Tauco is a fermented soybean paste like miso but typical from Indonesia, commonly used as umami seasoning. This study objective was to evaluate the nutritional composition of diverse tauco products found in Indonesia and to determine a potential tauco product characterized by certain amino acids related to umami peptides (Asp, Glu, Gly, Ser, Thr, Val). Proximate composition, NaCl salt, total sugars, total acids, and total amino acid profiles of 24 tauco products, collected from 24 producers in 7 provinces in Indonesia, were analyzed. The proximate composition mapped tauco into solid (containing 29.2–35.3% moisture on wet weight basis or ww), semisolid (42.2–54.5% ww moisture), and liquid (56.2–68.1% ww moisture). Solid tauco composed of 54.7–65.2% carbohydrates, 10.0–22.8% ashes, 7.13–16.9% NaCl salt, 9.72–11.9% crude proteins, 11.9–27.0% total sugars, 6.98–23.4% crude fats, 1.24–2.18% total acids, and 11.4–17.5% total amino acids on dry weight basis (dw). In contrast, liquid tauco consisted of 8.47–56.5% dw carbohydrates, 17.1–73.8% dw ashes, 10.7–68.4% dw NaCl, 20.4–30.9% dw crude proteins, 4.45–29.9.5% dw total sugars, 1.30–18.9% dw crude fats, 1.65–5.36% dw total acids, and 14.1–24.6% dw total amino acids. All tauco had a slight acidic pH, ranged from 4.38 to 5.91. Glu and Asp were the dominant amino acids of tauco, comprising 25–40% of total amino acids. Mapping of total amino acid profile mentioned a tauco product, characterized by the highest concentration of amino acids related to umami peptides. This finding leads to the exploration of umami peptides to reveal the science behind its traditional use as umami seasoning.
Fermentation is one of the oldest food-processing technologies, not only can improve the storage characteristics of foods but also improve the sensory, nutritional, and functional properties of foods [1, 2]. Traditional fermented foods are generally considered safe and healthy foods that the local communities have consumed for a long time [3, 4]. Soybean is a commodity commonly used to produce various types of fermented foods by using fungi, yeasts, and bacteria. One of fermented soybean products is soybean paste, characterized by a thick liquid or slurry, essentially made from soybean mixed with a lesser amount of cereals than soybean, fermented by fungi at first stage, followed by yeasts and bacteria with the addition of salt solution at relatively high concentration (Table 1). Table 1 shows many types of fermented soybean paste worldwide. Different countries in Asia have different names for their fermented soybean paste, depending on the production process and the type of microbial strain used . In Japan, fermented soybean paste is known as miso ; in Korean, doenjang ; in China, dajiang ; in Thailand, thua-nao ; and in Indonesia, tauco . Each product has unique flavor characteristics. Similar to fermented soy sauce in Indonesia, tauco as fermented soybean paste was originated from China and introduced into Indonesia by Chinese population mostly from Hokkien ethnic several ages ago. Tauco is then popular in Cianjur, West Java, and other coastal area in Indonesia (Fig. 1) where Chinese population is present.
Tauco has long been used as an ingredient to season many dishes in Indonesia because it provides a delicious umami/savory taste. Traditional dishes such as tauge goreng in Bogor (West Java), bubur sop ayam in Cirebon near Tegal (West Java), soto pekalongan (Central Java), sambal tauco (West Java and Central Java), also sayur tauco and ikan tauco (different regions in Indonesia) have used tauco as a typical seasoning for its desired aroma and umami taste. Tauco comes from its pronunciation in the Hokkien dialect of Chinese. The Chinese tauco character is written as “豆醬,” which consists of tau “豆,” which means beans, and co “ 醬,” which means thick soy sauce . According to Shurtleff and Aoyagi, tauco is still associated with the Chinese spice, Jiang. Jiang is thought to have originated before the Chou Dynasty (722–481 BC), so it can be claimed as the oldest seasoning known to humans . From historical records, tauco in Cianjur (West Java) was first recorded in Indonesia in the nineteenth century due to the acculturation of Chinese culinary culture with the Sundanese Cianjur ethnic group. However, tauco has become part of the identity of the Cianjur community. At present, tauco in Indonesia is developing in various regions. There are at least five known types of tauco in Indonesia: tauco Cianjur, tauco Kalimantan, tauco Pekalongan, tauco Bangka, and tauco Medan. Recent studies have also shown that tauco may have health benefits as it contains phenolic compounds that have antioxidant activity [13, 14].
