a vigorous alcoholic fermentation in the middle stage; and Torulopsis sp. or Candida sp. (a salt-resistant yeast), which grows at the last stage to produce some phenolic compounds and add some aroma to soy sauce. The moromi fermentation period is varied from 3 to 8 mo to 1 to 3 y (Yokotsuka 1960, 1983, 1986; Fujiwara and others 1962a; Kuroshima and others 1969; Whitaker 1978; Hesseltine and Wang 1980; Noda and others 1980; Fukushima 1981, 1985, 1989; Inamori and others 1984; Chou and Ling 1998; Kinoshita and others 1998). The final product of this step is called moromi which contained 0.5% to 2.5% w/v of total nitrogen (Table 1). The nitrogenous compounds consist of 40% to 50% free amino acids and 40% to 50% small peptides (Yokotsuka 1960, 1983, 1986; Whitaker 1978; Fukushima 1985, 1989). Among the free amino acids, glutamic acid, aspartic acid, and leucine are the major components (Fujiwara and others 1962b, 1962c; Kaneko and others 1994; Chou and Ling 1998). Lactic acid and glucose become the main organic acid and carbohydrate, respectively, in the fermented soy sauce (Fukushima 1981, 1985, 1989; Kaneko and others 1994). The production scheme of shoyu is shown in Figure 1. At the last step of soy sauce production, moromi is filtered and pasteurized at 80 °C to give a commercial soy sauce (Hesseltine and Wang 1980; Fukushima 1981).

image

Figure 1—. The production scheme of Japanese soy sauce (Hesseltine and Wang 1980).

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Indonesian soy sauce

The ingredient for Indonesian soy sauce production is only soybeans, whether black or yellow soybeans (Judoamidjojo 1987; Apriyantono and others 2004). The principle of fermentation in this type of soy sauce is the same as in Japanese soy sauce production, that is, 2 steps fermentation, consisting of koji or bungkil fermentation and moromi or baceman fermentation. Aspergillus oryzae mixed with another Aspergillus sp. and Rhizopus sp. are usually found in the traditional koji fermentation of Indonesian soy sauce due to the use of mixed starter (Judoamidjojo 1987; Röling and others 1994). This 1st step fermentation is done from 2 to 3 d to 1 to 2 wk at room temperature. After koji fermentation, the koji is mixed with 3 to 4 times volume of salt solution to obtain substrates with a salt concentration of 20% for moromi fermentation (Judoamidjojo 1987). The 2nd step fermentation occurs spontaneously for 4 mo or less subjected to tropical conditions (Judoamidjojo 1987; Röling and others 1996).

During moromi fermentation, amino acid production by the enzymes of koji at the earlier stage and growth of lactic acid bacterium Pediococcus halophilus (or currently named Tetragenococcus halophila) at the next stage are observed (Röling and others 1996). However, very little or no obvious yeast fermentation is observed in this fermentation step due to the sugar-poor ingredients used in the production (Röling and others 1994, 1996). Compared to Japanese soy sauce production, alcoholic fermentation by yeasts is not important because after filtration the moromi is thoroughly boiled and even added with a high portion of caramelized sugar to produce a thick, sweet soy sauce, called kecap manis, in addition to common soy sauce product or salty soy sauce, called kecap asin (Judoamidjojo 1987; Röling and others 1994).

The chemical composition of Indonesian soy sauce (kecap asin) is presented in Table 1. As in Japanese soy sauce, lactic acid is the dominant organic acid of Indonesian soy sauce (Judoamidjojo 1987). However, the microbiology and biochemistry of Indonesian soy sauce clearly differ from those of Japanese soy sauce as described previously.

Umami or Savory Taste Compounds

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

Umami, a word of Japanese term, means delicious. Therefore, umami taste means delicious taste. Umami taste terminology also closes to the terms of “meaty taste,”“bouillon-like taste,” and “savory taste” because the delicious or umami taste is usually used to describe the taste of meat extract/broth, meat and fish products, seafood products, and vegetable products made from tomatoes and mushrooms, which are all called as savory foods.

