Mealtimes are often a great pleasure in daily life, especially for the elderly [1]. However, many elderly persons suffer from taste and/ or smell impairment caused by senescence itself, medication, and/or diseases, and they are thus unable to enjoy many foods [2,3]. According to the research on the taste sensitivity of patients with taste impairments, a reduction in umami taste sensitivity showed a strong association with the reduction of food palatability [4,5]. Umami taste-which is one of the five basic tastes along with sweet, salty, sour and bitter tastes-is a major taste quality of soup stock, the so-called “savory” taste. The most typical umami substance is L-glutamate (Glu), a non-essential amino acid [6]. In the present study, we investigated the Glu concentration in miso soup served in hospitals and nursing homes for the elderly in Japan. miso soup is a very popular Japanese traditional savory soup and is served at least once a day in such institutions. miso soup is made by boiling various ingredients such as vegetables in soup stock (dashi) and seasoning the stock with fermented soybean paste (miso). We collected miso soup from institutions for the elderly all over Japan, because there are many local varieties of dashi and miso that are used as the source of umami substances for miso soup. In addition to the Glu concentration in miso soup, we analyzed the concentrations of Inosine 5’-Monophosphate (IMP) [7] and Guanosine 5’-Monophosphate (GMP) [8], which are other types of umami substance, nucleotides. The sodium (Na) concentrations of the soup samples were also analyzed.

Sample collection

Ten miso soups were collected from a typical elderly hospital in Kyushu, a southern island in Japan (Institution A), and three miso soups were collected from a typical elderly nursing home in the same area (Institution B). We also collected 2 or 3 miso soups from each of the five institutions from each prefecture in Japan. The miso soup samples were frozen immediately after being prepared at the originating institution and thawed just before our analysis.

Analysis of Glu concentration

The chunky ingredients in each miso soup sample were removed with a strainer, and the supernatant of the soup was collected. The supernatant was diluted 20-fold with ultrapure water and then filtrated using a 0.45-μm filter and a 10-kDa centrifugal ultrafiltration unit. The amino acid concentration in the filtrate was analyzed by an L-8900 amino acid analyzer (Hitachi High-Technologies Corp., Japan).

Analysis of IMP and GMP concentrations

Quantitative analyses of IMP and GMP concentrations of the filtrate of miso soup were performed by an HPLC method (Separation: CAPCELL PAK NH2 UG 80 (Shiseido Co., Ltd., Japan), 30 mM (NH₄)₂HPO₄ (pH 3.0 w/H₃PO₄)/CH₃CN = 95/5, Detection: λ=254 nm).

Analysis of Na concentration

The Na concentration of the supernatant was analyzed by an ICPS-8100 inductively coupled plasma atomic emission spectrometer (Shimadzu Corp., Japan).

Statistical analysis

Analysis of variance (ANOVA) and calculation of correlation coefficients were performed using SPSS 16.0 (SPSS Japan).

Variance of Glu concentration in miso soup within institutions for the elderly

We first analyzed the variation of the Glu concentration of miso soup within two institutions, using samples from Institutions A and B (Figure 1). The Glu concentration of miso soup at Institution A was 36 ± 4.0 mg/100 ml (Av ± SD, n=10), and that of Institution B was 98 ± 7.0 mg/100 ml (n=3). Despite the variation in ingredients, the variance of Glu within each institution was relatively small, with an approx. 10% coefficient of variation (CV) in both institutions. Thus, the Glu concentration in miso soup might strongly depend on the dashi and miso used, and might depend less on other ingredients. Therefore, we decided to collect 2 or 3 types of miso soup from each institution to determine the Glu concentration in the miso soup served in such institutions for the elderly in Japan. Although every institution for the elderly serves several special diets (e.g. low-salt diets for patients with hypertension, low-sugar diets for patients with diabetes), we collected only miso soups used for patients receiving a regular diet for the comparison.

Figure 1: Variance of Glu concentration within two institutions. The dotted lines represent the average for institution A and institution B.

Concentrations of umami substances, Glu, and nucleotides in miso soup served in institutions for the elderly in Japan

We collected 658 samples of miso soup served as part of the regular patient diet from 220 institutions, which represented about five institutions per prefecture in Japan. The samples’ average concentrations of amino acids including Glu are shown in Table 1. Glu was the most abundant protein amino acid in free form; however, its concentration showed a large CV (Av±SD: 156.3 ± 101.3 mg/100 g (range: 15.7– 697 mg/100 g, CV: 64.8%)).

Table 1: Concentration of amino acids, nucleotides, and Na. Amino acids are shown in abbreviation of IUPAC.

In the one-way ANOVA, the main effect of institution was significant (F(219, 438)=6.110 (p=0.000)). The main effects of dashi (five categories: dried bonito (katsuo-bushi), dried sardine (iriko), ready-made, natural ingredients + ready-made, and non-classifiable dashi) (F(4, 653)=37.538 (p=0.000)), and miso (seven categories: soybeans only, soybeans + rice, soybeans + wheat/barley, mixed, umami seasoning-added, umami seasoning-added + mixed, and nonclassifiable miso) (F(6, 651)=5.071 (p=0.000)) were significant. That is, because each institution had its own usage of dashi and miso for miso soup, the Glu concentration of miso soup differed by institution, but the Glu concentration was almost constant within each institution, as in the cases of Institution A and Institution B.