The production process of tauco is similar to the other fermented soybean pastes, which is divided into two stages of fermentation, koji/meju fermentation and brine/moromi fermentation  with microbials presented in Table 1. The detailed production process of tauco involves several steps: soybeans cooking, soybean mixing with roasted flour, koji mold fermentation, and followed by brine fermentation. Cooked soybeans mixed with roasted flour (2:1) were inoculated with a starter containing a mixture of molds (mostly Rhizopus oligosporus, Rhizopus oryzae, and Aspergillus oryzae) and fermented for 3–5 days at 30 °C. The mold fermented beans were broken into pieces and sun-dried (called as koji or meju). Koji was then mixed thoroughly with a high concentration of brine solution (c.a. 17–20%) at a ratio of 1:3 and stored at room temperature for several weeks. Raw tauco obtained can be cooked with palm sugar solution (c.a. 50%) and filled into packaging (plastic, bottle, or bamboo container). Tauco can also be dried in the oven at 60–80 °C to produce products with the desired moisture content. This process allows the tauco products to have a different consistency and forms [14,15,16].
Although the fermentation process conducted on an industrial scale has used starter culture, the fermentation process in the tauco production still occurs on a household and small-enterprise scale that involve spontaneous fermentation by various types of microorganisms originally present in the raw food materials . The complex and diverse strains lead to the non-uniform quality of the tauco products, including their nutritional value even from the same region, because they are closely related to the breakdown of the substrate, making it interesting to evaluate. Tauco, as local food of Indonesia has no detailed information on this matter. The studies that have explained the differences in the nutritional value of tauco products in Indonesia were still limited, including the characteristic differences based on the type of tauco (solid, semisolid, and liquid tauco). In fact, this study is needed as a reference in developing potential research related to tauco in the future. Therefore, the objective of this study was to evaluate different tauco characteristics through multivariate analysis with its function as savory seasoning, and to map tauco with other similar soybean paste from different countries.
Materials and methods
Materials and chemicals
Tauco samples (Fig. 2) were obtained from 24 producers in 7 provinces of Indonesia (West Java, Central Java, Bangka, North Sumatera, West Sumatera, South Sumatera, and West Kalimantan). The abbreviated brand names of them were CMC and CBC (from Cianjur, West Java); MNB (Bogor, West Java); SAP, PDP, TKP, MMP, LEP, and CIP (Pekalongan, Central Java); SKT (Tegal, Central Java); SAB, CBB, and BEB (Bangka); CGM, SIM, HAM, SUM, and CBM (Medan, North Sumatera); BMP (Padang, West Sumatera); MIP (Palembang, South Sumatera); and CDS, KES, MIS, and CKS (Singkawang, West Kalimantan). Each sample was pre-packaged in a plastic bottle, glass bottle, or bamboo container, and for analysis, 3 packages of each sample were purchased and then homogenized as a mixture. The visual appearance of the homogenized tauco samples can be seen in Fig. 3A. The homogenized tauco samples were further categorized based on their moisture content as solid tauco (MIS, CKS, SIM), semisolid tauco (CBC, CBM, MNB, SAP, PDP, TKP, MMP, CIP, LEP, SKT), and liquid tauco (CMC, SAB, CBB, CDS, KES, BMP, MIP, CGM, HAM, BEB, SUM). Representative of the three tauco groups is figured in Fig. 3B. Since the shelf-life of tauco is generally over 6 month, it was difficult to find the same shelf-life for 24 samples. All samples were obtained before their expiration dates. Detail of the tauco samples and lists of ingredients of each tauco are presented in Table 2. Samples were sent to the laboratory and immediately stored at −18 to (−20) °C upon receipt until the time of evaluation for maximum 1 week. Prior to analysis, samples were thawed overnight in a refrigerator (4–5 °C) and crushed and homogenized using blender for 1 min from 3 replicates of sampling (same brand/producer, but different packaging) to obtain a composite sample used for chemical characterizations.