In Japanese culture, the use of ingredients such as bonito flakes and a specific tangle seaweed (called konbu) can produce such an umami taste in many Japanese dishes (Maga 1983). This evidence led to the finding of monosodium L-glutamate, abbreviated as MSG, at the 1st time in the tangle seaweed more than 100 y ago (Ikeda 1908), as the key compound of the umami taste. Recently, the taste of MSG is used as an umami standard quality in a lot of sensory researches with the wide ranging topics such as food science, psychophysical study, molecular biology, psychometry, electrophysiology, and others (Nishimura and Kato 1988; Bellisle 1999). The typical umami taste is now recognized as the one of basic tastes, therefore, it is different from the other 4 basic tastes, that is, sweet, bitter, sour, and salty. This statement is strengthened by the recent finding that 2 taste receptor proteins, namely T1R1 and T1R3, are able to form a universal heterodimeric sensor for L-glutamate (Zhao and others 2003; Ozeck and others 2004).

Monosodium L-glutamate (MSG) is the sodium salt form of free L-glutamic acid, an acidic amino acid. Acidic amino acids such as L-glutamic acid and L-aspartic acid has a sour taste in dissociated state, but their sodium salts dissociated in aqueous solutions can elicit an umami taste, although the umami taste intensity of monosodium L-aspartate is about 4 times lower than that of MSG (Kato and others 1989) or possesses 7% of the potency of MSG (Yamaguchi and others 1971). Therefore free L-glutamic acid in a sodium salt form or in the presence of salt is the essential umami substance. This amino acid can be found in natural foods, either vegetable or animal origins (Maga 1983; Kato and others 1989). The umami taste threshold value of MSG is 6.25 × 10−4 M (Maga 1983) or 1.5 mM at pH 5/7 (Soldo and others 2003). Besides MSG, the other amino acids, which are discovered in mushrooms and not commonly found in animal system, are also known to have an umami taste. They are the derivatives of oxyglutamic acid, that is, ibotenic acid and tricholomic acid, which have umami taste intensities of 4 to 25 times that of MSG (Solms 1969; Kato and others 1989).

Umami taste is also perceived by purine-5′-nucleotides such as adenosine-5′-monophosphate (AMP), inosine-5′-monophosphate (IMP), and guanosine-5′-monophosphate (GMP) (Maga 1983; Kato and others 1989). These nucleotides are easily found in meat, fish, seafoods, and mushrooms (Maga 1983). It is known that the umami taste intensity of GMP is stronger than that of IMP or MSG. In a comparison study of the taste threshold of MSG, IMP, and GMP using the same panel, the respective values of 0.014%, 0.012%, and 0.0035% were reported (Maga 1983). Nowadays, the blends of IMP and GMP have been commercially produced with the trade name Ribotide (Maga 1983). In practice, they are used together with MSG to produce a more intense delicious or umami taste.

Several di- and tripeptides having L-glutamic acid at the N-terminal site can elicit a brothy or umami taste in aqueous solution containing sodium chloride (NaCl) at pH 6, although their umami taste intensity are much less than that of MSG. This glutamyl oligopeptides were isolated from a proteinase-hydrolyzed soybean protein (Arai and others 1972). Their primary structures are Glu-Asp, Glu-Gly-Ser, Glu-Ser, and Glu-Glu. It is also known that some of them have a sour taste in the absence of NaCl. Noguchi and others (1975) also reported umami oligopeptides containing glutamyl residue and hydrophilic amino acid residues. These peptides were isolated from L-Glu free-acidic fraction with a molecular weight of less than 500 Da, obtained from fish proteins that were hydrolyzed by Pronase, protease produced by Kaken Kagaku Co. (Japan). Their primary structures are Glu-Glu, Glu-Asp, Thr-Glu, Glu-Ser, Glu-Gly-Ser, Ser-Glu-Glu, Glu-Gln-Glu, Glu-Asp-Glu, and Asp-Glu-Ser. The acidic peptides with APL (average peptide length) less than 5 were also observed in miso, a Japanese fermented soybean product, and known to contain aspartyl, glutamyl, and glycyl residues (Kirimura and others 1969). However, their umami taste intensities are much less than that of MSG as well. Therefore, the individual umami peptide is not expected to be served as a seasoning like MSG.

