The Flavor of Soy Sauce

Considering that most soy sauces contain up to 18% NaCl, the most immediate and obvious flavor one detects is saltiness. However, the flavor of soy sauce is far more complex than simply saltiness. In fact, the shoyu products listed in Table 12-2 all contain between 16% and 19% salt, yet their flavor profiles, and the concentrations of volatile flavor compounds, can vary considerably. Nearly 200 volatile flavor components have been identified in shoyu, using GC or GC/MS analysis. Several of these, in particular, various furanones and phenolic compounds, are considered to be the most important, based on their relatively high concentrations and their characteristic soy sauce-like flavor (Table 12-4).

Furanones include 4-hydroxy-2-(or 5) ethyl-5 (or 2) methyl-3-(2H) furanone (HEMF) and 4-hydroxy-5-methyl-3-(2H) furanone (HMF), which confer sweet or roasted flavor notes, respectively. HEMF has been considered to be the main "character-impact" flavor of most

Box 12—1. Taking Salt out of Soy Sauce and Other Soy-fermented Foods

Salt is an essential ingredient in many of the Asian-type fermented foods. For example, shoyu and other soy sauces contain between 14% and 18% salt. Likewise, miso contains as much as 13% to 14% salt. Even when used in moderation, these high-salt foods may still account for a relatively high intake of salt.A single 1 tablespoon serving of soy sauce (about 17 g) contains about 950 mg of sodium or 2.4 grams of salt, and a cup of miso soup contains nearly 2 g of salt.

Not surprisingly, those countries that regularly consume these products have high salt consumption rates. In Japan, for example, per capita salt consumption has averaged (from 1975 to 1997) around 13 g per person per day, 30% more than the 10 g per day that is recommended as a maximum. In contrast, salt consumption in the United States is somewhat less, about 10 g per person per day, but still nearly twice that recommended by health authorities.

The problem with high-salt diets is that there is a strong association between sodium or salt consumption and various disease states, including hypertension, stroke, and gastric cancer.This has led public health agencies to recommend that salt intake be reduced, especially for at-risk individuals. Since soy sauce and related products account for as much as 20% of total salt intake in Japan and other Asian countries, decreasing their salt content could contribute to a significant overall reduction in salt consumption.

There are a number of possible ways to reduce the sodium content in soy sauce types of products.The most common means is to manufacture the product in the usual way (i.e.,adding the same amount of salt to the mash as would be done for a conventional fermentation), followed by removal of a portion of the salt via one of several separation technologies.The latter include ion exchange chromatography using de-salting resins, ion-exchange membrane processing, and electrolysis. Potassium chloride can then be added to satisfy many of the functions played by sodium chloride (e.g., preservation and flavor).

A second way to make low-salt soy products is to replace some of the sodium chloride with potassium chloride prior to the fermentation. The potassium salt will still influence the microflora, as does sodium chloride, by selecting for salt-tolerant lactic acid bacteria and yeast. The downside to both of these approaches, however, is that potassium imparts an undesirable bitter or metallic flavor that consumers generally dislike.Another approach is simply to reduce the salt content and add another antimicrobial agent, such as ethanol, to control the microflora (Chiou et al., 1999).

A quite different route to reduce the salt content has been to identify other compounds that contribute "saltiness" (without affecting the overall soy sauce flavor) and then to use those substances as salt mimics. Researchers in Japan recently discovered that several amino acid derivatives and peptides, such as ornithyltaurine, glycine ethyl ester hydrochloride, and lysine hydrochloride, were effective substitutes for potassium chloride (Kuramitsu, 2004).

In addition to affecting flavor, reducing the salt content of soy sauce and other fermented soy products may also have an effect on the microbiological stability of these products. However, pathogen challenge studies on low-salt miso revealed that even when salt levels were reduced to less than 3%, Staphylococcus aureus, Clostridium botulinum, Salmonella typhimurium, and Yersinia enterocolitica were unable to grow or survive, and toxin production was absent (Tanaka et al., 1985). Still, as an additional barrier, some manufacturers add ethanol to the product to control spoilage or pathogenic organisms.

References

Chiou, R.Y.-Y., S. Ferng, and L.R. Beuchat. 1999. Fermentation of low-salt miso as affected by supplementation with ethanol. Int. J. Food Microbiol. 48:11-20. Kuramitsu, R., 2004. Quality assessment of a low-salt soy sauce made of a salty peptide or its related com-

pounds.Adv. Exp.Med. Biol. 542:227-238. Tanaka, N., S.K. Kovats, J.A. Guggisberg, L.M. Meske, and M.P. Doyle. 1985. Evaluation of the bacteriological safety of low-salt miso. J. Food Prot. 48:435-437.

Figure 12-3. Chemical structure of HEMF. Adapted from Nunomura and Sasaki, 1992.

Japanese-type soy sauce (Figure 12-3). However, although it has been suggested that fura-nones are derived from fermentation, their presence and concentration in shoyu depends mostly on the composition of the raw material. For example, HEMF occurs in koikuchi, but not in tamari, indicating that wheat is necessary for its synthesis. Among the phenolic compounds that contribute to soy sauce or shoyu flavor are 4-ethylguaiacol (4-EG) and 4-ethylphenol (4-EP).They are also produced during fermentation by yeasts, but are derived from precursors formed by koji molds grown on wheat. They confer typical soy sauce-like flavors.

In addition to the furanone and phenols, a large number of acids, aldehydes, alcohols, and esters are produced during the shoyu fermen-tation.Although the concentrations of some of these compounds, i.e., ethanol and lactic and acetic acids, and 2,3-butanediol, can reach very high levels, most are in the order of parts per million. They are generally produced by lactic acid bacteria and yeasts. Other important flavors are generated, along with color, via the Maillard reaction. Although the formation of Maillard reaction products is accelerated by heating (i.e., pasteurization conditions), many end products are also formed during the mashing and moromi aging steps.

Another group of flavor constituents exists that is very important in soy sauce products, but whose flavor is not so easily de scribed. Included in this group are several nitrogenous compounds, especially the amino acid glutamic acid, and the 5' nucleotides in-osine 5'-monophosphate (IMP) and guano-sine 5'-monophosphate (GMP).The flavor or taste of these compounds (and their sodium salts) fall outside what are ordinarily considered to be the four basic flavors—sweet, sour, bitter, and salty. Rather, sodium glutamate, sodium inosinate, and sodium guany-late impart a totally unique flavor known as umami, a Japanese term meaning "delicious-ness." Umami has been described as conferring a meaty, savory, brothy flavor to foods. The glutamate salt monosodium glutamate is also considered to be a flavor enhancer. And although umami is usually associated with Asian cuisines, it is present in a wide array of foods, including cured ham, Parmesan cheese, and shitake mushrooms.

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