The average Glu concentration by prefecture (Figure 2) was higher in the middle region of Japan and lower in the southern region of the country. Most of the institutions in the southern region used dried sardines for dashi (iriko-dashi) and soybeans and wheat/barley miso (mugi-miso), which are relatively low in Glu content. In the middle part of Japan, such as the Kanto district where Tokyo is located, many institutions used ready-made dashi in which Glu and IMP/GMP are supplemented as umami seasoning. Several dietitians at institutions with low-Glu content in high-Glu districts, such as Kanto district, reported that elderly patients had complained about the unpalatability of miso soup with a diluted taste. This might be partly because the elderly people in such high-Glu districts were used in foods with high Glu contents.

Figure 2: Average Glu concentration by prefecture. The dotted line represents the average.

We also measured the concentration of another type of umami compound, the 5’-purinemononucleotides IMP and GMP, which show synergistic umami enhancement when mixed with Glu (Table 1). These nucleotides are present in meat products such as dried fish used for dashi, e.g., dried bonito and dried sardines. We found that the concentration of IMP was 8.8 ± 8.5 mg/100 ml, and that for GMP was 1.6 ± 3.1 mg/100 ml. However, miso contains phosphatase, which degrades nucleotides into nucleosides (compounds lacking umami taste) during cooking and storage, and thus our measurements of the concentrations of IMP and GMP of miso soup might be less reliable.

Relationship between concentrations of Glu and Na

The Na concentration fell within a narrower range than that of Glu, between 142 and 596 mg/100 g (328.5 ± 70.1 mg/100 g, CV: 21%), as shown in Table 1. The relationship between Glu and Na is shown in Figure 3. A significant positive correlation was found between the Glu and Na concentrations (r=0.534 (p=0.000)). miso contains high Glu and Na because it is made with salted and fermented soybeans. Some ready-made dashi contains high Glu and Na because its major ingredients are sometimes NaCl and umami compounds. These aspects of miso might explain the positive correlation between the concentrations of Glu and Na in miso soup.

Figure 3: Glu and Na concentration within each institute. The line pattern of each symbol shows the type of dashi, and the shape of each symbol represents the type of miso, as shown under the graph. One symbol represents an average value (2 or 3 miso soups) of each institution. Therefore, each symbol represents the major dashi and miso values of each institution. Concentrations are represented in logarithmic axes, because taste intensity is proportionate to the logarithmic value of the concentration of each tastant.

Umami taste in foods for the elderly

Most people perceive that overall flavor is enhanced when umami compound is added to foods appropriately. For example, when an umami compound, Monosodium L-Glutamate (MSG), was added to a diluted beef broth, the flavor characteristics such as thickness, mouth-fullness, impact, and overall taste were improved [9]. In another study, umami compounds were important for the characteristic flavor of fish, crab, clam, and other seafoods [10,11]. Thus, because umami compounds intensify flavor, Glu salts are sometimes used for compensation for the bland taste of salt-reduced foods [12-18]. Several reports showed that salt content could be reduced by as much as 30% by using MSG, though it contains Na (0.12 g Na/1 g MSG: cf. 0.40 g Na/1 g NaCl) [12,15].

However, the elderly often have appetite loss due to low umamitaste sensitivity [4], and it may be possible to improve their appetite by intensifying the umami taste via the fortification of the appropriate seasonings. We might be able to intensify the savory taste of Japanese miso soup by partial substitution of miso or ready-made dashi with a small amount of umami seasonings such as MSG, reducing Na at the same time to reduce the risk of hypertension. Many Glu-rich traditional fermented seasonings around the world [19] tend to have high Na content because NaCl is indispensable for bacteriostasis during longterm fermentation. In Asia, there are various traditional local high-salt sauces or pastes made with soybeans, fish, shrimp, and/or meat. Elderly people tend to prefer traditional dishes cooked with such traditional fermented seasonings; e.g., the miso soup made with fermented salted soybean paste (miso) examined in the present study. In such scenarios, the partial substitution of fermented seasonings by umami substances might be effective for cooking for the elderly, especially in persons with reduced umami-taste sensitivity.

We measured the concentration of a typical umami compound, Glu, of miso soup, a popular Japanese soup served in institutions for the elderly in Japan. The Glu concentration was the highest among amino acids (Av ± SD: 156.3 ± 101.3 mg/100 g). However, its range was very large (15.7–697 mg/100 g, CV: 64.8%), unlike that of the miso soup Na concentration.

We thank the institutions for their supply of miso soup samples and the staff of Ajinomoto Pharmaceuticals Co., Ltd. and Ajinomoto Nutrition Foods Co., Ltd. for their support in contacting respective institutions.