Higher peptides such as tetrapeptide Asp-Asp-Asp-Asp from enzymatically hydrolyzed beer yeast (Matsushita and Ozaki 1994) and octapeptide Lys-Gly-Asp-Glu-Glu-Ser-Leu-Ala from papain-hydrolyzed beef meat (Yamasaki and Maekawa 1978), were also reported to have an umami taste. The umami taste of the octapeptide was even reported to possess the higher intensity than that of MSG, indicated by its threshold value of 1.41 mM (Tamura and others 1989) or 0.53 mM (Wang and others 1996) compared to the MSG threshold value of 1.56 mM (Tamura and others 1989). Later, this octapeptide was called as the “beefy meaty peptide” or BMP (Tamura and others 1989) and then as savory taste-enhancing peptide (STEP) suggested by Spanier and others (1997). The elimination of 2 amino acid residues at the N-terminal site, that is Lys-Gly, led to the disappearance of the savory taste and changed it into a sour taste (Tamura and others 1989). Therefore, the combination of the basic N-terminal region with the acidic region of the molecule seems to be important for eliciting an umami taste (Tamura and others 1989; Nakata and others 1995). However, van Wassenaar and others (1995) and Hau and others (1997) gave a controversial report that the octapeptide did not have any umami taste and tasted a strong acid and astringent. These researchers even questioned the presence of umami peptides.

Beef broth which has a savory or brothy taste has been investigated, and the isolation of N-(1-methyl-4-hydroxy-3-imidazolin-2,2-ylidine)alanine as a new brothy taste compound was reported (Shima and others 1998). It seems that umami compounds belong to broad classes. This is also proved by the other recent findings of new classes of umami compounds. For an example, glutamate glycoconjugates from a Maillard reaction between glutamic acid and reducing sugars have been identified and recognized to have an umami taste (Beksan and others 2003). They are N-glycoside (namely N-glucosyl glutamate) and the corresponding Amadori compound (namely N-deoxyfructosyl glutamate), which have umami taste threshold values of 1.6 and 1.8 mM, respectively. These values are comparable to that of MSG (1.5 mM). The Amadori compound is known to occur in the processed foods such as dried tomatoes, celery, asparagus, cauliflower, carrots, red pepper as well as dark malts. Furthermore, Schlichtherle-Cerny and Amadò (2002) together with Schlichtherle-Cerny and others (2004) found pyroglutamyl peptides from an enzymatic hydrolysate of deamidated wheat gluten. These peptides were present in the 2 fractions with a pronounced glutamate-like taste. Moreover, Rotzoll and others (2005) has successfully isolated a morel-derived glycoside, namely (S)-malic acid 1-O-D-glucopyranoside or (S)-morelid, from morel mushrooms, which had an umami, slightly sour taste above 6.0 mM.

Taste Interaction Involving Umami Taste Compounds (Umami Tastants)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

Taste interaction may occur in tastant mixtures or food system by either potentiation/enhancement or suppression way. The composition of the tastant mixtures and the concentration level of each tastant in the mixture seem to be important factors for such an interaction. Umami compound or umami tastant, that is well known and widely studied by physiologists, psychophysicists, and food scientists, is monosodium L-glutamate (MSG). This compound at a level of less than 20 mM possesses a purely umami taste, whereas at more than 20 mM it exhibits both salty and umami tastes (Keast and others 2004). Therefore, the interaction between MSG, at concentrations below than 20 mM, and other tastants is interesting and subjected to be discussed in this part.

The most prominent phenomenon of taste interactions is a large umami enhancement of MSG by the other umami compounds, that is, inosine-5′-monophosphate (IMP), adenosine-5′-monophosphate (AMP) and guanosine-5′-monophosphate (GMP), which has been investigated more than 30 y ago. Yamaguchi (1967) investigated umami interaction between MSG and IMP using the MSG point of subjective equality method for the mixtures of MSG and IMP of 0.05% w/v total concentration. She revealed that the enhancement could occur synergistically, especially in the range of 0% to 20% and 80% to 100% of IMP proportion in the mixtures. Because the umami threshold value of IMP is 0.025% w/v, it is obvious that there is subthreshold enhancement of MSG taste by IMP. The synergism can be explained that in the case of MSG solution alone, the umami taste intensity increases linearly with concentration, but the taste intensity of MSG/IMP mixture increases exponentially. Bellisle (1999) reviewed that the synergism between IMP and MSG was reflected in the observation that IMP could lower the threshold value of MSG solution by more than 50-fold. Comparing with the other 5′-nucleotides, the synergistic effect between IMP and MSG possesses 5 times higher than the effect between AMP and MSG (Fuke and Shimizu 1993), but 2.3 times lower than that between GMP and MSG (Yamaguchi 1967; Yamaguchi and others 1971). Rifkin and Bartoshuk (1980) reported that the synergistic effect between MSG and 5′-nucleotides, studied using GMP, was a true synergism, not an addition of perceived intensities.