The chemicals used for proximate composition analysis were of analytical grade, including hexane, potassium sulfate, mercury oxide, sulfuric acid, sodium hydroxide, sodium thiosulfate, boric acid, and hydrochloric acid (Merck KGaA, Darmstadt, Germany). Potassium chromate and silver nitrate of analytical grade (Merck) were used for salt analysis. D-glucose standard and anthrone reagent (Merck) were used for total sugars analysis. Standard buffer solutions (pH 4.00 and 7.00) were used for pH measurement, while potassium hydrogen phthalate, natrium hydroxide, phenolphthalein, and ethanol (Merck) were used for total acids analysis. The chemicals for total amino acids analysis were amino acids standard mixtures, hydrochloric acid, sodium acetate, Na-EDTA, tetrahydrofuran (THF), o-phthalaldehyde reagent (OPA reagent) (Sigma-Aldrich, St. Louis, MO, USA), methanol and acetonitrile HPLC grade (Merck), Whatman No. 1 filter paper, and 0.45 µm filter membrane (Advantech Co., Ltd., Tokyo, Japan). Distilled water for chemical characterizations and redistilled water (IPHA Laboratories, Bandung, Indonesia) for HPLC analysis were used in this study.
Proximate composition consisting of moisture, ashes, crude proteins, crude fats, and carbohydrate (by difference) was carried out in accordance with the AOAC procedure . Moisture content by drying 1–2 g of samples at 105 °C in hot air oven, ash content by igniting 2–3 g of sample using an electric furnace (Furnace 48,000, Thermolyne, USA) at 550 °C, crude fats by extracting dried sample using n-hexane with a Soxhlet extraction unit, crude proteins by using a micro-Kjeldahl system to determine total nitrogen and then using a conversion factor of 6.25 to determine crude proteins, and carbohydrates by difference were determined for 24 tauco products. All results were reported in the form of mean ± standard deviation of triplicate analysis for moisture content and ash content, and of duplicate analysis for crude proteins and fats, as % dry weight basis (% dw) or g/100 g dry matters of tauco, except moisture content as % wet weight basis (% ww) org/100 g fresh tauco (as it is).
Salt content, total sugars, total acids, and pH measurement
Salt content (expressed as sodium chloride) was analyzed using Mohr method with modification by dissolving the ashes of the sample in distilled water and then titrated with 0.01 M AgNO3 as the titrant and potassium chromate as an indicator . Total sugars were quantified by spectrophotometry (UV–Vis 160 spectrophotometer) using D-glucose standard and anthrone reagent, which determined total reducing and non-reducing sugars. Total acids were titratable acids, analyzed by volumetric titration with 0.1 N NaOH as titrant and phenolphthalein as an indicator . The total acids were calculated as lactic acid equivalent, since lactic acid was used as the major indicator of organic acids in a similar fermented soybean product . The pH value was determined by dissolving 5 g of sample in 45 mL distilled water, then measured using a digital pH meter (MW-801, Milwaukee, Romania), which was calibrated by pH 4 and 7 buffer standards . All results were reported in the form of mean ± standard deviation of triplicate analysis, except for total sugars of duplicate analysis, all results were expressed in % dw, except the pH value (no unit).
Total amino acid profiles
The total amino acid composition or total amino acid profile was analyzed by IPB University’s Integrated Lab using an in house method after acid hydrolysis. Analysis was carried out by pre-column derivatization using o-phthalaldehyde (OPA) reagent and HPLC instrument (Shimadzu SCL 10-A). A total of 1–5 g of sample was weighed in a closed container and hydrolyzed with the addition of 2 mL of 6 N HCl. The sample was then streamed with nitrogen gas for 0.5–1 min and put in an oven at 110 °C for 24 h. The hydrolyzed sample was cooled at room temperature, quantitatively transferred to a rotary evaporator flask, and dried. The dried sample was added with precisely 10 mL of 0.01 N HCl.