A taste interaction has also been observed among 3 taste components, that is, amino acids (other than L-Glu and L-Asp), 5′-nucleotides (IMP or GMP), and L-Glu or L-Asp in the presence of sodium salt (1% NaCl) (Tanaka and others 1969; Yokotsuka and others 1969). The ternary synergism was observed for L-Ala, L-Cys, L-Gly, L-His, L-Met, L-Pro, L-Ser, L-Thr, DL-Try, and L-Val. Interestingly, although the individual taste of the amino acids are different from each other, the taste imparted in the ternary synergism (among L-Glu/L-Asp, other amino acids and IMP/GMP) was the same, that is, umami. However the umami taste was not potentiated in the binary mixtures of L-Asp/L-Glu and other amino acids, or other amino acids and IMP/GMP (Fuke and Shimizu 1993). In contrast, recently Kawai and others (2002) found the interaction between sweet taste-amino acids (L-Ala, L-Ser, and Gly) and IMP, and observed that umami was the only perceived quality of the interaction result.

MSG is also known to be able to interact with NaCl. MSG can enhance the taste sensitivity to NaCl, so that the use of NaCl in foods can be reduced (Fuke and Shimizu 1993; Bellisle 1999). In this case, MSG appear to enhance the perceived saltiness (Fuke and Ueda 1996; Keast and Breslin 2002). Kemp and Beauchamp (1994) revealed that MSG increased the saltiness of 25 mM NaCl solution as MSG concentration increased, and this was thought to be caused by the addition of salty component (sodium ion of MSG), not by potentiation. Moreover, Yamaguchi and Takahashi (1984) have evaluated the sensory interactions of MSG and NaCl in a clear soup. They suggested that to obtain the maximum palatability score at low MSG concentration, more NaCl was needed, whereas at higher MSG concentration less NaCl was needed.

The Fact of Umami Taste in Soy Sauce

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

The research on soy sauce taste was intensely conducted 4 to 5 decades ago with the emphasis on Japanese soy sauce (Oka and Nagata 1974a, 1974b). In this period, the umami taste in soy sauce has never been investigated in detail, eventhough several peptides were identified in some soy sauce fractions and a less volatile flavor component, namely 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H) furanone or HEMF, was found to give a bodied taste (sweet and caramel like taste) to Japanese soy sauce at the occurrence concentrations of 100 to 200 ppm (Yokotsuka 1986; van der Sluis and others 2001). Just started from 2004, the taste of soy sauce was then investigated further through the characterization of their low molecular weight fractions obtained by stepwise ultrafiltration on Indonesian soy sauce (Apriyantono and others 2004; Lioe and others 2004a, 2004b). Soy sauce fraction with a molecular weight of less than 500 Da was reported to contain the majority of the taste compounds (Apriyantono and others 2004; Lioe and others 2004a, 2004b). The fraction is tasted more umami than those with molecular weights more than 500 Da.

Investigation further of the umami fraction by conducting several chromatographic procedures, as well as capillary zone electrophoresis (CZE) profiles and taste profiles to screen and identify the umami or savory compounds, revealed that the major components in the fraction were free amino acids, especially free L-glutamic acid, and sodium salt (Lioe and others 2004b). The interesting fact was found when umami taste was also elicited by fraction containing free glutamic acid and salt with more hydrophobic amino acids. These bitter-tasted amino acids were actually present at their subthreshold concentrations in Indonesian soy sauce. This phenomenon lead to the study that proved the taste relationship among acidic amino acid, hydrophobic amino acid, and sodium salt (Lioe and others 2005). The umami enhancement activity of subthreshold L-phenylalanine and L-tyrosine is also responsible for the intense umami taste of Indonesian soy sauce (Lioe and others 2004b, 2005). This newly identified effect of umami enhancement may provide a new insight into umami taste perception in sensory science.