Derivatization with OPA reagent was carried out first on the hydrolyzed sample before being analyzed with an HPLC column. OPA reagent was prepared by dissolving 50 mg OPA powder in 1.25 mL ethanol. The OPA solution was then added with 50 µL 2-mercaptoethanol and 11.2 mL sodium borate buffer 0.1 M (pH 9.5). For sample preparation, potassium borate buffer pH 10.4 was added into the hydrolyzed sample in ratio 1:1. A total of 50 µL of this mixture was added with 250 µL of OPA reagent and vortexed for 1 min until complete derivatization. The sample was filtered using a 0.45 µm membrane filter and then injected with the volume of 5 µL into the HPLC instrument (Shimadzu SCL 10-A), equipped with a 195 DGU-20A degasser and a CBM-20A controller, with the following conditions: Thermo Scientific ODS-2 Hypersil (150 mm × 4,6 mm, 5 µm) as a column (Thermo Fisher Scientific, Los Angeles, USA), the mobile phase at flow rate 1 mL/min that consisted of Na-acetate pH 6.5 198 (0.025 M), Na-EDTA (0.05%), methanol (9.00%), THF (1.00%) in deionized water for solvent A, and methanol (95%) and deionized water (5%) for solvent B. For 35 min, the mobile phase was programmed to 0–2 min, 5% B; 2–13 min, from 5 to 35% B; 13–15 min, 35% B; 15–20 min, from 35 to 70% B; 20–22 min, from 70 to 90% B; 22–25 min, from 90 to 100% B; 25–28 min, from 100 to 0% B; 28–35 min, 0% B. The fluorescence detector was set in excitation and emission wavelengths of 350 and 450 nm, respectively. Individual total amino acid was calculated based on its corresponding amino acid standard provided by a standard mixture of amino acids. Total amino acids were also calculated by the sum of individual total amino acids for each sample.
One-way analysis of variance (ANOVA) with SPSS v.22 (IBM, NY, USA) was used to determine the differences in the analysis results between samples. Further tests using Duncan’s test at a 95% confidence interval were carried out if there was a significant difference. The principal component analysis (PCA) method was performed to map tauco samples based on proximate composition, salt content, total sugars, total acids, and total amino acids, and also based on total amino acid profiles by SIMCA v.17 (Umetrics, Sweden). Pre-treatment of the data using scaling on the tested variables can also be selected to obtain a fit model. The tested variables can have different ranges of values, so it is necessary to determine one coordinate with a length determined by scaling. The scaling used in this study was unit variance (UV-scaling).
Results and discussion
Nutritional and chemical composition of tauco
The nutritional and chemical composition of tauco samples is presented in Table 3. Twenty-four tauco samples had proximate composition, consisting of moisture content ranged at 29.2–68.1% ww, ash content at 10.0–73.8% dw, crude proteins at 9.72–33.6% dw, crude fats at 0.92–23.4% dw, and carbohydrates by difference at 6.59–68.4% dw. The wide range of salt content at 7.13–68.4% dw, total sugars at 3.56–42.3% dw, and total acids at 1.58–6.70% dw, were also observed. The salt content described the ash content of tauco samples because most of the ash is in the form of salt which is intentionally added at high concentrations (c.a. 17–20%) in the tauco production process .
Tauco samples were mapped with PCA based on the proximate composition (Fig. 4). The four PCs included in the model produced R2X and Q2 of 1 and 0.986, respectively. A good model will have an R2X value greater than the Q2 value, Q2 value greater than 0.5, and the difference between the R2X and Q2 value is not too large. In this model, PCA has been quite good in presenting the analyzed data. PCA biplot based on the proximate composition of tauco contributed 66.60% of the total variation for the data set, with PC1 and PC2 accounting for 42.20% and 24.40% of variance explained, respectively (Fig. 4). Satisfactory separation was observed for most of tauco samples and categorized into 3 clusters, namely liquid (green color in Fig. 4), semisolid (red color), and solid (orange color) tauco, except for CMC which had moisture content at 59.2 ± 0.5% ww but classified as semisolid sample, due to its higher carbohydrate content than liquid samples. However, CMC was categorized as a liquid sample in this study (Table 3). Among the proximate composition, moisture content best described the tauco grouping. The grouping of tauco products by their proximate composition to determine their physical consistency is firstly reported.
According to Wolfe and Dutton , moisture content affects the textural properties of fermented soybean products. High moisture content produces a product with a liquid consistency and vice versa. Based on the geographical distribution, tauco products produced in North Sumatera consist of various types of tauco, either solid, semisolid, or liquid. In contrast to North Sumatera, tauco products were only found in a liquid form in West Sumatera, South Sumatera, and Bangka, and in a semisolid form in Central Java. Tauco products produced in West Java, a central of tauco production in Indonesia, were found in semisolid and liquid forms. Moreover, tauco produced in West Kalimantan was found in solid and liquid forms. Tauco products from Cianjur (CBC and CMC) which is popular in West Java (inhabited by one-fifth of Indonesian population) are grouped as liquid tauco (CMC) and semisolid tauco (CBC).
In comparison with other fermented soybean paste, chemical characteristics of tauco were comparable to doenjang and miso. Doenjang had moisture content ranged from 48.5 to 63.1% ww, ash content at 21.6–36.9% dw, crude proteins at 18.8–33.0% dw, crude fats at 3.10–21.9% dw, carbohydrates by difference at 14.9–56.5% dw, salt content at 17.5–39.2% dw, total acids at 2.97–7.46% dw , total sugars at 6.88–33.1% dw, and pH values at 5.0–5.8 . Meanwhile, the characteristics of miso as follows: moisture content at 44.0–50.0%, crude proteins at 14.3–24.0%, crude fats at 3.57–12.0%, and salt content at 8.93–26.0% . PCA can be used to categorize fermented soybean paste products (tauco, doenjang, thua-nao, miso, and dajiang) based on the similarities of their proximate composition patterns (Fig. 5). Cumulatively, the first 3 PCs in the model produce goodness of fit (R2X) and goodness of prediction (Q2) of 0.980 and 0.665, respectively. The total diversity of data explained by the model was 80.10%, with PC1 at 59.30% and PC2 at 20.80%. This figure is the first report to compare tauco, which mimics a Japanese miso, to other similar products. Tauco in the liquid form had a similar proximate composition to dajiang and traditional doenjang, while semisolid form was similar to commercial doenjang. On the negative side of PC1, solid tauco had a similar proximate composition to miso in Japan, and both had fairly high crude fats. This is the first report to compare tauco with other soybean paste products according to the proximate composition.
In the present study, based on the three groups of tauco products, the chemical composition of all groups is summarized in Fig. 6. Moisture contents of the tauco products, which had a range of 29.2–35.3% ww for solid, 42.2–54.5% ww for semisolid, and 56.2–68.1% ww for liquid tauco (Table 3), showed a significant difference at p < 0.05 (Fig. 6). Liquid tauco had the highest moisture content, twice higher than solid form. Meanwhile, semisolid tauco had a moisture content of 1.5 times higher than solid form.
Tauco products contained a wide range of ash content, 10.0–73.8% dw (Table 3). Most of the ash is sodium salt, which was added in a high concentration of brine solution during the fermentation process. The salt content ranged from 7.13 to 68.4% dw (Table 3) or comprising 40–94% of the ash contained in all tauco products. The salt content reached 130–580 times salty taste threshold, that is, 0.0327% . Due to the relatively strong salty taste, tauco is not suitable for direct consumption, but is commonly used as a seasoning ingredient . Solid tauco contained salt ranged from 7.13 to 16.9% dw, while salt in semisolid and liquid tauco ranged from 10.9 to 24.8% dw and 10.7–68.4% dw. Liquid tauco had a significantly higher salt content (p < 0.05) than those of solid and semisolid form (Fig. 6). The relatively high salt contents in the tauco products were due to the use of salt solution at high concentrations for the second step of tauco fermentation that could inhibit the growth of undesirable microbes.