The taste characteristics of the low molecular weight fractions were also then investigated for Japanese soy sauces. The fractions with molecular weights less than 500 Da obtained from the 3 types of Japanese shoyu, koikuchi-shoyu, tamari-shoyu, and shiro-shoyu, by ultrafiltration were also tasted umami (Lioe and others 2007). The umami fractions obtained by further separation using gel filtration chromatograph contained relatively high free L-glutamic acid, salt, and peptides. The relatively high amount of peptides was questioned for their contribution to the intense umami taste of Japanese soy sauces. The precise contribution of small peptides to umami taste of food has become of interest since several decades ago. Oka and Nagata (1974a, 1974b) and Takeuchi and others (1962) found some peptides in soy sauce; however, they have not investigated further for their exact contribution to the soy sauce taste.

Takeuchi and others (1962) reported that abundance of small peptides in tamari-shoyu is 19% of the total nitrogenous compounds, about 2 times higher than that in koikuchi-shoyu. The average peptide length in tamari-shoyu is 3 to 4, whereas in koikuchi-shoyu 2 to 3. However, the contribution of peptides to the taste of soy sauce could not be proved on the basis of their concentrations and taste intensities. Three glucodipeptides and 8 dipeptides identified in neutral fraction of soy sauce had no taste (Oka and Nagata 1974a), whereas 4 peptides, 10 glycodipeptides, and 2 tripeptides identified in acidic fraction had an umami taste but their intensities were very low, that is, less than 12% of the monosodium glutamate taste (Oka and Nagata 1974b). The other 2 peptides isolated by Kirimura and others (1969) were reported to have bitter and astringent tastes. Nevertheless, the peptides present in Indonesian soy sauce have also made researchers being interested to know the contribution of them to the delicious taste of the soy sauce. Apriyantono and others (2004) have investigated Indonesian soy sauce ultrafiltration fractions to obtain the peptide fraction. Fraction with a molecular weight of less than 500 Da is the most delicious one and contained small peptides with a relatively low free glutamic acid content compared to the other fractions. The peptides in this fraction were expected to have a contribution to the umami taste of Indonesian soy sauce.

The exact role of peptides in Japanese soy sauce fractions, which have an intense umami taste was finally investigated by separating further the umami fractions with the 2nd step gel filtration chromatograph and reversed phase-HPLC linked to taste dilution analysis (Lioe and others 2006). Taste dilution analysis has been proved in many studies as a powerful method to trace the taste active compounds (Frank and others 2003; Ottinger and others 2003; Ottinger and Hofmann 2003). The high-performance liquid chromatography (HPLC) fractions containing peptides with glutamic acid residue from koikuchi-shoyu and tamari-shoyu have a much lower umami taste intensity (taste dilution factors found <1 to 4) compared to those containing free amino acids and salt (taste dilution factors found 16 to 32). Therefore, the contribution of peptides in eliciting an intense umami taste was clearly not significant, on the other hand, free amino acids and sodium salt are the key components of the intense umami taste of soy sauce. Interestingly, the free amino acids present in the fractions consisted of umami tasted and sweet tasted amino acids. The concentrations of umami tasted amino acids were actually much lower than their supposed concentrations for exhibiting the equal umami intensities found in their fractions (Lioe and others 2006). This fact give an insight that the intense umami taste in soy sauce is also resulted from the interaction between umami components and other tastants.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References

The different types of soy sauce have the same characteristic taste, called as umami or savory taste. Because they are produced from protein source ingredient and undergo a deep fermentation process, free amino acids were found as the most abundant nitrogenous component. Of which, free L-glutamic acid was the one of key compounds responsible for the soy sauce taste, in the presence of sodium salt. However the intense umami taste of soy sauce cannot be explained only by the concentration of each key component, since some taste interactions between the key components and others may occur to elicit a higher umami intensity, eventhough in one fact, a significant amount of small peptides present in the soy sauce was having a very low, not significant umami taste.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Soy Sauce
  5. Umami or Savory Taste Compounds
  6. Taste Interaction Involving Umami Taste Compounds (Umami Tastants)
  7. The Fact of Umami Taste in Soy Sauce
  8. Conclusions
  9. References
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