Liquid and semisolid tauco had a comparable range of crude proteins, 20.4–30.9% dw and 15.1–32.6% dw, respectively. Meanwhile, crude proteins in solid tauco were relatively lower, ranging from 9.72 to 11.9% dw (Fig. 6). These crude proteins distinguished solid tauco from the other types. Low crude proteins content in solid tauco (in dw) are due to the higher contents of other solid content, carbohydrates, which is discussed below. The proteins found in tauco could be associated with an increase in the number of the fermenting microorganisms , or due to proteolytic enzymes produced by the fermenting microorganisms [26, 27], or as a result of the synthesis of protein by microorganisms during fermenting substrates . The protein contents can also be influenced by soybean varieties used as raw materials .
The crude fats of solid, semisolid, and liquid tauco ranged at 6.98–23.4% dw, 0.92–14.0% dw and 1.30–18.9% dw, respectively. The lowest crude fats were found in semisolid tauco, and these were significantly different to other tauco groups (Fig. 6). Fats in raw materials could be used as an energy source for microorganisms , and thus, the lower crude fats in semisolid tauco may mention the more rigorous growth of microorganisms during the production of the semisolid tauco.
Carbohydrates content of solid, semisolid, and liquid tauco ranged at 54.7–65.2% dw, 33.7–68.4% dw, and 8.47–56.5% dw; meanwhile, their average and standard deviation were at 59.4 ± 5.3%, 49.4 ± 9.6% and 26.2 ± 12.9% dw, respectively. Carbohydrates by difference also distinguished liquid tauco from other types of tauco. Liquid tauco contained the lowest carbohydrates content, while solid tauco had the highest carbohydrates content. In solid tauco, carbohydrates seem to be present not only from the raw material used for tauco fermentation but also by later carbohydrate addition used as a thickener or texturizer. Sugars are among soluble carbohydrates which was found in 24 tauco products, ranging from 4.45 to 42.3% dw. The total sugars reached 30–90% of the crude carbohydrates. From this study, there was no significantly difference (p > 0.05) between total sugars in solid, semisolid, and liquid tauco.
The tauco products contained total acids (expressed as lactic acid equivalent) at very low concentrations compared to other chemical components, ranging from 1.24 to 5.43% dw. Solid, semisolid, and liquid tauco contained total acids ranging from 1.24 to 2.18% dw, 1.30–5.43% dw, and 1.65–5.36% dw, respectively. The total acids between semisolid and liquid tauco did not show a significant difference (p > 0.05), and the acid concentrations of both samples reached more than twice higher as those of solid tauco (Fig. 6). All tauco samples in this study had a slight acidic pH value, ranging from 4.38 to 5.91 (with an average and standard deviation at 4.94 ± 0.39). The pH values of solid, semisolid, and liquid tauco, respectively, ranged from 4.38 to 5.35, 4.48–5.34, and 4.41–5.91. The acid contents in tauco were due to the role of acid-producing microorganisms in the condition of relatively high salt concentration at the second stage of tauco fermentation . The acid-producing microorganisms are mostly from lactic acid bacteria as presented in Table 1.
Amino acid profiles of tauco
The amino acid composition of tauco is shown in Table 4. Twenty-four tauco products contained total amino acids in the range of 11.5–24.6% dw (17.1 ± 3.50% dw), figuring the content of crude proteins. Glutamic acid (Glu) was the primary total amino acid present in tauco (4.00 ± 1.01% dw), followed by aspartic acid (Asp) at 2.07 ± 0.43% dw, and leucine (Leu) at 1.64 ± 0.31% dw. Glu, Asp, and Leu were also the most abundant amino acids in Chinese fermented soybean food, sufu, prepared with 8% w/w salt contents . The final salt content in most soybean paste products, especially red and gray sufu, was still more than 10% dw . In this present study, the salt contents of tauco products ranged at 4.38 – 18.7% ww or 7.13 – 68.4% dw, which were higher compared to those of sufu and the other similar products, such as doenjang (17.5 – 39.2% dw)  and miso (8.93 – 26.0% dw) . The relatively high salt content in tauco and other similar products are due to salt addition during the second stage of fermentation to provide a selection for halophilic bacteria and yeasts which their growths are desired for the products. The presence of salt in the final products is also important for the desired taste because it could intensify the umami taste of free amino acids (glutamic acid and aspartic acid) contained in final products, as revealed in other salt and amino acids containing products such as soy sauce [34, 35], cheese , and tofuyo . These studies imply that amino acids together with salt may potentiate the umami taste of tauco. However, total amino acids of tauco (17.1 ± 3.50% dw) showed at lower concentrations compared to those in soybeans, 33.9 ± 2.8% dw, with the dominant amino acids in soybeans including Glu at 7.34 ± 0.06% dw, Asp at 3.88 ± 1.04% dw, arginine (Arg) at 2.84 ± 0.437% dw, Leu at 2.78 ± 0.22% dw, and lysine (Lys) at 2.15 ± 0.30% dw [38, 39]. This change especially happened after the second stage of fermentation. The study by Hong et al.  showed that soybean fermentation with A. oryzae which mimics the first stage of tauco fermentation did not affect the contents of amino acids as shown by the not significant increase from 39.0% dw to 41.4% dw, but it could provide small-size peptides, which is important for microbials growth at the later stage of fermentation, such as brine fermentation in tauco production. Another study, according to Han et al. , also stated that the fermentation of soybeans curd with the mold Actinomucor elegans in the manufacture of sufu did not show a significant difference in the total amino acids content (from 54.7% dw to 55.1% dw). However, subsequent fermentation with salt could cause a significant decrease in the total amino acids (from 54.7 to 35.1% dw). These results showed that mold fermentation did not affect a significant decrease in the total amino acid content, but brine fermentation provides a significant decrease in the total amino acid content due to the microbial community, dominated by halophilic yeasts and lactic acid bacteria that metabolize short chain peptides into amino acids and utilize amino acids in their metabolism.
During tauco fermentation, especially in brine fermentation, involved microorganisms (yeast and lactic acid bacteria) contribute to the metabolic activity that governs tauco quality. Among fermentation metabolites, small-size peptides and free amino acids are essential that play a role in the metabolism by microorganisms or for nutritional requirements in microbial growth through proteolytic activities [39,40,41,42]. Small-size peptides formed by enzymatic reaction during the koji/meju fermentation by fungi and then followed by shorter peptides or free amino acids formed at the early stage of brine/moromi fermentation by the activity of proteolytic enzymes produced by the fungi  may promote the growth of lactic acid bacteria observed through proteolytic activities at the later stage [41, 43]. These proteolytic activities are responsible for the organoleptic properties of the final products [44, 45]. Microbial communities found in tauco and similar products to tauco at koji/meju fermentation and at brine/moromi fermentation stages are shown in Table 1.
The free amino acids also contribute directly to taste perception and impart several basic tastes, such as umami taste from Glu and Asp, sweet taste from alanine (Ala), glycine (Gly), serine (Ser), and threonine (Thr), and also bitter taste from valine (Val), methionine (Met), isoleucine (Ile), leucine (Leu), tyrosine (Tyr), phenylalanine (Phe), histidine (His), and arginine (Arg) [46, 47]. In some fermented soybean products, the presence of free amino acids during fermentation, such as Glu and Asp (umami), also Phe and Tyr (bitter) at their subthreshold concentrations, and in the presence of salt, might also play an important role in impressing umami taste of Indonesian soy sauce . On the other hand, the abundant presence of free Glu and Asp and several sweet-taste eliciting amino acids (Ala, Ser) were considered to be the main contributors to the umami taste of shoyu [20, 35] and tofuyo .
Accordingly, Table 4 shows that bitter amino acids were in relatively high concentrations, contributing 37–45% of the total amino acids, while umami amino acids account for 25–40%. However, the percentage of bitter amino acids were for a combination of 8 amino acids, while umami amino acids are a combination of 2 amino acids. Sweet amino acids were in relatively lower concentrations than the others, contributing 18–22% of the total amino acids. The ratio of umami to bitter amino acids in the tauco was 1:1, while their ratio to sweet amino acids was higher, approaching 2:1. This ratio was comparable to sufu , which may give tauco a pleasant taste . The evaluation of nutritional composition of tauco related to its desired taste is firstly reported. Figure 7 shows no significant difference (p > 0.05) between solid, semisolid, and liquid tauco in sweet amino acids content. Meanwhile, semisolid tauco contained umami, sweet, and bitter amino acids content were not significantly different (p > 0.05) from the liquid tauco. Perhaps, the taste intensities of these tauco products were not different.
Figure 8 shows the PCA model performed on the amino acid profiles of tauco products. The first 2 PCs in the model produced R2X of 0.846 and Q2 of 0.687. The total variance of the data set was 84.60%, a combination of PC1 and PC2, which was 77.20% and 7.40%, respectively. Based on amino acid profiles, the scores plot was divided tauco products into 3 clusters, but different from those discussed above on their proximate composition. One sample of semisolid tauco, MNB sample, separated as a cluster (orange color). The MNB sample had the highest concentration of amino acids compared to other samples, either for umami, sweet, or bitter amino acids. The sample scores were also close to loadings of Glu and Asp (umami amino acids), Gly, Ser, and Thr (sweet amino acids), and Phe, Val, Ile, and Leu (bitter amino acids). Glu and Asp as well as Gly, Ser, Thr, and Val have been reported as umami-related amino acids which were found in many umami peptides [48,49,50]. Another study also showed that Arg, Asp, Glu, Gln, and Tyr as amino acid residues of umami peptides were on the binding sites to T1R1/T1R3 receptor, an umami receptor [5, 51, 52]. Therefore, MNB was an important tauco for further investigation to find umami peptides. The umami peptides may also have bioactivities such as antioxidant, antidiabetic, and antihypertensive activities as found in the studies of umami peptides [53, 54]. These could lead to the exploration of umami peptides and bioactive peptides from tauco.
The present study showed that nutritional composition especially the proximate composition best distinguished 24 tauco products produced throughout Indonesia. First, multivariate analysis of proximate composition could classify tauco products into solid, semisolid, and liquid forms, with moisture content best determined this category. Besides this, liquid tauco and semisolid tauco were characterized by crude proteins content, which is important for their desired taste; meanwhile, solid tauco was characterized by carbohydrates content. Second, PCA mapping based on total amino acid profiles showed a semisolid tauco which had a different amino acid profile from the others, highlighted by the high concentrations of amino acids related to umami peptides. This gives an insight for future research on umami peptides as well as bioactive peptides from tauco, since some umami peptides could exhibit bioactivities.
Availability of data and materials
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Principal component analysis
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The authors thank the anonymous reviewers for their valuable inputs and comments.
This article was written with the financial support from Ministry of Education and Culture of Republic of Indonesia through PMDSU research schemes (grant numbers 200/SP2H/PMDSU/DRPM/2020), under Hanifah Nuryani Lioe as principal investigator.
Authors and Affiliations
Department of Food Science and Technology, Faculty of Agricultural Engineering and Technology, IPB University, Bogor, 16680, Indonesia
VTH collected and analyzed the data as well as performed manuscript drafting. HNL devised the main conceptual idea, designed the research, and supervised this study as well as performed manuscript drafting and incorporating revisions. DRA and HDK supervised data analysis and data interpretation as well as performed manuscript drafting. All authors read and approved the final manuscript.
All authors declared that they have no competing interest arisen from this present study.
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